Cell
Cell
The cell is the smallest living component of organisms and is the basic unit of life. A cell contains the genetic material that supplies the coded instructions for the manufacture of a new cell, as well as the other materials necessary for the cell’s growth and survival.
In multicellular living things, a collection of cells that performs a similar function is called a tissue. Various tissues that perform coordinated functions form organs, and organs can work together to perform the general processes of body systems. As one example, the human digestive system is composed of various organs, including the stomach, pancreas, and the intestines. The tissue that lines the intestine is called epithelial tissue. Epithelial tissue, in turn, is composed of special cells called epithelial cells. In the small intestine, these epithelial cells are specialized for their absorptive function: each epithelial cell is covered with thousands of small projections called microvilli. The numerous microvilli greatly increase the surface area of the small intestine through which nutrients can be absorbed into the bloodstream.
Types of cells
Multicellular organisms contain an array of highly specialized cells. For example, plants contain root cells, leaf cells, and stem cells. In another example, humans have different cells that comprise the skin, nerves, and heart. Each kind of cell is structured to perform a highly specialized function. Examining a cell’s structure can reveal a great deal about its function in the organism. For instance, epithelial cells in the small intestine are specialized for absorption due to the numerous microvilli that crowd their surfaces. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long processes, called axons, that conduct nerve impulses. Dendrites are shorter processes that receive nerve impulses.
Sensory cells—the cells that detect sensory information from the outside environment and transmit this information to the brain—often have unusual shapes and structures that contribute to their function. The rod cells in the retina of the eye are structurally unique. Shaped like a rod, these cells have a light-sensitive region that contains numerous membranous disks. Within each disk is embedded a special light-sensitive pigment that captures light. When the pigment receives light from the outside environment, nerve cells in the eye are triggered to send a nerve impulse in the brain. In this way, humans are able to detect light.
Cells also exist as single-celled organisms. Protists are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas. Bacteria are also single-celled organisms.
Prokaryotes and eukaryotes
Prokaryotes (literally, “before the nucleus”) are cells that have no distinct nucleus. Instead, the genetic material is disperded in the interior material of the cell (the cytoplasm). Most prokaryotic organisms are single-celled, such as bacteria and algae. In contrast,
eukaryotic (literally, “true nucleus”) organisms have a distinct nucleus and a highly organized internal structure. Also present are membrane-bound structures that perform specific functions (an example is the lysosome; compartmentalized enzymes that digest compounds). Prokaryotes lack these membrane-bound regions; digestive enzymes and other functional molecules are dispersed in the cytoplams of the prokaryotic cell.
Cell size and numbers
An adult human body contains about 60 trillion (60 x 1012) cells. Most of these cells are so small that a microscope is necessary to see them. The small size of cells fulfills a distinct purpose in the functioning of the body. If cells were larger, many of the processes that cells perform could not occur efficiently. Such a large cell has a large volume, which is much larger than surface area. Since nutrients enter the cell via the surface, only a relatively small amount of nutrients could enter the cell. Put another way, a cell would likely starve, since the nutrient supply could not keep pace with the nutrient demand of the cell. In a small cell, the correspondingly smaller volume means that the available nutrient level is usually sufficient to support cell survivial and growth.
Another reason for the small size of cells is that control of cellular processes is easier in a small cell than in a large cell. Cells are dynamic, living things. Cells transport substances from one place to another, reproduce themselves, and produce various enzymes and chemicals for export to the extracellular environment. All of these activities are accomplished under the direction of the nucleus, the control center of the cell. If the nucleus had to control a large cell, then this direction might break down. Substances transported from one place to another would have to traverse great distances to reach their destinations; reproduction of a large cell would be an extremely complicated endeavor; and products for export would not be as efficiently produced. Smaller cells, because of their more manageable size, are much more efficiently controlled than larger cells.
Cell structure and function
Prokaryotic and eukaroytic cells share structural similarities. Both cell types have a plasma membrane through which substances pass into and out of the cell. With the exception of a few minor differences, plasma membranes are the same in prokaryotes and eukaryotes. The interior of both kinds of cells is called the cytoplasm. Finally, both types of cells contain small structures called ribosomes that function in protein synthesis. Composed of two protein subunits, ribosomes are not bounded by membranes; therefore, they are not considered organelles. In eukaryotes, ribosomes are either bound to an organelle, the endoplasmic reticulum, or exist as “free” ribosomes in the cytoplasm. Prokaryotes contain only free ribosomes.
An example of a typical prokaryote is the bacterial cell. Bacteria can be shaped like rods, spheres, or corkscrews. All prokaryotes are bounded by a plasma membrane and contain a rigid layer called the peptidoglycan. Some bacteria also contain another membrane, which sandwiches the peptidoglycan between the membranes. As well, some bacteria have a sugary outer coating called a capsule. Many bacteria that cause illness in animals have capsules. The capsule provides an extra layer of protection against host immune molecules like antibodies. Bacteria with capsules generally cause a more severe disease than those without capsules.
Attached to the cell wall of some bacteria are flagella, whip-like structures that provide for movement. Some bacteria also have pili, which are short, finger-like projections that assist the bacteria in attaching to tissues. Pili are also important disease determinants. Bacteria that cause pneumonia, for instance, attach to the tissues of the lung. Bacterial pili greatly facilitates this attachment to tissues, and thus, like capsules, bacteria with pili can be a greater disease problem than those without.
Eukaryote structure
The organelles found in eukaryotes include the membrane system, consisting of plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; cytoskeleton; and mitochondria. In addition, plant cells have special organelles not found in animals cells. These organelles are the chloroplasts, cell wall, and vacuoles.
The membrane system of a cell performs many important functions. This system controls the entrance and exit of substances into and out of the cell, and also provides for the manufacture and packaging of substances within the cell. The membrane system of the cell consists of the plasma membrane, which encloses the cell contents; the endoplasmic reticulum, which manufactures lipids and proteins; the Golgi body, which packages substances manufactured within the cell; and various vesicles, which perform different functions.
The plasma membrane of the cell is selectively permeable; that is, the membrane is designed so that only certain substances are allowed to cross unaided. Other compounds that cross require specialized transport proteins, or pass through specific channels. The plasma membrane is composed of two layers of molecules called phospholipids. Each phospholipid molecule consists of a phosphate “head” and two fatty acid chains that dangle from the head.
The orientation of these two sections of the phospholipid molecule is crucial to the function of the plasma membrane. The phosphate region is hydrophilic (literally, “water-loving”). The fatty acid region is hydrophobic (literally, “water-hating”) and repels water. In the phospholipid bilayer of the plasma membrane, the phospholipid layers are arranged so that the two phosphate hydrophilic regions face outward, towards the watery extracellular environment, and inward, towards the cellular cytoplasm, which also contains water. The two hydrophobic fatty acid portions of the chains face each other, forming a water-tight shield. The plasma membrane functions both as a boundary between the cell’s contents and the external cellular environment, yet also allows the transport of water-containing and other substances across its boundaries.
Embedded within the plasma membranes of eukaryotes are various proteins. These proteins serve several distinct functions in the cell. Some proteins are pumps or channels for the import and export of substances. Other proteins, called antigens, serve as identification markers for the cell. Still other proteins help the cell form attachments with other cells. Because these membrane proteins often protrude out of the cell membrane into the extracellular environment, they too have hydrophobic and hydrophilic regions. Portions of the proteins that are embedded within the plasma membrane are hydrophobic, and portions of the proteins that extend outward into the extracellular environment are hydrophilic.
Scientists studying plasma membranes use the term “fluid-mosaic model” to describe the structure of plasma membranes. The “mosaic” portion of the model describes the way proteins are embedded within the plasma membrane. The “fluid” part of the model explains the fluid nature of plasma membranes. Rather than being fixed in one place within the plasma membrane, experiments have shown that the phospholipids exhibit some movement within the plasma membranes, sometimes moving laterally, sometimes (although rarely), flip-flopping from one phospholipid layer to another. The membrane proteins also may drift within the plasma membrane, albeit more slowly than the phospholipids.
Endoplasmic reticulum
The endoplasmic reticulum (meaning “within the cytoplasm” and “net”) consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell’s cytoplasm. This internal system of membrane is continuous with the double membrane that surrounds the cell’s nucleus. Therefore, the encoded instructions that the nucleus sends out for the synthesis of proteins flow directly into the endoplasmic reticulum. Within the cell, the endoplasmic reticulum synthesizes lipids and proteins. The proteins that the endoplasmic reticulum synthesizes, such as enzymes, are exported from the cell to perform various functions in the body.
