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• Photosynthesis


  The process by which plants and other organisms use sunlight to synthesize nutrients from carbon dioxide and water
• Photosynthesis


  • All free energy needed by biological systems arises from solar energy that is trapped by photosynthesis
  • Globally, photosynthesis stores about 10^18 kJ of free energy every year
  • That is about 200 billion tons of carbon fixed into carbohydrates and other organic compounds
  • The 'light' reactions produce reducing power (NADPH) and ATP
  • The waste product is oxygen! About 140 billion tons a year
• Stomata


  Openings in the leaf surface that allow exchange of air
• Photosynthesis


  1. Energy capture

  2. Light dependent reactions

  3. Energy expenditure

  4. Light independent reactions
• Light dependent reactions


  1. Capture of photons

  2. Conversion of light energy to ATP and NADPH
• Light independent reactions


  3. Fixing or capturing of CO2

  4. Making bigger molecules can be moved round the plant or be stored
• Blue light


  Has more energy than red light
• Energy of a photon


  Depends on its wavelength
• Chlorophyll


  A Mg2+ porphyrin: heme is a Fe porphyrin

  Contains phytol, highly hydrophobic

  Chlorophyll b, β-carotene
• The Hill reaction


  Isolated chloroplasts evolve O2

  2. Reduce an electron acceptor (e.g. ferricyanide)

  Showed the primary event in photosynthesis is the light-driven energy requiring transfer of an electron from one substance to another
• Light harvesting complex


  • Thylakoid membrane

  Protein bound antenna chlorophylls and auxiliary pigments

  Reaction centres: Chlorophylls

  Chemical reaction happens here
• Reaction centre


  Photon eventually arrives at reaction centre chlorophylls

  Energy is used for charge separation

  Excited electron can be transferred elsewhere

  Electron can now be transported to other molecules in a series of redox reactions

  Energy released slowly and captured as proton gradient

  Two reaction centres: Photosystem I and Photosystem II
• The "red drop": Increase in wavelength decreases energy, but photosynthetic rate in the presence of both 670 nm and 700 nm light is greater than the sum of the individual rates at 600 and at 700 nm
• Conversion of light energy to ATP and NADPH


  Transport of electrons – terminal acceptor is NADP+ resulting in NADPH

  Proton gradient created by electron transport used to make ATP

  O2 produced as waste product
• The Z Scheme


  Redox potential diagram showing the electron transport chain in the light reactions
• Photosystem II (P680)


  • Water-plastoquinone oxidoreductase

  Only enzyme in Nature able to split water

  Possible due to high redox potential of P680

  Problems: Photons remove 1 e- at a time, Plastoquinone (the acceptor) takes 2 e-, Water needs to donate 4e- to make one molecule of oxygen
• Photosystem II


  H2O -> O2 + 4H+ + 4e-

  Mn cluster oxidises water one electron at a time
• Cytochrome bf complex


  • Similar to complex III in mitochondria

  Probably common ancestor

  Cytochrome so only 1e- carrier (heme protein)

  PQH2 unloads e- via Q cycle

  2H+ transferred to lumen
• Photosystem I


  Excited electron has low redox potential

  1. is transferred to ferredoxin (Fd), 1 e- carrier

  Electron is replaced by plastocyanin

  NADP+ is the final electron acceptor

  Needs two electrons to reduce to NADPH
• Resemblance to electron transport in mitochondria


  Energy, Reducing potential, Proton gradient, ATP production
• The Z Scheme: cyclic mode (Cyclic Photophosphorylation)


  What happens if we need more ATP than NAPDH?
• The light independent reactions


  Fixing of CO2

  Carbon dioxide is turned into organic molecules
• Calvin's experiment


  Pulse-chase labelling on Chlorella, a unicellular alga

  Feed Chlorella with 14CO2, give light

  Visualise labelled products

  The first 14C labelled product to be detected was 3-phosphoglycerate
• Calvin cycle


  CO2 fixation into sugars

  CO2 + 4H+ + 4e- → CH2O + H2O

  Energy and electrons supplied by NADPH and ATP

  First product is 3-phosphoglycerate
• Carbon fixation: the first step


  CO2 is added to ribulose 1,5-bisphosphate (RuBP) to form two 3-phosphoglycerate (3PG) molecules

  Catalysed by RuBisCo
• Calvin cycle - the next steps


  Carbon fixation

  2. Reduction of 3PG to Glyceraldehyde-3-phosphate (G-3-P)

  3. Regeneration of starting molecule: RuBP (Reverse of PPP)
• Calvin cycle - stochiometry


  To fix 6 CO2 and make an hexose (e.g. glucose):

  12 ATP and 12 NADPH spent from 3PG to G-3-P

  6 ATP spent from G-3-P to RuBP
• Rubisco - the world's worst enzyme?
• C4 Plants - how to avoid photorespiration


  CO2 is fixed by PEP carboxylase insensitive to oxygen

  C4 compound (malate) carry CO2 from mesophyll cells to bundle sheath cells

  ATP is used
• Making of bigger molecules for storage or movement around the plant


  Calvin cycle coupled to gluconeogenesis

  Sucrose: mobile sugar for distribution

  Starch: energy storage in stroma of chloroplasts and in seeds/tubers
• Photosynthesis: Efficiency