The Key Stages Of Photosynthesis

Photosynthesis is an ENDOTHERMIC reaction (takes in heat energy). There are two different photosystems, each containing a slightly different version of the CHLROPHYLL molecule.

(a-blue green pigment and b yellow-green pigment)

Photosystem I (PSI) in which the peak absorption spectrum is 700nm

Photosystem II (PSII) in which the peak absorption spectrum is 680nm

The light dependent reactions take place in the THYLOKOID membranes.

Stage 1:
PHOTONS of light energy are harvested by PSII. The ELECTRONS in PSII are raised to a higher energy level, released and captured by an electron ACCEPTOR. PSII has been OXIDISED, and so receives 2 electrons from the PHOTOLYSIS of water.
The photolysis of water is: H2O 2e- + 2H+ + O2
The O2 combines with another and is released as O2 gas.

Stage 2
Electrons in the electron acceptor pass along an electron TRANSPORT chain to PSI. This transfer of electrons is via a series of redox reactions. Energy is released and enables ADP to join with inorganic phosphate (Pi) and form ATP. Light energy has therefore been converted to and stored as CHEMICAL bond energy in ATP. This is non-cyclic PHOTOPHOSPHORYLATION.
Stage 3
Photons of light energy are harvested by PSI. The electrons in PSI are raised to a higher energy level and captured by another electron acceptor. PSI has been oxidised, and so takes electrons from the previous electron transport chain.
Stage 4
Some electrons pass back to the first electron transport chain and brought back to PSI. This generates a molecule of ATP and is called CYCLIC photophosphorylation.
Stage 5
Electrons transfer from electron acceptor along a chain of electron carriers (via redox reactions) to NADP. They combine with the protons (H+) from photolysis and NADP is reduced to NADPH.

Light Independent Reactions
These reactions occur in the STROMA of the chloroplasts. They occur whether or not light is available. These reactions are known as the CALVIN Cycle.

Stage 1
Carbon dioxide (in solution) diffuses through the plasma membrane, through the chloroplasts and into the stroma. Here is combines with a 5 CARBON compound ribulose bisphosphate (RuBP). This produces an unstable 6-carbon molecule that instantly splits into two 3-carbon molecules of glycerate-3-phosphate (GP). The reaction is CATALYSED by the enzyme ribulose bisphosphate carboxylase (RUBISCO).
Stage 2
GP is REDUCED to triose phosphate (TP) by NADPH (which was produced by the light-dependent reactions). This reaction is ENDOTHERMIC, and driven by the ATP produced via photophosphorylation. NADP is OXIDISED and available for further use in the light-dependent reactions.
Stage 3
The combination of two molecules of TP produces a molecule of GLUCOSE. The glucose is made of carbon, oxygen and hydrogen. The carbon and oxygen have been obtained from carbon dioxide, and the hydrogen has been obtained from water (via photolysis)
Stage 4
Most molecules of TP are recycled to REGENERATE RuBP for the cycle to continue.
TP and Glucose are the starting point for the SYNTHESIS of other carbohydrates and lipids in the plant. The addition of NITROGEN forms amino acids (and therefore proteins).