Light Compensation Point and Pasteur’s Effect | Plants

The below mentioned article provides a study-note on the light compensation point and Pasteur’s effect.

Light Compen­sation Point:

It is well known that in green plants respiration occurs continuously for 24 hours while photosynthesis occurs only during the day when light is available. Some of the organic matter produced during photosynthesis is oxidised during respiration while rest is stored by the plant. It is implied from this that the photosynthesis is faster process than respira­tion.

We also know that the rate of photosynthesis is higher at about noon when light intensity is higher and is slower during morning or evening when light intensity is low. Obviously, there will be a time either in the morning or evening when the rate of photo­synthesis will be as low as to equal the rate of respiration. This is called as light compen­sation point.

Pasteur’s Effect:

In his studies on alcoholic fermentation Pasteur in 1861 found that under anaerobic conditions much more sugar was taken up per quantity of yeast present than was consumed in the presence of O₂ (or aerobic conditions). This inhibition by O₂ of the rate of carbohydrate breakdown is often called as Pasteur’s effect. Previously the existence of this effect was known only in yeasts and animal tissue but now this is also known to be operative in a variety of tissues of higher plants e.g., barley leaves, apple fruits, potato tubers, carrot roots etc.

Although, the rate of the breakdown of carbohydrates (i.e., respiratory substrates) is decreased by a shift to aerobic conditions, the energy made available in the form of ATP mol­ecules is much more. This is because of the greater efficiency of the aerobic rather than anaero­bic respiration as an energy source in the cells. (One molecule of glucose for example yields 36 ATP molecules in aerobic respiration while only two ATP molecules are synthesized per glu­cose molecule oxidised anaerobically).

In other words, to perform a given amount of work, a tissue operating aerobically might be expected to oxidize much less glucose than in anaerobic work. It has in fact been observed that under anaerobic condition the rate of consumption of carbohydrates by muscle tissue is app. 5—8 times to that observed under aerobic condition indicating thereby that the operation of aerobic mechanism of carbohydrate breakdown inhib­its the rate of conversion of glucose to pyruvate.

The Pasteur’s effect therefore, appears to be an expression of the close interrelation between the cellular mechanisms of anaerobic glycolysis (responsible for the conversion of glucose to pyruvate) and of the citric acid cycle which causes the generation of ATP by aerobic breakdown of pyruvate through oxidative phosphorylation. This viewpoint has been supported by the findings that the Pasteur’s effect is counteracted in yeast and other organisms by the ‘uncoupling’ agent 2, 4—dinitrophenoi which is a well known inhibitor of mitochondrial oxidative phosphorylation.

One way in which the close interrelation of mitochondrial oxidative phosphorylation and glycolytic process may be mediated lies in their common dependence upon ADP as an accep­tor of phosphate. Thus inhibition of mitochondrial phosphorylation of ADP would result in a higher concentration of this compound available to the ADP requiring glycolytic process. Simi­larly, there is competition between these two processes for inorganic phosphate, the latter hav­ing important direct effects upon glycolytic systems.