Before considering individual compartments within plant cells where plant metabolites are synthesized and stored, we must first examine how a typical plant cell is organized and how its various components are related to one another. For this purpose, we shall refer to the cell illustrated in Figure 2.2. The "jacket" that encloses this cell is the cell wall. This is the primary site for polymerization of cell wall polysaccharides (cellulose, hemicellulose, and pectic polysaccha-rides), lignin, and amorphous silica gel in plants that accumulate this polymer. Within the cell wall is the plasma membrane that encloses the nucleus, cytoplasm, and various cellular organelles [chloroplasts or other plastids, mitochondria, endoplasmic reticulum, golgi apparatus (also called dictyosome), peroxy-somes or glyoxysomes, and vacuoles]. The nucleus is the information center of the cell. It is surrounded by a double lipid membrane and contains genetic information (DNA) needed to create proteins within the cell. Thus, within the nucleus of each cell is all the information needed to create the entire organism. The cytoplasm is the liquid phase of the cell that contains (1) the majority of the ribosomes involved in protein synthesis (the other main location of ribosomes is on the endoplasmic reticulum); (2) microtubules and microfilaments that provide a physical skeleton for the cell and also act in cellular trafficking of proteins and organelles; and (3) all the soluble enzymes of the cell not found within organelles or cellular membranes.
Each kind of organelle has many biochemical functions but the generally accepted function of each major class of organelle is as follows. The chloroplasts (Figure 2.3) of a plant cell are organelles bounded by a double lipid membrane which contains the enzymes and pigments (such as chlorophyll) that perform photosynthesis. There are, however, other types of plastids, such as those found in the petals of flowers (chromoplasts or leucoplasts) that do not develop the "machinery" to perform photosynthesis, yet still act as locations for the pro-
duction of many plant products. Mitochondria (Figure 2.4) are also surrounded duction of many plant products. Mitochondria (Figure 2.4) are also surrounded
by two lipid membranes. These organelles are the location of the TCA cycle, the respiratory chain, and oxidative phosphorylation all of which produce ATP. The endoplasmic reticulum (ER) (Figure 2.5) is a system of membrane-bound tubes and flattened sacs that spread throughout the cell and work in conjunction with dictyosomes (on Golgi bodies) to produce and secrete various compounds as well as to deliver specific proteins and membrane lipids to their proper
locations within the cell. Peroxysomes (Figures 2.3 and 2.6) are microbody organelles that have the very important function of housing the formation of toxic peroxides that are necessary for other metabolic mechanisms but would otherwise kill the cell. Peroxysomes also break down fats and participate in photorespiration (an important metabolic pathway coupled to photosynthesis). Glyoxysomes are specialized peroxysomes found only in the early stages of plant development. They contain the enzymes necessary for the conversion of stored lipids to carbohydrates in such processes as seed germination where photosyn-
thesis is not yet possible. Finally, the vacuole is a liquid-filled cavity in the plant cell enclosed by a single membrane. Vacuoles play a wide variety of roles in cellular metabolism, some being digestive chambers, some storage chambers, some "waste bins", but they also play a very important role as a support structure. Water is pumped into the vacuole of each plant cell causing a build up of pressure, called turgor, which allows non-woody plants to remain standing. Without an adequate supply of water a plant will wilt.
Remember that what you see in Figure 2.2 represents only a static view of the cell at only one point in time. Most of the cell's contents are in a continuous state of motion, called cyclosis, and each molecule within the cell is, at a molecular level, experiencing Brownian movement. So, each cell of even the most solid looking of plants is actually a dynamic system of complex biochemical pathways which, when linked together, not only result in the organisms that we see but also define and regulate the interaction that the plant has with its environment. We shall now take a look at the individual components of a plant cell and some of the kinds of biosynthetic pathways known to occur in each structure.
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WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.