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4.5.7 LIGHT DEPENDANT STEP OF PHOTOSYNTHESIS

Figure 5.33 – in the light dependant reactions of photosynthesis, the energy from the sun excites chlorophyll molecules. High energy electrons leave the chlorophyll and pass along a series of carrier proteins in the electron transport chain. The ionised chlorophyll causes the photolysis of water; hydrogen is pulled off water molecules raising the local hydrogen ion concentration within the thylakoid. ATP is also formed as a phosphate group is added to the ADP.

LIGHT DEPENDENT STEP:

1. Chlorophyll absorbs light (remember chlorophyll is the trap in the bottom of the photosystem)

2. Chlorophyll emits electrons

3. Electrons are received by electron carrier proteins in the thylakoid membrane (electron transport chain)

4. Electron transport chain uses high energy electrons to power the following conversions;

5. Water is split (photolysis) to produce replacement electrons for the photosystems, H+ for the reduction of NADP and O2 which is excreted.

The purpose of the light dependent step is to produce ATP and NADPH. ATP provides the energy for converting CO2 into glucose and NADPH provides the H for glucose.