Cells

Plants are multicellular organisms composed of millions of plant cells. Although individual cells may differ greatly from each other in mature structure, all plant cells share the same basic eukaryotic organization. That is, each plant cell possesses a nucleus and cytoplasm with subcellular organelles. In addition to these components, all plant cells possess a cell wall of cellulose. Although plant cells begin their life with a full complement of cellular components, some specialized cell types lose their nuclei and all or part of their cytoplasm as they mature.

Components of a Cell

Cell Wall.

Plant cells, unlike animal cells, are surrounded by a relatively thin but mechanically strong cell wall. Plant cell walls consist of a complex mixture of polysaccharides , proteins, and phenolic polymers that are secreted by the protoplast and then assembled into an organized network linked together by covalent and hydrogen bonds. Cell walls function in the support of plant tissues and in mechanical protection from insects and pathogens . Plant cell walls are made up of cellulose microfibrils embedded in an amorphous matrix, an organization analogous to that of fiberglass or steel-reinforced concrete. Cellulose microfibrils consist of linear chains of glucose, with each chain composed of two thousand to twenty-five thousand glucose units. About fifty such chains are linked side by side through hydrogen bonds to form one microfibril. Hydrogen bonding between adjacent glucose units forms highly crystalline regions within the cellulose microfibril, giving cellulose its stiffness and high tensile strength.

Cellulose microfibrils are embedded in a matrix composed of pectin, hemicelluloses, and proteins. Pectins and hemicelluloses are shorter chain polysaccharides that are either branched or unbranched and form cross-links between the cellulose microfibrils. In the presence of calcium ions, pectins form a highly hydrated gel (purified pectin is used in jam and jelly making). Cell wall carbohydrates are covalently linked to cell wall proteins that are rich in the rare amino acid hydroxyproline. Cell wall structural proteins vary greatly in their composition but are thought to provide strength, particularly in cells that are growing.

Specialized types of plant cells such as sclerenchyma fibers and xylem vessels require a hard, rigid cell wall in order to function. These cells synthesize a thick, inner wall layer called a secondary cell wall. The secondary cell wall is impregnated with a polymer of phenolic units called lignin. Lignins are much-branched, long chain phenolic compounds that form many cross-links with other wall components and give secondary cell walls great strength and rigidity.

Cell Membrane.

The portion of the plant cell inside the cell wall is called the protoplast. The protoplast is bounded by a membrane known as the

cell membrane or the plasma membrane. Like all biological membranes, it consists of a double layer of phospholipids in which proteins are embedded. Phospholipids are a class of lipids in which glycerol is covalently linked to two fatty acids and to a phosphate group. The hydrocarbon chains of the fatty acids are nonpolar and form a region that is highly hydrophobic . The proteins associated with the lipid layer are of two types: integral and peripheral. Integral proteins span the entire thickness of the lipid layer. For example, the cellulose synthase enzymes that catalyze the synthesis of cellulose are integral proteins. They extend across the cell membrane, taking up glucose precursors on the inner side and extruding a cellulose microfibril on the outer side. Peripheral proteins are attached to one surface of the lipid layer. Peripheral proteins on the inner surface of the plasma membrane often function in interactions between the membrane and components of the cytoskeleton. Some peripheral proteins on the outer surface of the plasma membrane function in hormone perception and signaling.

All plant cell membranes share the same basic structure but differ in the makeup of specific components. All membranes also share the important property of semi-impermeability. Small molecules such as water move readily across the membrane, but larger molecules can move only if the appropriate integral proteins are present.

Nucleus.

The nucleus is the most prominent structure within the protoplast and contains the genetic information responsible for regulating cell metabolism, growth, and differentiation. The nucleus contains the complex of deoxyribonucleic acid (DNA) and associated proteins, known as chromatin in the uncondensed state and as chromosomes in the condensed state. The chromatin is embedded in a clear matrix called the nucleoplasm. Nuclei also contain a densely granular region, called the nucleolus, that is the site of ribosome synthesis. The nucleus is bounded by a double membrane, the nuclear envelope. The two membranes of the nuclear envelope are joined at sites called nuclear pores. Each nuclear pore is an elaborate structure that allows macromolecules such as ribosomal subunits and ribonucleic acid (RNA) to pass between the nucleus and the cytoplasm.

