Light and Dark Reactions in Photosynthesis

Question

LOOKING FOR INFORMATION ABOUT  Light and Dark Reactions in Photosynthesis. ANY HELP PLEASE

Answer 1

Hi Bob,

 

Photosynthesis is a process in which light energy is converted to chemical energy.  Over all reaction of photosynthesis is:

 

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

 

In plants, photosynthesis takes place in the chloroplasts.  Look up the structure of chloroplasts to understand where light and dark reactions of photosynthesis take place.  Light reactions takes place in the thylokoids and dark reaction takes place in the stroma of the chloroplasts.  

 

Thylokoids are membrane structures consisting of photosystems I and II.. Photosystems  I and II contain the pigments chlorophylls and carotenoids that capture the light energy, pass the energy on to a reaction center from which a high energy electron will be released.  Reactions that follow will result in the synthesis of ATP and NADPH. To replace the electron lost from the photosystem, electrons will be pulled from water, which results in the release of oxygen from water.

These are light reactions.  

 

From the over all reaction of photosynthesis above, notice that input of light energy, water and output of oxygen are accomplished in the light reaction.  

 

High energy molecules ATP and NADPH synthesized in the light reaction diffuse to stroma where they are used in the dark reaction to synthesize glucose/carbohydrate using carbon dioxide from the atmosphere.   This is carried out by the enzyme RUBISCO in a series of reactions called Calvin-Benson cycle.  Presence of light is not necessary for these reactions. Refer to the overall reaction of photosynthesis and notice that CO₂ reduction to C₆H₁₂O₆ takes place in the dark reaction.

 

Hope it helps.