Factors Affecting Photosynthesis

The External factors that affect the rate of photosynthesis would include the availability of sunlight, temperature, CO₂ concentration and water. As a plant photosynthesises, all these factors will simultaneously affect its rate. 

When several factors affect any [bio] chemical process, Blackman’s (1905) Law of Limiting Factors comes into effect. This states the following: 

“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”.

For example, despite the presence of a green leaf and optimal light and CO₂ conditions, the plant may not photosynthesise if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesising.

CO₂ is limiting in clear sky but light becomes limiting in cloudy days and in dense forest or for plants growing in shade.

We need to distinguish between light quality, light intensity and the duration of exposure to light, while discussing light as a factor that affects photosynthesis. There is a linear relationship between incident light and CO₂ fixation rates at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting.

Light saturation occurs at 10% of the full sunlight.

Except for plants in shade or in dense forests, light is rarely a limiting factor in nature. Increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.

1.0Temperature

The dark reactions being enzymatic are temperature controlled. Though the light reactions are also temperature sensitive they are affected to a much lesser extent.

C₃ plants have a much lower temperature optimum (20° – 25° C). 

The Temperature optimum for photosynthesis of different plants also depends on the habitat that they are adapted to. Tropical plants have a higher temperature optimum than the plants adapted to temperate climates.

2.0CO₂ Concentration

Carbon dioxide is the major limiting factor for photosynthesis. The concentration of CO₂ is very low in the atmosphere (between 0.03 and 0.04 percent). Increase in concentration upto 0.05 percent can cause an increase in CO₂ fixation rate, beyond this the levels can become damaging over longer periods.

3.0Water

Even though water is one of the reactants in the light reaction, the effect of water as a factor is more through its effect on the plant, rather than directly on photosynthesis. Water stress causes the stomata to close hence reducing the CO₂ availability. Besides, water stress also makes leaves wilt, thus, reducing the surface area of the leaves and their metabolic activity as well.

4.0Where Are the ATP And NADPH Used? 

Dark Reaction / Biosynthetic Phase

(i) In this process, CO₂ is reduced to sugar. 

(ii) It is known as dark reaction but it doesn't mean that it occurs in dark. 

(iii) This process does not directly depend on the presence of light but is dependent on the products of the light reaction like ATP and NADPH + H⁺. So immediately after light becomes unavailable, the biosynthetic process continues for some time and then stops. If then light is made available, the synthesis starts again.

The Calvin Cycle

• Calvin and his co-workers worked out the whole pathway and explained that the pathway is operated in a cyclic manner. 
• The Calvin cycle occurs in stroma of chloroplast of mesophyll cells.
• 6 turns of Calvin cycle are required for the formation of one glucose as 6 CO₂ are required for the synthesis of one hexose.
• 12 NADPH + H⁺ & 18 ATP are required as assimilatory power to produce one Glucose in dark reaction in C₃ cycle.
• I^st stable compound of Calvin cycle is 3carbon compound–PGA (Phosphoglyceric acid or phosphoglycerate) thus Calvin cycle is called C₃–cycle. 
• RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is main enzyme in C₃–cycle which is present in stroma. RuBisCO is the most abundant enzyme and protein on earth.
• For ease of understanding, the Calvin cycle can be described under three stages: carboxylation, reduction and regeneration.

Carboxylation

Carboxylation is the fixation of CO₂ into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA. Since this enzyme also has an oxygenation activity it would be more correct to call it RuBP carboxylase-oxygenase or RuBisCO.

Reduction

These are a series of reactions that lead to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO₂ molecule fixed. The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the formation of one molecule of glucose from the pathway. 

Regeneration

Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.

• Hence for every CO₂ molecule entering the Calvin cycle, 3 molecules of ATP and 2 of NADPH are required. It is probably to meet this difference in number of ATP and NADPH used in the dark reaction that the cyclic phosphorylation takes place.