The Physiology of Flowering Plants - KHAM PHA MOI

higher rate of ATP synthesis. The result is a higher rate of respirationand a higher turnover of ATP and ADP. The ratio ATP : ADP seems tobe fairly constant in cells.The effects of ATP and ADP are not only simple concentrationeffects. The glycolytic enzyme phosphofructokinase is inhibited byATP, which is one of its substrates; pyruvate kinase is also inhibitedby ATP. On the other hand, ADP activates pyruvate kinase. Theseeffects would result in slowing glycolysis by an increase in ATPconcentration and a speeding up by a rise in the concentrationof ADP.Accumulation of intermediates of the respiratory pathwayscauses inhibition of enzymes acting earlier in the pathways – thewell-known phenomenon of feedback inhibition. Phosphofructokinase,one ofthe first enzymes of the glycolytic pathway, is inhibitedby phosphoglycerate and by phosphoenolpyruvate. Citrate (fromthe Krebs cycle) inhibits both phosphofructokinase and pyruvatekinase. A fall in the demand for ATP would slow down the mitochondrialelectron transport; citrate metabolism would slow down correspondingly,being dependent on the mitochondrial terminaloxidation. The accumulating citrate would cause slowing down ofglycolysis. Similarly, a fall in the demand for biosynthetic intermediateswould result in a build-up of these intermediates, and in aninhibition of preceding steps. Any change in requirement for ATPor/and metabolic intermediates would result in an adjustment inrespiration rate, until a new equilibrium between supply anddemand was established. For the PPP, the ratio of NADP + : NADPHis a regulatory factor.When metabolic pathways were first elucidated, rate control waspostulated to be concentrated at a few key steps catalysed by ‘pacemaker’enzymes. For glycolysis, phosphofructokinase and pyruvatekinase were considered to be pacemakers, these being the two glycolyticenzymes for which the reactions are irreversible underphysiological conditions. Several metabolites are moreover knownto regulate the activities of these enzymes, as noted in the previousparagraph. But changing the amounts of individual enzymes in cellsby genetic manipulation has shown that large changes in the activitiesof phosphofructokinase and pyruvate kinase have little effect onrespiration rate. It now appears that all steps in glycolysis and theKrebs cycle contribute to regulation of respiration rate, though notall to the same degree. From the viewpoint of the cell, the larger thenumber of control points, the more opportunities there are for finetuningand interactions between pathways. From the viewpoint ofthe investigator, it makes the study of rate control exceedingly complex.There is nevertheless no doubt regarding the basic principle:respiration rate is integrated with cellular activity so that a depletionof respiratory products (ATP and metabolites) leads to an increase inthe rate of respiration. An accumulation of the same leads to adecrease in the respiration rate and in each case a new steady stateis achieved.RESPIRATION AND PLANT ACTIVITY 55