Respiracion Vegetal.

C H A P T E R

Enzymes, Energetics, and Respiration
INTRODUCTION 135 ENZYMES AND ENERGETICS 135 Enzymes 135 Energetics 137 RESPIRATION 137 Maintenance Respiration and Growth Respiration 138 Respiration Measurement 140 General Characteristics 140 Cellular Respiration 140 Biological Oxidations 140 ATP 140 Other High-Energy Compounds 141 Glycolysis and the Krebs Cycle 141 Electron Transfer andOxidative Phosphorylation 141 Other Oxidases 142 The Pentose Shunt 142 Anaerobic Respiration 142 Respiratory Quotient 142 Photorespiration 143 RESPIRATION OF PLANTS AND PLANT PARTS 143 Amount of Food Used in Respiration 143 Respiration of Entire Trees 143 Respiration of Various Plant Parts 143 Seasonal Variations 148 Scaling of Respiration to the Ecosystem Level 148 Respiration of Harvested Fruits149 FACTORS AFFECTING RESPIRATION 150 Age and Physiological Condition of Tissues 150 Available Substrate 150 Hydration 150 Temperature 150 Composition of the Atmosphere 152 Soil Aeration 152 Mechanical Stimuli and Injuries 153 Chemicals 154 Air Pollutants 154 ASSIMILATION 156 SUMMARY 157 GENERAL REFERENCES 158

134

ENZYMES AND ENERGETICS

135 shunt occur principally in the cytoplasm. Someextracellular enzymes even occur on the extemal surfaces of cells or diffuse out into the surrounding medium. The basic structure of an enzyme is a protein molecule. Enzymes vary because of differences in the sequence of amino acids in their proteins. Some enzymes such as urease and papain consist only of protein molecules, but many require a nonprotein constituent, often termed a cofactor orcoenzyme, closely associated with or bound to the protein molecule. Cofactors that are integral parts of an enzyme, such as the copper in tyrosinase and ascorbic acid oxidase and the iron in catalase, are known as prosthetic groups. Many enzymes are active only in the presence of ions such as Mg2+, Mn^^-, Ca^^, and K"^. These are known as metal activators. The most important function of themicronutrient elements in plants and animals is as prosthetic groups or metal activators of enzymes. Some enzymes are active only in the presence of complex organic molecules, which if tightly bound are called prosthetic groups but if loosely bound are termed cofactors or coenzymes. Several vitamins, especially those of the B complex, play important roles as enzyme cofactors. Pyridine nucleotides are ofparamount importance as cofactors of enzymes involved in metabolic energy transfers. NAD+ (nicotinamide adenine dinucleotide) and NADP"*" (nicotinamide adenine dinucleotide phosphate) are essential coenzymes of the enzymes involved in oxidation-reduction systems of living cells, both in respiration and in photosynthesis. In their reduced form, NADH or NADPH, they are high-energy compounds that supplyreducing power in such processes as electron transport and the reduction of carbon in the process of photosynthesis. Two other important enzymes involved in oxidation-reduction reactions are the flavin nucleotides FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide), which are derived from one of the B2 vitamins, riboflavin. Thiamine pyrophosphate is derived from vitamin Bi (thiamine)and serves as a coenzyme for various decarboxylases, oxidases, and transketolases. Pyridoxal, pyridoxine, and pyridoxamine constitute the vitamin B^ complex, from which is derived pyridoxal phosphate, an important coenzyme in reactions in amino acid synthesis. Another vitamin, pantothenic acid, is a precursor of coenzyme A, which plays an important role in metabolism (see Figs. 6.1 and 6.20).Thus, several vitamins are essential because of their roles as coenzymes in important metabolic reactions. The complement of enzymes produced in a plant is determined primarily by its genotype, and occasionally metabolic disorders are caused by gene mutations that eliminate specific enzymes. Some genetic control also exists outside the nucleus, especially in the chloroplasts. Among the most...