Two types of endoplasmic reticulum are found in the eukaryotic cell. Rough endoplasmic reticulum is studded with ribosomes on its outer face. These ribosomes are the sites of protein synthesis. Once a protein is synthesized on a ribosome, it is enclosed within a vesicle, a bubble-type structure. The vesicle travels to another organelle called the Golgi body (see below for a description). By fusing with the Golgi body membrane, the contents of the vesicle can be released into the Golgi body. Within the Golgi body, the proteins within the vesicle are further modified before they are exported from the cell. Cells that specialize in protein secretion contain large amounts of rough endoplasmic reticulum. For instance, cells of the pancreas that produce the protein insulin have abundant rough endoplasmic reticulum. Plasma cells, white blood cells that secrete immune proteins called antibodies, are so crowded with rough endoplasmic reticulum it is difficult to distinguish other organelles within the cytoplasm.
The other type of endoplasmic reticulum is smooth endoplasmic reticulum. Smooth endoplasmic reticulum does not have ribosomes and is the site of lipidmetabolism. Here, macromolecules containing lipids are broken down into their constituent parts. In addition, smooth endoplasmic reticulum functions in the synthesis of lipid-containing macromolecules. Smooth endoplasmic reticulum is not as common in cells as rough endoplasmic reticulum. Large amounts of smooth endoplasmic reticulum are found in cells that specialize in lipid metabolism. For instance, liver cells remove alcohol and drugs from the bloodstream. Liver cells have an impressive network of smooth endoplasmic reticulum. Similarly, cells of the ovaries and testes, which produce the lipid-containing hormones estrogen and testosterone, contain large amounts of smooth endoplasmic reticulum.
The Golgi body
Named for its discoverer, the nineteenth century Italian scientist Camillo Golgi, the Golgi body is one of the most unusually shaped organelles. Looking somewhat like a stack of pancakes, the Golgi body consists of stacked, membrane-bounded, flattened sacs. Surrounding the Golgi body are numerous, small, membrane-bounded vesicles. The Golgi body and its vesicles function in the sorting, modifying, and packaging of macromolecules that are secreted by the cell or used within the cell for various functions.
One portion of the Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles. A portion of the Golgi body located on the opposite side is the site from which modified and packaged macromolecules are transported to their destinations.
Within the Golgi body, as the macromolecules move from the receiving to the transporting faces of the Golgi body, various chemical groups are added to the macromolecules so ensure that they reach their proper destination. For example, cells called goblet cells in the lining of the intestine secrete mucus. The protein component of mucous, called mucin, is modified in the Golgi body by the addition of carbohydrate groups. From the Golgi body, the modified mucin is packaged within a vesicle (a small, membranous sphere). The vesicle containing its mucus cargo fuses with the plasma membrane of the goblet cell, and is released into the extracellular environment.
Vesicles
Vesicles are small, membrane-bounded spheres that contain various macromolecules. Lysosomes are vesicles that contain enzymes involved in cellular
KEY TERMS
Actin filament— A type of cytoskeletal filament that has can contract.
Amyloplast— A starch-storing portion of a plant cell.
Centriole— Paired structures consisting of microtubules; in animal cells, the centriole has a vital role in cell division.
Chloroplast— Green organelle in higher plants and algae in which photosynthesis occurs.
Cilia— Short tubular projections that cover the surface of some cells and stimaulate the movement of molecules over the surface.
Crista— pl., cristae; The folds of the inner membrane of a mitochondrion.
Cytoskeleton— A network of assorted protein filaments attached to the cell membrane and to various organelles that comprises the framework for cell shape and movement.
Deoxyribonucleic acid (DNA)— The genetic material of a cell that contains encoded instructions for the synthesis of proteins and other molecules.
Endoplasmic reticulum— The network of membranes that extends throughout the cell, and which is involved in protein synthesis and the use of lipids.
Eukaryotic cell— A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
Flagellum— Thread-like appendage of certain cells, such as sperm cells and some bacteria, which controls their locomotion.
Golgi body— The organelle that manufactures, sorts, and transports macromolecules within a cell.
Hydrophilic— “Water-loving;” describes the charged phosphate portion of a phospholipid.
Hydrophobic— “Water-hating;” describes the uncharged fatty acid portion of a phospholipid.
Lysosome— A vesicle that contains digestive enzymes.
Mitochondrion— The power-house of the cell; contains the enzymes necessary for the oxidation of food into energy.
Nuclear envelope— The double membrane that surrounds the nucleus.
Nuclear pore— Tiny openings in the nuclear envelope that permit movement of molecules into and out of the nucleus.
Nucleolus— The darker region within the nucleolus where ribosomal subunits are manufactured.
Peroxisome— A vesicle that oxidizes fats and other substances and stores hydrogen peroxide.
Phospholipid bilayer— The double layer of phospholipids that compose the plasma membrane.
Photosynthesis— The process in which carbon dioxide and water are converted to sugars utilizing the energy of sunlight.
Pili— Short projections that assist bacteria in attaching to tissues.
Prokaryote— A cell without a true nucleus.
Ribonucleic acid— RNA; the molecule translated from DNA in the nucleus that directs protein synthesis in the cytoplasm. It is also the genetic material of many viruses.
Ribosome— A protein composed of two subunits that functions in protein synthesis.
Stroma— The material that bathes the interior of chloroplasts in plant cells. Tubulin—A protein that comprises microtubules.
Vacuole— Membrane-enclosed structure within cells which store pigments, water, nutrients, and wastes.
Vesicle— A membrane-bound sphere that contains a variety of substances in cells.
digestion and the digestion of cellular components that are chemically targeted for destruction. In a process called phagocytosis, protists surrounds a food particle and engulfs it within a vesicle. This food containing vesicle is transported within the protist’s cytoplasm until it is contiguous with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break the food down into smaller parts for use by the protist.
Peroxisomes contain hydrogen peroxide. Peroxisomes function in the oxidation of many materials, including fats. In oxidation, oxygen is added to a molecule. When oxygen is added to fats, hydrogen peroxide is formed. The chemical is lethal to cells.
Therefore, the oxidation of fats takes place within the membranes of peroxisomes so that the harmful chemical does not leak out into the cell’s cytoplasm.
Mitochondria
The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane; the inner membrane of a mitochondrion is folded into compartments called cristae (meaning “box”). The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy.
Cells contain hundreds to thousands of mitochondria. An interesting aspect of mitochondria is that they contain their own DNA sequences, although not in the profusion that the nucleus contains. The presence of this separate DNA, along with the resemblance of mitochondria to single-celled prokaryotes, has led to a theory that postulates that mitochondria were once free-living prokaryotes that became engulfed within other prokaryotes. Instead of being digested, the mitochondrial prokaryotes remained within the engulfing cell and performed its energy-releasing functions. Over millions of years, this symbiotic relationship fostered the evolution of the eukaryotic cell.
Plant organelles
Plant cells have several organelles not found in animal cells. Plastids are vesicle-type organelles that perform a variety of functions in plants. Amylopasts store starch, and chromoplasts store pigment molecules that give some plants their vibrant orange and yellow colors. Chloroplasts are plastids that carry out photosynthesis, a process in which water and carbon dioxide are transformed into sugars. The interior of chloroplasts contains an elaborate membrane system. Thylakoids bisect the chloroplasts, and attached to these platforms are stacks of membranous sacs called grana. Each granum contains the enzymes necessary for photosynthesis. The membrane system within the chloroplasts is bathed in a fluid called stroma, which also contains enzymes.
Like mitochondria, chloroplasts resemble some ancient single-celled prokaryotes and also contain their own DNA sequences. Their origin within eukaryotes is thought to have arisen from the endosymbiotic relationship between a photosynthetic single-celled prokaryote that was engulfed and remained within another prokaryotic cell.
Vacuoles
Plant vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90% of the cellular space. They perform a variety of functions in the cell, including storage of organic compounds, waste products, pigments, and poisonous compounds, as well as digestive functions.
Resources
BOOKS
Alberts, Bruce, Alexander Johnson, Julian Lewis, Martin Raff, Dennis Bray, Karen Hopkin, Keith Roberts, and Peter Walter. Essential Cell Biology, Second Edition. New York: Garland Science/Taylor & Francis Group,.
Dyer, Betsey D. A Field Guide to Bacteria. Cornell: Cornell University Press, 2003.
Gilbert, Scott F. Developmental Biology. Sunderland, MA: Sinauer Associates, 2006.
Lodish, Harvey F. Molecular Cell Biology. New York:W.H. Freeman & Company, 2003.
Padilla, Michael J., Ioannis Miaoulis, and Martha Cyr.Scienc Explorer: From Bacteria To Plants. New York: Pearson Prentice Hall, 2004.
Kathleen Scogna
Cell
Cell
The cell is the smallest living component of organisms and is the basic unit of life. In multicellular living things, a collection of cells that work together to perform similar functions is called a tissue ; various tissues that perform coordinated functions form organs; and organs that work together to perform general processes form body systems. The human digestive system , for example, is composed of various organs including the stomach, pancreas, and the intestines. The tissue that lines the intestine is called epithelial tissue. Epithelial tissue, in turn, is composed of special cells called epithelial cells. In the small intestine, these epithelial cells are specialized for their absorptive function: each epithelial cell is covered with thousands of small projections called microvilli. The numerous microvilli greatly increase the surface area of the small intestine through which nutrients can be absorbed into the bloodstream.
