Light independant reactions

Synthesis of organic compounds such as glucose from inorganic carbon dioxide and water using light as an energy source.
This is also known as photoautotrophic nutrition.
Carbon dioxide is an energy poor compound which is reduced during photosynthesis
There are two main stages:
1. The light dependent reactions that occur on the granal membranes. Pigments are required to absorb and transduce the light energy. Membrane proteins act as electron carriers.
2. The light independent reactions that occur in the stroma of the chloroplast.
Chlorophyll and other pigments are grouped together to form a photosystem.

There are two photosystems, photosystem II(P700) and photosystemI(P680)

Each photosystem acts as a light harvesting system; light energy is passed from the accessory pigments to the chlorophyll in the reaction centre of the photosystem.

Light energy strikes the chloroplast.
The energy is channelled towards the chlorophyll molecule in the reaction centre of the photosystem. Two electrons from the chlorophyll in the reaction centre of photosystem II are ‘excited’ and raised to a higher energy level (the chlorophyll is oxidised)
The electrons are accepted by an electron carrier.
The excited electrons from photosystem II are accepted by a carrier and pass down a series of electron carriers losing energy.
The energy ‘lost’ is used to synthesise ATP from ADP + iP.
The electrons pass into photosystem 1
Water is broken up using light energy into protons, electrons and oxygen which is evolved.
The electrons released from the water replace those lost from photosystem II.
Light energy strikes photosystem I at the same time as it strikes photosystem II and the energy is passed from the accessory pigments to the chlorophyll in the reaction centre of photosystem I.
Two electrons from the chlorophyll in the reaction centre of photosystem I are raised to a higher energy level (the chlorophyll is oxidised)
The electrons passing along the carriers from PSII replace those lost from PSI.
The electrons ‘lost’ from PSI pass to an electron acceptor and pass down a series of carriers
These electrons and the protons from the breakdown of the water combine with NADP to form reduced NADP
This sequence of events is called
Non cyclic photophosphorylation
Sometimes the electrons ‘lost’ from PSI pass along carriers back into PSI
This generates more ATP and is called cyclic photophosphorylation
One of the electron carriers is a proton pump
The energy ‘lost’ by the electrons is used to actively transport protons across the thylakoid membrane into the thylakoid space
The concentration of protons in the thylakoid space increases
The protons now pass through a membrane protein that is the enzyme ATP synthase
The protons make the enzyme rotate releasing energy
The energy is used to synthesise ATP
Tutorial 8.2 Photophosphorylation http://highered.mcgraw-hill.com/olc/dl/120072/bio13.swf The reduced NADP and the ATP are required for the light independent reactions. (Calvin Cycle)
The ATP has two functions:
It provides the energy needed to reduce Glycerate 3 phosphate (GP) to triose phosphate (TP)
It provides iP to convert ribulose phosphate to ribulose bisphosphate

The reduced NADP provides the hydrogen needed to reduce GP to TP