Factors Affecting Photosynthesis

According to Blackman's law of limiting factors, if a chemical process is affected by more than one factor, then the rate of reaction will be determined by the factor which is nearest to its minimal value.

Photosynthesis is affected by factors like light intensity, carbon dioxide concentration, temperature and water. 

1. Light: if light increases, photosynthesis also increases upto a certain level. Then it remains constant. Plants need only 10% of the available light intensity of sunlight. Therefore, light is never a limiting factor except in case the plant is in shade or in dense forest. 

If the light intensity increases too much, it starts destroying the chlorophyll, thus, photosynthesis reduces. 

2. Carbon dioxide: if amount of carbon dioxide increases, photosynthesis also increases upto CO2 concentration of 0.05 %. Then photosynthesis reduces. 

Saturation level or optimum level of carbon dioxide for C4 plants is 360 microlitre per litre of air; and for C3 plants is 450 microlitre per litre of air. Thus, carbon dioxide is a limiting factor more for C3 plants. 

3. Temperature: if temperature increases, photosynthesis also increases upto a certain level. Then photosynthesis reduces. C3 plants have lower optimum temperature (18 to 24 degree celsius). C4 plants have a little higher optimum temperature (30 to 40 degree celsius). 

4. Water: if available water is more, photosynthesis increases up to a certain level. Then there is no effect. If water available is less, the stomata do not open fully. So absorption of carbon dioxide is less. Thus, photosynthesis will also be less. 

C4 Cycle

Some tropical plants like maize, sugarcane, many grasses, sorgam, millet, many plants of asteraceae, many plants of brassicaceae etc show a special type of photosynthesis called C4 cycle or C4 pathway. In this type of pathway, the first product of carbon dioxide fixation (malic acid or aspartic acid) is formed in the cells of mesophyll. It is a 4 carbon compound. 

Malic acid is transferred to the bundle sheath cells where it is converted into pyruvic acid. Pyruvic acid then takes part in the C3 cycle or Calvin cycle.

Class 11 Photosynthesis

In the mesophyll cells of the leaves, there are a large number of chloroplasts. The membranes of granna inside chloroplast are the site for the light reaction while the dark reaction takes place in the matrix of the chloroplast ie stroma. 

In the light reaction, the light energy is absorbed by the pigment present in the antenna and funneled to special chlorophyll a molecules called Reaction Centre chlorophylls. 

There are two photo systems, PS1 and PS2. PS1 has a 700 NM absorbing chlorophyll called Chlorophyll a P700 molecule at its reaction centre, while PS2 has a P680 reaction centre that absorbs red light at 680 NM. After absorbing light, electrons are excited and transferred through PS2 and PS1 and finally to NAD forming NADH. 

Initially, in the dark reaction, carbon dioxide is added by the enzyme RuBisCO to a 5 carbon compound RuBP that is converted to two molecules of 3 carbon PGA. This is then converted to glucose. In this process, ATP and NADPH synthesized in the light reaction are utilised. 

Note: The above notes in the image(s) are taken from a personal notebook which was initially not intended to be shared online. Please excuse the rough handwriting and spelling errors, if any.