5. Energy transfers within and between organisms

• Light-dependent reactions occur in the thylakoids of chloroplasts
• Light-independent reactions occur in the stroma of chloroplasts
• Role of light in photoionisation:

  • Chlorophyll molecules absorb energy from photons of light
  • Energy excites 2 electrons, raising them to a higher energy level and causing them to be released from the chlorophyll
• 2 main stages involved in ATP production in the light-dependent reaction:

  1. Electron transfer chain

  2. Chemiosmosis
• Electron transfer chain (ETC):

  • Electrons released from chlorophyll move down carrier proteins in the thylakoid membrane
  • Undergo a series of redox reactions, releasing energy
• Proton concentration gradient established during chemiosmosis:

  • Energy from ETC couples to active transport of H+ ions from stroma into thylakoid space
• Chemiosmosis produces ATP in the light-dependent stage by:

  • H+ ions moving down their concentration gradient from thylakoid space into stroma via ATP synthase
  • ATP synthase catalysing ADP + Pi → ATP
• Role of light in photolysis:

  • Light energy splits molecules of water into 4H+, 4e-, and O2
• Products of photolysis of water:

  • H+ ions: move out of thylakoid space via ATP synthase to reduce NADP
  • e-: replace electrons lost from chlorophyll
  • O2: used for respiration or diffuses out of leaf as waste gas
• Reduced NADP produced in the light-dependent reaction:

  • NADP + 2H+ + 2e- → reduced NADP
  • Catalysed by dehydrogenase enzymes
  • Produced in the stroma of chloroplasts
• H+ ions and electrons used to reduce NADP come from:

  • H+ ions: photolysis of water
  • Electrons: NADP acts as the final electron acceptor of the electron transfer chain
• 3 main stages in the Calvin cycle:

  1. Carbon fixation

  2. Reduction

  3. Regeneration
• Carbon fixation:

  • Reaction between CO2 & ribulose bisphosphate (RuBP) catalysed by rubisco
  • Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP)
• Reduction in the Calvin cycle:

  • 2 x GP are reduced to 2 x triose phosphate (TP)
  • Requires 2 x reduced NADP & 2 x ATP
  • Forms 2 x NADP & 2 x ADP
• Light-independent reaction produces useful organic substances by:

  • Converting some TP into useful organic molecules
• Regeneration in the Calvin cycle:

  • After 1C leaves the cycle, the 5C compound RuP forms
  • RuBP is regenerated from RuP using 1x ATP
  • Forms 1x ADP
• Sequence of events in the Calvin cycle:

  • Carbon fixation → Reduction → Regeneration
• Roles of ATP & (reduced) NADP in the light-independent reaction:

  • ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP
  • (reduced) NADP: coenzyme transports electrons needed for reduction of GP to TP
• Number of carbon atoms in RuBP, GP & TP:

  • RuBP: 5
  • GP: 3
  • TP: 3
• Structure of a chloroplast:

  • 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
• Structure of the chloroplast maximises the rate of the light-dependent reaction by:

  • ATP synthase channels within granal membrane
  • Large surface area of thylakoid membrane for ETC
  • Photosystems position chlorophyll to enable maximum absorption of light
• Structure of the chloroplast maximises the rate of the light-independent reaction by:

  • Own DNA & ribosomes for enzyme synthesis
  • High concentration of enzymes & substrates in stroma
• The 4 main stages in aerobic respiration and where they occur:

  • Glycolysis: cytoplasm
  • Link reaction: mitochondrial matrix
  • Krebs cycle: mitochondrial matrix
  • Oxidative phosphorylation via electron transfer chain: membrane of cristae
• Limiting factor:

  • Factor that determines the maximum rate of a reaction, even if other factors change to become more favourable
• 4 environmental factors that can limit the rate of photosynthesis:

  • Light intensity
  • CO2 levels
  • Temperature
  • Mineral/magnesium levels
• Stages of glycolysis:

  1. Glucose is phosphorylated to glucose phosphate by 2x ATP

  2. Glucose phosphate splits into 2x triose phosphate (TP)

  3. 2x TP is oxidised to 2x pyruvate

  Net gain of 2x reduced NAD & 2x ATP per glucose
• Common agricultural practices to overcome the effect of limiting factors in photosynthesis:

  • Artificial light, especially at night
  • Artificial heating
  • Addition of CO2 to greenhouse atmosphere
• How pyruvate from glycolysis enters the mitochondria:

  • Via active transport
• Reasons why farmers try to overcome the effect of limiting factors:

  • To increase yield
  • Balancing additional cost with yield for maximum profit
• What happens during the link reaction:

  1. Oxidation of pyruvate to acetate

  • Per pyruvate molecule: net gain of 1x CO2 (decarboxylation) & 2H atoms (used to reduce 1x NAD)

  2. Acetate combines with coenzyme A (CoA) to form acetylcoenzyme A
• Investigating the effect of a named variable on the rate of photosynthesis:

  • Dependent variable: rate of O2 production/CO2 consumption
  • Use a potometer
  • Place balls of calcium alginate containing green algae in hydrogencarbonate indicator
• Purpose and principle of paper chromatography:

  • Separates molecules based on their relative attraction to the mobile vs stationary phase
• Summary equation for the link reaction:

  Pyruvate + NAD + CoA → acetyl CoA + reduced NAD + CO2
• Method for extracting photosynthetic pigments:

  • Grind a leaf with an extraction solvent e.g. propanone
• What happens in the Krebs cycle:

  • Series of redox reactions produces:
  • ATP by substrate-level phosphorylation
  • Reduced coenzymes
  • CO2 from decarboxylation
• Stages of the Krebs cycle:

  • NB: the 6C compound is citrate
• Using paper chromatography to separate photosynthetic pigments:

  • Spot pigment extract onto pencil 'start line'
  • Place chromatography paper in solvent
  • Allow solvent to run until pigments move different distances
• What is the electron transfer chain (ETC):

  • Series of carrier proteins embedded in membrane of the cristae of mitochondria
  • Produces ATP through oxidative phosphorylation via chemiosmosis during aerobic respiration
• Rf values:

  • Ratios that allow comparison of how far molecules have moved in chromatograms
  • Calculated as distance between origin and centre of pigment spot / distance between origin and solvent front
• What happens in the electron transfer chain (ETC):

  • Electrons released from reduced NAD & FAD undergo successive redox reactions
  • Energy released is coupled to maintaining proton gradient or released as heat
  • Oxygen acts as final electron acceptor