Plants Are Autotrophs

At night, there is no photosynthesis and respiratory energy becomes as important in plants as in other organisms. The starch that accumulated during the day is metabolized by the activation ofphosphorylase and some glycolytic enzymes to triose phosphate (Fig. 1.7), 

The dark reaction (Calvin cycle) uses the NADPH and ATP to make glyceraldehyde 3-phosphate (triose phosphate), which is metabolized initially to starch, sucrose, and cellulose. Starch and sucrose are the major plant storage products. Starch is synthesized from ADP-glucose in the chloroplast, sucrose from fructose 6-phosphate and UDP-glucose in the leaf cytosol. 

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Plants must be especially versatile in their handling of carbohydrates, for several reasons. First, plants are autotrophs, able to convert inorganic carbon (as C02) into organic compounds. Second, biosynthesis occurs primarily in plastids, membrane-bounded organelles unique to plants, and the movement of intermediates between cellular compartments is an important aspect of metabolism. Third, plants are not motile they cannot move to find better supplies of water, sunlight, or nutrients. They must have sufficient metabolic flexibility to allow them to adapt to changing conditions in the place where they are rooted. Finally, plants have thick cell walls made of carbohydrate polymers, which must be assembled outside the plasma membrane and which constitute a significant proportion of the cell s carbohydrate. 

The other two kingdoms, Plantae and Animalia, are also composed of multicelled organisms. Plants, including seaweeds, trees, and dandelions, do not move around but get their food by converting the Sun s energy into simple carbon compounds. Therefore, plants are autotrophs. Animals, on the other hand, cannot make their own food. These organisms are heterotrophs, and they include fish, whales, and humans, all of which must actively seek the food they eat. 

Animals and bacteria are heterotrophs they obtain carbon in various forms as food and metabolize many forms of it to provide energy and body structure. Plants are autotrophs all their carbon comes from C02 powered by photosynthesis. Photosynthesis occurs within the thylakoid membranes of chloroplasts in plant leaves, and it is mediated by chlorophyll. The light reaction splits water into 02, electrons, and protons (H+). NADPH is produced by electron transport and ATP synthesis by associated proton transport. 

Tobacco plants, like other higher plants, are autotrophs possessing the capacity to synthesize all of its complex organic materials provided that carbon dioxide, water, minerals, and the proper physical environment are available. Their chemical composition is influenced by environmental factors such as light, temperature, moisture, soil type, and cultural practices, as well as inorganic nutrition. 

In this section and sections H - K the general principles and strategy of synthesis of the many carbon compounds found in living things will be considered. Since green plants and autotrophic bacteria are able to assemble all of their needed carbon compounds from C02, let us first examine the mechanisms by which this is accomplished. We will also need to ask how some organisms are able to subsist on such simple compounds as methane, formate, or acetate. 

Despite its importance in ecosystem C fluxes, soil respiration has limitations as a constraint on SOM turnover, for two main reasons. First, it is difficult to partition soil respiration into its two sources (1) decomposition of SOM by microbes (heterotrophic respiration) and (2) respiration from live plant roots (autotrophic respiration) (Kuzyakov, 2006). As a result, an increase in soil respiration may indicate not only an increase in SOM decomposition but also an increase in root respiration. Second, it is likely that in most soils only a small fraction of total SOM contributes to heterotrophic respiration. As a result, respiration measurements provide information about the dynamic fraction of SOM (particularly when combined with 14C measurements of respiration) but do not provide information about the large, stable pools unless they are destabilized and contribute to respiration (detectable with 14C02 respiration measurements). Attributing the sources of respiration from different SOM reservoirs, which may respond differently to climatic variables, is not... 

Figure 10.12. A. Carbon isotopic composition of major groups of higher plants and autotrophic microorganisms compared with oxidized carbon (CO2, HCO3, CC>32 )- The triangles are mean values. (After Holser et al 1988 Schidlowski, 1988). B. Sulfur isotopic composition of bacteriogenic sulfide in modern marine anaerobic sediments (open bars, 1-6) and in the Permian Kupferschiefer (black bar, 7) compared with oxidized sulfate of modem (1-6) and Permian seawater (+11 %o). The black triangles are mean values. (After Holser et al., 1988.)...

Marine plants fall into two major types macroalgae and vascular plants. Macroalgae, or seaweeds as they are commonly called, are multicellular autotrophs with unique adaptations for their marine habitats. Seaweeds occur in three different colors green, red, and brown, depending on the pigments they contain. Many types of macroscopic algae are found in the intertidal zone, whereas vascular plants are rare there and are primarily represented by a few species of marine grasses. 

Symbiontst many apparent autotrophic plants are associated... 

Autotrophy An organism that produces organic matter from mineral nutrients by photosynthesis or chemosynthesis. Plants and many microbes are autotrophs. 

Although higher plants being autotrophs are not dependent on external organic compounds for existence, the use of labeled substrates has shown that they are capable of many of the catabolic reactions found in other organisms. This chapter will examine the catabolism of the protein amino acids in this context of comparative biochemistry. 

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