Page 11 - LN-PHOTOSYNTHESIS IN PLANTS

is created across thylakoid membrane, which led to decrease in the pH at the side
the lumen.
The gradient is broken down due to the movement of protons across the membrane
to the stroma through the transmembrane channel of the F 0 portion of the ATPase
enzyme.
Therefore, the proton gradient is important as it is the gradient whose breakdown
leads to the release of energy (ATP).
ATPase Enzyme
The enzyme ATPase consists of the following two parts
i. F 0 Particle
This portion remains embedded in the membrane and forms a transmembrane
channel, which carries out facilitated diffusion of protons across the membrane.
ii. F 1 Particle
This portion protudes towards the outer surface of the thylakoid membrane which
faces the stroma. Conformational change occurs in F 1 particle of ATPase, which
caused due to the breakdown of the gradient, which allows the enzyme to synthesise
several molecules of ATP.
Thus, the chemiosmosis, for its functioning, requires a membrane, a proton pump, a
proton gradient and ATPase enzyme. The ATP thus, produced will be used
immediately in the biosynthetic reaction (in stroma), responsible for the fixing of
CO 2 and synthesis of sugar.
Dark Reaction (Biosynthetic Phase)
This phase does not require direct sunlight but it depends on the products of the
light reaction, i.e., ATP and NADPH beside CO 2 and water that drive the processes
leading to the synthesis of food more accurately the sugars.
The O 2 thus, produced in the light reaction of photosynthesis diffuses out of the
chloroplast
As soon as the light becomes unavailable, the biosynthetic process continues for
sometime and then eventually stops and starts again if the light is made available
again.
Originally, this process is known as carbon-fixation or Photosynthetic Carbon
Reduction (PCR) cycle.
These reactions are sensitive to temperature change, but are independent of light,
hence called dark reaction. This takes place in the stroma of chloroplast.
Thus, assimilation of CO 2 during photosynthesis is of two main types
i. C 3 Pathway This pathway is followed by the plants when first product of
CO 2 fixation is a C 3 acid, i.e., PGA.
ii. C 4 Pathway This pathway is followed or shown by the plants in which first product
ofCO 2 fixation is a C 4 acid, i. e., OAA.
Calvin Cycle (C 3-Pathway)
This is a cycle biochemical pathway of reduction of CO 2 or photosynthetic carbon,
cycle, which was discovered by Calvin. The Calvin cycle runs in all photosynthetic
plants, no matter they shows C 3, C 4 or any other pathways. It occurs in stroma of the
chloroplast
Primary Acceptor of CO 2 in C 3 Pathway
After a long research and conducting many experiments it was concluded by the
scientists that in C 3 pathway, the acceptor molecule is a 5-carbon ketose sugar, i.e.,
Ribulose 5-phosphate (5 RuBP).