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.