Respiration

Abstract

Aerobic respiration is a process, where energy rich compounds are enzymatically hydrolyzed and oxidized. A part of the energy released is trapped in the form of ATP and made available for use in energy consuming processes of plant cells. The general process can be summarized as under: n (CH2O) + n O2 + n H2O --> n CO2 + 2n H2O + Energy (ATP) The first step in aerobic respiration occurs when glucose (or other storage carbohydrate) is metabolized through glycolisis or oxidative pentose phosphate pathway, where intermediates (i.e. amino acids, nucleotides) and NADPH are produced (see Chapter 20). In plants, malate and pyruvate are the end products of glycolisis and malate is most important. Malate and pyruvate are transported from the cytosol to the mitochondria where they are oxidised in the TAC cycle. Complete oxidation of one molecule of malate yields 5 molecules of NADH and one molecule of FADH2, then one molecule of ATP. NADH and FADH2 donate their electrons to the electron transport chain. Aerobic respiration occurs in both light and darkness. It can cause a considerable decrease in dry weight of plant tissues by itself. Another respiration occurs only in green tissues and is light-dependent. It involves enzymatic oxidation of glycolic acid generated in photosynthesis to CO2: Glycolate oxidase CH2OH-COOH + O2 -----> CHO-COOH + H2O2 Glycolic acid Glyoxylic acid CHO-COOH + H2O2 ---> HCOOH + CO2 + H2O Glyoxylic acid Formic acid Energy released during this process, known as photorespiration, is not useful for the cell. Whole photorespiration process occurs in chloroplast, peroxisomes and mitochondria and is not the subject of this chapter. Nevertheless, several comprehensive texts on photorespiration and protocols to study it are available. Tissues of higher plants can respirate in the absence of oxygen through a process called fermentation and CO2 is also one of the end products. However, organic molecules respired are not completely oxidized to CO2 and H2O and many reduced end products are generated, but ATP production is very small. This chapter provides techniques to measure aerobic respiration and fermentation in plant tissues.