RuBisCO is the most abundant enzyme in the world. Its active site can bind to both CO2 and O2. But RuBisCO has a much greater affinity for CO2 than O2. The relative concentration of O2 and CO2 determines which of them will bind to the enzyme.
In C3 plants, some oxygen binds to RuBisCO and hence CO2 fixation is decreased. The RuBP; in this case; binds with oxygen to form one molecule of PGA and phosphogylcolate. This happens in a pathway called photorespiration. In the photorespiratory pathway, neither the sugar nor the ATP is synthesized. Rather utilization of ATP results in the release of CO2. Even NADPH is not synthesized in the photorespiratory pathway. Thus, it is a wasteful process.
Photorespiration does not occur in C4 plants. They have a mechanism which increases CO2 concentration at the enzyme site. This happens when the C4 acid from the mesophyll is broken down in the bundle cells to release CO2. Thus, intracellular concentration of CO2 is increased. This ensures that the RuBisCO functions as a carboxylase; with minimum oxygenase activity.
FACTORS AFFECTING PHOTOSYNTHESIS
Blackman’s (1905) Law of Limiting Factors:
“If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed.”
Light: At low intensities of light, there is a linear relationship between incident light and carbon fixation. At high intensities of light, the rate does not show further increase because other factors become limiting. Increase in incident light beyond a point results in the breakdown of chlorophyll and a decrease in photosynthesis.
Carbon dioxide Concentration: Carbon dioxide is a major limiting factor for photosynthesis. It is important to remember that carbon dioxide is in very low concentration in the atmosphere (0.03 – 0.04%). Increase in concentration up to 0.05% can increase carbon fixation. An increase beyond this level can be damaging over longer periods.
Temperature: The dark reactions are controlled by temperature because they are enzymatic. Light reactions are also sensitive to temperature but to a much lesser extent. The C4 plants have a much higher temperature optimum than C3 plants. The temperature optimum for a particular plant also depends on it natural habitat. Tropical plants have a higher temperature optimum than temperate plants.
Water: The effect of water is more on the plant rather than directly on photosynthesis. Water stress results in closing of stomata and; thus in reduced availability of CO2. Water stress also leads to wilting of leaves. Wilting of leaves means reduced surface area and reduced metabolic activity in leaves.