By Hans-Walter Heldt
Academic Press Copyright © 2008
Spektrum Akademischer Verlag Heidelberg
All right reserved. ISBN: 978-0-12-384987-8
Chapter One A leaf cell consists of several metabolic compartments
In higher plants photosynthesis occurs mainly in the mesophyll, the chloroplast-rich tissue of leaves. Figure 1.1 shows an electron micrograph of a mesophyll cell and Figure 1.2 shows a schematic presentation of the cell structure. The cellular contents are surrounded by a plasma membrane called the plasmalemma and are enclosed by a cell wall. The cell contains organelles, each with its own characteristic shape, which divide the cell into various compartments (subcellular compartments). Each compartment has specialized metabolic functions, which will be discussed in detail in the following chapters (Table 1.1). The largest organelle, the vacuole, usually fills about 80% of the total cell volume. Chloroplasts represent the next largest compartment, and the rest of the cell volume is filled with mitochondria, peroxisomes, the nucleus, the endoplasmic reticulum, the Golgi bodies, and, outside these organelles, the cell plasma, called cytosol. In addition, there are oil bodies derived from the endoplasmic reticulum. These oil bodies, which occur in seeds and some other tissues (e.g., root nodules), are storage organelles for triglycerides (see Chapter 15).
The nucleus is surrounded by the nuclear envelope, which consists of the two membranes of the endoplasmic reticulum. The space between the two membranes is known as the perinuclear space. The nuclear envelope is interrupted by nuclear pores with a diameter of about 50 nm. The nucleus contains chromatin, consisting of DNA double strands that are stabilized by being bound to basic proteins (histones). The genes of the nucleus are collectively referred to as the nuclear genome. Within the nucleus, usually off-center, lies the nucleolus, where ribosomal subunits are formed. These ribosomal subunits and the messenger RNA formed by transcription of the DNA in the nucleus migrate through the nuclear pores to the ribosomes in the cytosol, the site of protein biosynthesis. The synthesized proteins are distributed between the different cell compartments according to their final destination.
The cell contains in its interior the cytoskeleton, which is a three-dimensional network of fiber proteins. Important elements of the cytoskeleton are the microtubuli and the microfilaments, both macromolecules formed by the aggregation of soluble (globular) proteins. Microtubuli are tubular structures composed of α and β tubuline monomers. The microtubuli are connected to a large number of different motor proteins that transport bound organelles along the microtubuli at the expense of ATP. Microfilaments are chains of polymerized actin that interact with myosin to achieve movement. Actin and myosin are the main constituents of the animal muscle. The cytoskeleton has many important cellular functions. It is involved in the spatial organization of the organelles within the cell, enables thermal stability, plays an important role in cell division, and has a function in cell-to-cell communication.
1.1 The cell wall gives the plant cell mechanical stability
The difference between plant cells and animal cells is that plant cells have a cell wall. This wall limits the volume of the plant cell. The water taken up into the cell by osmosis presses the plasma membrane against the inside of the cell wall, thus giving the cell mechanical stability. The cell walls are very complex structures; in Arabidopsis about 1,000 genes were found to be involved in its synthesis. Cell walls also protect against infections.
The cell wall consists mainly of carbohydrates and proteins
The cell wall of a higher plant is made up of about 90% carbohydrates and 10% proteins. The main carbohydrate constituent is cellulose. Cellulose is an unbranched polymer consisting of D-glucose molecules, which are connected to each other by β-1,4 glycosidic linkages (Fig. 1.3A). Each glucose unit is rotated by 180° from its neighbor, so that very long straight chains can be formed with a chain length of 2,000 to 25,000 glucose residues. About 36 cellulose chains are associated by interchain hydrogen bonds to a crystalline lattice structure known as a microfibril. These crystalline regions are impermeable to water. The microfibrils have an unusually high tensile strength, are very resistant to chemical and biological degradations, and are in fact so stable that they are very difficult to hydrolyze. However, many bacteria and fungi have cellulose-hydrolyzing enzymes (cellulases). These bacteria can be found in the digestive tract of some animals (e.g., ruminants), thus enabling them to digest grass and straw. It is interesting to note that cellulose is the most abundant organic substance on earth, representing about half of the total organically bound carbon.
Hemicelluloses are also important constituents of the cell wall. They are defined as those polysaccharides that can be extracted by alkaline solutions. The name is derived from an initial belief, which later turned out to be incorrect, that hemicelluloses are precursors of cellulose. Hemicelluloses consist of a variety of polysaccharides that contain, in addition to D-glucose, other carbohydrates such as the hexoses D-mannose, D-galactose, D-fucose, and the pentoses D-xylose and L-arabinose. Figure 1.3B shows xyloglycan as an example of a hemicellulose. The basic structure is a β-1,4-glucan chain to which xylose residues are bound via α-1,6 glycosidic linkages, which in part are linked to D-galactose and D-fucose. In addition to this, L-arabinose residues are linked to the 2'OH group of the glucose.
