Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Membranes and Cell Walls
Plant Cell Walls

The thick plant Cell wall (Figs. 1–3) has an exceptionally complex Structure [113–116]. This intricate architecture provides plants with strength and rigidity, while enabling their Cells to elongate rapidly during growth. Northcote [113] compared The plant cell wall to fibreglass, a Glass-fiber-reinforced plastic. Specifically, The Cell wall contains microfibrils composed of Cellulose and other Polysaccharides embedded in a matrix, which is also predominantly polysaccharide-rich. During the Cytology/cytology/16.html">Early stages of green plant growth, the primary cell wall is laid down; it contains freely interlaced cellulose fibers approximately 10 nm in diameter, with a crystalline core of about ~4 nm. Such cellulose fibers contain 8,000–12,000 glucose residues.

As the plant grows, the secondary cell wall forms beneath the primary wall. It is built from numerous layers of densely packed microfibrils oriented in different directions in adjacent layers. In green plants, these microfibrils most commonly consist of cellulose, though they may also contain other polysaccharides. The fibrils of certain Algae are rich in Xylan and mannan.

The COMPOSITION OF THE matrix changes during plant growth. In the early developmental stages, the matrix consists primarily of pectic substances (polygalacturonic acid derivatives), but xylans and various neutral polysaccharides ("hemicellulose") appear later. The primary cell wall of dicotyledonous plants contains xyloglucans (comprising a linear glucan chain with side chains of xylose, galactose, and fucose residues), arabinogalactans, and rhamnogalacturonans (consisting of straight chains of galacturonic acid residues interspersed with rhamnose rings at specific intervals). These three complex polysaccharides are covalently linked to one another and to the cellulose microfibrils [114] (Figs. 5–12).

CARBOHYDRATES are the Main Components of Plant Cell Walls. However, In addition to carbohydrates, they contain small amounts of a glycoprotein known as extensin [117]. Similar to the Collagen of the Extracellular matrix in animal Tissues, this compound is rich in 4-hydroxyproline. It also incorporates arabinose and galactose attached as Oligosaccharides (consisting of approximately 9 residues) to the hydroxyl groups of the protein. At later developmental stages, when the walls lignify and become woody, significant amounts of lignins are deposited in the plant cells (Chap. 14, Sec. 3.6). These chemically stable polymers contain a high number of aromatic rings.

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FIG. 5–12. Proposed STRUCTURE OF THE plant cell wall. The relative sizes of the various wall components are not drawn to scale, but their proportions are close to physiological reality. The spacing between bundles of cellulose fibers is enlarged for clarity to show the interconnecting structures. The primary cell wall of a single cell may contain from 10 to 100 bundles of cellulose fibers [115]

The Biosynthesis of Pectins and hemicelluloses likely occurs within Golgi vesicles, from which these compounds are transported outward via exocytosis. Cellulose fibers, on the other hand, are presumably extruded across The Plasma Membrane. An interesting property of primary plant cell walls is their capacity for extremely rapid elongation during cell growth. This expansion of the cell walls, as well as the overall increase in cell volume, is driven by internal turgor pressure. After a certain developmental stage, this expansion is strictly regulated; under METABOLISM/18.html">The Influence of various Plant HORMONES, cells begin to elongate in a single direction only. Gibberellins exhibit the most pronounced activity of this kind (Chap. 12, Sec. 3.1), capable of inducing very rapid cell elongation.

Plant cell elongation is undoubtedly associated with the breaking of old bonds and The formation of new ones between polysaccharides, while preserving the integrity of the cellulose fibers and their ability to slide past one another [114].

The cell walls of Yeasts and Fungi consist of glucans, Chitin, and a mannan-protein complex. Certain highly branched mannan chains act as species-specific Antigens [118]. Much like the surface antigens of animal and bacterial cells, plant cell antigens exhibit immense structural diversity, which is of great Biomedical Importance. Yeasts serve as a convenient model Organism for studying the GENETIC ASPECTS OF the biosynthesis of Enzymes involved in mannan synthesis. They can be cultured in both haploid and hybrid-diploid forms, which greatly facilitates genetic analysis.

