IMAT 2016 Q38 [Light-­Dependent Reactions in Photosynthesis]

Which option correctly identifies the site of the light-­dependent reactions in photosynthesis (S), the hydrogen carrier (H) used and its end state (N)?

A. S: stroma; H: NAD; N: reduced
B. S: granum; H: NAD; N: reduced
C. S: granum; H: NADP; N: reduced
D. S: stroma; H: NADP; N: oxidised
E. S: stroma; H: FAD; N: oxidised

The correct answer is C.

What is photosynthesis ?

Photosynthesis, like the name suggests, is the synthesis of organic matter under the influence of light. The bio products are oxygen and glucose, which is a type of sugar.

It is the process that allows plants to create its tissues, combined with the extraction of minerals from the soil.

There are 3 key elements to photosynthesis :

  1. light (solar energy)
  2. water (H2O) from the soil
  3. carbon dioxide (CO2) from the air

The main condition for photosynthesis to happen is the presence of light : under its influence, carbon dioxide is coupled with water and then split into a carbohydrate (glucose) and oxygen.

What are the light dependent reactions ?

First, we have to talk about the chloroplast. It is a plant organelle that is composed of different structures :

  1. the stroma is the inside of the chloroplast
  2. it has 2 membranes : one outer and one inner
  3. thylakoids are compartiments inside the chloroplast
  4. a stack of thylakoids is a granum

The thylakoid is a structure in which light dependent reactions occur, also known as the electron transport chain. It results in the production of an ATP molecule.

Therefore, the light dependent reactions occur in the stroma and not the granum.

Here is a diagram of the electron transport chain :

(it is P680 and not P850)

It is composed of 2 photosystems :

  1. PSI or P700
  2. PSII or P680

Both photosystems contain pigments ; their role is to collect light energy to ensure photophosphorylation (ADP is phosphorylated into ATP).

The light energy increases the electrons’ free energy and ultimately allows the production of NADPH = the reduced form of NADP.