Photorespiration

Photorespiration

Photo-respiration is light dependent uptake of oxygen and output of carbon dioxide. It is not related to the normal respiration and resembles only in that oxygen is used and carbon dioxide produced.

Warburg Effect

Photosynthesis is believed to have evolved in atmosphere much richer than carbon dioxide than it is today, but one containing little oxygen, probably about 0.02% oxygen compared with 21% today. In early 1920s a German biochemist Otto Warburg (1920) found that photosynthesis in algae is inhibited by oxygen concentration in air. Later, it was found that this inhibition occurs in aH C-3 species. This is known as Warburg effect. McAlister and Myers showed the effect of high and low concentrations of oxygen on the process of photosynthesis. High concentration of oxygen inhibited the process. In early 1950s it was found that rate of respiration in some plants is greater in light than in darkness. Additional work in the late 1960s and 1970s revealed that the rate of respiration, as measured by 02 consumption or CO₂ liberation from the leaves of C-3 plants, was often as much as two times greater in light than in darkness. Further investigations showed that this light respiration is similar to true aerobic respiration, however no energy liberation occurs. This type of respiration was called photo-respiration.

Orgen And Bowes

In 1971 Orgen and Bowes provided an explanation of Warburg effect. They showed that oxygen displace carbon dioxide at the active site of the enzyme RuBP carboxylase and combine with 1,5-bisphosphate to produce phosphoglycolic acid, a 2-C compound. High oxygen concentration favors ttlis combination. Oxygen, therefore, competes for the RuBP with carbon dioxide and enzyme operates as an oxygenase catalyzing the oxidation of RuBP to phosphoglycolic acid.

At high light intensity, high oxygen and high temperature, the mesophyll cells in the leaves of all C-3 plants exhibit high rates photo-respiration. C-4 plants, however, exhibit low yield from photosynthesis in C-3 plants by up to 50%.

Significance of Photosynthesis

Photosynthesis is one of the most important of all biochemical processes because almost all living things depend upon photosynthesis, directly or indirectly, for their organic nutrients. It is significant in the following respects:

Photosynthesis And Plant Metabolism

The plant metabolism depends upon products of photosynthesis, the sugars and other compounds, the products are transported from the chloroplasts to all parts of the plant. These products are used in living cells as the building blocks for all substances the plant requires.

Photosynthesis And Energy

The product of photosynthesis also provide energy needed to carry out chemical changes. Energy is held in ATP made during respiration of sugars and by Photophosphorylation during the light reaction of photosynthesis.

Photosynthesis And Food Chain

The survival of life depends upon photosynthesis. This is because all organisms get their nutrients from green plants, either directly or indirectly. The feeding relationship between organisms is represented in a food chain. Food chain commences with green plants or phytoplankton (producers). All other organisms depend directly or indirectly upon food stored in the producers.

Photosynthesis And the Composition of Air

The carbon dioxide is added to the atmosphere by respiration of animals, plants and micro-organisms, and by the combustion of fossil fuels. The release of the carbon dioxide from combustion is now on increase and contribute to a possible warming of our planet. Photosynthesis re-uses carbon dioxide released into the atmosphere (carbon cycle) lowering the risk of global warming. Similarly, photosynthesis is the only natural process that releases oxygen to the atmosphere that helps to keep the oxygen concentration constant.

Principle of Limiting Factors

It states that when a chemical process is affected by more than one factor its rate limited by the factor which is nearest its minimum value (slowest factor). The slowest factor means a factor which is present in quality or intensity that is lesser than which is required for the process.

Blackmann (1905)

Blackmann in 1905 explained the principle of limiting factor as followings:

Suppose a leaf is exposed to a light intensity which is sufficient to decompose 5 mg of CO₂ per hour. If only 1 mg of gas enters the leaf per hour, photosynthesis goes on at certain rate. In such a situation increase in the intensity of light does not cause any increase in rate of photosynthesis because light is already in excess of that required for the decomposition of 1 mg of CO₂. If the supply of carbon dioxide is increased to 2 mg per hour, the rate of photosynthesis increases with same light intensity. Thus, rate of reaction is determined by carbon dioxide and not light, therefore carbon dioxide is the limiting factor. The rate of photosynthesis would go on increasing till the supply of CO₂ reach 5 mg per hour. At this point two factors are equally balanced and each has a maximum effect on Photosynthesis. Further increase in the supply of CO₂ will have no effect on the rate of photosynthesis because light intensity is just enough to decompose 5 mg of CO₂ per hour. This means that light has now become the limiting factor and an increase in the rate of photosynthesis would be possible only with an increase in the light factor. The above discussions suggest that when photosynthesis is under the simultaneous influence of a number of factors, an increase in that factor and that factor alone which is limiting will bring about an increase in the rate of photosynthesis.

In plains, the light intensity and temperature are high and carbon dioxide content of an air often act as a limiting factor. In cold and foggy countries temperature and light are often limiting factors in photosynthesis.