Endomembrane System.

The cytoplasm of plant cells has a continuous network of internal membranes called the endomembrane system. The nuclear envelope forms part of this system and is continuous with another component, the endoplasmic reticulum (ER). The ER consists of flattened sacs or tubes known as cisternae . There are two types of ER, smooth and rough, that are interconnected but carry out different functions. Rough ER tends to be lamellar (formed into flattened sacs) and is covered with ribosomes. Rough ER functions in protein synthesis and in the processing and storage of proteins made on the outer surface. In contrast, smooth ER tends to be tubular and is a major site of the synthesis of lipids such as those making up membranes.

Another major component of the endomembrane system is the Golgi apparatus (or dictyosome ). The Golgi apparatus consists of a stack of flattened membrane cisternae and associated vesicles . The two primary functions of the Golgi apparatus are the modification of proteins synthesized on the rough

ER and packaging of processed proteins and carbohydrates to be secreted outside the plasma membrane. The Golgi apparatus is a very dynamic part of cell structure. Vesicles carrying newly synthesized proteins or other precursors fuse with a cisterna on the forming face of the Golgi apparatus. As its contents are processed, a cisterna moves through the stack until it reaches the maturing face of the stack. Here the cisterna breaks up into separate vesicles that release their contents at the plasma membrane. Golgi apparatus are very numerous in secretory cells such as those of nectaries or root caps, and they also play a role in the secretion of cell wall matrix polysaccharides.

Vacuole.

The vacuole is a conspicuous component of the cytoplasm in most plant cells. It may occupy more than 90 percent of cell volume in unspecialized parenchyma cells. The vacuole is surrounded by a membrane called the tonoplast that, because of the high density of integral proteins that are ion channels, plays an important role in the osmotic relationships of the cell. The vacuole stores a wide range of inorganic and organic substances such as the compounds that give beets their color (the water soluble red pigment anthocyanin), apples their sweetness (sucrose), lemons their sourness (citric acid), and tea its bitterness (tannin). In some plants, the vacuoles function as part of the plants' defense systems: it may be filled with sharp crystals of calcium oxalate that help deter herbivores .

Chloroplast.

Chloroplasts are organelles that function in photosynthesis and are another feature that distinguish plant from animal cells. Chloroplasts are bounded by a double membrane, the chloroplast envelope. The inner membrane of the envelope is invaginated (folded) to form flattened sacs within the chloroplast called thylakoids. Thylakoid membranes take two forms: stacks called grana, and sheets that connect the grana, called stroma thylakoids. Granal thylakoids contain photosynthetic pigments such as chlorophyll and carotenoids, as well as the proteins associated with the light reactions of photosynthesis. The carbon fixation reactions of photosynthesis take place within the amorphous portion of the chloroplast called the stroma. Chloroplast DNA is found in discrete regions within the stroma. The stroma also contains chloroplast ribosomes and other components required for protein synthesis. Therefore the chloroplast is semiautonomous, relying on the nuclear genome for only some of its proteins. Green chloroplasts are just one of several types of plastids that share the same basic structure. Chromoplasts are red or orange plastids that contain large amounts of carotenoid pigments and give fruits such as tomatoes and oranges their color. The brilliant colors of autumn leaves results from both the conversion of chloroplasts to chromoplasts and the formation of anthocyanin in the vacuole. Amyloplasts, such as those found in a potato tuber, are plastids that store starch.

Other Organelles.

Mitochondria are small organelles with a double membrane that function in cellular respiration. The inner membrane of the mitochondrial envelope is infolded to form cristae that are the sites of the electron transfer system. The inner membrane encloses the matrix region, the location of the Krebs cycle. Like plastids, mitochondria possess their own DNA, ribosomes, and protein-synthesizing machinery. Proteins encoded by the mitochondria genome include ribosomal proteins and components of the electron transfer system.

