Krebs Cycle in Aerobic Respiration (With Diagram) | Botany

In this article, we will discuss about the Krebs cycle in aerobic respiration.

Pyruvic acid is therefore the pivot. When aerobic respiration occurs, pyruvic acid is converted to another organic acid which in turn is changed to a third one, etc. Although these organic acids are of widespread occurrence in plant tissues, it is only rela­tively recently that we have gained an understanding of their function or the ways in which they are synthesised by the plant. We now realise that the plant organic acids are intermediates in the respiratory oxidation of pyruvic acid.

The pathway of the oxi­dative degradation of pyruvic acid molecule takes place in several steps to CO₂ and H₂O, passing through a series of changes converting it successively into 6, 5 and 4-carbon acids. This cyclic series is known as Krebs cycle [or citric acid or tricarboxylic acid (TCA) cycle], after the English biochemist H. A. Krebs (Nobel Prize winner, 1953) who first formulated and proposed the mechanism.

The respiratory significance of this cycle is that it provides a mechanism for the oxidation of pyruvic aid, the glycolytic product of sugar breakdown (Fig. 721).

The specific enzymes of the Krebs cycle are all located on the inner walls of the cristae of mitochondria (subcellular organelles). These organelles can be regarded as efficient low temperature furnaces where organic molecules are burnt with oxygen. Most of the relea­sed energy is converted into high energy bonds of ATP. Within each mitochondrion, there are the particles capable of performing Krebs cycle reactions including the oxi­dative phosphorylation and containing the complete set of enzymes and co-factors. These particles have rightly been called elementary particles or oxysomes.

Recently the cells of higher organisms have been found to contain another type of subcellular organelles, called peroxisomes, whose major function is thought to be the protection of cells from free molecular oxygen. Tolbert and his colleagues (1968, 1969) isolated peroxisomes from a number of leaves.

The peroxisomes contain the enzymes that catalyse the direct reduction of oxygen molecules, forming H₂O₂, through the oxidation of metabolites, such as amino acids and acids of the Krebs cycle. In addition to catalase, the leaf peroxisomes contain glycolate oxidase, glutamate, glyoxylatc, amino trans­ferase, malate dehydrogenase, etc.

The classical dark respiratory pathway may be referred to as mitochondrial respiration in contrast to peroxisomal respiration to refer to the light-induced or light dependent pathway.

The rate of reduction of oxygen by the peroxisomes increases proportionately with oxygen concentration, so that an excessive amount of oxygen in the cell increases the rate of its reduction to H₂O₂ or H₂O by peroxisomes.

The basic difference between the mitochondrial oxidation reactions and those of peroxisomes, is that in the oxisomes, the steps of oxidation are not coupled to the synthesis of ATP. The energy released in peroxisomes is thus lost to the cell; apparently the func­tion of the peroxisome is primarily to protect the cells against the destructive effects of free molecular oxygen which can sometimes be disastrous.

Now leaving aside, peroxisomes and photorespiration, for the present, the mito­chondria, in general, perform all of the reactions of Krebs cycle including the forma­tion of ATP molecules, when atmospheric oxygen is finally reduced by electrons obtained from acid substrates.

The labelling of the mitochondria as the power-houses of cell and the classical Krebs cycle as mitochondrial cycle seem appropriate enough and totally justified. Mitochondrial cycle or energy-trans-mutating cycle is certainly the energy transformer of the cell whereby energy inherent in chemical compounds is ultimately modulated to the biolo­gical energy of the ATP molecules, which is essential for the organism.

The pyruvic acid formed by glycolysis reacts with CoA, forming acetyl CoA, before it enters the cycle, as written below:

CH₃CO.COOH + RSH + NAD—CH₃CORS + NADH₂ + CO₂

where RSH is coenzyme A; it contains S and pantothenic acid.

A molecule of CO₂ is evolved here and the acetyl coenzyme A, thus formed enters the cycle proper and in presence of water, and condensing enzymes, react with oxalacetic acid obtained from the cycle, to produce 6-carbon citric acid and making the cycle ‘go’ normally in a clockwise manner.

With appropriate enzyme systems with their activators available for the sequence of cyclic reactions, the TCA cycle moves on and energy, originally contained in the sugar, is apportioned and ATP molecules are synthesised as the electrons released run through the coupled electron transport redox system, ultimately forming water.

In the sequence of changes in the cycle proper (Fig. 721) ten organic acids take part of which three are ketoacids—oxalosuccinic(?) not shown in the figure, α-oxoglutaric and oxalacetic acids. There may be an alternative pathway through the formation of glyoxylic acid, by-passing succinic and fumaric acids, directly to malic acid (see Fig. 721).