Another major constituent of the cell wall is pectin, a mixture of polymers from sugar acids, such as D-galacturonic acid, which are connected by α-1,4 glycosidic links (Fig. 1.3C). Some of the carboxyl groups are esterifi ed by methyl groups. The free carboxyl groups of adjacent chains are linked by Ca++ and Mg++ ions (Fig. 1.4). When Mg++ and Ca++ ions are absent, pectin is a soluble compound. The Ca++/Mg++ salt of pectin forms an amorphous, deformable gel that is able to swell. Pectins function like glue in sticking neighboring cells together, but these cells can be detached again during plant growth. The food industry makes use of this property of pectin when preparing jellies and jams.
The structural proteins of the cell wall are connected by glycosidic linkages to the branched polysaccharide chains and belong to the class of proteins known as glycoproteins. The carbohydrate portion of these glycoproteins varies from 50% to over 90%.
For a plant cell to grow, the very rigid cell wall has to be loosened in a precisely controlled way. This is facilitated by the protein expansin, which occurs in growing tissues of all flowering plants. It probably functions by breaking hydrogen bonds between cellulose microfibrils and cross-linking polysaccharides. Cell walls also contain waxes (Chapter 15), cutin, and suberin (Chapter 18).
In a monocot plant, the primary wall (i.e., the wall initially formed after the growth of the cell) consists of 20% to 30% cellulose, 25% hemicellulose, 30% pectin, and 5% to 10% glycoprotein. It is permeable for water. Pectin makes the wall elastic and, together with the glycoproteins and the hemicellulose, forms the matrix in which the cellulose microfibrils are embedded. When the cell has reached its final size and shape, another layer, the secondary wall, which consists mainly of cellulose, is added to the primary wall. The microfibrils in the secondary wall are arranged in a layered structure like plywood (Fig. 1.5).
The incorporation of lignin in the secondary wall causes the lignification of plant parts and the corresponding cells die, leaving the dead cells with only a supporting function (e.g., forming the branches and twigs of trees or the stems of herbaceous plants). Lignin is formed by the polymerization of the phenylpropane derivatives cumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, resulting in a very solid structure (section 18.3). Dry wood consists of about 30% lignin, 40% cellulose, and 30% hemicellulose. After cellulose, lignin is the most abundant natural compound on earth.
Plasmodesmata connect neighboring cells
Neighboring cells are normally connected by plasmodesmata thrusting through the cell walls. Plant cells often contain 1,000–10,000 plasmodesmata. In its basic structure plasmodesmata allow the passage of molecules up to a molecular mass of 800 to 1,200 Dalton, but, by mechanisms to be discussed in the following, plasmodesmata can be widened to allow the passage of much larger molecules. Plasmodesmata connect many plant cells to form a single large metabolic compartment where the metabolites in the cytosol can move between the various cells by diffusion. This continuous compartment formed by different plant cells (Fig. 1.6) is called the symplast. In contrast, the spaces between cells, which are often continuous, are termed the extracellular space or the apoplast (Figs. 1.2, 1.6).
Figure 1.7 shows a schematic presentation of a plasmodesm. The tube like opening through the cell wall is lined by the plasma membrane, which is continuous between the neighboring cells. In the interior of this tube there is another tube-like membrane structure, which is part of the endoplasmatic reticulum (ER) of the neighboring cells. In this way the ER system of the entire symplast represents a continuous compartment. The space between the plasma membrane and the ER membrane forms the diffusion pathway between the cytosol of neighboring cells. There are probably two mechanisms for increasing this opening of the plasmodesmata. A gated pathway widens the plasmodesmata to allow the unspecific passage of molecules with a mass of up to 20,000 Dalton. The details of the regulation of this gated pathway remain to be elucidated. In the selective trafficking the widening is caused by helper proteins, which are able to bind specifically macromolecules such as RNAs in order to guide these through the plasmodesm. This was first observed with virus movement proteins encoded by viruses, which form complexes with virus RNAs to facilitate their passage across the plasmodesm and in this way enable the spreading of the viruses over the entire symplast. By now many of these virus movement proteins have been identified, and it was also observed that plants produce movement proteins that guide macromolecules through plasmodesmata. Apparently this represents a general transport process of which the viruses take advantage. It is presumed that the cell ' s own movement proteins, upon the consumption of ATP, facilitate the transfer of macromolecules, such as RNA and proteins, from one cell to the next via the plasmodesmata. In this way transcription factors may be distributed in a regulated mode as signals via the symplast, which might play an important role during defense reactions against pathogen infections.
