Question

Give a general description of the light and dark reactions of photosynthesis. Light – light energy is converted into chemical energy (ATP) Dark – Carbon fixation – carbon is fixed into organic molecules (sugars) using ATP and enzymes

Answer 1

Photosynthesis is a complex process of synthesis of organic food materials.  

It is a oxidation - reduction process. Here water is oxidised and carbon dioxide is reduced to carbohydrates.

Photosynthesis takes place in two steps: i) light reaction ii) dark reaction.

Light Reaction is the first stage in photosynthesis where water is broken in the chlorophyll molecule into H+ ions and OH- ions in the presence of light resulting in the formation of assimilatory powers such as NADPH2 and ATP . It occurs in the grana of chloroplast.

Dark Reaction is the second step in the mechanism of photosynthesis. The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of the chloroplast.

The Dark reaction of photosynthesis is purely enzymatic and it is slower than the light reaction.

In dark reactions, two types of cyclic reaction occur. They are Calvin cycle or C3 Cycle and Hatch Slack Cycle or C4 cycle.

In dark reaction, sugars are synthesised from CO2. The energy poor carbon dioxide is fixed to energy rich carbohydrates using the energy rich compound ATP and the assimilatory power NADPH2 of light reaction. This process is called carbon fixation.

Blackman first demonstrated the existence of dark reaction .Hence it is known as Blackman’s reaction.

Light Reaction vs Dark Reaction

Light Reaction	Dark Reaction
Occurs : Grana of the chloroplasts.	Occurs: Stroma of the chloroplast
It is a light dependent process Involves two photosystems: PS I and PS II	This process does not require light. No photosystem is required.
Photolysis of water takes place and oxygen is liberated.	Photolysis of water does not take place. Carbon dioxide is absorbed.
ATP and NADPH is produced and they are used to drive the dark reaction.	Glucose is produced. Reduced NADP is oxidised