Types of cells
Multicellular organisms contain a vast array of highly specialized cells. Plants contain root cells, leaf cells, and stem cells . Humans have skin cells, nerve cells, and sex cells. Each kind of cell is structured to perform a highly specialized function. Often, examining a cell's structure reveals much about its function in the organism . For instance, as we have already seen, epithelial cells in the small intestine are specialized for absorption due to the numerous microvilli that crowd their surfaces. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long processes, called axons, that conduct nerve impulses. Dendrites are shorter processes that receive nerve impulses.
Sensory cells—the cells that detect sensory information from the outside environment and transmit this information to the brain—often have unusual shapes and structures that contribute to their function. The rod cells in the retina of the eye , for instance, look like no other cell in the human body. Shaped like a rod, these cells have a light-sensitive region that contains numerous membranous disks. Within each disk is embedded a special light-sensitive pigment that captures light . When the pigment receives light from the outside environment, nerve cells in the eye are triggered to send a nerve impulse in the brain . In this way, humans are able to detect light.
Cells, however, can also exist as single-celled organisms. The organisms called protists, for instance, are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas.
Prokaryotes and eukaryotes
Two types of cells are recognized in living things. Prokaryotes (literally, "before the nucleus") are cells that have no distinct nucleus. Most prokaryotic organisms are single-celled, such as bacteria and algae . Eukaryotic (literally, "true nucleus") organisms, on the other hand, have a distinct nucleus and a highly organized internal structure. Distinct organelles, the small structures that each perform a specific set of functions, are present within eukaryotes. These organelles are bound by membranes. Prokaryotes, in addition to their lack of a nucleus, also lack these membrane-bound organelles.
Cell size and numbers
It is estimated that an adult human body contains about 60 trillion cells. Most of these cells, with some exceptions, are so small that a microscope is necessary to see them. The small size of cells fulfills a distinct purpose in the functioning of the body. If cells were larger, many of the processes that cells perform could not occur efficiently. To visualize this concept, think about the intestinal epithelial cells discussed earlier. What if the intestinal epithelium were composed of one, large cell instead of thousands of small cells? A large cell has a large volume , or contents. The surface area, or membrane , of this large cell is the site through which nutrients enter the small intestine for delivery to the bloodstream. Because the volume of this large cell is so large, the surface area, by comparison, is relatively small. Large cells, therefore, have a small surface area to volume ratio . Only so many nutrients can pass through the limited membrane area of this large cell. With a small surface area to volume ratio, the amount of substances passing into and out of the cell is severely restricted.
However, if the intestinal epithelium is divided into thousands of smaller cells, the volume stays the same, but the surface area—the number of cell membranes—greatly increases. Many more nutrients can pass through the intestinal epithelium cells. Small cells, therefore, have a large surface area to volume ratio. The large surface area to volume ratio of small cells makes the transport of substances into and out of cells extremely efficient.
Another reason for the small size of cells is that control of cellular processes is easier in a small cell than in a large cell. Cells are dynamic, living things. Cells transport substances from one place to another, reproduce themselves, and produce various enzymes and chemicals for export to the extracellular environment. All of these activities are accomplished under the direction of the nucleus, the control center of the cell. If the nucleus had to control a large cell, then this direction might break down. Substances transported from one place to another would have to traverse great distances to reach their destinations; reproduction of a large cell would be an extremely complicated endeavor; and products for export would not be as efficiently produced. Smaller cells, because of their more manageable size, are much more efficiently controlled than larger cells.
The structure and function of cells
The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have a plasma membrane through which substances pass into and out of the cell. With the exception of a few minor differences, plasma membranes are the same in prokaryotes and eukaryotes. The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles; the cytoplasm of prokaryotes contains no organelles. Finally, both types of cells contain small structures called ribosomes that function in protein synthesis. Composed of two protein subunits, ribosomes are not bounded by membranes; therefore, they are not considered organelles. In eukaryotes, ribosomes are either bound to an organelle, the endoplasmic reticulum, or exist as "free" ribosomes in the cytoplasm. Prokaryotes contain only free ribosomes.
The structure of prokaryotes
An example of a typical prokaryote is the bacterial cell. Bacterial cells can be shaped like rods, spheres, or corkscrews. All prokaryotes are bounded by a plasma membrane. Overlying this plasma membrane is a cell wall, and in some bacteria, a capsule consisting of a jelly-like material overlies the cell wall. Many bacteria that cause illness in animals have capsules. The capsule provides an extra layer of protection for the bacteria, and often pathogenic bacteria with capsules cause much more severe disease than those without capsules.
Within the cytoplasm of prokaryotes is a nucleoid, a region where the genetic material (DNA) resides. This nucleoid is not a true nucleus because it is not bounded by a membrane. Also within the cytoplasm are numerous ribosomes. These ribosomes are not attached to any structure and are thus called "free" ribosomes.
Attached to the cell wall of some bacteria are flagella , whip-like structures that provide for movement. Some bacteria also have pili, which are short, finger-like projections that assist the bacteria in attaching to tissues. Bacteria cannot cause disease if they cannot attach to tissues. Bacteria that cause pneumonia , for instance, attach to the tissues of the lung. Bacterial pili greatly facilitates this attachment to tissues, and thus, like capsules, bacteria with pili are often more virulent than those without.
The structure of eukaryotes
The organelles found in eukaryotes include the membrane system consisting of plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; cytoskeleton; and mitochondria. In addition, plant cells have special organelles not found in animals cells. These organelles are the chloroplasts, cell wall, and vacuoles.
The membrane system
The membrane system of a cell performs many important functions. This system controls the entrance and exit of substances into and out of the cell, and also provides for the manufacture and packaging of substances within the cell. The membrane system of the cell consists of the plasma membrane, which encloses the cell contents; the endoplasmic reticulum, which manufactures lipids and proteins ; the Golgi body, which packages substances manufactured within the cell; and various vesicles, which perform different functions.
The plasma membrane
The plasma membrane of the cell is often described as "selectively permeable;" that is, the plasma membrane is designed so that only certain substances are allowed to traverse its borders. The plasma membrane is composed of two layers of molecules called phospholipids. Each phospholipid molecule consists of a phosphate "head" and two fatty acid chains that dangle from the head.
The orientation of these two sections of the phospholipid molecule is crucial to the function of the plasma membrane. The phosphate region is hydrophilic (literally, "water-loving") and attracts water . The fatty acid region is hydrophobic (literally, "water-hating") and repels water. In the phospholipid bilayer of the plasma membrane, the phospholipid layers are arranged so that the two phosphate hydrophilic regions face outward, towards the watery extracellular environment, and inward, towards the cellular cytoplasm, which also contains water. The two hydrophobic fatty acid portions of the chains face each other, forming a water-tight shield. The plasma membrane, then, is both water-proof and water-attracting. It functions both as a boundary between the cell's contents and the external cellular environment, yet also allows the transport of water-containing and other substances across its boundaries.
Embedded within the plasma membranes of eukaryotes are various proteins. These proteins serve several distinct functions in the cell. Some proteins are pumps or channels for the import and export of substances. Other proteins, called antigens, serve as identification markers for the cell. Still other proteins help the cell form attachments with other cells. Because these membrane proteins often protrude out of the cell membrane into the extracellular environment, they too have hydrophobic and hydrophilic regions. Portions of the proteins that are embedded within the plasma membrane are hydrophobic, and portions of the proteins that extend outward into the extracellular environment are hydrophilic.
Scientists studying plasma membranes use the term "fluid-mosaic model" to describe the structure of plasma membranes. The "mosaic" portion of the model describes the way proteins are embedded within the plasma membrane. The "fluid" part of the model explains the fluid nature of plasma membranes. Rather than being fixed in one place within the plasma membrane, experiments have shown that the phospholipids exhibit some movement within the plasma membranes, sometimes moving laterally, sometimes (although rarely), flip-flop-ping from one phospholipid layer to another. The membrane proteins also move within the plasma membrane, albeit more slowly than the phospholipids.
Endoplasmic reticulum
The endoplasmic reticulum (meaning "within the cytoplasm" and "net") consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell's cytoplasm. This internal system of membrane is continuous with the double membrane that surrounds the cell's nucleus. Therefore, the encoded instructions that the nucleus sends out for the synthesis of proteins flow directly into the endoplasmic reticulum. Within the cell, the endoplasmic reticulum synthesizes lipids and proteins. The proteins that the endoplasmic reticulum synthesizes, such as enzymes, are exported from the cell to perform various functions in the body. Proteins that are made in the cell for use by the cell-for instance, as channels in the plasma membrane-are made by the free ribosomes that dot the cytoplasm.
