H J

-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.)

Tonoplast Membrane
Figure 3.14 A small portion of a root cap cell of tobacco, x100,000. V = vacuole; T = vacuolar membrane (tonoplast); G = dic-tyosome with vesicles (arrows); M = mitochondrion; ER = endoplasmic reticulum; PM = plasma membrane; CW = cell wall. (Electron-micrograph courtesy John Z. Kiss)

Microbodies

Various small bodies distributed throughout the cytoplasm tend to give it a granular appearance. Examples of such components include types of small, spherical organelles called microbodies, which contain specialized enzymes and are bounded by a single membrane. Peroxisomes, for instance, contain enzymes needed by some plants to survive during hot conditions in a process called photorespiration (discussed in Chapter 10), whereas glyoxisomes contain enzymes that aid in the conversion of fats to carbohydrates during, for example, the germination of seeds containing fats. If present, peroxi-somes are generally found associated with chloroplasts, and glyoxisomes usually are located near mitochondria. During a plant's life cycle, peroxisomes and glyoxisomes may increase in number at stages when the need for them is greatest.

At one time, lipid, fat, or wax droplets commonly found in cytoplasm were believed to be bounded by a membrane; recent evidence, however, suggests no membrane is present, and some, therefore, do not consider them true organelles. Another organelle, called a lysosome, stores enzymes that digest proteins and certain other large molecules but is apparently confined to animal cells. The digestive activities of lysosomes are similar to those of the vacuoles of plant cells (discussed next).

Vacuoles

In a mature living plant cell, as much as 90% or more of the volume may be taken up by one or two large central vacuoles that are bounded by vacuolar membranes (tonoplasts) (Fig. 3.14). The vacuolar membranes, which constitute the inner boundaries of the living part of the cell, are similar in structure and function to plasma membranes.

The vacuole evidently received its name because of a belief that it was just an empty space; hence its name has the same Latin root as the word vacuum (from vacuus— meaning "empty"). Vacuoles, however, are filled with a watery fluid called cell sap, which is slightly to moderately acidic. Cell sap, which helps to maintain pressures within the cell (see the discussion of osmosis in Chapter 9), contains dissolved substances, such as salts, sugars, organic acids, and small quantities of soluble proteins. It also frequently contains water-soluble pigments. These pigments, called anthocyanins, are responsible for many of the red, blue, or purple colors of flowers and some reddish leaves. In some instances, anthocyanins accumulate to a greater extent in response to cold temperatures in the fall. They should not be confused, however, with the red and orange carotenoid pigments confined to the chromoplasts. Yellow

Chapter 3

Chapter 3

Small Flowering Shrubs
Figure 3.15 A small portion of a plant cell wall with microtubules more or less perpendicular to it, x100,000. (Electron micrograph courtesy John Z. Kiss)

carotenoid pigments (carotenes) also play a role in fall leaf coloration (discussed in Chapter 7).

Sometimes, large crystals of waste products form within the cell sap after certain ions have become concentrated there. Vacuoles in newly formed cells are usually tiny and numerous. They increase in size and unite as the cell matures. In addition to accumulating the various substances and ions mentioned above, vacuoles are apparently also involved in the recycling of certain materials within the cell and even aid in the breakdown and digestion of organelles, such as plastids and mitochondria.

The Cytoskeleton

The cytoskeleton is involved in movement within a cell and in a cell's architecture. It is an intricate network constructed mainly of two kinds of fibers—microtubules and microfilaments.

Microtubules apparently control the addition of cellulose to the cell wall (Fig. 3.15). They are also involved in cell division, movement of cytoplasmic organelles, controlling the movement of vesicles containing cell-wall components assembled by dictyosomes, and movement of the tiny whiplike flagella and cilia possessed by some cells (see the section on plant movements in Chapter 11).

Microtubules are unbranched, thin, hollow, tubelike structures that resemble tiny straws. They are composed of proteins called tubulins and are of varying lengths, most being between 15 and 25 nanometers in diameter. They are most commonly found just inside the plasma membrane. Microtubules are also found in the special fibers that form the spindles and phragmo-plasts of dividing cells discussed later in this chapter.

