AN OVERVIEW OF PLANT RESPIRATION
Aerobic (oxygen-requiring) respiration is common to nearly all eukaryotic organisms, and in its broad outlines, the respiratory process in plants is similar to that found in animals and lower eukaryotes. However, some specific aspects of plant respiration distinguish it from its animal counterpart.
Aerobic respiration is the biological process by which reduced organic compounds are mobilized and subsequently oxidized in a controlled manner.
During respiration, free energy is released and transiently stored in a compound, ATP, which can be readily utilized for the maintenance and development of the plant.
C12H22O11 + 12 O2 → 12 CO2 + 11 H2O
This reaction is the reversal of the photosynthetic process; it represents a coupled redox reaction in which sucrose is completely oxidized to CO2 while oxygen serves as the ultimate electron acceptor, being reduced to water.
To prevent damage (incineration) of cellular structures, the cell mobilizes the large amount of free energy released in the oxidation of sucrose in a series of step-by-step reactions. These reactions can be grouped into four major processes:
glycolysis, the citric acid cycle, the reactions of the pentose phosphate pathway, and oxidative phosphorylation.
Glycolysis
involves a series of reactions carried out by a group of soluble enzymes located in both the cytosol and the plastid. A sugar—for example, sucrose—is partly oxidized via six-carbon sugar phosphates (hexose phosphates) and three-carbon sugar phosphates (triose phosphates) to produce an organic acid—for example, pyruvate. The process yields a small amount of energy as ATP, and reducing power in the form of a reduced pyridine nucleotide, NADH.
Pentose phosphate pathway
In the pentose phosphate pathway, also located both in the cytosol and the plastid, the six-carbon glucose6-phosphate is initially oxidized to the five-carbon ribulose-5-phosphate. The carbon is lost as CO2, and reducing power is conserved in the form of two molecules of another reduced pyridine nucleotide, NADPH. In the following near-equilibrium reactions, ribulose-5-phosphate is converted into three- to seven-carbon sugars.
Citric acid cycle
In the citric acid cycle, pyruvate is oxidized completely to CO2, and a considerable amount of reducing power (16 NADH + 4 FADH2 equivalents per sucrose) is generated in the process. With one exception (succinate dehydrogenase), these reactions involve a series of enzymes located in the internal aqueous compartment, or matrix, of the mitochondrion. As we will discuss later, succinate dehydrogenase is localized in the inner of the two mitochondrial membranes.
Oxidative phosphorylation
In oxidative phosphorylation, electrons are transferred along an electron transport chain, consisting of a collection of electron transport proteins bound to the inner of the two mitochondrial membranes. This system transfers electrons from NADH (and related
species)—produced during glycolysis, the pentose phosphate pathway, and the citric acid cycle—to oxygen. This electron transfer releases a large amount of free energy, much of which is conserved through the synthesis of ATP from ADP and Pi (inorganic phosphate) catalyzed by the enzyme ATP synthase. Collectively the redox reactions of the electron transport chain and the synthesis of ATP are called oxidative phosphorylation. This final stage completes the oxidation of sucrose.