Photosynthesis

Photosynthesis is the fixation of carbon dioxide and its subsequent reduction to carbohydrate, using hydrogen from water.

• it transfers light energy into chemical potential energy of organic molecules
• it takes place inside the chloroplast
• two sets of reactions are involved; light dependent reactions and light independent reactions
• the light dependent reactions only take place in the presence of suitable pigments that absorb certain wavelengths of light 
• light is neccesary for:
  • photolysis
  • to provide chemical energy in the form of ATP, for reduction of CO2 to carbohydrate in LDR
• the photosynthetic pigments involved fall into two categories: primary and accessory pigments
• the pigments are arranged in light-harvesting clusters called photosystems of which there are I and II
• photosystems consist of several hundred accessory pigment molecules surrounding a primary pigment molecule, light energy absorbed by the different pigments is passed to the primary pigment
• the primary pigments act as reaction centres and are two forms of chlorophyll 

Light Dependent Reactions

The light dependednt reactions consist of photolysis and the synthesis of ATP in photophosphorylation

Photophosphorylation of ADP to ATP can be cyclic or non-cyclic depending on electron flow pattern in one or both types of photosystem

Cyclic photophosphorylation

• it involves only photosystem I
• light is absorbed by photosystem I and passed to the primary pigment
• an electron in the chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule. This is photoactivation.
• the excited electron is captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron carriers
• during this enough energy is released to synthesise ATP from ADP and Pi by chemiosmosis
• the ATP then passes to the light independent…