B11 photosynthesis

• the overall equation for photosynthesis is:

  6CO2 + 6H2O -> C6H12O6 + 6O2
• photosynthesis has three main stages:

  • capture of light energy
  • light-dependent reaction
  • light-independent reaction
• the first stage of photosynthesis is capture of light energy, which takes place in photosynthetic pigments such as chlorophyll, in chloroplasts
• the second stage of photosynthesis is the light-dependent reaction, where some energy is conserved in chemical bonds, and light on photosynthetic pigments causes photolysis of water, splitting it into protons, electrons and oxygen
• the third stage of photosynthesis is the light-independent reaction, where protons are used to produce sugars and organic molecules
• the light-dependent reaction takes place in the thylakoid membranes in the chloroplasts
• the light-dependent reaction has the following stages:

  • photoionisation
  • photolysis
  • chemiosmosis
  • ATP synthesis
• in the first stage of the light-dependent reaction, photoionisation, photons hit chlorophyll molecules, exciting their electrons
• in the second stage of the light-dependent reaction, photolysis, water is split with light, producing 4 protons, 4 electrons and 1 oxygen, the oxygen diffuses away, the electrons replace the missing ones in chlorophyll, and protons move into the stroma
• in the third stage of the light-dependent reaction, chemiosmosis, electrons move down a chain of protein complexes by a series of redox reactions, releasing energy as they move which is used to pump protons from stroma to thylakoid, eventually electrons combine with protons to form hydrogen which is used to reduce NADP to form reduced NADP
• in the fourth stage of the light-dependent reaction, ATP synthesis, concentration of protons in thylakoid > stroma, so protons move across the membrane by facilitated diffusion through an ATP synthase channel, causing a conformational change which catalyses the formation of ATP from ADP and Pi
• the light-independent reaction takes place in the stroma in the chloroplasts
• the light-independent reaction has the following stages:

  • carbon dioxide fixation
  • reduction
  • regeneration of RuBP
  • organic molecule production
• in the first stage of the light-independent reaction, carbon dioxide fixation, carbon dioxide diffuses in through the stomata and is fixed with 5-carbon ribulose biphosphate RuBP, this reaction is catalysed by the enzyme rubisco, and forms two molecules of 3-carbon glycerate-3-phosphate GP
• in the second stage of the light-independent reaction, reduction, reduced NADP reduces GP to two molecules of triose phosphate TP, using energy supplied by ATP, this reforms NADP which can be re-used
• in the third stage of the light-independent reaction, regeneration of RuBP, most TP is used to regenerate RuBP, using energy supplied by the rest of the ATP
• in the fourth stage of the light-independent reaction, organic molecule production, two molecules of TP combine to form hexose sugars such as glucose
• to produce 1 molecule of glucose, the Calvin cycle must be done 6 times