Plant Cell Photomicrograph

Electron Photomicrograph Chloroplast

Figure 3.4 Anatomy of a young leaf cell. A. Generalized drawing. B. Transmission electron micrograph of a small leaf cell with cross sections of two chloroplasts visible, x20,000. (A. From Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved. B. © Newcomb/Wergin/BPS.)

Figure 3.4 Anatomy of a young leaf cell. A. Generalized drawing. B. Transmission electron micrograph of a small leaf cell with cross sections of two chloroplasts visible, x20,000. (A. From Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved. B. © Newcomb/Wergin/BPS.)

Chapter 3

Chapter 3

Electron Micrograph Young Plant CellAlgal Cell Mcgraw Hill

Figure 3.5 Anatomy of a plant cell. A. Scanning electron micrograph, x20,200. B. Diagram showing interpretation of structures in the micrograph. (Electron micrograph © Gary T. Cole/BPS) (From Moore, Clark, and Vodopich, Botany, 2nd edition. © 1998 The McGraw-Hill Companies. All rights reserved.)

Full-grown organisms have astronomical numbers of cells. For example, it has been calculated that a single mature leaf of a pear tree contains 50 million cells and that the total number of cells in the roots, stem, branches, leaves, and fruit of a full-grown pear tree exceeds 15 trillion. Can you imagine how many cells there are in a 3,000-year-old redwood tree of California, which may reach heights of 90 meters (300 feet) and measure up to 4.5 meters (15 feet) in diameter near the base?

Some cells are boxlike with six walls, but others assume a wide variety of shapes, depending on their location and function. The most abundant cells in the younger parts of plants and fruits may be more or less spherical, like bubbles, when they are first formed, but as they press against each other, they commonly end up with an average of 14 sides by the time they are mature. These cell types are discussed in Chapter 4.

The Cell Wall

A novelty song of more than 50 years ago listed food items the writer said he disliked. Each verse ended with the line, "But I like bananas because they have no bones!" Indeed, bananas and all plants differ from larger animals in having no bones or similar internal skeletal structures. Yet large trees support branches and leaves weighing many tons. They can do this because most plant cells have either rigid walls that provide the support afforded to animals by bones or semirigid walls that provide flexibility. At the same time, the walls protect delicate cell contents within. When millions of these cells function together as a tissue, their collective strength is enor mous. The redwoods and Tasmanian Eucalyptus trees, which are the largest trees alive today, exceed the mass and volume of the largest land animals, the elephants, by more than a hundred times. The wood of one giant redwood tree could support the combined weight of a thousand elephants.

The first cell structure discovered by Robert Hooke in 1665 was the cell wall, and among plant cell structures observed with a microscope, the cell wall is the most obvious because it defines the shape of the cell. Many of the prepared specimens observed with a microscope in plant biology are merely stained remnants of once-living cells. But the vast diversity of cell walls within and among species tells a story about the structure and function of each cell. For instance, epidermal cells, which form a thin layer on the surfaces of all plant organs, often have unusual shapes and sizes. Some such cells form hairs that may secrete substances that discourage animals from grazing on the plants producing them. Thin-walled cells found beneath the epidermis of leaves are specialized for their function of photosynthesis (discussed in Chapter 10); and thick-walled cells of wood help to transport water without collapsing. The structure and function of different plant cells, and the tissues they form, are addressed in Chapter 4.

The main structural component of cell walls is cellulose, which is composed of 100 to 15,000 glucose monomers in long chains, and is the most abundant polymer on earth. As a primary food source for grazing animals and at least indirectly for nearly all other living organisms, it could be said that most life on earth relies directly or indirectly on the cell wall. Humans also depend on cell walls because they provide clothing, shelter, furniture, paper, and fuel.

Cells

Microfibrils Diagrams
Figure 3.6A A small portion of a cell wall of the green alga Chaetomorpha melagonium, showing how cellulose microfibrils are laid down. Each microfibril is composed of numerous molecules of cellulose, x24,000. (Electron micrograph courtesy Eva Frei and R. D. Preston)

In addition to the cellulose, cell walls typically contain a matrix of hemicellulose (a gluelike substance that holds cellulose fibrils together), pectin (the organic material that gives stiffness to fruit jellies), and glycoproteins (proteins that have sugars associated with their molecules).

A middle lamella, which consists of a layer of pectin, is first produced when new cell walls are formed. This middle lamella is normally shared by two adjacent cells and is so thin that it may not be visible with an ordinary light microscope unless it is specially stained. A flexible primary wall, consisting of a fine network of cellulose, hemicellulose, pectin, and glycoproteins, is laid down on either side of the middle lamella (Fig. 3.6A). Reorganization, synthesis of new molecules, and insertion of new wall polymers lead to rearrangement of the cell wall during growth. Secondary walls, which are produced inside the primary walls, are derived from primary walls by thickening and inclusion of lignin, a complex polymer.

Secondary cell walls of plants generally contain more cellulose (40% to 80%) than primary walls. As the cell ages, wall thickness can vary, occupying as little as 5% to more than 95% of the volume of the cells. During secondary wall formation, cellulose microfibrils become embedded in lignin, much like steel rods are embedded in concrete to form prestressed concrete (Fig. 3.6B).

Communication Between Cells

Cells that store, manufacture, or process food have thin walls, while those involved in support usually have relatively thick walls. Although each living cell is capable of independently carrying on complex activities, it is essential that these activities be coordinated through some means of communication among all the living cells of an organism. Fluids and dissolved substances can pass through primary walls of adjacent cells via plasmodesmata (singular: plas-modesma), which are tiny strands of cytoplasm that extend between the cells through minute openings (see Fig. 3.20). The translocation of sugars, amino acids, ions, and other substances occurs through the plasmodesmata. The middle lamellae and most cell walls are, however, permeable and permit slower movement of water and dissolved substances between cells.

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