Photosynthesis

• Light-dependent reactions


  Occur in the thylakoids of chloroplasts
• Light-independent reactions


  Occur in the stroma of chloroplasts
• Light Dependent Stage

  1. Photoionisation

  2. Photolysis

  3. Chemiosmosis

  4. Electron Transfer Chain
• Photoionisation


  Chlorophyll absorbs (protons from) light, exciting (2) electrons and causing them to be released from the chlorophyll
• Photolysis


  Photolysis of water produces oxygen, protons & electrons

  • H+ = move out of thylakoid space via ATP synthase & used to reduce NADP.
  • e- = replace electrons lost from chlorophyll
  • 02 = used for respiration or diffuses out of leaf as water gas
• Chemiosmosis


  H+ ions move down their concentration gradient from the thylakoid space into the stroma via ATP synthase, which catalyses ADP + Pi → ATP
• Chemiosmosis: Establishment of a proton concentration gradient

  
  

  Some energy released from the ETC is coupled to the active transport of H+ ions from the stroma into the thylakoid space
• Electron transfer chain (ETC)


  Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which releases energy (ATP)

  
  

  NADP acts as the final electron acceptor
• Reduced NADP


  Produced in the light-dependent reaction, catalysed by dehydrogenase enzymes
• Light independent reaction / Calvin cycle


  1. Carbon fixation

  2. Reduction

  3. Regeneration
• Carbon fixation


  Carbon dioxide reacts with RuBP to form 2 molecules of glycerate 3-phosphate (GP)- catalysed by rubisco
• Reduction


  ATP & reduced NADP from the LDR are used to reduce GP to triose phosphate (TP) - Net gain of (2X) ATP & (2X) NADP

  
  

  Some of the TP used to regenerate RuBP. Some is converted to useful organic substances.
• Regeneration


  • After 1C leaves the cycle. RuP forms
  • RuBP is regenerated from RuP using ATP
  • Forms (1x) ADP
• LIR: Useful organic substances

  • 1C leaves cycle every cycle (some of the carbon from TP is converted to useful organic molecules)
  • After 6 cycles, a hexose sugar can be formed
• Number of carbon atoms: RuBP (5), GP (3), TP (3)
• Chloroplast structure


  • Usually disc-shaped; Double membrane (envelope)
  • Thylakoids: flattened discs stack to form grana
  • Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
  • Stroma: fluid-filled matrix
• Chloroplast structure maximises rate of light-independent reaction


  • Own DNA & ribosomes for synthesis of enzymes e.g. rubisco
  • Concentration of enzymes & substrates in stroma is high
• Chloroplast structure maximises rate of light-dependent reaction


  • ATP synthase channels within the granal membrane
  • Large SA of thylakoid membrane for ETC
  • Photosystems position chlorophyll to enable maximum absorption of light
• Environmental factors that can limit the rate of photosynthesis

  • Light intensity (LDR - in photolysis & photoionisation)
  • CO2 levels (light-independent stage)
  • Temperature (enzyme-controlled steps - LIR)
  • Mineral/ magnesium levels (maintain normal functioning of chlorophyll)
• Farmers try to overcome the effect of limiting factors to increase yield & as additional cost must be balanced with yield to ensure maximum profit
• Common agricultural practices used to overcome the effect of limiting factors in photosynthesis

  • Artificial light, especially at night
  • Artificial heating
  • Addition of CO2 to greenhouse atmosphere
• How to investigate the effect of a named variable on the rate of photosynthesis

  1. Use a potometer

  2. Place balls of calcium alginate containing green algae in the hydrogencarbonate indicator (colour change orange → magenta as CO2 is consumed & pH increases)