Compare and contrast the cyclic and noncyclic electron transport pathways of photosynthesis. What ...

Answer to q. 1

Both cyclic and noncyclic electron transport involve photosynthetic pigments embedded in
the thylakoid membranes as well as the flow of electrons down an electron transport chain
resulting in the generation of ATP via chemiosmosis. Cyclic electron transport does not
involve the reduction of NADP.+ Instead, electrons flow in a cyclical pathway from
photosystem I to the electron transport chain and back to photosystem I. By contrast,
noncyclic electron flow involves the linear flow of electrons from water to photosystem II,
down an electron transport chain to photosystem I, and finally to NADP+. Consequently, the
noncyclic pathway results in the formation of O2, but the cyclic pathway does not.

Answer to q. 2

The proton gradient is established in the thylakoid lumen as electrons flow down the
photosynthetic electron transport chain. As the electrons lose energy some of that energy is
used to pump protons across the thylakoid membrane into the lumen. As the protons
accumulate, they form a gradient, with the proton concentration in the lumen higher than in
the stroma. Eventually the protons flow downhill through the ATP synthase complex in the
thylakoid membrane. The exergonic process of diffusion down this gradient is used to power
the endergonic process of ATP synthesis.

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