Two types of endoplasmic reticulum are found in the eukaryotic cell. Rough endoplasmic reticulum is studded with ribosomes on its outer face. These ribosomes are the sites of protein synthesis. Once a protein is synthesized on a ribosome, it is enclosed within a vesicle, a small, membrane-bound "bubble." The vesicle travels to another organelle, the Golgi body. Within the Golgi body, the proteins within the vesicle are further modified before they are exported from the cell. Cells that specialize in protein secretion contain large amounts of rough endoplasmic reticulum. For instance, cells of the pancreas that produce the protein insulin , have abundant rough endoplasmic reticulum. Plasma cells, white blood cells that secrete immune proteins called antibodies, are so crowded with rough endoplasmic reticulum it is difficult to distinguish other organelles within the cytoplasm.
The other type of endoplasmic reticulum is smooth endoplasmic reticulum. Smooth endoplasmic reticulum does not have ribosomes and is the site of lipid metabolism . Here, macromolecules containing lipids are broken down into their constituent parts. In addition, smooth endoplasmic reticulum functions in the synthesis of lipid-containing macromolecules. Smooth endoplasmic reticulum is not as common in cells as rough endoplasmic reticulum. Large amounts of smooth endoplasmic reticulum are found in cells that specialize in lipid metabolism. For instance, liver cells remove alcohol and drugs from the bloodstream. Liver cells have an impressive network of smooth endoplasmic reticulum. Similarly, cells of the ovaries and testes, which produce the lipid-containing hormones estrogen and testosterone, contain large amounts of smooth endoplasmic reticulum.
The Golgi body
Named for its discoverer, nineteenth century Italian scientist Camillo Golgi, the Golgi body is one of the most unusually shaped organelles. Looking somewhat like a stack of pancakes, the Golgi body consists of stacked, membrane-bounded, flattened sacs. Surrounding the Golgi body are numerous, small, membrane-bounded vesicles. The Golgi body and its vesicles function in the sorting, modifying, and packaging of macro-molecules that are secreted by the cell or used within the cell for various functions.
The Golgi body can be compared to the shipping and receiving department of a large company. Each Golgi body within a cell has a cis face, which is analogous to the receiving division of the department. Here, the Golgi body receives macromolecules synthesized in the endoplasmic reticulum encased within vesicles. The trans face of the Golgi body is analogous to the shipping division of the department, and is the site from which modified and packaged macromolecules are transported to their destinations.
Within the Golgi body, various chemical groups are added to the macromolecules so ensure that they reach their proper destination. In this way, the Golgi body attaches an "address" to each macromolecule it receives. For example, cells called goblet cells in the lining of the intestine secrete mucous. The protein component of mucous, called mucin, is modified in the Golgi body by the addition of carbohydrate groups. From the Golgi body, the modified mucin is packaged within a vesicle. The vesicle containing its mucous cargo fuses with the plasma membrane of the goblet cell, and is released into the extracellular environment.
Vesicles
Vesicles are small, membrane-bounded spheres that contain various macromolecules. Some vesicles, as we have seen, are used to transport macromolecules from the endoplasmic reticulum to the Golgi body, and from the Golgi body to various destinations. Special kinds of vesicles perform other functions as well. Lysosomes are vesicles that contain enzymes involved in cellular digestion. Some protists, for instance, engulf other cells for food. In a process called phagocytosis, the protist surrounds a food particle and engulfs it within a vesicle. This food containing vesicle is transported within the protist's cytoplasm until it is contiguous with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break the food down into smaller parts for use by the protist.
Lysosomes, however, are found in all kinds of cells. In all cells, lysosomes digest old, worn-out organelles. They also play a role in the self-destruction of old cells. Although scientists do not understand the trigger mechanism of this self-destruction, cells that are not functioning properly due to old-age apparently self-digest by means of lysosomes. Cell death is also a component of normal developmental processes . For instance, a human fetus has web-like hands and feet. As development progresses, the cells that compose these webs slowly self-destruct, freeing the fingers.
Peroxisomes, as their name implies, contain hydrogen peroxide . Peroxisomes function in the oxidation of many materials, including fats. In oxidation, oxygen is added to a molecule. When oxygen is added to fats, hydrogen peroxide is formed. As anyone who has treated a cut with hydrogen peroxide knows, this substance is lethal to cells. Therefore, the oxidation of fats takes place within the membranes of peroxisomes so that the harmful chemical does not leak out into the cell's cytoplasm.
The nucleus
The nucleus is the control center of the cell. Under a microscope, the nucleus looks like a dark blob, with a darker region, called the nucleolus, centered within it. The nucleolus is the site where the subunits of ribosomes are manufactured. Surrounding the nucleus is a double membrane called the nuclear envelope. The nuclear envelope is studded all over with tiny openings called nuclear pores.
The nucleus directs all cellular activities by controlling the synthesis of proteins. The nucleus contains encoded instructions for the synthesis of proteins in a helical molecule called deoxyribonucleic acid (DNA) . The cell's DNA is packaged within the nucleus in a structural form called chromatin . Chromatin consists of DNA wound tightly around spherical proteins called histones. When the cell prepares to divide, the DNA unwinds from the histones and assumes the shape of chromosomes, the X-shaped structures visible within the nucleus prior to cell division . Chromatin packaging of DNA allows all of the cell's DNA to fit into the combined space of the nucleus. If DNA was not packaged into chromatin, it would spill out over a space about 100 times as large as the cell itself.
The first step in protein synthesis begins in the nucleus. Within the nucleus, DNA is translated into a molecule called messenger ribonucleic acid (mRNA). mRNA then leaves the nucleus through the nuclear pores. Once in the cytoplasm, mRNA attaches to ribosomes (either bound to endoplasmic reticulum or free in the cytoplasm) and initiates protein synthesis. Proteins made for export from the cell function as enzymes that participate in all the body's chemical reactions . Because enzymes are essential for all the body's chemical processes-from cellular respiration to digestion-direction of the synthesis of these enzymes in essence controls all the activities of the body. Therefore, the nucleus, which contains the instructions for the synthesis of these proteins, directs all cellular activities and thus all body processes.
The cytoskeleton
The cytoskeleton is the "skeletal" framework of the cell. Instead of bone, however, the cell's skeleton consists of three kinds of protein filaments that form networks. These networks give the cell shape and provide for cellular movement. The three types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.
Microtubules are 25 nanometers in diameter and consist of protein subunits called tubulin. Each micro-tubule is composed of eleven pairs of these tubulin subunits arranged in a ring. In animal cells, microtubules arise from a region of the cell called the microtubule organizing center (MTOC) located near the nucleus. From this center, microtubules fan out across the cell, forming a network of "tracks" over which various organelles move within the cell. Microtubules also form small, paired structures called centrioles within animal cells. These structures are not considered organelles because they are not bounded by membranes. Scientists once thought that centrioles formed the microtubules that pull the cell apart during cell division; now it is known that each centriole with the pair move apart during cell division and indicate the plan along which the cell divides.
Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Flagella and cilia both have the same structure: a ring of nine tubulin triplets arranged around two tubulin subunits. The difference between flagella and cilia lies in their movement and numbers. Flagella are attached to the cell by a "crank"-like apparatus that allows the flagella to rotate. Usually, a flagellated cell has only one or two flagella. Cilia, on the other hand, are not attached with a "crank," and beat back and forth to provide movement. Ciliated cells usually have hundreds of these projections that cover their surfaces. For example, the protist Paramecium moves by means of a single flagellum, while the protist Didinium is covered with numerous cilia. Ciliated cells also perform important functions in the human body. The airways of humans and other animals are lined with ciliated cells that sweep debris and bacteria upwards, out of the lungs and into the throat. There, the debris is either coughed from the throat or swallowed into the digestive tract, where digestive enzymes destroy harmful bacteria.
Actin filaments are 8 nanometers in diameter and consist of two strands of the protein actin that are wound around each other. Actin filaments are especially prominent in muscle cells, where they provide for the contraction of muscle tissue.
Intermediate filaments are 10 nanometers in diameter and are composed of fibrous proteins. Because of their relative strength, they function mainly to anchor organelles in place within the cytoplasm.
Mitochondria
The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane; the inner membrane of a mitochondrion is folded into compartments called cristae (meaning "box"). The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy .
Cells contain hundreds to thousands of mitochondria. An interesting aspect of mitochondria is that they contain their own DNA sequences, although not in the profusion that the nucleus contains. The presence of this separate DNA, along with the resemblance of mitochondria to single-celled prokaryotes, has led to a theory of eukaryotic evolution called the endosymbiotic theory. This theory postulates that mitochondria were once separate prokaryotes that became engulfed within other prokaryotes. Instead of being digested, the mitochondrial prokaryotes remained within the engulfing cell and performed its energy-releasing functions. Over millions of years, this symbiotic relationship fostered the evolution of the eukaryotic cell.