Questions and Problems

1. Stearic acid (1.16 g) was dissolved in 100 ml of ethanol. A 10 µl aliquot of the resulting solution was pipetted onto the clean surface of a dilute HCl solution (in a shallow tray) to form a stearic acid monolayer. This film was compressed (by moving a Teflon barrier) until the surface pressure π rose sharply to reach a value of 20 dyn∙cm^-1. Note that π = γ₀—γ, where γ is the surface tension measured in the presence of the film, and γ₀ is the higher surface tension of pure Water. The compressed film covered an area of 20×24 cm. Calculate the cross-sectional area of the alkyl chain in stearic acid. (See Davenport J. B., in: Biochemistry and Methodology of Lipids (Johnson A. S., Davenport J. B., eds.), pp. 47–83. Wiley-Interscience, New York, 1971; and Phillips M. C., in: Progress in Surface and Membrane Science (Danielli J. F., Rosenberg M. D., Cadenhead D. A., eds.), Vol. 5, pp. 139–221, Academic Press, New York, 1972.)

2. In 1925, E. Gorter and F. Grendel (J. Exp. Med., 41, 439) described experiments in which they extracted lipids with acetone from Erythrocyte membranes, prepared a monolayer from them, and measured the area of the compressed monolayer. They then estimated the surface area of the erythrocytes and calculated that The ratio of the lipid area (in the form of a monolayer) to the erythrocyte surface area is 1.9–2.0. Later experiments [11] established that each erythrocyte membrane contains 4.5∙10^-16 mol of Phospholipids and 3.1∙10^-16 mol of Cholesterol.

a. What area would the monolayer occupy if we assume that the cross-sectional areas of a phospholipid molecule and a cholesterol molecule in the membrane are 0.70 and 0.38 nm², respectively?

b. What would be the ratio of the area calculated in (a) to the surface area of an erythrocyte, given that its measured surface area is 167 µm²?

c. How would you account for the discrepancy between your calculated value and the corresponding value reported by Gorter and Grendel (see [11])?

3. Below are experimental data concerning the Structure and function of Introduction/36.html">Biological Membranes. Explain how these data relate to membrane Organization.

a. Many macrocyclic Antibiotics (such as nonactin and valinomycin) exhibit high selectivity in binding to alkali Metal Ions, forming 1 : 1 complexes. These complexes are readily soluble in nonpolar organic Solvents. These antibiotics increase both the electrical conductivity and the permeability of artificial phospholipid membranes to alkali metal ions. Valinomycin increases the electrical conductivity of chloroplast thylakoid membranes in the presence of K⁺ ions, but not Na⁺ ions; it also uncouples Oxidative Phosphorylation in Mitochondria (see Chapter 10).

b. Treatment of intact Chloroplasts with galactolipase releases galactose from galactosyldiglycerides. Treatment of erythrocyte ghosts with phospholipase C releases approximately 75% of the lipid phosphorus in a water-soluble form. In neither case is the structural integrity of the membrane disrupted.

c. Using sphingomyelin as a hapten, specific Antibodies against this lipid can be raised. It can be demonstrated that these antibodies interact with erythrocyte ghosts, but only on one side of the membrane.

4. Describe The structure of biological membranes and the specific Functions of their lipid, protein, and carbohydrate components. What are the differences between the inner and outer surfaces of the membrane?

5. Compare the Intracellular Distribution of triglycerides, phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, Glycolipids, and cholesterol. Evaluate the differences between the Two Sides of cell membranes.

6. What chemical principles underlie the modification of cell surface components using the following Reagents: (a) lactoperoxidase, (b) galactose oxidase, (c) formylmethionylsulfonemethylphosphate, (d) diiodosulfanilic acid diazonium salt, (e) fluorescent antibodies, and (f) ferritin-conjugated antibodies? Write the corresponding chemical reaction equations. Indicate which surface groups are modified. List the advantages of each reagent.

7. Which detergent would be more effective in the pH range from 2 to 3 — sodium lauryl sulfonate or sodium laurate? Why?

8. How much Energy is required to transport 3 equivalents of Na⁺ out of the cell and 2 equivalents of K⁺ into the cell, given that the intracellular concentrations of these ions are 10 mM and 100 mM, respectively, while their extracellular concentrations are 100 mM and 5 mM? Compare this value with the ∆G' of ATP Hydrolysis at pH 7. Assume that the membrane is permeable to Cl^- ions.

9. An E. coli cell is estimated to contain approximately 10⁵ molecules of envelope protein with a Molecular Weight of 36,500. What fraction of the bacterial surface would be covered by this protein if the molecules are spherical and packed closely in a hexagonal array? What is the diameter of the protein molecule? What would the cross-sectional area of the protein molecule have to be if 10⁵ molecules were to cover the entire surface? Propose a shape for the protein molecule that would satisfy this requirement.

10. Trace the movement of Ca²+ ions within the cell that trigger myofibril contraction, starting from the moment a Nerve Impulse reaches a Muscle cell (in a mammalian Skeletal Muscle) and ending with the completion of the contraction process. Name the specific Proteins and (or) structures involved in these processes.