Photosynthesis

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  • Created by: Bethfr
  • Created on: 01-06-15 22:48

Light dependent: non cyclic photophosphorylation

1. Photosynthetic pignments in the antenna complex of PSII aborb light energy, which is then transferred to the reaction centre where it excites 2 electrons in chlorophyll a molecule. These electrons lost are replaced by electrons from a water molecule. Loss of electrons causes water to dissociate into protons and oxygen - photolysis,

2. Excited electrons are boosted to a higher energy level, where they leave the chlorophyll a molecule and are recieved by an electron acceptor.

3. The electrons are passed along the electron transport chain in a series of redox reactions to PSI which is at a lower energy level. Energy lost by electrons used to convert ADP+Pi --> ATP.

4. Light absorbed by PSI boosts 2 electrons from the chlorophyll a molecule to an even higher energy level where they are recieved by another electron acceptor.

5. Electrons and H+ from the photolysis of water are used to reduce the final electron acceptor NADP to NAPH2.

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Light dependent: cyclic photophosphorylation

1. Photosynthetic pignments in the antenna complex of PSI aborb light energy, which is then transferred to the reaction centre where it excites 2 electrons in chlorophyll a molecule.

2. Excited electrons are boosted to the highest energy level, where they leave the chlorophyll a molecule and are recieved by an electron acceptor.

3. The electrons are passed along the electron transport chain in a series of redox reactions before returning to PSI. Energy released from redox reactions used to phosphorylate ATP.

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Chemiosmotic theory of photophosphorylation

AKA the synthesis of ATP during the light dependent stage...

Energy lost by electrons as they pass along the ETC is used to pump protons from the stroma, across the thylakoid membrane and into the thylakoid space.

The protons accumulate so that steep concentration and electrochemical gradients are established across the the thylakoid membrane. These gradients also maintained by:

photolysis of water in the thylakoid space, increasing H+ conc

reduction of NADP in the stroma, reducing H+ conc

Protons diffuse back into stroma through the chemiosmotic channel proteins where ATP synthase enzyme is located. Flow of protons through AT synthase provides energy for ADP+Pi-->ATP 

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The light independent stage

Enzymes for Calvin cycle located in the stroma.

1. Carbon dioxide combines with ribulose bisphosphate to form two molecules of glycerate-3-phosphate (a carboxylatio reaction catalysed by Rubisco enzyme) (carbon converted from inorganic form to organic form so also carbon fixation).

2. NADPH2 used to reduce the two glycerate-3-phosphate molecules into two molecules of triose phosphate. ATP hydrolysis provides energy for this reaction.

3. Most (5 out of 6) triose phosphate molecules are converted, by a series of reactions, back into RuBP. ATP supplies the required energy and phosphate.

4. Some (1 out of 6) of the triose phosphate molecules are rapidly convert to glucose, other carbs, amino acids, lipids and nucleic acids.

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