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Photosynthesis as an Energy Transfer Process
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Photosynthesis as an Energy Transfer Process



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  • 1. PhotosynthesisPhotosynthesis as an energy transfer process ALBIO9700/2006JK
  • 2. • Photosynthesis transfers light energy into chemical potential energy of organic molecules• This energy can then be released for work in respiration• Photoautotrophs – green plants, the photosynthetic prokaryotes and both single-celled and many-celled protoctists (including the green, red and brown algae)• Chemoautotrophs – nitrifying bacteria (obtain their energy from oxidising ammonia to nitrite, or nitrite to nitrate) ALBIO9700/2006JK
  • 3. ALBIO9700/2006JK
  • 4. Outline of the process• Photosynthesis is the trapping (fixation) of CO2 and its subsequent reduction to carbohydrate, using H from H2O• Overall equation for photosynthesis in green plants is: light energy nCO2 + nH2O (CH2O)n + nO2 chlorophyll• Hexose sugars and starch are commonly formed: light energy 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll ALBIO9700/2006JK
  • 5. • 2 sets of reactions involved: – Light-dependent reactions (light energy necessary) • Only takes place in the presence of suitable pigments which absorb certain wavelengths of light • Light energy is necessary: – for the splitting of water into hydrogen and oxygen – to provide chemical energy (ATP) for the reduction of CO2 to carbohydrate in the light-independent reactions – Light-independent reactions (light energy not needed) ALBIO9700/2006JK
  • 6. The light-dependent reactions• Include the synthesis of ATP in photophosphorylation and the splitting of water by photolysis to give H+• H+ + NADP NADPH• ATP and NADPH - passed from the light- dependent to the light-independent reactions• Photophosphorylation of ADP to ATP: – Cyclic – Non-cyclic ALBIO9700/2006JK
  • 7. • Cyclic photophosphorylation – Only photosystem I – Light absorbed by photosystem I and passed to chlorophyll a (P700) – An e- in the chlorophyll a molecule is excited and emitted – Captured by an e- acceptor and passed back to a chlorophyll a (P700) molecule via a chain of electron carriers – Synthesis of ATP – ATP passes to light-independent reactions ALBIO9700/2006JK
  • 8. • Non-cyclic photophosphorylation – ‘Z scheme’ – Light absorbed by both photosystem and excited e- emitted from the primary pigments of both reaction centres (P680 and P700) – e- absorbed by e- acceptors and pass along chains of e- carriers leaving the photosystems positively charged – The P700 of photosystem I absorbs electrons from photosystem II – P680 receives replacement e- from the splitting (photolysis) of water – ATP synthesised ALBIO9700/2006JK
  • 9. • Photolysis of water: – Photosystem II includes a water-splitting enzymes which catalyses the breakdown of water: H2O → 2H+ + 2e- + ½O2 – H+ combine with e- from photosystem I and the carrier molecule NADP to give reduced NADP 2H+ + 2e- + NADP → reduced NADP – This passes to the light-independent reactions and is used in the synthesis of carbohydrate ALBIO9700/2006JK
  • 10. ALBIO9700/2006JK
  • 11. Light-independent reactions• The fixation of CO2• CO2 combines with a 5C sugar {ribulose biphosphate (RuBP)} 2 molecules of a 3C compound {glycerate-3-phosphate (GP/PGA)}• GP is reduced to triose phosphate (3C sugar) in the presence of ATP and NADPH• Some condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetyl CoA to make amino acids and lipids• Others regenerate RuBP• The enzyme ribulose biphosphate carboxylase (rubisco), catalyses the combination of CO 2 and RuBP ALBIO9700/2006JK
  • 12. Calvin cycle ALBIO9700/2006JK
  • 13. Leaf structure and function• Has a broad, thin lamina, a midrib and a network of veins, leaf stalk (petiole)• To perform its function the leaf must: – Contain chlorophyll and other photosynthetic pigments arranged in such a way that they can absorb light – Absorb CO2 and dispose of the waste product O2 – Have a water supply and be able to export manufactured carbohydrate to the rest of the plant• Large surface area of lamina makes it easier to absorb light and thinness minimises diffusion pathway for gaseous exchange• Upper epidermis is made of thin, flat, transparent cells which allow light through to the cells of the mesophyll , where photosynthesis takes place• A waxy transparent cuticle provides a watertight layer• Cuticle and epidermis together form a protective layer ALBIO9700/2006JK
  • 14. • Stomata are pores in the epidermis through which diffusion of gases occurs• Each stoma is bounded by 2 sausage- shaped guard cells• Changes in turgidity cause them to change shape so that they open and close the pore• Guard cells gain and loss water by osmosis ALBIO9700/2006JK
  • 15. ALBIO9700/2006JK
  • 16. • The palisade mesophyll is the main site of photosynthesis (many chloroplasts per cell than in the spongy mesophyll)• Adaptations for light absorption: – Long cylinders arranged at right-angles to the upper epidermis (reduces number of light-absorbing cross walls in the upper part of the leaf so that as much light as possible can reach the chloroplasts) – Large vacuole with a thin layer of cytoplasm (restricts chloroplasts to a layer near the outside of the cell where light can reach them most easily) – Chloroplasts can be moved within cells (to absorb the most light or to protect it from excessive light intensities)• Adaptations for gaseous exchange: – Cylindrical cells pack together with long, narrow air spaces between them (large surface area of contact between cell and air) – Cell walls are thin (gases can diffuse through them easily)• Spongy mesophyll is adapted as a surface for the exchange of CO2 and O2 – Smaller number of chloroplasts – Photosynthesis only at high light intensities – Irregular packing and large air spaces produced provide a large surface area of moist cell wall for gaseous exchange• Veins in leaf help to support large surface area of leaf – contains xylem and phloem ALBIO9700/2006JK
  • 17. ALBIO9700/2006JK
  • 18. Investigation of limiting factors• External factors: – Light intensity – Temperature – CO2 concentration• Light intensity – Rate initially increases as the light intensity increases – Rate reaches a plateau at higher light intensities• Temperature – At high light intensities the rate of photosynthesis increases as the temperature is increased over a limited range – At low light intensities, increasing the temperature has little effect on the rate of photosynthesis• Photochemical reactions are not generally affected by temperature• Since temperature affects rate, there must be 2 sets of reactions – Light-dependent photochemical stage – Light-independent, temperature-dependent stage• Limiting factor ALBIO9700/2006JK
  • 19. ALBIO9700/2006JK