Energy-Yielding Processes

Organisms can gain energy by the following three processes  

Plants cannot always get the energy that they need from sunlight. During the dark, they must use stored food. Plant cells, like animal cells, contain mitochondria in which stored food is converted to energy by cellular respiration. 

Plant cells, which use sunlight for energy and CO2 for carbon, are said to be autotrophic. In contrast, animal cells must depend upon organic material manufactured by plants for their food. These are called heterotrophic cells. They act as middlemen in the chemical reaction between oxygen and food material, using the energy from the reaction to carry out their life processes. 

The chapter summary below is presented in a programmed format to review the main points covered in this chapter. It is used most effectively by filling in the blanks, referring back to the chapter as necessary. The correct answers are given at the end of the summary. 

The biomolecules that constitute matter in living organisms are often high-molecular-mass  

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The potentials found for nitrate reductases [96] vary with the role of the particular enzyme. Assimilatory nitrate reductase, found in plants, algae, and fungi, is involved in the first step in nitrogen assimilation and has a molybdenum center that operates at around 0 mV. Respiratory (dissimilatory) nitrate reductase, utilized by bacteria in energy yielding processes, has a molybdenum center that operates at around +200 mV [97,98],... 

Oxidation, we know, is an energy-yielding process, and the cell must make use of the energy released during oxidation of the glyceraldehyde-3-phosphate to make a molecule of ATP. 

Without question, photosynthesis is the most important biochemical process on the Earth. With a few minor exceptions, photosynthesis is the only mechanism by which an external source of energy is harnessed by the living world. As with other energy-yielding processes, photosynthesis involves oxidation-reduction reactions. Water is the source of electrons and protons that reduce C02 to form organic compounds. Chapter 13 is devoted to a discussion of the principles of photosynthetic processes. The relationship between photosynthetic reactions and the structure of chloroplasts and the relevant properties of light are emphasized. 

Fermentations are principally free energy-yielding processes (AG < 0) and/or are exothermic, AH < 0. At present but few data are available in the literature. 

Table 1.2 Energy yielding processes used by microorganisms to oxidize acetate in the presence of different terminal electron acceptors (after [11] and [12])...

In certain bacteria the initial steps of oxidation of tryptophan may be an important energy-yielding process. Stanier (personal communication) has estimated that the content of enzymes concerned with the reactions leading to anthranilic acid may be 100 to 1000 times as great in the strains of Pseudomonas oxidizing tryptophan as in liver. 

On the basis of his studies with photosynthetic bacteria van Niel (362-364) reached the conclusion that the mechanism for COi fixation was separate and distinct from the energy-yielding process. Support for this hypothesis comes from the work of Hill and Scaris-brick (161), who obtained O evolution without the reduction of COj. Evidence that light was not immediately essential for COi fixation was provided by Ruben, Hassid, and Kamen (304) when they observed the formation of carbohydrate from CXlz in the dark intervals immediately following illumination of barley leaves. 

There is one item on the intake side which is usually overlooked and that is the ipaler of oxidation. As we have seen, the respiratory chain is the most important energy-yielding process. It constantly produces water in the normally nourished man, about 300 gm per day. In some organisms the water of oxidation can fill the total requirement for water (desert animals, clothes moths). In these cases, the excretion of water is reduced to a minimum. 

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