Peroxisomes are small, single-membrane-bound organelles that function in photorespiration, a process that consumes oxygen and releases carbon dioxide. These peroxisomes are often found in association with chloroplasts in green leaf tissue. Other peroxisomes, called glyoxysomes, function in the conversion of stored fats to sucrose and are common in the tissues of germinating seeds.

Cytoskeleton.

All living plant cells possess a cytoskeleton, a complex network of protein filaments that extends throughout the cytosol. The cytoskeleton functions in mitosis , cytokinesis, cell growth, and cell differentiation. The plant cell cytoskeleton has two major components: hollow cylinders called microtubules that are composed of tubulin protein, and solid microfilaments composed of actin protein. Microtubules guide chromosome movements during mitosis and the orientation of cellulose microfibrils during cell wall synthesis. The contractile microfilaments play a role in chromosome movement and in cytoplasmic streaming.

see also Carbohydrates; Cell Cycle; Cells, Specialized Types; Cellulose; Cell Walls; Chloroplasts; Plastids.

Nancy G. Dengler

Bibliography

Purves, William K., Gordon H. Orians, H. Craig Heller, and David Sadavai. Life: The Science of Biology. Sunderland, MA: Sinauer Associates/W. H. Freeman, 1998.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1998.

Cells

All living organisms are comprised of one or more cells. Most animal cells range in size between 10 and 100 micrometers and have several key elements.

The outer layer of a cell, the cell membrane, consists of a phospholipid bilayer, which serves primarily as a barrier from the external environment, and integral proteins which function mainly to regulate transport. The cell membrane creates an enclosed space in which the chemical processes, or metabolism, of the cell can occur. It also serves as a gatekeeper for the cell, carefully regulating what passes in and out. For example, nutrients pass in, and metabolic waste passes out.

The inside of an animal cell contains several organelles , specialized structures that perform specific functions for the cell. These organelles are suspended in a thick aqueous solution, the cytoplasm. The coordinated activity of the organelles is required for the survival of the cell. Many organelles are enclosed by membranes that generate separate compartments within the cytoplasm. One of the most important compartments is the nucleus. The nucleus contains genetic material, which acts as a blue print for the production of the proteins that perform most cellular functions. Protein synthesis is performed by ribosomes and takes place in the cytoplasm. Ribosomes attach to the endoplasmic reticulum (ER) during synthesis of those proteins that are to be exported, incorporated into the membrane, or placed into organelles. These proteins are then shipped from the ER to another compartment, the Golgi apparatus, where they are modified and then shipped to their final destinations. In recycling compartments, known as the lysosomes, old proteins and other molecules are broken down so that their components can be reused. Diverse chemical processes (e.g. the synthesis of the gas Nitric Oxide that functions as an important signal between cells) that produce toxic molecules (peroxides) as side products, take place in specialized chemical compartments of the cell which are called peroxisomes. As a result, the peroxides can be broken down safely within the peroxisomes without harming the cell.

The transport of cargo between individual compartments through the cytoplasm is the job of transport vesicles, tiny membrane-enclosed compartments that contain the cargo and transport it through the cytoplasm. Every cell produces many transport vesicles, and each type is specialized for a distinct shipping route within the cell, for a kind of cargo, or even for the storage of substances (e.g., neurotransmitters for communication between cells).

The energy for all the chemical processes in the cell is generated in compartments called mitochondria , which can be considered the cell's powerhouses. Mitochondria produce ATP, the energy source of the cell, using sugars and oxygen in a process called oxidative phosphorylation.

The shape of cells is maintained by a cytoskeleton, or cell skeleton, made of three membrane-free organelles—microtubules, actin filaments, and intermediate filaments. Together these organelles form a network of molecular cables and struts that stabilizes the cell shape.

In contrast to plant cells, which are further stabilized by a cell wall that surrounds the outer cell membrane, animal cells are stabilized by a cytoskeleton and an extracellular matrix made mostly of glycoproteins . However, because they are less rigid, animal cells can change their shape more easily and even use these shape changes to move. Animal cells such as the infection-fighting white blood cells can be sophisticated "crawlers." In the absence of a rigid cell wall, they assemble and disassemble parts of their cytoskeleton in such a way that specific shape changes leading to cell movement will occur. Microfilaments are also responsible for the movement of specific organelles within the cell, and microfilaments and microtubules together are essential for cell division. While microtubules ensure the distribution of duplicated chromosomes to the two daughter cells, the microfilaments will finish the separation of the original cell by pinching in the outer cell membrane.

