Cyclic and Non-Cyclic Photophosphorylation and Photosystems

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for explaining this so well and in a way that is easy to understand for an A2 Biology student

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Cyclic and Non-Cyclic
Cambridge International
Examinations: A2 Biology…read more

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Background Information
· The first reactions that take place during
photosynthesis need light to be able to work
· Therefore they are called light-dependent reactions
· The purpose of these reactions is to produce ATP
(adenosine triphosphate) from ADP (adenosine
diphosphate) and inorganic phosphorous
· Also to collect hydrogen so that CO² can be reduced
(hydrogen added) to form a carbohydrate in the
second series of reactions
· The production of ATP using light is called
photophosphorylation…read more

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What is a Photosystem
· The pigments in chlorophyll are arranged in photosystems
· Photosystems are funnel shaped and sit on the thylakoid membranes in the
· Each photosystem has several hundred pigment molecules called accessory
pigments are arranged in clusters around one pigment molecule in particular
called the primary pigment
· Each of the different accessory pigments absorbs light at a different wavelength
and pass the light's energy down the photosystem
· Eventually the energy reaches the primary pigment which is like the reaction
· There are two types of photosystem:
­ Photosystem 1 (PSI): its primary pigment is a molecule of chlorophyll a with an
absorption peak at 700nm; it is called P700
­ Photosystem 2 (PSII): its primary pigment is a molecule of chlorophyll b with an
absorption peak at 680nm; it is called P680…read more

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· When light is absorbed by PSI and PSII it boosts the energy levels of the chlorophyll molecules
· So, because they have all this extra energy they are emitted from the photosystem and passed on
to electron carriers
· When PSII loses electrons it causes water to split
· The water then gives its electrons to PSII to fill the gap
· So oxygen and hydrogen ions are given off as waste from the splitting of water
· The electrons that were emitted from PSII pass through a series of electron carriers
· Each electron carrier has a lower energy level than the one before it
· So the electrons release all their extra energy and this is used to make ATP from ADP and P
· This happens by pumping hydrogen ions from the stoma across the thylakoid membrane to create
an electrochemical gradient
· The hydrogen ions flow through proteins as they diffuse down the concentration gradient
· Part of each of these proteins is ATP synthetase
· The energy that is released as the hydrogen ions diffuse down the concentration gradient is used
to synthesise ATP
· The electrons eventually reach PSI and fill in the gap there…read more

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Non-Cyclic Photophosphorylation
· The electrons from PSI may also pass
onto an electron carrier and combine with
the hydrogen ions from the water to
reduce NADP to NADPH (by adding
hydrogen ions)
· NADP + 2H+ + 2e- NADPH + H
· This reduced NADP is used in the next
series of reactions…read more

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Cyclic Photophosphorylation
· If there is plenty of NADPH then a different process occurs:
· The electron from PSI is passed to the electron carriers in
· ATP is formed and the electrons return to PSI to fill the gap
· This means that PSII isn't doing anything because we don't
need any electrons from it to fill the gap in PSII
· So:
­ No PSII involved
­ No oxygen involved
­ No NADPH made
­ BUT; ATP is still made…read more


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