The process of photosynthesis

Photosynthesis is a complex process of synthesis of organic food materials in which water is oxidised and CO₂ is reduced to carbohydrates.

This process is summarised by the flow diagram below.

Light-dependent reaction

Light energy results in the excitation of electrons in the chlorophyll.
These electrons are passed along a series of electron acceptors in the thylakoid membranes, collectively known as the electron carrier system.
Energy from excited electrons funds the production of ATP (adenosine triphosphate).
The final electron acceptor forms NADP⁺
Electron loss from chlorophyll causes the splitting of water (photolysis)
- H₂O = H⁺ + OH^– then 4OH^– = 2H₂O + O₂ + 4e^–
Oxygen is produced, water to re-use, and electrons stream back to replace those lost in the chlorophyll.
Hydrogen ions (H⁺) from photolysis, together with NADP⁺ form NADPH₂

Light-independent reaction

Two useful substances are produced by the light-dependent stage, ATP and NADPH₂. These are needed to drive the light-independent stage.
They react with glycerate-3-phosphate (GP) to produce a triose sugar – triose phosphate.
Triose phosphate is used either to produce a 6C sugar or to form ribulose bisphosphate (RuBP).
The conversion of triose phosphate (3C) to RuBP begins Calvin’s cycle and utilises ATP, which supplies the energy required.
A RuBP molecule (5C) together with a carbon dioxide molecule (1C) forms two GP molecules (2 3C) to complete Calvin’s cycle.
The GP is then available to react with ATP and NADPH₂ to synthesise more

triose sugar or RuBP.

Calvin Cycle (C3 Cycle)

Observed by Melvin Calvin in chlorella (single-cell green algae).
Carbon dioxide is converted into sugar so it is a process of carbon fixation.
First stable compound produced is 3-Phosphoglyceric acid (a 3-carbon molecule), often referred to as PGA.

Hatch & Slack Cycle (C4 Cycle)

Discovered by Marshall Davidson Hatch and C.R. Slack in Australia in 1966.
Alternative pathway of the Calvin Cycle, taking place during the dark phase of photosynthesis.
First stable compound formed is Oxaloacatic acid (a 4-carbon molecule).
Only produced in C₄ plants (about 7,600 species), often possessing a leaf anatomy called kranz anatomy.
Evolved from the C3 process to withstand drought, high temperatures and CO₂ limitations.

How do the photosystems contribute to photosynthesis?

This can be explained in terms of the Z scheme shown below.

Non-cyclic photophosphorylation

Light reaches the chlorophyll of both photosystems (P680 and P700) which results in the excitation of electrons.
Electron acceptors receive these electrons (accepting electrons is reduction!).
P680 and P700 have become oxidised (loss of electrons is oxidation!).
P680 receives electrons from the lysis (splitting) of water molecules and becomes neutral again.
Lysis of water molecules releases oxygen which is given off.
Electrons are elevated to a higher energy level by P₆₈₀ to electron acceptor A and are passed along a series of electron carriers to P₇₀₀.
Passage along the electron carrier system funds the production of ATP.
The electrons pass along a further chain of electron carriers to NADP, which becomes reduced, and at the same time this combines with H⁺ ions to form NADPH₂.

Cyclic photophosphorylation

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