Plant organelles
Plant cells have several organelles not found in animal cells. These are plastids, vacuoles, and a cell wall.
Plastids
Plastids are vesicle-type organelles that perform a variety of functions in plants. Amylopasts store starch, and chromoplasts store pigment molecules that give some plants their vibrant orange and yellow colors.
Chloroplasts are plastids that carry out photosynthesis , a process in which water and carbon dioxide are transformed into sugars. The interior of chloroplasts contains an elaborate membrane system. Thylakoids bisect the chloroplasts, and attached to these platforms are stacks of membranous sacs called grana. Each granum contains the enzymes necessary for photosynthesis. The membrane system within the chloroplasts is bathed in a fluid called stroma, which also contains enzymes.
Like mitochondria, chloroplasts resemble some ancient single-celled prokaryotes and also contain their own DNA sequences. Their origin within eukaryotes is thought to have arisen from the endosymbiotic relationship between a photosynthetic single-celled prokaryote that was engulfed and remained within another prokaryotic cell.
Vacuoles
Plant vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90% of the cellular space. They perform a variety of functions in the cell, including storage of organic compounds, waste products, pigments, and poisonous compounds, as well as digestive functions.
Cell wall
All plant cells have a cell wall that overlies the plasma membrane. The cell wall of plants consists of a tough carbohydrate substance called cellulose laid down in a matrix or network of other carbohydrates. The cell wall provides an additional layer of protection between the contents of the cell and the outside environment. The crunchiness of an apple, for instance, is attributed to the presence of these cell walls.
See also Cellular respiration; Chloroplast; Chromosome; Enzyme; Eukaryotae; Flagella; Gene; Meiosis; Mitosis; Neuron; Nucleus, cellular; Organ; Ribonucleic acid (RNA); Tissue.
Resources
books
Barritt, Greg J. Communication within Animal Cells. Oxford: Oxford University Press, 1992.
Bittar, F. Edward, ed. Chemistry of the Living Cell. Greenwich, CT: JAI Press, 1992.
Bray, Dennis. Cell Movements. New York: Garland Press, 1992.
Carroll, Mark. Organelles. New York: Guilford Press, 1989. The Cell Surface. Plainview, NY: Cold Spring Harbor Laboratory Press, 1992.
periodicals
Maddox, John. "Why Microtubules Grow and Shrink." Nature 362 (March 18, 1993): 201.
Pante, Nelly, and Ueli Aebi. "The Nuclear Pore Complex." TheJournal of Cell Biology 122 (September 1993): 5-6.
Scott, J. D. and T. Pawson. "Cell Communication: The Inside Story." Scientific American 282 (June 2000): 54-61.
Shay, Jerry W., and Woodring E. Wright. "Hayflick, His Limit, and Cellular Aging." Nature Reviews/Molecular Cell Biology (October 1, 2000): 72-76.
Kathleen Scogna
KEY TERMS
- Actin filament
—A type of cytoskeletal filament that has contractile properties.
- Amyloplast
—A plant cell plastid that stores starch.
- Centriole
—Paired structures consisting of micro-tubules; in animal cells, directs the plane of cell division.
- Chloroplast
—Green organelle in higher plants and algae in which photosynthesis occurs.
- Chromoplast
—A plant cell plastid that contains yellow and orange pigments.
- Cilia
—Short projections consisting of microtubules that cover the surface of some cells and provide for movement.
- Cisface
—The side (or "face") of the Golgi body that receives vesicles containing macromolecules.
- Crista
—pl., cristae, the folds of the inner membrane of a mitochondrion.
- Cytoplasm
—All the protoplasm in a living cell that is located outside of the nucleus, as distinguished from nucleoplasm, which is the protoplasm in the nucleus.
- Cytoskeleton
—A network of assorted protein filaments attached to the cell membrane and to various organelles that makes up the framework for cell shape and movement.
- Deoxyribonucleic acid (DNA)
—The genetic material of a cell that contains encoded instructions for the synthesis of proteins
- Endoplasmic reticulum
—The network of membranes that extends throughout the cell; involved in protein synthesis and lipid metabolism.
- Endosymbiotic theory
—A theory that proposes that mitochondria, chloroplasts, and other eukaryotic organelles originally arose within cells by symbiosis between a single-celled prokaryote and another prokaryote.
- Eukaryotic cell
—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.
- Flagellum
—Thread-like appendage of certain cells, such as sperm cells, which controls their locomotion.
- Fluid-mosaic model
—The model that describes the nature of the plasma membrane; the "mosaic" portion describes the proteins embedded within the plasma membrane, and the "fluid" portion describes the fluidity of the plasma membrane.
- Golgi body
—The organelle that manufactures, sorts, and transports macromolecules within a cell.
- Granum
—Sacs within a chloroplast that contain photosynthetic enzymes.
- Hydrophilic
—"Water-loving;" describes the phosphate portion of a phospholipid.
- Hydrophobic
—"Water-hating;" describes the fatty acid portion of a phospholipid.
- Intermediate filament
—A type of cytoskeletal filament that anchors organelles.
- Lysosome
—A vesicle that contains digestive enzymes.
- Matrix
—The inner space of a mitochondrion formed by cristae.
- Microtubule
—A type of cytoskeletal filament; the component of centrioles, flagella, and cilia.
- Mitochondrion
—The power-house of the cell; contains the enzymes necessary for the oxidation of food into energy.
- Nuclear envelope
—The double membrane that surrounds the nucleus.
- Nuclear pore
—Tiny openings that stud the nuclear envelope.
- Nucleoid
—The region in a prokaryote where the cell's DNA is located.
- Nucleolus
—The darker region within the nucleolus where ribosomal subunits are manufactured.
- Nucleus
—The control center of a cell; contains the DNA.
- Organelle
—A membrane-bounded cellular "organ" that performs a specific set of functions within a eukaryotic cell.
- Peroxisome
—A vesicle that oxidizes fats and other substances and stores hydrogen peroxide.
- Phospholipid
—A molecule consisting of a phosphate head and two fatty acid chains that dangle from the head; the component of the plasma membrane.
- Phospholipid bilayer
—The double layer of phospholipids that compose the plasma membrane.
- Photosynthesis
—In plants, the process in which carbon dioxide and water are converted to sugars.
- Pili
—Short projections that assist bacteria in attaching to tissues.
- Plasma membrane
—The membrane of a cell.
- Plastid
—A vesicle-like organelle found in plant cells.
- Prokaryote
—A cell without a true nucleus.
- Protist
—A single-celled eukaryotic organism.
- Ribonucleic acid
—RNA; the molecule translated from DNA in the nucleus that directs protein synthesis in the cytoplasm; it is also the genetic material of many viruses.
- Ribosome
—A protein composed of two subunits that functions in protein synthesis.
- Stroma
—The material that bathes the interior of chloroplasts in plant cells.
- Surface area to volume ratio
—The relationship between the surface area provided by the plasma membrane to the volume of the contents of a cell.
- Thylakoid
—A membranous structure that bisects the interior of a chloroplast.
- Transface
—The side (or "face") of a Golgi body that releases macromolecule-filled vesicles for transport.
- Tubulin
—A protein that comprises microtubules.
- Vacuole
—Membrane-enclosed structure within cells which store pigments, water, nutrients, and wastes.
- Vesicle
—A membrane-bound sphere that contains a variety of substances in cells.
Cell
Cell
The cell is the basic unit of a living organism. In multicellular organisms (organisms with more than one cell), a collection of cells that work together to perform similar functions is called a tissue. In the next higher level of organization, various tissues that perform coordinated functions form organs. Finally, organs that work together to perform general processes form body systems.
Types of cells
Multicellular organisms contain a vast array of highly specialized cells. Plants contain root cells, leaf cells, and stem cells. Humans have skin cells, nerve cells, and sex cells. Each kind of cell is structured to perform a highly specialized function. Often, examining a cell's structure reveals much about its function in the organism. For instance, certain cells in the small intestine have developed microvilli (hairs) that promote the absorption of foods. Nerve cells, or neurons, are another kind of specialized cell whose form reflects function. Nerve cells consist of a cell body and long attachments, called axons, that conduct nerve impulses. Dendrites are shorter attachments that receive nerve impulses.
Sensory cells are cells that detect information from the outside environment and transmit that information to the brain. Sensory cells often have unusual shapes and structures that contribute to their function. The rod cells in the retina of the eye, for instance, look like no other cell in the human body. Shaped like a rod, these cells have a light-sensitive region that contains numerous disks. Within each disk is embedded a special light-sensitive pigment that captures light. When the pigment receives light from the outside environment, nerve cells in the eye are triggered to send a nerve impulse in the brain. In this way, humans are able to detect light.
