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Summary Photosynthesis Revision Notes A2-2
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Completed revision notes that go by the A-Level Biology specification for CCEA
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Photosynthesis: Carbon Dioxide + Water -> Glucose + Oxygen
Chloroplast Structure:
Chloroplasts contain fluid-filled sacs called thylakoids.
Thylakoids are stacked up like pancakes to form structures which we
call grana.
Each granum is connected together by pieces of thylakoid membrane called lamellae.
The gel-like substance which surrounds the thylakoids is called the stroma.
Light Dependent Reaction (LDR):
Located in the thylakoids
Light energy is absorbed by PSII and excites the electrons within PSII and causes them to move into
a higher energy state (photoactivation). The electrons are then passed onto a series of electron
carriers within the electron transport chain to PSI.
The electrons which have been lost from PSII need to be replaced. Light energy causes a water
molecule to split apart and release hydrogen ions, electrons, and oxygen (photolysis). The electrons
from water replace the electrons lost from PSII.
As the electrons move along the electron transport chain, they move from high to low energy. The
energy lost by the electrons is used to pump hydrogen ions from the stroma into the thylakoids.
This generates a proton gradient across the thylakoid membrane.
Protons flow down their concentration gradient through ATP synthase. The energy from the
movement of protons is used to phosphorylate ADP to ATP (photophosphorylation) in a process
called chemiosmosis.
Light is absorbed by PSI causing another electron to become excited and be passed along the rest of
the electron transport chain.
The electron is passed onto NADP to form
reduced NADP (NADPH). NADPH is an electron
carrier which transfers electrons from one
molecule to another.
The ATP and reduced NADP move into the stroma
for the next stage of photosynthesis, the light
independent reaction.
Light Independent Stage (Calvin Cycle):
Located in the Stroma
Carbon dioxide is ‘fixed’ by adding it to a 5-carbon molecule called
ribulose bisphosphate (RuBP), forming a 6-carbon molecule. This
reaction is catalysed by an enzyme called Rubisco
The 6C molecule is unstable and immediately breaks down to form
two 3-carbon compounds called glycerate-3-phosphate (GP)
An isomerisation reaction occurs which converts GP into a different
3-carbon compound called triose phosphate (TP).
This reaction requires energy, so ATP (from the light-dependent
reaction) is hydrolysed into ADP.
The reaction also requires electrons from the electron carrier NADPH
(also from the LDR), transferring electrons to GP, reducing it to TP.
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