3.2 The light-dependent reaction


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The light-dependant reaction

Oxidation and reduction


Oxidation is loss (electrons)

Reduction is gain (electrons) 

Oxidation results in energy being given out whereas reduction results in being taken in. 

Oxidation and reduction always take place together. 

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Photolysis of water

2H20 --> 4H+   +   4e-   + O2

Water --> protons + electrons + oxygen

The loss of electrons when light strikes a chlorophyll molecule leaves it short of electrons.  If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced.

The replacement electrons are provided from water molecules that are split using light energy.  This photolysis of water also yields hydrogen ions (protons).

These hydrogen ions (protons) are taken up by an electron carrier called NADP.  On taking up the hydrogen ions (protons) the NADP becomes reduced.  The reduced NADP then enters the light-independant reaction along with the electrons from the chlorophyll molecules.  

The reduced NADP is important because it is a further potential source of chemical energy to the plant.  The oxygen by-product from the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product. 

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Site of the light-dependent reaction

The light-dependant reaction of photosynthesis takes place in the thylakoids of chloroplasts. The thylakoids are disc-like structures that are stacked together in groups called grana. 

Chloroplasts are structurally adapted to their function of capturing sunlight and carrying out the light-dependent reaction of photosynthesis in the following ways:

- The thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enxymes that carry out the light-dependent reaction.

- A network of proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorption of light

- The granal membranes have enzymes attached to them, which help manufacture ATP.

- Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins needed for the light dependent-reaction.  

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