4.3 Photosynthesis

Photosynthesis in Plants What is the simple word equation for photosynthesis?

Chloroplasts Thylakoids are a phospholipid bilayer membrane-bound compartment. A granum is a stack of thylakoids folded on top of one another. The stroma is the fluid space within the chloroplast. The lumen is the fluid filled space within a thylakoid. An average green plant cell has between 10 – 50 chloroplast

Chlorophyll Chlorophyll is actually made up of 5 pigments Chlorophyll a & b Carotenoids (carotene and xanthophyll) Phaeophytin (a grey breakdown product of the others)

There are 2 distinct chlorophyll complexes Photosystem I (PSI) Photosystem II (PSII) Each system has a different combination of pigments They therefore absorb light in different areas of the spectrum PSI absorbs around 700 nm PSII absorbs around 680 nm Each system has a different role in photosynthesis

Biochemistry of Photosynthesis Photosynthesis is a two stage process involving a series of reactions. Light-dependent stage can only take place during daylight. It produces products which are used by the light-independent stages. The whole process occurs all the time during daylight hours. But the light-independent stage can also occur in the dark.

Light-Dependent Takes place on thylakoid membranes 2 functions: Produce ATP Split water in a photochemical reaction providing hydrogen ions to reduce CO2 and make carbohydrates Light is made of photons, photons hit chlorophyll and excite them (give them more energy) excited electrons are picked up by electron acceptors ATP can then be made by photophosphorylation via an electron transport chain.

Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation NADP (electron acceptor) +2H+ Reduced NADP (used in light-independent reaction) Electron transport chain

Light-independent Stage CO2 Glucose Ribulose bisphosphate (RuBP) 6C 2 GP (glycerate 3 phosphate) 2 GALP (glyceraldyde 3 phosphate) RUBISCO Calvin Cycle 1st step CO2 combines with a 5C molecule RuBP, carbon is “fixed” using Enzyme ribulose bisphosphate carboxylase/oxidase (RUBISCO). Makes a theoretical 6C compound That splits to make 2, 3C compounds (GP) This is reduced to make a 3C sugar (GALP) using reducedNADP and ATP GALP then used to replace used up RuBP, some left over to stick 2 together and make glucose.