Photosynthesis- the Light Dependent Reactions

Question 1

The light dependent stage of photosynthesis consists of two types of photophosphorylation reactions, what are they?

Answer 1

Cyclic and non-cyclic photophosphorylation reactions.
Cyclic and non-cyclic photophosphorylation proceed simultaneously.

Question 2

List the events which occur during non-cyclic photophosphorylation.

Answer 2

1. Water molecules are split when light energy strikes them. 2H2O—->O2 (releasing 2e and 4H+)
2. Electrons from photosystem II become excited when light strikes them and are accepted by electron acceptor X.
3. Electrons from the splitting of water are donated to photosystem II to replace those electrons that moved out to acceptor X. (The electrons from the splitting of water move along two electron carriers to photosystem II). While they are being transferred along the carriers, energy is released and used to synthesise ATP.
4. The electrons that are taken up by electron acceptor X move along a series of electron carriers through the positively charged photosystem I. While they are being transferred along the carriers energy is released and ATP is synthesized.
5. These electrons continue to move along from PSI to electron acceptor Y and then along another series of electron carriers to NADP.
6. NADP then becomes negatively charged and attracts the H+ which were split off from water. NADP acts as the final acceptor of electrons during non-cyclic photophosphorylation.
   The products of non-cyclic photophosphorylation are reduced NADP, ATP and O2(waste product).

Question 3

List the events which occur during cyclic photophosphorylation.

Answer 3

1. Light strikes PSI and electrons become excited moving out of their orbitals.
2. The electrons are taken up by electron acceptor Y and then cycled back to PSI via a series of electron carriers. During their transfer along the carriers energy is released and used to synthesize ATP.
3. Initial donor of electrons during cyclic photophosphorylation is PSI.
   Final acceptor of electrons is PSI.
   Product is ATP only.

Question 4

Compare cyclic and non-cyclic photophosphorylation.

Answer 4

1. In cyclic photophosphorylation only PSI is involved whereas in non-cyclic PSI and PSII are involved.
2. In cyclic photophosphorylation water is not involved whereas in non-cyclic photophosphorylation photolysis of water is needed.
3. Initial donor of electrons in cyclic photophosphorylation is PSI whereas in non-cyclic photophosphorylation the initial donor of electrons is water.
4. During cyclic photophosphorylation the final acceptor of electrons is PSI whereas in non-cyclic photophosphorylation the final acceptor of electrons is NADP.
5. O2 does not evolve during cyclic photophosphorylation whereas O2 does evolved during non-cyclic photophosphorylation.
6. The only product of cyclic photophosphorylation is ATP whereas the products of non-cyclic photophosphorylation are reduced NADP, O2 and ATP.
7. Cyclic photophosphorylation utilizes electron acceptor Y only whereas non-cyclic photophosphorylation utilizes electron acceptors X and Y.