Whereas in single-celled organisms all life functions are performed by a single cell, in multicellular organisms, such as animals, division of labor and specialization among cells occurs. For example, humans have about 200 different cell types that differ in structure and in function. In all but the simplest animals, the sponges, specialized cells that have a similar structure and function are arranged together into tissues. Although there are many different types of animal cells, scientists group them all into only four general tissue types—epithelial tissue, muscle tissue, nervous tissue, and connective tissue . The cells in a tissue may be held together by the extracellular matrix that makes the cells sticky or ties them together.

In all animals except sponges and jellyfishes, different tissue types may form a functioning unit called an organ. Organs may also be part of an organ system, such as the digestive system and reproductive system, where several organs function together. Each organ system has a different function, but just like the organelles within an individual cell, the function of each organ system must be regulated and coordinated to ensure the survival of the whole animal.

see also Cell Division; Homeostasis.

Kathrin F. Stanger-Hall

Bibliography

Barnes-Svarney, Patricia. New York Public Library Desk Reference. New York: Macmillan USA, 1998.

Ingber, D. E. "The Architecture of Life." Scientific American 278 (1998):48-57.

Rhoades, Rodney, and Richard Pflanzer. Human Physiology, 3rd ed. Fort Worth, TX: Saunders College Publishing, 1996.

Rothman, James E. "Budding Vesicles in Living Cells." Scientific American 274 (1996):70-75.

72. Cells

See also 44. BIOLOGY .

auxesis

growth, especially owing to an increase in cell size. Cf. merisis . —auxetic, adj.

basophile

a cell or tissue that stains easily. —basophilic, basophilous, adj.

chromatolysis

the breakdown of the protoplasm that contains the genes in the cell nucleus.

cytochemistry

the branch of cytology that deals with the chemistry of living cells. —cytochemical, adj.

cytology

the branch of biology that studies the structure, function, multiplication, and life history of cells. —cytologist, n. —cytologie, cytological, adj.

cytolysis

the degeneration of cells. —cytolytic, adj.

cytoplasm

the protoplasm of a cell, not including the nucleus. —cytoplasmic, adj.

cytotechnology

the study of human cells, especially to detect signs of cancer. —cytotechnologist, n. —cytotechnologic, adj.

ectoplasm

the outer part of the cytoplasm of a cell. Cf. endoplasm . —ectoplasmic, adj.

embryogeny

the formation and growth of an embryo. —embryogenic, —embryogenetic, adj.

endoplasm

the inner part of the cytoplasm of a cell. Cf. ectoplasm . —endoplasmic, adj.

epigenesis

the formation of a cell as a new product and not as the result of development from some existing cell. —epigenetic, adj.

karyology

a branch of cytology dealing with the structure of cell nuclei, especially chromosomes. —karyologic, karyological, adj.

karyoplasm

the substance forming the nucleus of a cell. —karyoplasmic, karyoplasmatic, adj.

karyotype

the aggregate of morphological characteristics of the chromosomes in a cell. —karyotypic, karyotypical, adj.

lysis

the destruction of cells by the action of certain lysins. See also 197. HEALTH . — lytic , adj.

merisis

any form of growth, especially as a product of cell division. Cf . auxesis .

mitosis

the normal process of cell division. —mitotic , adj.

monad

any simple, single-cell organism. —monadic, monadical, monadal , adj.

neutrophile

a cell or tissue that accepts a stain from a neutral solution. —neutrophilous , adj.

osmosis

the process by which fluids pass through a semipermeable membrane into a solution of lower concentration to equalize the concentration on both sides of the membrane. —osmotic , adj.

phagocytosis

the action of phagocytes in ingesting and destroying cells.

trophoplasm

the form of protoplasm that constitutes the nutritive element of a cell. —trophoplasmic, trophoplasmatic , adj.

trophotropism

the movement of cells in relation to food or nutritive matter. —trophotropic , adj.