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Photosynthesis

Chloroplasts

• Chloroplasts are large organelles, usually in a biconvex shape, and about 4-10μm long and 2-3μm wide.
• Within plants most chloroplasts are found in the mesophyll cells of leaves.
• The stages of photosynthesis take place within the chloroplast, stage 1 within the granum and stage 2 within the stroma.

Stage 1 - Light Dependent

1. PSII - Light energy is trapped in PSII and boosts electrons to a higher energy level.
2. The electrons are received by an electron acceptor.
3. The electrons are passed from the electron acceptor along a series of electron carriers to PSI which is at a lower energy level. The energy lost by the electrons is captured by converting ADP to ATP. Light energy has thereby been converted to chemical energy.
4. Light energy absorbed by PSI boosts the electron to an even higher energy level.
5. The electrons are received by another electron acceptor.
6. The electrons which have been removed from the chlorophyll are replaced by pulling in other electrons from a water molecule.
7. The loss of electrons from the water molecule causes it to dissociate into protons and oxygen gas.
8. The protons from the water molecule combine with the electrons from the second acceptor and these reduce NADP.
9. The Cyclic reaction - This causes the electrons to be recycled and is used for forming more ATP.

Stage 2 - Light Independent

1. CO₂ diffuses into the leaf through the stomata. It then through and into the stroma of the chloroplast.
2. The CO₂ the combines with a five carbon ribulose bisphosphate forming an unstable six carbon intermediate.
3. The six carbon intermediate breaks down into two, three carbon glycerate 3 phosphate molecules.
4. Some of the ATP and NADPH₂ produced during the LDS is used to convert glycerate 3 phosphate into triose phosphate.
5. Pairs of triose phosphate molecules can combine to produce an intermediate hexose sugar, this can then be polymerised to form lipids, amino acids, sugars and starch.
6. A portion of the triose phosphate can be regenerate the original CO₂