Cellular Components

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Most chemical reactions that take place in cells occur in the protoplasm, as part of a dynamic series of events that make the plant a living entity. Each organelle within the protoplast has a primary function, and the flow of metabolites (products of chemical synthesis or breakdown) from one organelle to another is necessary for a balance of events that take place.

Envision a journey through the plant cell as an exciting voyage in which information is stored primarily in the nucleus, processed in the cytoplasm, and sent on to different parts of the cell. This information can bring about the synthesis of proteins in the cytoplasm where they become involved in metabolic reactions (see Chapter 10), or they may be destined for use in other cellular locations. The

Chapter 3

Cellulose Microfibrils Function

11 nu cwjo hydrogen bonds lignin hemicellulose hemicellulose cross link

Figure 3.6B Secondary cell wall structure. Components are arranged so that the cellulose microfibrils and hemicellulose chains are embedded in lignin. (Reproduced by permission of the Oklahoma Academy of Science; Figure provided by Dr. James E. Bidlack.)

cellulose

11 nu cwjo hydrogen bonds lignin hemicellulose hemicellulose cross link

Figure 3.6B Secondary cell wall structure. Components are arranged so that the cellulose microfibrils and hemicellulose chains are embedded in lignin. (Reproduced by permission of the Oklahoma Academy of Science; Figure provided by Dr. James E. Bidlack.)

cellulose packaged proteins may be incorporated in membranes or organelles, and other compounds may be manufactured in specific organelles or enter from an adjacent cell.

The Plasma Membrane

The outer boundary of the living part of the cell, the plasma membrane, is roughly eight-millionths of a millimeter thick. To get an idea of how incredibly thin that is, consider that it would take 12,500 such membranes neatly stacked in a pile to achieve the thickness of an ordinary piece of writing paper. Yet this delicate, semipermeable structure is of vital importance in regulating the movement of substances into and out of the cell. While the plasma membrane may inhibit movement of some substances, it can otherwise allow free movement and can even control movement of other substances into and out of the cell. As a result, the proportions and makeup of chemicals within a cell become quite different from those outside the cell. The plasma membrane is also involved in the production and assembly of cellulose for cell walls.

Evidence obtained since the early 1970s indicates that the plasma membrane and other cell membranes are composed of phospholipids arranged in two layers, with proteins interspersed throughout (Fig. 3.7). Covalent bonds link carbohydrates to both lipids and proteins on the outer surfaces

Cells

Cell Membrane Mcgraw Hill Fig

Figure 3.7 A model of a small portion of a plasma membrane, showing its fluid-mosaic nature. The proteins, which are coiled chains of polypeptides, are either embedded or on the surfaces. Some of the embedded proteins extend all the way through and may serve as conduits for diffusion of certain ions. In cells, and other places where there are watery fluids, a double layer of phospholipids forms. The heads point outward toward the water. The tails, which are long-chain fatty acids, are hydrophobic (i.e., they "dislike" water) and point inward away from the water. The membrane is about 8 nanometers thick. (Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

Figure 3.7 A model of a small portion of a plasma membrane, showing its fluid-mosaic nature. The proteins, which are coiled chains of polypeptides, are either embedded or on the surfaces. Some of the embedded proteins extend all the way through and may serve as conduits for diffusion of certain ions. In cells, and other places where there are watery fluids, a double layer of phospholipids forms. The heads point outward toward the water. The tails, which are long-chain fatty acids, are hydrophobic (i.e., they "dislike" water) and point inward away from the water. The membrane is about 8 nanometers thick. (Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

of membranes. Some proteins extend across the entire width of the membrane, while others are embedded or apparently are loosely bound to the outer surface.

The remainder of cell contents usually push the plasma membrane up against the cell wall because of pressures developed by osmosis (see Chapter 9), but the membrane is quite flexible and often forms folds, which may, in turn, become little hollow spheres or vesicles that float off into the cell. In fact, experiments have shown that by adding detergents to a continuous membrane, it can be broken up and dispersed, yet it can partially reform when the detergents are removed. The membrane may even shrink away from the wall temporarily, but if it ever ruptures, the cell soon dies.

The Nucleus

The nucleus is the control center of the cell. In some ways, it functions like a combination of a computer program and a dispatcher that sends coded messages or "blueprints" originating from DNA in the nucleus with information that will ultimately be used in other parts of the cell. In other words, the DNA in the nucleus provides the original information needed to fulfill the cell's needs. This nuclear information contributes toward growth, differentiation, and the myriad activities of the complex cell "factory." The nucleus also stores hereditary information, which is passed from cell to cell as new cells are formed.