Microfilaments, which play a major role in the contraction and movement of cells in multicellular animals, are present in nearly all cells. They are three or four times thinner than microtubules and consist of long, fine threads of protein with an average diameter of 6 nanometers. They are often in bundles and appear to play a role in the cytoplasmic streaming (sometimes referred to as cyclosis) that occurs in all living cells. When cytoplasmic streaming is occurring, a microscope reveals the apparent movement of organelles as a current within the cytoplasm carries them around within the walls. This streaming probably facilitates exchanges of materials within the cell and plays a role in the movement of substances from cell to cell. The precise nature and origin of cytoplasmic streaming is still not known, but there is evidence that bundles of microfilaments may be responsible for it. Other evidence suggests that it may be related to the transport of cellular substances by microtubules.

CELLULAR REPRODUCTION The Cell Cycle

When cells divide, they go through an orderly series of events known as the cell cycle. This cycle is usually divided into interphase and mitosis, mitosis itself being subdivided into four phases (Fig. 3.16). The length of the cell cycle varies with the kind of organism involved, the type of cell within an organism, and with temperature and other environmental factors. In most instances, however, interphase may occupy up to 90% or more of the time it takes to complete the cycle. The relatively small amount of time involved in actual division explains why most cells viewed with a microscope are in interphase, and cells in stages of mitosis can be hard to find.

Interphase

Living cells that are not dividing are said to be in interphase, a period during which chromosomes are not visible with light microscopes. It is such cells that have been discussed up to this point.

Cells

Time Period Cell Cycle
Figure 3.16 A diagram of a cell cycle showing relative amounts of time involved in interphase, mitosis, and cytokinesis.

For many years, immature cells were considered to be "resting" when they were not actually dividing, but we know now that three consecutive periods of intense activity take place during interphase. These intervals are designated as gap (or growth) 1, synthesis, and gap (or growth) 2 periods, usually referred to as G1, S, and G2, respectively.

The G1 period is relatively lengthy and begins immediately after a nucleus has divided. During this period, the cell increases in size. Also, ribosomes, RNA, and substances that either inhibit or stimulate the S period that follows are produced. During the S period, the unique process of DNA replication (duplication) takes place. Details of this process and of DNA structure are discussed in Chapter 13. In the G2 period, mitochondria and other organelles divide, and microtubules and other substances directly involved in mitosis are produced. Coiling and condensation of chromosomes also begin during G2.

Mitosis

All organisms begin life as a single cell. This cell usually divides almost immediately, producing two new cells. These two cells, in turn, divide, with each of them producing two more cells. This process, called mitosis (Fig. 3.17) occurs in an organism until it dies. It ensures that the two new cells (daughter cells) resulting from a cell undergoing mitosis each have precisely equal amounts of DNA and certain other substances duplicated during interphase. Strictly speaking, mitosis refers to the division of the nucleus alone, but with a few exceptions seen in algae and fungi (discussed in Chapters 18 and 19), the division of the remainder of the cell, called cytokinesis, normally accompanies or follows mitosis. Both processes will be considered together here.

In flowering plants, conifers, and other higher plants, mitosis occurs in specific regions, or tissues, called meristems (see Fig. 4.1). Meristems are found in the root and stem tips and also in a thin, perforated, and branching cylinder of tissue called the vascular cambium (often referred to simply as the cambium), located in the interior of some stems and roots a short distance from the surface. In some herbaceous and most woody plants, a second meristem similar in form to the cambium lies between the cambium and the outer bark. This second meristem is called the cork cambium. These specific tissues are discussed in Chapters 4, 5, and 6.

When mitosis occurs, the number of chromosomes in the nucleus, whether small or large, makes no difference in the way the process takes place. The daughter cells that result from mitosis each have exactly the same number of chromosomes and distribution of DNA as the parent cell. Mitosis is a continuous process, which may take as little as 5 minutes or as long as several hours from start to finish. Typically, however, it takes from 30 minutes to 2 or 3 hours.

