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Photosynthesis

AQA A-level biology photosynthesis year 13

TermDefinition
Compensation point Photosynthesis & respiration normally occur at different rates depending on light intensity, but at a certain light intensity they occur at the same rate
How to tell when compensation point is reached Measuring how much oxygen or carbon dioxide is produced & how much is used at different light intensities. The compensation point is where the gas is being used as quickly as it’s produced
Light Dependent Reaction (LDR) - How plants use light from the sun They contain pigments which absorb different wavelengths. The main pigment is chlorophyll which absorbs red & blue but reflects green which is why most plants appear green to us. Other pigments can be used too which is why there are different colours
Where the Light Dependant Reaction (LDR) takes place (+internal structures of chloroplasts) Membrane of thylakoid discs in the chloroplasts. Thylakoids are stacked into a granum (grana for plural). They’re connected by a membrane called a lamella & surrounded by a watery nutrient solution called stroma
Photoionisation Chlorophyll absorbs visible wavelengths of light & an electron pair becomes excited enough to move onto an electron carrier molecule causing the chlorophyll to oxidise to a positive ion
Photolysis The chlorophyll cation reacts with water molecules which act as reducing agents & by donating their electron pair to chlorophyll are oxidised to oxygen gas & H+ ions
What happens to the products of photolysis They pass through an electron transport chain similar to respiration where the electrons power the pumping of H+ ions into the thylakoid space where they pass through ATP synthase & bind to NADP to form NAPH+ & then are reduced to NADPH by the electrons
Antenna complex Clusters of 100s of photosynthetic pigment molecules which funnel high energy electrons from photolysis down to reaction centres surrounded by electron transport proteins which carry them to the thylakoid membranes for the electron transport chain
Photosystem II Antenna complex which captures light energy & transfers it to photosystem 1, this is
Photosystem I Cyclic system where electrons which don’t reduce NADP+ to NADPH bind to photoionised chlorophyll & are reused in the electron transport chain to generate more ATP
Cyclic photophosphorylation The process occurring in photosystem I where electrons from the ETC which have lost energy overtime return to the reaction centre in photosystem II to be re-excited & continue to make ATP for the Calvin cycle
What a Z diagram means Vertical axis = energy of electrons, start low but are excited in the photosystems then used in ETC. Horizontal axis = progression of electron transport through the photosystems. Goes from low to high then low to high again, cycling back to photosystem I
Structure of chlorophyll Quaternary protein structure with 4 subunits coordinately bonded to their own magnesium ions which allows the correct colour wavelength of light to be captured
Light Independent Reaction (LIR) - Calvin cycle Series of biochemical reactions in the stroma which synthesises organic molecules such as glucose by fixing carbon from carbon dioxide either in the atmosphere or from mitochondria
How products of the LIR are used in the LIR ATP provides energy for the Calvin cycle through phosphorylation while NADPH provides reducing power
Calvin cycle: Step 1 Carbon dioxide binds to linear 5-carbon molecule (ribulose bisphosphate) using the enzyme rubisco to form an unstable intermediate which immediately hydrolyses to 2 3-carbon glycerate-3-phosphate molecules
Calvin cycle: Step 2 Each 3-carbon molecule is phosphorylated by ATP & reduced by NADPH to form triose phosphate
Calvin cycle: Step 3 1 carbon is removed (fixed) from one of the triose phosphate and the 2-carbon molecule binds with a triose phosphate to regenerate ribulose bisphosphate using another molecule of ATP
How many times the Calvin cycle must be repeated This cycle must be repeated 6 times in order for there to be enough carbon to form glucose
Created by: Study_B
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