Introduction

Photosynthesis is a process that converts light energy into biochemical energy which is then used to drive the assimilation of low energy inorganic carbon (CO₂) into high energy organic biochemicals. Photosynthesis takes place in the chloroplast of photoautotrophic eukaryotic organisms such as green plants and algae, and some prokaryotes (some bacteria and cyanobacteria). The general equation shows that photosynthesis is the reverse of cellular respiration.

Chloroplast and chlorophyll

The chloroplast is a double membraned organelle that consists of grana (stacks of thylakoids) and the stroma (soluble matrix). Chlorophyll a is the primary pigment that gives leaves their green colouration and absorbs light energy that is channelled into reaction centres in the chlorophyll molecules embedded in the thylakoid membranes as discrete organised units: Photosystems I and II. Colourful accessory pigments such as carotenoids absorb excess energy, transfer electrons to the primary electron acceptor (chlorophyll a) and prevent damage to the photosytems.

The two phases of photosynthesis

Let's consider the reactants (H₂0 and CO₂) and how they are used to make the products (O₂ and C₆H₁₂O₆).

This takes place in two phases, the light-dependent phase (use of sunlight to split water) in the grana and the light-independent (dark) phase in the stroma (use of high energy biomolecules generated in the light phase to fix carbon dioxide into sugars). Note the light-independent phase does not take place in the dark!

The light-dependent phase is a physical phase where water (H₂0 ) is split using light energy (photolysis). This liberates oxygen (O₂) along with protons (H⁺ ions) and electrons (e^-). Oxygen is a waste product that is released to the atmopshere via stomata. (This is lucky for us and all other organisms that use oxygen, so every time you take a breath, say thank you to a plant!). The electrons (e^-) generated by photolysis of water enter the electron transfer chain (ETC) and are passed through a series of protein complexes (including Photosytems I and II) and mobile electron carriers - these generate NADPH. The protons (H⁺) generated by photolysis of water are released into the lumen of the thylakoid then diffuse down an electrochemical gradient through an enzyme called ATP synthase (also embedded in the thylakoid membrane) to generate the high energy molecule ATP.

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The light-independent (dark) phase is a chemical phase that uses the high energy molecules generated by the light phase (ATP and NADPH) to drive the Calvin Cycle where inorganic CO₂ is incorporated into high energy sugars. The Calvin cycle has three phases:

Fixation of CO₂ (catalysed by RuBisCO),
Reduction (using NADPH) and
Regeneration of Ribulose 1-5 bisphophate (RuBP), the initial CO₂ acceptor (using ATP).

The Calvin Cycle produces a 3 carbon molecule (glyceraldehyde-3-phosphate - G3P) which is subsequently packaged into sugars such as glucose (or sucrose, fructose).

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