Mitosis is initiated with the appearance of a ringlike pre-prophase band of microtubules just beneath the plasma membrane and is usually divided into four arbitrary phases, primarily for convenience. Descriptions of the phases follow.

Prophase

The main features of prophase (Fig. 3.17A) are (1) the chromosomes become shorter and thicker, and their two-stranded nature becomes apparent; (2) the nuclear envelope fragments, and the nucleolus disintegrates.

Prophase utilizes about as much time as the remaining three phases combined. Before prophase begins, a pre-prophase band, formed from microtubules and microfilaments inside the plasma membrane, develops in a narrow bundle around the nucleus. The beginning of prophase is marked by the appearance of the chromosomes as faint threads in the nucleus. These chromosomes gradually coil or fold into thicker and shorter structures, and soon, two strands, or chromatids, can be distinguished for each chromosome. The chromatids are themselves independently coiled and are identical to each other. The coils appear to tighten and condense until the chromosomes have become relatively short, thick, and rodlike, with areas called centromeres holding each pair of chromatids together.

The centromere is located at a constriction on the chromosome (Fig. 3.18). A kinetochore, which is a dense region composed of a protein complex, is located on the outer surface of each centromere; spindle fibers become attached to the kinetochore. When examined with a light microscope, the centromeres appear to be single structures, but they actually have become double by the G2 stage of interphase and simply function as a single unit at this point. They may be located almost anywhere on a chromosome but tend to be toward the middle. Sometimes other constrictions may appear on individual chromosomes, usually toward one end,

Stern-Jansky-Bidlack: I 3. Cells I Text I I © The McGraw-Hill

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Ninth Edition

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Stern-Jansky-Bidlack: I 3. Cells I Text I I © The McGraw-Hill

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Microscapes

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Microscapes

Scientists use the scanning electron microscope (SEM) to study the details of many different types of surfaces. Unlike the light microscope or even a transmission electron microscope that form images by passing either a beam of light or electrons through a thin slice of fixed tissue, the SEM's great advantage is its ability to allow us to look at surfaces of specimens and observe topographical detail not possible with other types of microscopy.

The basic concept of a scanning electron microscope is that a finely focused beam of electrons is scanned across the surface of the specimen. The high-velocity electrons from the beam create an energetic interaction with the surface layers. These electron-specimen interactions generate particles that are emitted from the specimen and can be collected with a detector and sent to a TV screen (cathode ray tube). Particles that form the typical scanning electron image are called secondary electrons because they come from the electrons in the specimen itself. The more electrons a particular region emits, the brighter the image will be on the TV screen. The end result, therefore, is brightness associated with surface characteristics and an image that looks very much like a normally illuminated subject. SEM images typically contain a good deal of topographical detail because the electrons that are emitted and produced on the TV screen represent a one-for-one correspondence with the contours of the specimen.

All scanning electron images have one very distinctive characteristic because of this feature of electron emission and display—the images are three dimensional rather than the flat two-dimensional images obtained from other types of microscopes. The images can be understood even by the lay person because the eye is accustomed to interpreting objects that are in three dimensions.

Take, for instance, a leaf surface, which looks smooth with an ordinary light microscope. But with a scanning electron microscope, the leaf surface is a rich composition of undulating cell walls, cells joined together like pieces of a jigsaw puzzle, squiggly ridges of waxes that look like frosting decorations on a cake, and lens-shaped stomata (Box Figure 3.1A). The stomata even provides a window into the interior of the leaf where deeper cellular layers are visible. Or look at the tentacles seen in Box Figure 3.1B that remind us of some sinister sea creature. No stinging tentacles here but rather the surface of a small flower of a common weed called mouse-ear cress (Arabidopsis thaliana). The "tentacles" are actually stigmatic papillae that serve to trap pollen grains, which are released from the pollen sacs of the flower (see Chapter 8 for details of flower structure).

Box Figure 3.1A A scanning electron micrograph of the surface of a sepal (modified leaf) from a flower of the mouse-ear cress, Arabidopsis thaliana, x2,000. (Electron micrograph by Daniel Scheirer)

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