Cells, however, can also exist as single-celled organisms. The organisms called protists, for instance, are single-celled organisms. Examples of protists include the microscopic organism called Paramecium and the single-celled alga called Chlamydomonas.
Prokaryotes and eukaryotes. Two types of cells are recognized in living things: prokaryotes and eukaryotes. The word prokaryote literally means "before the nucleus." As the name suggests, prokaryotes are cells that have no distinct nucleus. Most prokaryotic organisms are single-celled, such as bacteria and algae.
The term eukaryote means "true nucleus." Eukaryotes have a distinct nucleus and distinct organelles. An organelle is a small structure that performs a specific set of functions within the eukaryotic cell. These organelles are held together by membranes. In addition to their lack of a nucleus, prokaryotes also lack these distinct organelles.
The structure and function of cells
The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have an outer covering called a plasma membrane. The plasma membrane holds the cell together and permits the passage of substances into and out of the cell. With a few minor exceptions, plasma membranes are the same in prokaryotes and eukaryotes.
The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles. As noted above, the cytoplasm of prokaryotes contains no organelles. Finally, both types of cells contain small structures called ribosomes. Ribosomes are the sites within cells where proteins are produced. (Proteins are large molecules that are essential to the structure and functioning of all living cells.) Ribosomes are not bounded by membranes and are not considered, therefore, to be organelles.
Words to Know
Cell wall: A tough outer covering that overlies the plasma membrane of bacteria and plant cells.
Cilia: Short projections that cover the surface of some cells and provide for movement.
Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.
Cytoskeleton: The network of filaments that provide structure and movement of a cell.
DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.
Endoplasmic reticulum: The network of membranes that extends throughout the cell and is involved in protein synthesis and lipid metabolism.
Enzyme: Any of numerous complex proteins that are produced by living cells and spark specific biochemical reactions.
Eukaryote: A cell that contains a distinct nucleus and organelles.
Flagellum: A whiplike structure that provides for movement in some cells.
Golgi body: Organelle that sorts, modifies, and packages molecules.
Membrane: A thin, flexible layer of plant or animal tissue that covers, lines, separates or holds together, or connects parts of an organism.
Mitochondrion: The power-house of the cell that contains the enzymes necessary for turning food into energy.
Nuclear envelope: The double membrane that surrounds the nucleus.
Nuclear pore: Tiny openings that stud the nuclear envelope.
Nucleolus: The darker region within the nucleolus where ribosomal subunits are manufactured.
Nucleus: The control center of a cell that contains the DNA.
Organelle: A membrane-bounded cellular "organ" that performs a specific set of functions within a eukaryotic cell.
Pili: Short projections that assist bacteria in attaching to tissues.
Plasma membrane: The membrane of a cell.
Plastid: A vesicle-like organelle found in plant cells.
Prokaryote: A cell without a true nucleus.
Protein: Large molecules that are essential to the structure and functioning of all living cells.
Protist: A single-celled eukaryotic organism.
Ribosome: A protein composed of two subunits that functions in protein synthesis.
Vacuole: A space-filling organelle of plant cells.
Vesicle: A membrane-bound sphere that contains a variety of substances in cells.
The structure of prokaryotes. An example of a typical prokaryote is the bacterial cell. Bacterial cells can be shaped like rods, spheres, or corkscrews. Like all cells, prokaryotes are bounded by a plasma membrane. Surrounding this plasma membrane is a cell wall. In addition, in some bacteria, a jelly-like material known as a capsule coats the cell wall. Many disease-causing bacteria have capsules. The capsule provides an extra layer of protection for the bacteria. Pathogenic bacteria with capsules tend to cause much more severe disease than those without capsules.
Within the cytoplasm of prokaryotes is a nucleoid, a region where the cell's genetic material is stored. (Genes determine the characteristics passed on from one generation to the next.) The nucleoid is not a true nucleus because it is not surrounded by a membrane. Also within the cytoplasm are numerous ribosomes.
Attached to the cell wall of some bacteria are flagella, whiplike structures that make it possible for the bacteria to move. Some bacteria also have pili, short, fingerlike projections that help the bacteria to attach to tissues. Bacteria cannot cause disease if they cannot attach to tissues. Bacteria that cause pneumonia, for instance, attach to the tissues of the lung. Bacterial pili greatly facilitate this attachment to tissues. Thus, bacteria with pili, like those with capsules, are often more deadly than those without.
The structure of eukaryotes. The organelles found in eukaryotes include the membrane system, consisting of the plasma membrane, endoplasmic reticulum, Golgi body, and vesicles; the nucleus; cytoskeleton; and mitochondria. In addition, plant cells have special organelles not found in animals cells. These organelles are the chloroplasts, cell wall, and vacuoles. (See the drawing of a plant cell on page 435.)
Plasma membrane. The plasma membrane of the cell is often described as selectively permeable. That term means that some substances are able to pass through the membrane but others are not. For example, the products formed by the breakdown of foods are allowed to pass into a cell, and the waste products formed within the cell are allowed to pass out of the cell. Since the 1960s, scientists have learned a great deal about the way the plasma membrane works. It appears that some materials are able to pass
through tiny holes in the membrane of their own accord. Others are helped to pass through the membrane by molecules located on the surface of and within the membrane itself. The study of the structure and function of the plasma membrane is one of the most fascinating in all of cell biology.
Endoplasmic reticulum. The endoplasmic reticulum (ER) consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell's cytoplasm. The ER looks something like a very complex subway or highway system. That analogy is not a bad one, since a major function of ER is to transport materials throughout the cell.
Two kinds of ER can be identified in a cell. One type is called rough ER and the other is called smooth ER. The difference between the two is that rough ER contains ribosomes on its outside surface, giving it a rough or grainy appearance. Rough ER is involved in the process of protein synthesis (production) and transport. Proteins made on the ribosomes attached to rough ER are modified, "packaged," and then shipped to various parts of the cell for use. Some are sent to the plasma membrane, where they are moved out of the cell and into other parts of the organism's body for use.
Smooth ER has many different functions, including the manufacture of lipids (fatlike materials), the transport of proteins, and the transmission of nerve messages.
The Golgi body. The Golgi body is named for its discoverer, the nineteenth century Italian scientist Camillo Golgi (1843–1926). It is one of the most unusually shaped organelles. Looking somewhat like a stack of pancakes, the Golgi body consists of a pile of membrane-bounded, flattened sacs. Surrounding the Golgi body are numerous small membrane-bounded vesicles (particles). The function of the Golgi body and its vesicles is to sort, modify, and package large molecules that are secreted by the cell or used within the cell for various functions.
The Golgi body can be compared to the shipping and receiving department of a large company. Each Golgi body within a cell has a cis face, which is similar to the receiving division of the department. Here, the Golgi body receives molecules manufactured in the endoplasmic reticulum. The trans face of the Golgi body can be compared to the shipping division of the department. It is the site from which modified and packaged molecules are transported to their destinations.
Vesicles. Vesicles are small, spherical particles that contain various kinds of molecules. Some vesicles, as noted above, are used to transport molecules from the endoplasmic reticulum to the Golgi body and from the Golgi body to various destinations. Special kinds of vesicles perform other functions as well. Lysosomes are vesicles that contain enzymes involved in cellular digestion. Some protists, for instance, engulf other cells for food. In a process called phagocytosis (pronounced FA-go-sy-to-sis), the protist surrounds a food particle and engulfs it within a vesicle. This food-containing vesicle is transported within the protist's cytoplasm until it is brought into contact with a lysosome. The food vesicle and lysosome merge, and the enzymes within the lysosome are released into the food vesicle. The enzymes break down the food into smaller parts for use by the protist.
The nucleus. The nucleus is the control center of the cell. Under a microscope, the nucleus looks like a dark blob, with a darker region, called the nucleolus, centered within it. The nucleolus is the site where parts of ribosomes are manufactured. Surrounding the nucleus is a double membrane called the nuclear envelope. The nuclear envelope is covered with tiny openings called nuclear pores.
The nucleus directs all cellular activities by controlling the synthesis of proteins. Proteins are critical chemical compounds that control almost everything that cells do. In addition, they make up the material from which cells and cell parts themselves are made.
The instructions for making proteins are stored inside the nucleus in a helical molecule called deoxyribonucleic acid, or DNA. DNA molecules differ from each other on the basis of certain chemical units, called nitrogen bases, that they contain. The way nitrogen bases are arranged within any given DNA molecule carries a specific genetic "message." One arrangement of nitrogen bases might carry the instruction "Make protein A," another arrangement of bases might carry the message "Make protein B," yet a third arrangement might code for the message "Make protein C," and so on.
The first step in protein synthesis begins in the nucleus. Within the nucleus, DNA is translated into a molecule called messenger ribonucleic acid (mRNA). MRNA then leaves the nucleus through the nuclear pores. Once in the cytoplasm, mRNA attaches to ribosomes and initiates protein synthesis. The proteins made on ribosomes may be used within the same cell or shipped out of the cell through the plasma membrane for use by other cells.
