Energy transfers

• Photosynthesis


  The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen

  
• Stages of photosynthesis


  1. Light dependent reaction

  2. Light independent reaction
• Light dependent reaction


  • Occurs in thylakoid membrane of chloroplast
• Light independent reaction


  • Occurs in stroma of chloroplast
• Light dependent reaction


  1. Photoionisation

  2. Electron transfer chain

  3. Proton pumping

  4. ATP synthesis

  5. NADP reduction
• Photoionisation


  Chlorophyll absorbs light energy which excites its electrons to a higher energy level, causing electrons to be released from chlorophyll
• What happens after photoionisation

  1. Electrons move along electron transfer chain, releasing energy

  2. Energy used to pump protons into thylakoid

  3. Protons move down electrochemical gradient through ATP synthase to form ATP

  4. NADP accepts proton and electron to become reduced NADP
• Photolysis of water


  Water splits to produce protons, electrons and oxygen
• Light independent reaction (Calvin cycle)

  1. CO2 reacts with ribulose bisphosphate (RuBP)

  2. Forming 2 glycerate 3-phosphate (GP) molecules

  3. GP reduced to triose phosphate (TP)

  4. Some TP converted to organic substances

  5. Some TP used to regenerate RuBP

  
• As temperature increases


  Rate of photosynthesis increases
• Above optimum temperature


  Rate of photosynthesis decreases
• As light intensity increases

  Rate of photosynthesis increases
• Above certain light intensity

  Rate of photosynthesis stops increasing
• As CO2 concentration increases

  Rate of photosynthesis increases
• Above certain CO2 concentration

  Rate of photosynthesis stops increasing
• Agricultural practices to overcome limiting factors

  • Should increase rate of photosynthesis, leading to increased yield
  • But profit from extra yield should be greater than costs
• Paper chromatography to isolate leaf pigments

  1. Crush leaves with solvent

  2. Draw pencil line on paper

  3. Add extract to line

  4. Stand paper in solvent

  5. Allow solvent to move up, carrying dissolved pigments
• Rf value


  Distance moved by spot / distance moved by solvent front
• Measuring rate of dehydrogenase activity

  1. Extract chloroplasts

  2. Set up test tubes with DCPIP, water, chloroplasts

  3. Shine light and time how long for DCPIP to turn colourless

  4. Rate = 1 / time taken

  
• Respiration


  The process by which organisms release energy from food molecules
• Stages of aerobic respiration

  • Glycolysis
  • Link reaction
  • Krebs cycle
  • Oxidative phosphorylation
• Stages of anaerobic respiration

  • Glycolysis
  • NAD regeneration
• Glycolysis


  1. Glucose phosphorylated to glucose phosphate

  2. Hydrolysed to 2 triose phosphate

  3. Oxidised to 2 pyruvate

  
• Anaerobic respiration


  • Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast)
  • Oxidising reduced NAD to regenerate NAD
• Anaerobic respiration produces less ATP per molecule of glucose than aerobic respiration
• Aerobic respiration


  1. Glycolysis - cytoplasm (anaerobic)

  2. Link reaction - mitochondrial matrix

  3. Krebs cycle - mitochondrial matrix

  4. Oxidative phosphorylation - inner mitochondrial membrane
• Anaerobic respiration


  1. Glycolysis - cytoplasm

  2. NAD regeneration - cytoplasm
• Glycolysis


  1. Glucose phosphorylated to glucose phosphate using inorganic phosphates from 2 ATP

  2. Hydrolysed to 2 x triose phosphate

  3. Oxidised to 2 pyruvate, 2 NAD reduced, 4 ATP regenerated (net gain of 2)

  
• Anaerobic respiration after glycolysis

  1. Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast)

  2. Oxidising reduced NAD → NAD regenerated

  3. So glycolysis can continue (which needs NAD) allowing continued production of ATP
• Aerobic respiration after glycolysis


  Pyruvate is actively transported into the mitochondrial matrix

  
• Link reaction


  1. Pyruvate oxidised (and decarboxylated) to acetate, CO2 produced, Reduced NAD produced

  2. Acetate combines with coenzyme A, forming Acetyl Coenzyme A
• Krebs cycle


  1. Acetyl coenzyme A (2C) reacts with a 4C molecule, releasing coenzyme A and producing a 6C molecule that enters the Krebs cycle

  2. In a series of oxidation-reduction reactions, the 4C molecule is regenerated and 2 x CO2 lost, Coenzymes NAD & FAD reduced, Substrate level phosphorylation (direct transfer of Pi from intermediate compound to ADP) → ATP produced

  
• Oxidative phosphorylation


  1. Reduced NAD/FAD oxidised to release H atoms → split into protons (H+) and electrons (e-)

  2. Electrons transferred down electron transfer chain (chain of carriers at decreasing energy levels) by redox reactions

  3. Energy released by electrons used in the production of ATP from ADP + Pi (chemiosmotic theory): Energy used by electron carriers to actively pump protons from matrix → intermembrane space, Protons diffuse into matrix down an electrochemical gradient, via ATP synthase (embedded), Releasing energy to synthesise ATP from ADP + Pi

  4. In matrix at end of ETC, oxygen is final electron acceptor (electrons can't pass along otherwise), so protons, electrons and oxygen combine to form water
• Other respiratory substrates


  • Breakdown products of lipids
  • Breakdown products of amino acids
• Fatty acids from hydrolysis of lipids → converted to Acetyl Coenzyme A
• Amino acids from hydrolysis of proteins → converted to intermediates in Krebs cycle
• Measuring rate of aerobic respiration using a respirometer

  Add a set mass of single-celled organism and a set volume/concentration of substrate, Add a buffer to keep pH constant, Add a chemical that absorbs CO2, Place in water bath at a set temperature and allow to equilibrate, Measure distance moved by coloured liquid in a set time
• Why the liquid moves in a respirometer

  Organisms aerobically respire → take in O2, CO2 given out but absorbed by sodium hydroxide solution, So volume of gas and pressure in container decrease, So fluid in capillary tube moves down a pressure gradient towards organism
• Leaving the respirometer apparatus open for 10 minutes allows it to equilibrate and the respiration rate of organisms to stabilise
• The respirometer apparatus must be airtight to prevent air entering or leaving, which would change the volume and pressure and affect the movement of the liquid