Cell

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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.

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.