STPM S1: Light-dependant Reactions

During photosynthesis, which of the following processes release electrons that return the chlorophyll molecules to their reduced state?

• Activation of photosystem 1

• Oxidation of reduced NADP

• Phosphorylation of ADP

• Photolysis of water

Answer explanation

When photosystem II absorbs light, it releases electrons. The photoactivated chlorophyll molecules of PS II become positively charged. The water molecules activated by light energy dissociate to produce OH- ions and H+ ions as well as the activated electrons. The activated electrons released are accepted by the positively-charged chlorophyll molecules of PS II.

How does photorespiration differ from aerobic respiration?

I It occurs in the presence of light

II It produces carbon dioxide and water

III It consumes oxygen

IV It produces no ATP

• I and II

• I and IV

• II and III

• III and IV

Answer explanation

Photorespiration is a respiration that is initiated in the chloroplasts and occurs in light only. It is a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, wasting some energy produced by photosynthesis. It is a wasteful pathway that occurs when the enzyme acts on oxygen rather than carbon dioxide.

What are the products of light reaction in photosynthesis?

• Oxygen, ATP and NADP

• Water, ATP and NADPH

• Water, ATP and reduced NADP

• Oxygen, ATP and reduced NADP

Answer explanation

At the end of the light-dependent reaction, oxygen, ATP and reduced NADP are produced. ADP, NADP and carbohydrates are products of the light independent reactions.

During cyclic photophosphorylation

I no photosystem II is involved

II no oxygen is evolved

III no NADPH is produced

IV ATP is produced

• I, II and III

• I, II and IV

• II, III and IV

• I, II, III and IV

Answer explanation

Cyclic photophosphorylation only involves Photosystem I.

There is no

photolysis of water. Hence, no oxygen is produced. Only ATP is produced.

Which of the following statements is true regarding the light-dependent reaction of photosynthesis

• Water is required for oxygen formation and carbon dioxide reduction.

• ATP is required to transform carbon dioxide to triose phosphate.

• Light is required for oxidation of water and to reduce NADP to NADPH.

• ATP and NADPH are used to transform phosphoglyceric acid to hexose sugar.

Answer explanation

During light-dependent reaction, water molecules are split up and electrons are removed from water to replace the electrons released from Photosystem II when light is absorbed. At the end of the reaction, NADP+ accepts electrons and proton to form NADPH.