A2: Photosynthesis

A2: Photosynthesis

1. • Occurs in plants – photoautotrophic (self-feeding) • Metabolic reactions – series (2 stages) of anabolic reactions (‘building up’) • Conversion of energy – light energy → chemical (stored) energy • Food webs – plants are called producers (trophic level 1)

2. •Chlorophyll – a mixture of various light sensitive pigments • Overall equation: (inorganic) carbon dioxide + water (light energy input) → glucose and oxygen (organic) •Aerobic respiration (catabolic) – plants respire all the time •Glucose + oxygen → carbon dioxide and water •Chloroplast – double membrane controls molecules entering and leaving the cell. •Thylakoids are approximately 5nm in diameter. • Can contain up to 50 grana.

3. What are photosystems? •A photosystem is a collection of pigments located on the thylakoid membranes. •‘Chlorophyll’ is a mixture of various light sensitive (+ other) pigments → primary=chlorophyll a, accessory=chlorophyll b, B-carotene (orange) and xanthophyll (yellow). • Chlorophyll a is located in the reaction centre. Other pigments help to harvest light energy in the RC. •2 photosystems: PS I and PS II. •Visible light – wavelengths in nm. • 400 (violet) --------- 700 nm (red). • Some wavelengths are reflected, some are absorbed by various pigments.

4. Stage 1: Light-Dependent stage (LDS)

   • •Photolysis of water molecules: light energy → H2O = 2 H+ ions/protons, 2e-, oxygen gas (released from plant, used in mitochondria for aerobic respiration). Water is a source of protons used in photophosphorylation, a source of oxygen and keeps plant cells turgid. •Cyclic photophosphorylation – PS I. Product=ATP. • Non-cyclic photophosphorylation PS I and II. Products=ATP, NADPH (H+ ions from photolysis), oxygen. Electrons from photolysis replace those lost.

5. Background knowledge • Synthesis of ATP molecules=phosphorylation. ADP + iP (inorganic) → ATP. • Co-enzyme molecule=NADP – hydrogen atom carrier. •Oxidation: loss of electrons, addition of oxygen or removal of hydrogen. •Reduction: gain of electrons, addition of hydrogen or removal of oxygen.

6. The biochemistry of Photosynthesis •A series of reactions/enzyme controlled (in living organisms) • Occur on/in (photosystems I and II – cluster of pigments around chlorophyll a) the membranes=thylakoids. • Light energy is ‘harvested’ and ‘electrons’ in chlorophyll a become ‘excited’. •LDS can be split into 3 stages: Cyclic photophosphorylation, Non-cyclic photophosphorylation and Photolysis of water molecules.

7. Stage 2: Light-Independent stage (LIS)

   • • Products from LDS → ATP and NADPH molecules. • Inside chloroplasts, at various locations in the stroma. • Cycle of reactions, driven by ATP/NADPH (enzyme controlled)
     • The Calvin cycle is a metabolic pathway that occurs in the stroma of chloroplasts, where carbon is fixed, with the products of the LDS to make organic compounds. It also occurs in many photoautotrophic bacteria.
       • 1. Carbon dioxide binds with ribulose biphosphate (RuBP, a 5-carbon compound), catalysed by the enzyme RuBisCo. 2. RuBP has accepted a carboxyl group (COO-)and so it becomes carboxylated, forming an unstable intermediate 6-carbon compound which immediately breaks down. 3. The product of this breakdown is 2GP (glycerate-3-phosphate), fixing carbon dioxide. 4. GP is reduced to TP by hydrogen from reduced NADP. Energy from ATP is used to at the rate of 2ATP molecules:1xCO2 fixed in stage 3. 5. In every 10/12 TP molecules, the atoms are rearranged to regenerate 6xRuBP. This requires phosphate groups. Chloroplasts contain low levels of RuBP as it's continuously being converted into GP but also being continually regenerated. the remaining 2/12 molecules of TP are the products.
         • TP can be used to synthesise organic compounds i.e. some glucose is converted to sucrose, some to starch and some to cellulose and some TP is used to synthesise amino acids, fatty acids and glycerol. The rest of it is used to regenerate RuBP in a 5:3 ratio.

8. Factors affecting photosynthesis • Rate is measured by something per (divide by) time period. • Main factors: light intensity, temperature, CO2 concentration. • A limiting factor (for photosynthesis) is the one which is furthest from the optimum.

   • • Light intensity is the limiting factor as when the light intensity increases, the rate of photosynthesis increases. • As it starts to plateau, the rate of photosynthesis doesn’t increase as the light intensity increases as there is another limiting factor.
   • •The rate of photosynthesis increases as temperature increases until 25-30°C. •After 30°C, oxygen competes with carbon dioxide for the enzyme RuBisCO’s active site. This reduces the amount of GP and TP being produced. •Above 45°C, enzymes may become denatures so the rate of photosynthesis will decrease.

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