The cytoskeleton. The cytoskeleton is the skeletal framework of the cell. Instead of bone, however, the cell's skeleton consists of three kinds of protein filaments that form networks. These networks give the cell shape and provide for cellular movement. The three types of cytoskeletal fibers are microtubules, actin filaments, and intermediate filaments.
Microtubules are very thin, long tubes that form a network of "tracks" over which various organelles move within the cell. Microtubules also form small, paired structures called centrioles within animal cells. These structures are not considered organelles because they are not bounded by membranes. Centrioles are involved in the process of cell division (reproduction).
Some eukaryotic cells move about by means of microtubules attached to the exterior of the plasma membrane. These microtubules are called flagella and cilia. Cells with cilia also perform important functions in the human body. The airways of humans and other animals are lined with such cells that sweep debris and bacteria upwards, out of the lungs and into the throat. There, the debris is either coughed from the throat or swallowed into the digestive tract, where digestive enzymes destroy harmful bacteria.
Actin filaments are especially prominent in muscle cells, where they provide for the contraction of muscle tissue. Intermediate filaments are relatively strong and are often used to anchor organelles in place within the cytoplasm.
Mitochondria. The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane. The inner membrane of a mitochondrion is folded into compartments called cristae (meaning "box"). The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy.
Plant organelles. Plant cells have several organelles not found in animal cells. These include plastids, vacuoles, and a cell wall.
Plastids are vesicle-type organelles that perform a variety of functions in plants. For example, amyloplasts store starch and chromoplasts store pigment molecules that give some plants their vibrant orange and yellow colors. Chloroplasts are plastids that carry out photosynthesis, a process in which water and carbon dioxide are transformed into sugars.
Vacuoles are large vesicles bound by a single membrane. In many plant cells, they occupy about 90 percent of the cellular space. They perform a variety of functions in the cell, including storage of organic compounds, waste products, pigments, and poisonous compounds as well as digestive functions.
All plant cells have a cell wall that surrounds the plasma membrane. The cell wall of plants consists of a tough carbohydrate substance called cellulose laid down in a medium or network of other carbohydrates. (A carbohydrate is a compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.) The cell wall provides an additional layer of protection between the contents of the cell and the outside environment. The crunchiness of an apple, for instance, is attributed to the presence of these cell walls.
[See also Chromosome; Enzyme; Neuron; Nucleic acid; Protein; Reproduction; Respiration ]
Cell
Cell
A cell is the smallest unit of living matter. Cells were first identified in Europe in the seventeenth century by Antoni van Leeuwenhoek and others. They were named by Robert Hooke, an Englishman, who said they reminded him of the rooms or "cells" in a monastery. The cell theory describes some fundamental characteristics of all cells and is one of the unifying concepts in biology. It states that: (1) all organisms are made of cells, a cell is the structural and functional unit of organs, and therefore cells are organisms; and (2) cells are capable of self-reproduction and come only from preexisting cells.
Prokaryotic Cells
Cells come in many shapes and sizes and have different structural features. Bacteria are single-celled organisms approximately 1 to 10 micrometers (.00004 to .0004 inch) in size and can be spherical, rod-shaped, or spiral-shaped. They are known as prokaryotes (from the Greek pro, meaning "before" and karyon, meaning "kernel" or "nucleus") because they contain a nucleoid region rather than a true nucleus where their genetic material is found. All bacteria have cell walls that may be surrounded by a capsule and/or a gelatinous slime layer.
Beneath the cell wall is the plasma membrane responsible for regulating the flow of materials into and out of the cell's cytoplasm within the interior of the cell. The cytoplasm is composed of fluid known as cytosol and solid materials. Within the cytosol are ribosomes , granular bodies that direct the synthesis of all bacterial proteins . Some bacteria have whiplike appendages called flagella that enable them to move. The genetic material of bacteria is deoxyribonucleic acid (DNA), which is contained within a single circular chromosome in the nucleoid region and sometimes also in a smaller ring called a plasmid .
Eukaryotic Cells
Eukaryotic cells (from the Greek eu, meaning "true" and karyon, meaning "kernel" or "nucleus") are more complex than prokaryotic cells and are found in both unicellular organisms like the amoeba and multicellular organisms like sunflowers, mushrooms, and humans. They are generally larger than prokaryotic cells, ranging from about 10 to 100 micrometers (.0004 to .004 inch) in size. In multicellular organisms, there are many different types of cells that perform specialized functions. In animals, for instance, pancreatic cells make and secrete hormones , whereas red blood cells are specialized for transporting oxygen throughout the body. Cells with specialized functions such as these are called "differentiated."
All eukaryotic cells share specific structural characteristics. These include a true nucleus that is bounded by a double-layered membrane known as the nuclear membrane. Within the nucleus is housed the cell's genetic material in the form of linear chromosomes of DNA contained in thread-like structures called chromatin . All eukaryotic cells have a plasma membrane that encloses the cytoplasm. Cells of plants, fungi, and many protists have an additional outer boundary called a cell wall that differs significantly in structure and composition from that of a prokaryotic cell.
Eukaryotic cells have many different kinds of small membrane-bound structures called organelles that, with the exception of ribosomes, are absent from prokaryotic cells. Eukaryotic ribosomes (which are not enclosed by a membrane) float freely in the cytosol or are attached to another organelle known as the endoplasmic reticulum (ER). The ER is a series of membrane-bound, fluid-filled spaces in contact with the nuclear membrane. Its function is to synthesize and/or modify proteins, phospholipids, and cholesterol and to transport substances from the nucleus to the rest of the cell.
When the ER is studded with ribosomes it is called the rough ER. When ribosomes are absent it is called the smooth ER. The Golgi apparatus is a system of membrane-enclosed sacs responsible for transporting newly synthesized proteins and lipids from the ER to other organelles and the plasma membrane. It is also the site of polysaccharide synthesis and modification of proteins and lipids by addition of sugars.
Both animal and plant cells have mitochondria , power houses that convert energy stored in the chemical bonds of nutrients like carbohydrates , proteins, and fats into adenosine triphosphate (ATP), a high-energy chemical compound that is required for many cellular processes. Many plant cells also have chloroplasts, organelles that contain the pigment chlorophyll. Chloroplasts conduct photosynthesis, in which plants use sunlight, water, and carbon dioxide to synthesize the sugar glucose .
| Organelles in Eukaryotic Cells | |
| Structure | Function |
| Nucleus | Contains genetic material |
| Ribosomes | Protein synthesis |
| Endoplasmic reticulum | Synthesis/modification and transport of proteins and lipids |
| Golgi apparatus | Processing, distribution of proteins, lipids |
| Lysosomes | Digestion of substances in cell |
| Peroxisomes | Digestion and detoxification |
| Chloroplasts | Photosynthesis |
| Flagella/Cilia | Cell movement |
| Vacuole and vesicle | Storage of cellular substances |
| Centriole | Cytoskeletal organization |
Lysosomes are membrane-enclosed bodies in plant and animal cells that contain enzymes responsible for digesting substances within the cell. In animal cells, peroxisomes contain enzymes that metabolize lipids and alcohol. In plants, peroxisomes also convert fatty acids into molecules that are precursors of sugars. Both plant and animal cells have vacuoles, membranous sacs that store substances such as water, sugars, and salts. Protozoans, a type of unicellular protist, have specialized contractile vacuoles for removing excess water from the cell.
Most organelles do not flow freely in the cytoplasm but are anchored to a complex intracellular framework known as the cytoskeleton , which is made of three different types of protein fibers: microfilaments, intermediate filaments, and microtubules. The cytoskeleton is involved in maintaining cell shape and participates in cell movement and cell division. The centrosome contains a pair of organelles called centrioles close to the nucleus of animal cells. It is responsible for organizing some of the cytoskeletal components.
Some plant and animal cells have projections from the plasma membrane known as flagella or cilia that are capable of movement. For example, a single flagellum is responsible for the movement of sperm cells.
see also Cell Wall; Chloroplast; Cytoskeleton; DNA; Golgi; History of Biology: Cell Theory and Cell Structure; Mitochondrion; Nucleus; Ribosome; Vacuole
Michele D. Blum
Bibliography
Mader, Sylvia S. Biology, 6th ed. Boston: McGraw-Hill, 1998.
McFadden, Carol, H., and William T. Keeton. Biology: An Exploration of Life. New York: W. W. Norton and Company, Inc., 1995.
cell
It was the invention of the microscope, in the seventeenth century, that allowed scientists the first glimpses of individual cells. In particular, the Dutchman, Antoni van Leeuwenhoek described the extraordinary variety of motile single-celled organisms (which he called ‘animalcules’) present in pond water. The word ‘cell’ (from the Latin cella, ‘a small room’) was first coined in 1665, by the English physicist Robert Hooke, to refer to the microscopic structure of cork. Technical improvements in microscopy in the eighteenth and nineteenth centuries allowed more precise observation. It gradually became apparent that cells had a complicated internal structure, and that some features (for example, what we now refer to as the nucleus) were common to most cells, even though the appearance of the cells themselves varied enormously. This in turn hinted that a common basic organization might underlie all living matter.
