Nov. 16, 2019

Chemiosmotic theory factors aff photosynthesis:

Chemiosmotic Hypothesis

              The chemiosmotichypothesis has been put forward to explain the mechanism how actually ATP is synthesized in the chloroplast. Like in respiration, in photosynthesis too, ATP synthesis is linked to developed of a proton gradient across a membrane. This time these are membranes of the thylakoid. There is one difference though, here the proton accumulation is towards the inside of the membrane, i.e., in the lumen. In respiration, protons accumulate in the intermembrane space of the mitochondria when electrons move through the ETS,

               Chemiosmosis requires a membrane, a proton pump, a proton gradient and ATPase, Energy is used to pump protons across a membrane, to create a gradient or a high concentration of protons within the thylakoid's lumen. ATPase has a channel that allows diffusion of protons back across the membrane; this releases enough energy to activate ATPase enzyme that catalyses the formation of ATP.

               Along with the NADPH produced by the movement of electrons, the ATP will be used immediately in the biosynthetic reaction taking place in the stroma, responsible for fixing CO2, and synthesis of sugars.


               Law of limiting factors was given by F.F.Blackman (1905) and states 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 photosynthesis if the temperature is very low. This leaf, if given the optimal temperature, will start photosynthesizing.

               CO2 is usually a limiting factor in photosynthesis under field conditions. C4 plans show saturation at about 360 mlL-1 while C3 responds to increased CO2 concentration and saturation is seen only beyond 450 mlL-1 . Thus, current availability of CO2 levels is limiting to the C3plants. High O2 levels inhibit rate of photosynthesis and this inhibition of photosynthesis by increased O2 concentration is called Warburg's effect.

               High light intensity for longer duration causes degradation of chlorophyll molecule photoxidatio. Photo-oxidation of chlorophyll under very high intensity is called solarisation. The value of light saturation at which further increase is not accompanied by an increase in CO2 uptake is called as light saturation point. During compensation point there is no evolution of any gas. Photosynthesis is maximum in polychromatic light or white light. Rate of photosynthesis is maximum in red light (650 nm) then in blue light (440 nm) and minimum in green light.

              The temperature coefficient (Q10) is defined as the ratio of the velocity of a reaction at a particular temperature to 100C higher. For a physical process the value of Q10 is slightly greater than one. Q10 (temperature coefficient)value for photosynthesis is 2 upto about 400C and above it, denaturation of enzymes begins and hence rate of photosynthesis deceases.