Photosynthesis

During photosynthesis, light energy is trapped and used to synthesise organic compounds from CO2 and water; the chloroplast is the organelle involved in photosynthesis.

The rate of photosynthesis is affected by a number of factors including light intensity, carbon dioxide concentration and temperature; the rate is determined by the limiting factor.

The absorption spectrum is a graph that shows how much light a particular pigment absorbs at each wavelength. Different pigments absorb light of different wavelengths.

The action spectrum is a graph that shows the rate of photosynthesis that takes place at different wavelengths.

Light harvesting:

• Different chlorophyll and carotenoid molecules (clustered at photosystems on the lamellae) absorb light maximally at different wavelengths.
• This allows photosynthesis to take place over a range of wavelengths
• As pigment molecules are energised, energy passes by resonance to a primary pigment that emits high-energy electrons

Light-dependent stage:

• Electrons pass from PS II to PS I through an electron transport system within the lamellae that synthesises ATP (photophosphorylation).
• PS II regains electrons from the photolysis of water; oxygen is released.
• Energised electrons from PS I reduce NADP+ to NADPH

Light-independent stage:

• Occurs in the stroma, CO2 is fixed by RuBP forming glycerate phosphate.
• Using energy from ATP, this is reduced by NADPH to triose phosphate.