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The synthesis of ATP from ADP+Pi using light energy.
Describe the internal structure of a chloroplast (optional diagram)
- Chloroplasts are surrounded by a double membrane.
- In side there are Grana which are stacks of unto 100 disc-like structures called thylakoids. This is where the reactions of the light-dependent stage of photosynthesis takes place and where the photosynthetic pigments are located. And the Stroma which is the fluid filled interior where the light independent reactions take place
What are the main types of photosynthetic pigments?
- Chlorophyll A and B: which absorb light mainly in the red and blue regions of the spectrum.
- Carotenoids of which there are two main types: Carotenes and xanthophylls both of which act as accessory pigments
How is light harvested in the thylakoid membrane.
Chlorophyll and accessory pigments are grouped in clusters of several hundred molecules called an antenna complex. Special proteins associated with these pigments help to funnel photons of light energy entering the chloroplast. The accessory pigments pass the photons of light energy from one molecule to the next until it reaches the chlorophyll a molecule, which is a primary pigment molecule know as the reaction centre.
Describe the two types of reaction centre
- Photosystem 1 (PS1): is arranged around a chlorophyll a molecule with an absorption peak of 700nm. (The reaction centre is also called P700)
- Photosystem 2 (PS2): is arranged around a chlorophyll a molecule with an absorption peak of 680nm. (The reaction centre is also called P680)
The light- dependent stage of photosynthesis involves ?
- Photolysis, the splitting of water molecules by light, to produce hydrogen ions and electrons.
- The synthesis of ATP from ADP+Pi.
- The combination of hydrogen ions with NADP to produce reduced NADP.
Describe non-cyclic photophosphorylation
- Photons of light are absorbed by PS2 and passed to chlorophyll a in the reaction centre.
- This causes the displacement of two high energy electrons, which are raised to a higher energy level where they are picked up by electron acceptors. These electron acceptors pass the electron along a chain of carriers through a proton pump and to PS1.
- The energy lost by the electrons is used to convert ADP to ATP.
- Photons of light are absorbed by PS1 and passed to chlorophyll a in the reaction centre.
- This in turn caused two electrons to be emitted and raised to a higher energy level where they are picked up by another electron acceptor.
- PS2 is now unstable as it has lost electrons. They are replaced when electrons become available from the splitting of water by photolysis.
- During photolysis, the water molecule dissociates into electrons, hydrogen ions and oxygen.
- The second electron acceptor, which receives electrons from photosystem 1, passes some of these electrons to hydrogen ions outside of the thylakoid membrane, where they reduce NADP.
- Since these electrons are not recycled back into the chlorophyll, this method of ATP production is called non-cyclic photophosphorylation.
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