Why does photosynthesis need light?
Light provides energy for the synthesis of glucose from carbon dioxide and water during photosynthesis.
Photosynthesis is a photo-chemical reaction involving 2 main steps, i.e. Light reaction or Hill’s reaction and Dark reaction or Blackmann’s reaction.
Light reaction occurs in presence of light. Dark reaction can occur in absence of light but is dependent upon the end product of light reaction. Thus light reaction must precede dark reaction.
During light reaction, the chlorophyll entraps light and the solar energy is converted into chemical energy in the form of ATP molecules. This can happen because light energy is used to split water. The products of this reaction are oxygen (yeah for us heterotrophs!) and hydrogen ions. The hydrogen ions are used to make the ATP mentioned earlier.
Here is a video showing the formation of oxygen in the light reaction.
Video from: Noel Pauller
ATP molecules provide energy for the synthetic reactions during dark reaction of photosynthesis and are converted into ADP molecules.
ADP molecules, thus, formed during dark reaction are reconverted into ATP molecules during light reaction.
Light is, thus, necessary for the process of photosynthesis to provide energy for the synthetic reactions.
The synthesis of ATP molecules in presence of light is termed photo-phosphorylation.