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Summary Biology Notes: Photosynthesis, Cellular Respiration

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Follows the Singapore-Cambridge GCE A Level H2 Biology Syllabus 9744 (NEW syllabus). Compiled by student from Hwa Chong Institution (Graduated in 2018) with consistent A in H2 Biology. Received 90 RP for A level 2018.

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PHOTOSYNTHESIS

Light-dependent reaction

Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a​ light harvesting complex (LHC)​ and energy is
relayed via ​resonance energy transfer​ until it reaches one of the two ​special chlorophyll a
molecules (P680) ​in the ​PSII ​reaction centre
(a) An electron in P680 is excited to a higher energy state
(2) This photoexcited electron is captured by the ​primary electron acceptor​ in the reaction
centre
(3) Photolysis of water​ produces two electrons, two hydrogen ions and an oxygen atom per
water molecule; an electrons released is used to replenish the electron deficit in P680, while
the oxygen atom combines with another oxygen atom to release​ O​2​ ​as a by-product
(4) From the primary electron acceptor, the energised electron passes (unidirectionally through a
series of oxidation-reduction reactions) from PSII to PSI via a ​first electron transport chain
(ETC)​ consisting of ​plastoquinone (Pq)​,​ cytochrome (b-f) complex ​and ​plastocyanin (Pc)
(5) Free energy released from the exergonic flow of electrons is used to pump protons against
concentration gradient from the stroma into the thylakoid space, generating a proton
gradient used to drive ATP synthesis (photophosphorylation)
(a) Chemiosmosis​:
Thylakoid membrane is impermeable to H​+​; H​+​ ions accumulate in thylakoid space as
light reaction proceeds (pumping of H​+ ​by cytochrome complex, photolysis of water)
H​+​ diffuse down the gradient from thylakoid space across thylakoid membrane into
the stroma, through the ​ATP synthase complex​; ATP synthase catalyses the formation
of ATP (chemiosmosis)
(6) Meanwhile, another photon of light strikes a pigment molecule in a ​LHC ​of PSI, exciting an
electron of one of the two ​special chlorophyll a molecules ​in the ​PSI ​reaction centre (​P700​)
(7) This excited electron is captured by the ​primary electron acceptor​ in PSI
(8) The electron deficit in P700 is replenished by the electron from PSII
(9) The excited electron is passed from primary electron acceptor in PSI down a ​second ETC
through ​ferredoxin (Fd)
(10) NADP reductase ​transfers electrons from Fd to ​NADP​; two electrons are required to produce
NADPH

Non-cyclic phosphorylation
(1) A photon of light strikes a pigment molecule in a ​LHC ​of PSI, exciting an electron of one of the
two​ special chlorophyll a molecules ​in the ​PSI ​reaction centre (​P700​)
(2) This excited electron is captured by the ​primary electron acceptor​ in PSI
(3) The excited electron is passed to ​ferredoxin (Fd)​, cycled back to ​cytochrome (b-f) complex ​on
first ETC and eventually​ back to PSI
(4) As these electrons are passed along first ETC, enough energy is released to synthesise ATP
(​chemiosmosis​) for the light-independent reaction


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