The nucleus often is the most conspicuous object in a living cell, although in green cells, chloroplasts may obscure it. In living cells without chloroplasts, the nucleus may appear as a grayish, spherical to ellipsoidal lump, sometimes lying against the plasma membrane to one side of the cell or toward a corner. Some nuclei are irregular in form, and they can vary greatly in size. They are, however, generally from 2 to 15 micrometers or larger in diameter. Certain fungi and algae have numerous nuclei within a single extensively branched cell, but the cells of more complex plants usually have a single nucleus.

Each nucleus is bounded by two membranes, which together constitute the nuclear envelope. Structurally complex pores, about 50 to 75 nanometers apart, occupy up to one-third of the total surface area of the nuclear envelope (Fig. 3.8). Proteins that act as channels for molecules are embedded within the pores. The pores apparently permit only certain kinds of molecules (for example, proteins being carried into the nucleus and RNA being carried out) to pass between the nucleus and the cytoplasm.

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Nucleus Electron Micrograph

Figure 3.8 Drawing of the nucleus showing the inner and outer membranes, nuclear pores, and nucleolus. The electron micrographs show detail of the nuclear pores, which are about 60 nanometers in diameter. Larger photo courtesy E. G. Pollock; smaller photo courtesy Ron Milligan; Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.

Figure 3.8 Drawing of the nucleus showing the inner and outer membranes, nuclear pores, and nucleolus. The electron micrographs show detail of the nuclear pores, which are about 60 nanometers in diameter. Larger photo courtesy E. G. Pollock; smaller photo courtesy Ron Milligan; Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.

The nucleus contains a granular-appearing fluid called nucleoplasm, which is packed with short fibers that are about 10 nanometers in diameter; several different larger bodies are suspended within it. Of the larger nuclear bodies, the most noticeable are one or more nucleoli (singular: nucleolus), which are composed primarily of RNA and are bounded by a single membrane. Additional discussion of the activities of nucleoli is found in Chapter 13.

Other important nuclear structures, which are not apparent with light microscopy unless the cell is stained or is in the process of dividing, include thin strands of chro-matin. When a nucleus divides, the chromatin strands coil, becoming shorter and thicker, and in their condensed condition, they are called chromosomes. Chromatin is composed of protein and DNA (discussed in Chapters 2 and 13). Each cell of a given plant or animal species has its own fixed number and composition of chromosomes; the cells involved in sexual reproduction have half the number found in other cells of the same organism. The number of chromosomes present in a nucleus normally bears no relation to the size and complexity of the organism. Each body cell of a radish, for example, has 18 chromosomes in its nucleus, while a cell of one species of goldenweed has 4, and a cell of a tropical adder's tongue fern has over 1,000. Humans have 46 chromosomes in each body cell.

The Endoplasmic Reticulum

The outer membrane of the nucleus is connected and continuous with the endoplasmic reticulum. The endoplasmic reticulum facilitates cellular communication and channeling of materials. Many important activities, such as the synthesis of membranes for other organelles and modification of proteins from components assembled from elsewhere within the cell, occur either on the surface of the endoplasmic retic-ulum or within its compartments.

The endoplasmic reticulum (often referred to simply as ER) is an enclosed space consisting of a network of flattened sacs and tubes that form channels throughout the cytoplasm, the amount and form varying considerably from cell to cell. Transmission electron micrographs of sectioned ER give it the appearance of a series of parallel membranes that resemble long, narrow tubes or sacs, creating subcompart-ments within the cell.

Ribosomes (discussed in the section that follows) may be distributed on the outer surface (i.e., the surface in contact

Cells

Endoplasmic Reticulum Corn Leaf

Figure 3.9 A small portion of the endoplasmic reticulum and ribosomes in a young leaf cell of corn (Zea mays). x100,000. The ribo-somes are 20 nanometers in diameter. (Electron micrograph courtesy Jean Whatley; Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

Figure 3.9 A small portion of the endoplasmic reticulum and ribosomes in a young leaf cell of corn (Zea mays). x100,000. The ribo-somes are 20 nanometers in diameter. (Electron micrograph courtesy Jean Whatley; Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

with the cytoplasm) of the endoplasmic reticulum. Such endoplasmic reticulum is said to be rough and is primarily associated with the synthesis, secretion, or storage of proteins (Fig. 3.9; see also Chapter 13). This contrasts with smooth endoplasmic reticulum, which has few, if any, ribo-somes lining the surface, and is associated with lipid secretion. Both types of endoplasmic reticulum can occur in the same cell and can be interconverted, depending on the demands of the cell. Many enzymes involved in the process of cellular respiration are synthesized on the surface of the endoplasmic reticulum. The enzymes, however, enter other organelles (primarily mitochondria, which are discussed later in this chapter) without passing through the endoplas-mic reticulum. The endoplasmic reticulum also appears to be the primary site of membrane synthesis within the cell.