The first simple distinction had been between nucleus and cytoplasm — the rest of the cellular substance — but by the end of the nineteenth century the principal internal components of cells that we are familiar with today (sub-cellular structures or organelles) had been identified. These included the endoplasmic reticulum (an extensive network of membranes within the cell), mitochondria (cylindrical, membrane-limited structures) and the Golgi complex (a stack of flattened membrane sacs, named after the Italian anatomist who described those and other intracellular structures in 1898, and later shared a Nobel prize with Spaniard Ramón y Cajal). The true complexity of the internal structure of cells, however, only became apparent in the 1950s, when cells were examined with the newly-invented electron microscope, which had much greater resolving power than the conventional light microscope — magnifying 20–30 000 times. It was around this time that the field now known as cell biology began to come to prominence, with the goal of understanding how the various organelles acted together to allow the cell to carry out its many functions. As well as simply observing cell structure, cell biologists now began to take cells apart and purify the different organelles using high-speed centrifugation. It was also shown that the purified organelles could be made to work in isolation, which allowed a detailed study of their functions, and the identification of the mechanisms underlying them.
Our current view of the cell is as an organism-in-miniature. The blueprint is contained in the DNA, packaged into chromosomes in the nucleus. Parts of the DNA sequence are replicated into ‘messenger’ RNA, which exits the nucleus and specifies the sequences of the cell's proteins, which are constructed in the cytoplasm. The power-houses of the cell are the mitochondria, which use nutrients taken up from outside to generate ATP, the energy currency of the cell. (Plant cells have additional organelles, the chloroplasts, which contain chlorophyll, the molecule responsible for capturing the energy of sunlight and initiating the process of photosynthesis. This results in the production of carbon-containing molecules for use by the cell and the generation of oxygen, which is essential for the continuation of life on earth.)
Many cells are responsible for secreting substances which will have external effects. In the pancreas, for instance, some cells secrete enzymes into the gut, where they digest our food, whereas other cells secrete insulin into the bloodstream, which instructs cells in the rest of the body to take up glucose. Both the digestive enzymes and the insulin are packaged into the endoplasmic reticulum and are then transported to the surface of the cell via the Golgi apparatus. Thus although each organelle is a discrete structure, there is extensive communication between organelles. This intracellular trafficking system demands that there be strict controls on the movement of proteins between organelles, and that individual proteins be ‘tagged’ for delivery to particular destinations. Without this control, the organization of the cell would quickly disintegrate.
A single higher organism contains a huge variety of cell types: compare, for example, a neuron with a lymphocyte, or a skeletal muscle cell with a liver cell (hepatocyte). All of these cells were produced from a single fertilized egg, by processes including cell division, migration, differentiation, and death. We are only just beginning to understand how these processes are orchestrated to produce the complete organism. One aspect that is crucial to the development and maintenance of multicellular organisms is communication between cells. Cells are continually signalling to their neighbours through the release of molecules that are detected by specialized receptors on the surface of other cells. In the brain, for example, neurons ‘talk’ to each other by means of small molecules. These molecules, or ‘neurotransmitters’, are packaged in small sacs within the neurons, and are released when an electrical impulse passes to the end of its axon. The neurotransmitter then binds to receptors on the neighbouring neurons and changes the electrical properties of these neurons, making them more or less likely to initiate an electrical impulse themselves. In other parts of the body, neurons communicate in similar fashion with muscle cells, causing them to contract, or with glandular cells, causing them to secrete. Many drugs work by blocking or mimicking the action of these neurotransmitters. Again, some cells release molecules which travel in the blood: messengers which communicate with remotely distant cells that have the appropriate receptors on their surface.
Once tissues and organs have been formed it is essential that cell division be strictly controlled in order to maintain normal function. Many proteins are now known which control cell division, often in response to external stimuli. Mutations in these proteins can result in uncontrolled cell division. This can lead eventually to the formation of tumours, which can be life threatening.
We are now familiar with the idea that cells are produced by the division of progenitor cells. This idea, of course, begs the question as to how the first cell was produced. It has been shown that simple organic molecules can form under conditions believed to be similar to those that existed on earth in its early history. How these molecules became assembled into proteins, and more particularly how the self-replicating ‘blueprint’ molecules such as DNA came about, are fundamental unanswered questions.
Michael Edwardson
See also cell membranes; cell signalling.
cell
1665 | English physicist Robert Hooke (1635–1703) coins the word ‘cell’. |
1831 | Robert Brown discovers the nucleus in plant cells. |
1838 | German botanist Matthias Schleiden (1804–81) proposes that plants are composed of cells. |
1839 | Theodor Schwann states that animals are composed of cells and concludes that all living things are made up of cells. |
1846 | German botanist Hugo von Mohl (1805–72) coins the word ‘protoplasm’ for the living material of cells. |
1858 | German pathologist Rudolf Virchow (1821–1902) postulates that all cells arise from other cells. |
1865 | German botanist Julius von Sachs (1832–97) discovers the chlorophyll-containing bodies in plant cells later named chloroplasts. |
1876–80 | German cytologist Eduard Strasburger (1844–1912) describes cell division in plants and states that new nuclei arise from division of existing nuclei. |
1882 | German cytologist Walther Flemming (1843–1905) describes the process of cell division in animal cells, for which he coins the term ‘mitosis’. Strasburger coins the words ‘cytoplasm’ and ‘nucleoplasm’. |
1886 | German biologist August Weismann (1834–1914) proposes his theory of the continuity of the germ plasm. |
1887 | Belgian cytologist Edouard van Beneden (1846–1910) discovers that the number of chromatin-containing threadlike bodies (subsequently named chromosomes) in the cells of a given species is always the same and that the sex cells contain half this number. |
1888 | German anatomist Heinrich von Waldeyer (1836–1921) coins the word ‘chromosome’. |
1898 | Camillo Golgi discovers the Golgi apparatus. |
1901 | US biologist Clarence McClung (1870–1946) discovers the sex chromosomes. |
1911 | Thomas Hunt Morgan produces the first chromosome map. |
1949 | Canadian geneticist Murray Barr (1908–95) discovers Barr bodies. |
1955 | Belgian biochemist Christian de Duve (1917– ) discovers lysosomes and peroxisomes. |
1956 | Romanian-born US physiologist George Palade (1912– ) discovers the role of microsomes (later renamed ribosomes). |
cell
cell / sel/ • n. 1. a small room in which a prisoner is locked up or in which a monk or nun sleeps. ∎ a small compartment in a larger structure such as a honeycomb. ∎ hist. a small monastery or nunnery dependent on a larger one.2. Biol. the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane. ∎ an enclosed cavity in an organism. ∎ fig. a small group forming a nucleus of political activity, typically a secret, subversive one: the weapons may be used to arm terrorist cells. ∎ the local area covered by one of the short-range transmitters in a cellular telephone system.3. a device containing electrodes immersed in an electrolyte, used for current-generation or electrolysis. ∎ a unit in a device for converting chemical or solar energy into electricity.DERIVATIVES: celled adj. [in comb.] a single-celled organism.cell-like / -ˌlīk/ adj.
cell
cell
1. Small apartment of any sort, such as a room in a dormitory or inn, but especially a confined study-bedroom allotted to a monk or nun in a monastery. Also a penitential cell in the Middle Ages in which penitents were immured.
2. Secure room with bed or beds in a prison.
3. Any small cavity or room.
4. Cella or naos.
5. Web of a vault framed by the ribs, or one surface of a groin vault.
6. In timber-framed structures, one room or unit. A single-cell plan is one volume, while a two-cell plan may have a cross-entry or cross-passage, and a three-cell plan will have a cross-passage, cross-entry, or lobby-entry.
cell
1. An address, a location in memory, or a register, usually one capable of holding a binary number. It is sometimes a location capable of holding one bit.
2. The basic unit of a spreadsheet or some other table of text, formed by the intersection of a row and column. It contains a label, value, or formula with attributes such as size, font, and color.
3. The name given to a packet in one version of a packet switching system. Packet switching systems subdivide the data to be transmitted into a number of packets. In contrast to many systems, a cell is short – for instance 53 bytes in the case of an ATM cell – and its internal structure is fixed. Small size and fixed structure allow the cell to be switched using a very simple algorithm; the processing time required for switching is thus reduced, with a corresponding increase in the number of cells switched in a given time.
4. The coverage area provided by a base station to a mobile (wireless) phone. As the user moves geographically, the conversation is “handed over” to another cell at another base station.