Photosynthesis Edexcel A level Biology A1

A level Biology

1. Photosynthesis: light-dependent reaction Objectives Describe the structure of chloroplasts Explain where, specifically, the light dependent reactions occur Explain the role of light in photolysis and photoionisation Explain how photoexcited electrons move along the electron transfer chain, and how ATP and reduced NADP are produced Explain chemiosmosis and the role of ATP synthase in producing ATP Interpret energy level diagrams.during electron transfer.

2. ATP Adenosine triphosphate (ATP) is the main cellular energy storage molecule ATP: Is very soluble Allows energy to be released quickly Provides energy in a one-step reaction Stores less energy than glucose It is used to provide energy for most cellular processes e.g. muscle contraction, active transport The energy in ATP is released when it is hydrolysed by an ATPase enzyme: ATP → ADP + Pi

3. Adenine (purine base) Ribose (pentose sugar) 3x phosphate groups Molecular processes = 30.6 KJ/mol Respiration (and photosynthesis) + Pi

4. Chloroplast structure

5. Chlorophyll and other pigments Chloroplasts contain chlorophyll a + b, and other pigment molecules (e.g. carotenoids) The pigments absorb photons of light with different wavelengths Different species of plant have different combinations of photosynthetic pigments, giving rise to different coloured leaves.

6. Thin layer chromatogram (TLC) of an extract of thylakoid membranes. A drop of extract was laid at the bottom of a chromatography sheet. The sheet was then placed in a beaker of solvent. The picture shows the solubility of the extract in solvent. Carotene Pheophytin Chlorophyll A Chlorophyll B Carotenoids Solvent line Chromatography

7. Photosystems Chlorophyll and the other pigments are arranged in complexes called photosystems. Each photosystem contains ~200 chlorophyll molecules and ~50 molecules of accessory pigments, together with several enzymes Photosystems are located in the thylakoid membranes There are two kinds of photosystem: photosystem I (PSI) and photosystem II (PSII). These absorb light at different wavelengths and have slightly different functions

8. Light dependent reactions Use light energy to split water and synthesise/release: ATP Oxygen Energetic hydrogen atoms. Takes place within the thylakoid membranes of chloroplasts

9. Stage 1 Chlorophyll molecules in PSII absorb photons of light, exciting chlorophyll electrons to a higher energy level and a charge separation within PSII. This causes the splitting (photolysis) of water: 2H 2 O → O 2 + 4H + + 4e - The oxygen produced diffuses out of the chloroplast and eventually into the air The protons build up in the thylakoid lumen causing a proton gradient The electrons replace the excited electrons that have been ejected from chlorophyll.

10. Stage 2 The excited, high-energy electrons are passed along a chain of proteins in the thylakoid membrane In PSI more light energy is absorbed and passed to the electron The energy of the electrons is used to pump protons from stroma to lumen, creating a steeper proton gradient across the thylakoid membrane

11. Stage 3 The electrons are recombined with protons to form hydrogen atoms, These are taken up by molecules of NADP, reducing it to NADPH: NADP + H + + e - → NADPH

12. Stage 4 The proton gradient is used to synthesise ATP using the ATP synthase enzyme (Think of it like wind passing through a turbine) This synthesis of ATP is called photophosphorylation because it uses light energy to phosphorylate ADP: ADP + Pi → ATP

13. Photosynthesis: light-independent reaction Objectives Explain where the light independent reactions occur Explain the Calvin cycle and the molecules involved Explain the roles of reduced NADP and ATP Interpret experimental data about the light independent reaction.

14. Location Takes place in the chloroplast stroma

15. Glucose or starch

16. Stage 1 Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP) to form 2 molecules of the 3- carbon compound glycerate-3-phosphate (GP). This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco)

17. Stage 2 Glycerate-3-phosphate is reduced and activated to form triose phosphate (Six) ATP and NADPH molecules from the light-dependent reactions provide the energy for this step The ADP and NADP return to the thylakoid membrane for recycling

18. Stage 3 Most of the triose phosphate continues through a complex series of reactions to regenerate the RuBP and complete the cycle: 5 triose phosphate molecules (5 x 3C = 15 carbon atoms) combine to form 3 RuBP molecules (3 x 5C = 15 carbon atoms). This requires 3 ATP molecules (1 per reformed RuBP)

19. Stage 4 Every 3 turns of the Calvin Cycle 3 CO 2 molecules are fixed to make 1 new triose phosphate molecule Two new triose phosphate molecules combine to form one glucose molecule (i.e. every 6 turns one glucose molecule is formed) The glucose is transported out of the chloroplast and used to make all organic compounds the plant needs (cellulose, lipids, proteins, nucleic acids, etc)

20. Suggest an explanation for what happened to the amount of radioactive GP in the light. Why did the amount of GP increase in the dark?

21. GP rose at the start as carbon dioxide combined with RuBP to produce GP. Levelled out because although more is being made some is being converted into triose phosphate. Light is needed to convert GP into another substance . No light = GP builds up.

22. Photosynthesis: limiting factors Objectives Explain what is meant by rate limiting factors Identify environmental factors that limit the rate of photosynthesis Interpret graphs showing the rate of photosynthesis and explain graphs in terms of which factors are rate limiting Explain how farmers seek to maximise crop growth through knowledge of rate limiting factors, and how this is a balance between cost vs profit Evaluate data relating to common agricultural practices used to overcome the effect of these limiting factors.

23. Farmers and limiting factors At any given time there can only be one factor that is actually controlling the rate – the limiting factor. This is the factor that is in shortest supply, or furthest from its optimum. In a closed greenhouse the CO 2 concentration can fall very low, so increasing the CO 2 concentration in the greenhouse often increases the rate of photosynthesis. This is most efficiently done by burning a fuel, since this releases CO 2 and raises the temperature. Day length can be increased with artificial lighting.