Ribosomes

Ribosomes are tiny bodies that are visible with the aid of an electron microscope. They are typically roughly ellipsoidal in shape with apparently varied and complex surfaces. Each ribosome is composed of two subunits that are composed of RNA and proteins; the subunits, upon close inspection, can be differentiated by a line or cleft toward the center. Ribosomes average only about 20 nanometers in diameter in most plant cells. Unattached ribosomes often occur in clusters of 5 to 100, particularly when they are involved in linking amino acids together in the construction of the large, complex protein molecules that are a basic part of all living organisms.

Ribosomal subunits are assembled within the nucleolus, released, and in association with special RNA molecules, they initiate protein synthesis. Once assembled, complete ribosomes may line the outside of the endoplasmic reticulum but can also occur unattached in the cytoplasm, chloroplasts, or other organelles. About 55 kinds of protein are found in each ribosome of prokaryotic cells and a slightly higher number in those of eukaryotic cells (see the discussion of various types of RNA in Chapter 13). Unlike other organelles, ribo-somes have no bounding membranes, and because of this, some scientists prefer not to call them organelles.

Dictyosomes

Cells may contain from several to hundreds of groups of roundish, flattened-appearing sacs scattered throughout the cytoplasm. These sacs, which in plant cells are known as dictyosomes (Golgi bodies in animal cells), are often bounded by branching tubules that originate from the endoplasmic

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Chapter 3

Dictyosome Plants
Figure 3.10 A dictyosome from the alga Euglena, x40,000. (Electron micrograph courtesy John Z. Kiss; Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

reticulum but are not directly connected to it (Fig. 3.10). Frequently, 5 to 8 dictyosomes are organized in stacks, but stacks of up to 30 or more are common in simpler organisms.

Dictyosomes are involved in the modification of carbohydrates attached to proteins that are synthesized and packaged in the endoplasmic reticulum. Complex polysaccharides are also assembled within the dictyosomes and collect in small vesicles (tiny blisterlike bodies) that are pinched off from the margins. These vesicles migrate to the plasma membrane, fuse with it, and secrete their contents outside of the cell. Substances secreted by vesicles may include cell-wall poly-saccharides, floral nectars, and essential oils found in herbs.

The enzymes needed for the packaging of proteins are produced by the endoplasmic reticulum and further modified within the dictyosomes. One might describe dic-tyosomes as collecting, packaging, and delivery centers or, perhaps, as "post offices" of the cell.

Plastids

Most living plant cells have several kinds of plastids, with the chloroplasts (Fig. 3.11A) being the most conspicuous. They occur in a variety of shapes and sizes, such as the beautiful corkscrew-like ribbons found in cells of the green alga

Spirogyra (see Fig. 18.6) and the bracelet-shaped chloroplasts of other green algae, such as Ulothrix (see Figs. 18.2D and 18.5). The chloroplasts of higher plants, however, tend to be shaped somewhat like two Frisbees glued together along their edges, and when they are sliced in median section, they resemble the outline of a rugby football.

Although several algae and a few other plants have only one or two chloroplasts per cell, the number of chloroplasts is usually much greater in a green cell of higher plants. Seventy-five to 125 is quite common, with the green cells of a few plants having up to several hundred. The chloroplasts may be from 2 to 10 micrometers in diameter, and each is bounded by an envelope consisting of two delicate membranes. The outer membrane apparently is derived from endoplasmic reticulum, while the inner membrane is believed to have originated from the cell membrane of a cyanobac-terium (discussed in Chapter 17). Within is a colorless fluid matrix, the stroma, which contains enzymes. Genes in the nucleus dictate most of the activities of chloroplasts, but each chloroplast contains a small circular molecule of DNA that encodes instructions for production of proteins related to photosynthesis and other activities in the chloroplast and cell.

Grana (singular: granum), which are formed from membranes, have the appearance of independent stacks of coins with double membranes within each chloroplast.

