AQA A2 Biology: The Light-Dependent Reaction

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  • Created on: 20-09-13 20:35
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The Light-Dependent Reaction
The light-dependent reaction of photosynthesis involves the capture of light whose energy is used for
two purposes:
To add an inorganic phosphate molecule to ADP thereby making ATP
To split water by the process of photolysis, into H+ ions and OH- ions
The reaction needs light energy.
It takes place in the thylakoid membranes of the chloroplasts.
The light dependent reaction is useful for making three things:
Making ATP from ADP and inorganic phosphate
Making reduced NADP from NADP
Splitting water into protons, electrons and oxygen
The making of ATP
When a chlorophyll molecule absorbs light energy, it excites the electrons, raising them to a higher
energy level.
The electrons become so energetic that they leave the chlorophyll molecule altogether.
The electrons that leave the chlorophyll are taken up by a molecule called an electron carrier.
Having lost a pair of electrons, the chlorophyll molecule has been oxidised.
The electron carrier, which has gained electrons, has been reduced.
The electrons are now passed along a number of electron carriers in a series of oxidation-reduction
These electron carriers form a transfer chain that is located in the membranes of the thylakoids.
Each new carrier chain is at a slightly lower energy level than the previous one in the chain, and so the
electrons lose energy at each stage.
This energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make

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Photolysis of Water
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.
2H2O 4H+ + 4e- + O2
These hydrogen ions are taken up by an electron carrier called NADP.
On taking up the hydrogen ions, the NADP becomes reduced.…read more

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Light energy hits photosystem 2
This light energy excites the electrons and causes the chlorophyll to lose electrons
These electrons must be replaced
Light energy also causes water to split into protons, electrons and oxygen
The electrons from photolysis are used to replace the electrons lost from the chlorophyll
2. The excited electron that has come from the chlorophyll has a raised energy level
The electron is captured by an electron acceptor and enter an electron chain
3.…read more

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Chemiosmosis is the name of the process where the movement of H+ ions across a membrane generates
ATP ­ this also occurs in respiration.
The excited electrons lose energy as they move along the electron transport chain.
This energy is used to transport protons into the thylakoid so that the thylakoid has a higher
concentration of protons than the stroma.
This forms a proton gradient across the membrane.
Protons move down their concentration gradient, into the stroma via an enzyme ATP synthase.…read more


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