The plant cell wall, which is very rigid and resistant, can be lysed by cellulose and pectin hydrolyzing enzymes obtained from microorganisms. When leaf pieces are incubated with these enzymes, plant cells can be obtained without the cell wall. These naked cells are called protoplasts. Protoplasts, however, are stable only in an isotonic medium in which the osmotic pressure corresponds to the osmotic pressure of the cell fluid. In pure water the protoplasts, as they have no cell wall, swell so much that they burst. In appropriate media, the protoplasts of many plants are viable, they can be propagated in cell culture, and they can be stimulated to form a cell wall and even to regenerate a whole new plant.
1.2 Vacuoles have multiple functions
The vacuole is enclosed by a membrane, called a tonoplast. The number and size of the vacuoles in different plant cells vary greatly. Young cells contain a larger number of smaller vacuoles but, taken as a whole, occupy only a minor part of the cell volume. When cells mature, the individual vacuoles amalgamate to form a central vacuole (Figs. 1.1 and 1.2). The increased volume of the mature cell is due primarily to the enlargement of the vacuole. In cells of storage or epidermal tissues, the vacuole often takes up almost the entire cellular space.
An important function of the vacuole is to maintain cell turgor. For this purpose, salts, mainly from inorganic and organic acids, are accumulated in the vacuole. The accumulation of these osmotically active substances draws water into the vacuole, which in turn causes the tonoplast to press the protoplasm of the cell against the surrounding cell wall. Plant turgor is responsible for the rigidity of nonwoody plant parts. The plant wilts when the turgor decreases due to lack of water.
Vacuoles have an important function in recycling those cellular constituents that are defective or no longer required. Vacuoles contain hydrolytic enzymes for degrading various macromolecules such as proteins, nucleic acids, and many polysaccharides. Structures, such as mitochondria, can be transferred by endocytosis to the vacuole and are digested there. For this reason one speaks of lytic vacuoles. The resulting degradation products, such as amino acids and carbohydrates, are made available to the cell. This is especially important during senescence (see section 19.5) when prior to abscission, part of the constituents of the leaves are mobilized to support the propagation and growth of seeds.
Last, but not least, vacuoles also function as waste deposits. With the exception of gaseous substances, leaves are unable to rid themselves of waste products or xenobiotics such as herbicides. These are ultimately deposited in the vacuole (Chapter 12).
In addition, vacuoles also have a storage function. Many plants use the vacuole to store reserves of nitrate and phosphate. Some plants store malic acid temporarily in the vacuoles in a diurnal cycle (see section 8.5). Vacuoles of storage tissues contain carbohydrates (section 13.3) and storage proteins (Chapter 14). Many plant cells contain different types of vacuoles (e.g., lytic vacuoles and protein storage vacuoles next to each other).
The storage function of vacuoles plays a role when utilizing plants as natural protein factories. Genetic engineering now makes it possible to express economically important proteins (e.g., antibodies) in plants, where the vacuole storage system functions as a cellular storage compartment for accumulating high amounts of these proteins. Since normal techniques could be used for the cultivation and harvest of the plants, this method has the advantage that large amounts of proteins can be produced at low costs.
1.3 Plastids have evolved from cyanobacteria
Plastids are cell organelles which occur only in plant cells. They multiply by division and in most cases are maternally inherited. This means that all the plastids in a plant usually have descended from the proplastids in the egg cell. During cell differentiation, the proplastids can differentiate into green chloroplasts, colored chromoplasts, and colorless leucoplasts. Plastids possess their own genome, of which many copies are present in each plastid. The plastid genome (plastome) has properties similar to that of the prokaryotic genome, e.g., of cyanobacteria, but encodes only a minor part of the plastid proteins; most of the chloroplast proteins are encoded in the nucleus and are subsequently transported into the plastids. The proteins encoded by the plastome comprise enzymes for replication, gene expression, and protein synthesis, and part of the proteins of the photosynthetic electron transport chain and of the ATP synthase.
As early as 1883 the botanist Andreas Schimper postulated that plastids are evolutionary descendants of intracellular symbionts, thus founding the basis for the endosymbiont hypothesis. According to this hypothesis, the plastids descend from cyanobacteria, which were taken up by phagocytosis into a host cell (Fig. 1.8) and lived there in a symbiotic relationship. Through time these endosymbionts lost the ability to live independently because a large portion of the genetic information of the plastid genome was transferred to the nucleus. Comparative DNA sequence analyses of proteins from chloroplasts and from early forms of cyanobacteria allow the conclusion that all chloroplasts of the plant kingdom derive from a symbiotic event. Therefore it is justified to speak of the endosymbiotic theory.
Excerpted from Plant Biochemistry by Hans-Walter Heldt Copyright © 2008
by Spektrum Akademischer Verlag Heidelberg . Excerpted by permission of Academic Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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