Cells

Mesophyll Granum

Figure 3.11 Drawings of leaf mesophyll cell chloroplasts and transmission electron micrographs showing variation in chloroplast structure. A. A chloroplast x 20,000. B. Cutaway of a chloroplast. C. Grana x 40,000. D. A few thylakoids. (A. Courtesy Herbert W. Israel; C. Courtesy Blake Rowe; B., D., and Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

Figure 3.11 Drawings of leaf mesophyll cell chloroplasts and transmission electron micrographs showing variation in chloroplast structure. A. A chloroplast x 20,000. B. Cutaway of a chloroplast. C. Grana x 40,000. D. A few thylakoids. (A. Courtesy Herbert W. Israel; C. Courtesy Blake Rowe; B., D., and Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

There are usually about 40 to 60 grana linked together by arms in each chloroplast, and each granum may contain from 2 or 3 to more than 100 stacked thylakoids. In reality, thylakoids are part of an overlapping and continuous membrane system suspended in the stroma (Fig. 3.11B). The thylakoid membranes contain green chlorophyll and other pigments. These "coin stacks" of grana are vital to life as we know it, for it is within the thylakoids that the first steps of the important process of photosynthesis (see Chapter 10) occur. In photosynthesis, green plants convert water and carbon dioxide (from the air) to simple food substances, harnessing energy from the sun in the process. The existence of human and all other animal life depends on the activities of the chloroplasts.

There are usually four or five starch grains in the stroma, as well as oil droplets and enzymes. Some plastids (e.g., those of tobacco) store proteins. The stroma contains ribosomes in addition to the DNA molecule.

Chromoplasts are another type of plastid found in some cells of more complex plants. Although chromoplasts are similar to chloroplasts in size, they vary considerably in shape, often being somewhat angular. They sometimes develop from chloroplasts through internal changes that include the disappearance of chlorophyll. Chromoplasts are yellow, orange, or red in color due to the presence of carotenoid pigments, which they synthesize and accumulate. They are most abundant in the yellow, orange, or some red parts of plants, such as ripe tomatoes, carrots, or red peppers (Fig. 3.12). These carotenoid pigments, which are lipid soluble, are not, however, the predominant pigments in most red flower petals. The antho-cyanin pigments of most red flower petals are water soluble.

Leucoplasts are yet another type of plastid common to cells of higher plants. They are essentially colorless and include amyloplasts, which synthesize starches, and elaio-plasts, which synthesize oils. If exposed to light, some leu-coplasts will develop into chloroplasts, and vice versa.

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Chapter 3

Figure 3.12 Chromoplasts in the flesh of a red pepper, x100.

Mitochond

Figure 3.12 Chromoplasts in the flesh of a red pepper, x100.

Plastids of all types develop from proplastids, which are small, pale green or colorless organelles having roughly the size and form of mitochondria (discussed in the next section). They are simpler in internal structure than plastids and have fewer thylakoids, the thylakoids not being arranged in grana stacks. Proplastids frequently divide and become distributed throughout the cell. After a cell itself divides, each daughter cell has a proportionate share. Plastids also arise through the division of existing mature plastids.

,ria

Mitochondria (singular: mitochondrion) are often referred to as the powerhouses of the cell, for it is within them that energy is released from organic molecules by the process of cellular respiration (the role of mitochondria in respiration is further discussed in Chapter 10). This energy is needed to keep the individual cells and the plant functioning as a whole. Carbon skeletons and fatty acid chains are also rearranged within mitochondria, allowing for the building of a wide variety of organic molecules. Mitochondria are numerous and tiny, typically measuring from 1 to 3 or more micrometers in length and having a width of roughly one-half micrometer; they are barely visible with light microscopes. They appear to be in constant motion in living cells and tend to accumulate in groups where energy is needed. They often divide in two; in fact, they all originate from the division of existing mitochondria.

Mitochondria typically are shaped like cucumbers, paddles, rods, or balls. A sectioned mitochondrion resembles a scooped-out watermelon with inward extensions of the rind forming mostly incomplete partitions perpendicular to the surface (Fig. 3.13). The appearance of incomplete partitions results from the fact that each mitochondrion is bounded by two membranes, with the inner membrane forming numerous platelike folds called cristae. The cristae greatly increase the surface area available to the enzymes contained in a matrix fluid. The number of cristae, as well as the number of mitochondria themselves, can change over time, depending on the activities taking place within the cell. The matrix fluid also contains DNA, RNA, ribosomes, proteins, and dissolved substances.

-crista matrix (fluid)

Figure 3.13 A mitochondrion greatly enlarged and cut away to show the cristae (folds of the inner membrane). A mitochondrion is about 2 micrometers long. (Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.)

-outer membrane

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