The Light dependant stage

In depth notes on the light dependant stage, empty boxes are left for you to draw the diagrams (out of the edexcel A2 biology revision guide), have fun!

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The light dependant reaction
Photosynthesis can be split into two stages
In the Light dependant reaction ATP is made by Photophosphorylation
In the light dependant reaction, the light energy absorbed by the photosystems is used for three things:
1) Making ATP from ADP and inorganic phosphate. This reaction is called Photophosphorylation
2) Making reduced NADP from NADP
3) Splitting water into protons (H+ ions), electrons and oxygen. This is called photolysis
The light dependant reaction actually includes two types of Photophosphorylation ­ noncyclic and
cyclic. Each of these processes had different products.
Non-cyclic Photophosphorylation produces ATP, reduced NADP and O2
To understand the process you need to know that the photosystems (in the thylakoid membranes) are linked by
electron carriers. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers
from an electron transport chain ­ a chain of proteins through which excited electrons flow. All the processes in
the in the diagrams are happening at the same time; however they are split into 4 to break it down.

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Cyclic Photophosphorylation only produces ATP
Cyclic Photophosphorylation only uses PS1. Its called `cyclic' because the electrons from the chlorophyll molecule
aren't passed onto NADP, but are passed back to PS1 via electron carriers. This means the electrons are recycled and
can repeatedly flow through PS1. This process doesn't produce any reduced NADP or O2 ­ it only produces small
amounts of ATP.…read more


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