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Category: Crop Physiology Objectives:

1. be able students to describe the mechanism of photosynthesis

2. be able student to differentiate photosynthesis pathways (C3,C4 & CAM)

Target audience:   First Year Agriculture Undergraduates

Photosynthesis is the method that plants and photoautotrophes utilize light energy to produce ATP via photophosphorylation in order to anabolise sugars. It is an energy transfer process, and almost all energy transferred to ATP in all organisms is derived from light energy trapped by autotrophs. The equation for it is listed below;

6 CO2(g) + 12 H2O(l) + photons → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)

carbon dioxide + water + light energy → glucose + oxygen + water


The Z-scheme - representing the relative energy levels of the photosystems CYCLIC This type of photophosphorylation involves only photosystem I. When light is absorbed by photosystem I and passed to "chlorophyll a (P700)", an electron in this chlorophyll molecule is excited to a higher energy level and then captured by an electron acceptor. It is then passed back to a "chlorophyll a" molecule through a cycle of electron carriers (or electron transport chain/ETC), which at the meanwhile, release energy to synthesise ATP from ADP+Pi (phosphorylation) by a mechanism known as chemiosomosis. This ATP later enters the light independent stage.


See picture In the light-independent stage, RuBP (5-C) combines with one CO2 molecule, that then splits into 2 glycerate-3-phosphate (GP) molecules (3-C), which is finally reduced to 2 triose phosphates (3-C). 1 triose phosphates (3-C) feedback in to the cycle to regenerate RuBP (5-C), 1 is polymerised into starch. The products of this cycle are used to form glucose, amino acids or lipids. Source Text is available under the Creative Commons Attribution-ShareAlike License

Photosynthesis Rena Jones - Photosynthesis.mp3

A-level Biology/Central Concepts/Photosynthesis

Animation Photosynthesis


Light Reactions of Photosynthesis