A-Level Revision
Notes
A2 Biology
SAMPLE
‘Photosynthesis & The Chloroplast’
By Jonathan Reynolds
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Photosynthesis & The Chloroplast
The stroma is an area inside of the chloroplast where reactions occur and starches (sugars)
are created. One thylakoid stack is called a granum. The thylakoids have chlorophyll
molecules on their surface. That chlorophyll uses sunlight to create sugars. The stacks of sacs
are connected by stromal lamellae. The lamellae act like the skeleton of the chloroplast,
keeping all of the sacs a safe distance from each other and maximizing the efficiency of the
organelle.
The Photosystems
Here there are a large number of chlorophyll molecules
The process of photosynthesis:
Light Dependent Stage (thylakoid)
Light independent stage (Stroma)
Light Dependent Stage:
• This takes place in the grana of the chloroplast
• It involves the very fast movement of electrons and is not controlled by enzymes
• Light is absorbed by a range of pigment clusters found in the chloroplast (almost all
colours of light are absorbed but green is normally reflected)
• This light energy is transferred to the electron that has been passed to the chlorophyll
• This chlorophyll is in the pigment cluster and is strategically placed near an electron
acceptor
• The pigment cluster absorbs as much light energy as possible and passes it to the
chlorophyll which passes it to the electron
• The energised electron is passed to the electron acceptor which can the send it on one
of two pathways
In detail:
• The electrons pass from the first electron acceptor to a series of other electron
acceptors and back again to the chlorophyll
• As the electrons are passed around they lose energy
• This energy is used to join a phosphate to ADP to form high energy ATP
• Water is also formed in this process
• ADP + Energy + P
>>>>>>
ATP + Water
• The addition of phosphate to ADP is called phosphorylation
• Because the electron travel in a cycle and returns to its original chlorophyll this
process is called Cyclic Phophorylation
• 2 high energy electrons at a time are passed from chlorophyll to the electron acceptor
and then along another series of electron acceptors
• In this case the electrons do not return to the original chlorophyll
• They lose energy as they pass from electron acceptor to electron acceptor and this
energy is used to make more ATP
• Eventually the 2 electrons are passed to combine with NADP+ to form NADP• The chlorophyll molecule is now short of electrons and gains more from the splitting
of water
• The splitting of water using light energy is called Photolysis
•
•
The protons that were stored in the proton pool are attracted to NADP- and combine
with it to form NADPH
Because the electrons start at a chlorophyll and finish at NADPH and form ATP on
their way this pathway is known as Non cyclic photophosphorylation
Light Independent Stage
 Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate
(RuBP) to form a 6-carbon sugar.
 This process is known as carbon fixation and is catalysed by the enzyme ribulose
bisphosphate carboxylase (rubisco).
 This 6-carbon sugar is unstable and breaks down to form two 3-carbon sugars.
 These are converted into triose phosphates using the energy from ATP and using the
hydrogen from reduced NADP.
 Most of this triose phosphate is used to regenerate RuBP, but some is used to produce
6-carbon sugars from which complex carbohydrates, amino acids and other
substances are made.
What factors affect photosynthesis?
 Temperature
 Light Intensity
 Water
 Carbon Dioxide
Limiting Factors:
Q) Explain why the above three environmental factors are known as “limiting
factors”.
 If any one of the 3 above factors is in short supply the rate at which photosynthesis
occurs will slow down.
 The increase in any one of the above factors will cause the rate of photosynthesis to
increase.
Light Intensity
As the light intensity increases > rate of photosynthesis increases
Reason: More ATP and NADPH are formed.
Q) Why does graph level off?
1) Plant is light saturated (the chloroplasts cannot absorb any more sunlight energy)
2) There may be a shortage of CO2 in the air.
3) Temperature of the environment is not hot enough.
Compensation Point:
 The compensation point is the amount of light intensity on the light curve where the
rate of photosynthesis exactly matches the rate of respiration.
 At this point, the uptake of CO2 through photosynthetic pathways is exactly matched
to the respiratory release of carbon dioxide, and the uptake of O2 by respiration is
exactly matched to the photosynthetic release of oxygen.