Question

Which of the following statements about the light reactions of photosynthesis are true?

• A. The splitting of water molecules provides a source of electrons
• B. Chlorophyll (and other pigments) absorb light energy
• C. ATP is generated by photophosphorylation
• D. All of the above

Answer 1

The correct option is D All of the above

a) While photosystem I gain electrons from photosystem II, the electrons lost by photosystem II have not been replaced yet. Its reaction center acquires these electrons by splitting water. During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O$_2$), a waste product of photosynthesis.

b) Chlorophylls are arranged in photosystems which are in the thylakoid membranes of chloroplasts, The main function of chlorophyll is to absorb light energy and transfer it to the reaction center chlorophyll of the photosystem.

c) ATP synthesis in chloroplasts (photophosphorylation) proceeds according to a mechanism that is basically similar to that in mitochondria. The chemiosmotic coupling which links the movement of protons down an electrochemical potential gradient to ATP synthesis via an ATP synthase applies in both organelles. In chloroplasts, protons accumulate in thylakoid lumen and pass outwards through the ATP synthase into the stroma.