Topic 5

• Stages of photosynthesis

  1. Light dependent reaction

  2. Light independent reaction
• Photoionisation

  Chlorophyll absorbs light energy which excites its electrons (higher energy level), so electrons are released from chlorophyll (chlorophyll becomes positively charged)
• What happens after photoionisation in the light-dependent reaction

  1. Electrons move along electron transfer chain (electron carriers), releasing energy

  2. Energy used to actively pump protons from stroma into thylakoid

  3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase

  4. Energy used to join ADP and Pi to form ATP (photophosphorylation)

  5. NADP accepts a proton and an electron to become reduced NADP
• Photolysis of water

  Water splits to produce protons, electrons and oxygen (H2O → ½ O2 + 2e- + 2H+)
• Light-independent reaction of photosynthesis (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 useful organic substances (eg. glucose)

  5. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)

  
• As temperature increases

  Rate of photosynthesis increases
• Above an optimum temperature

  Rate of photosynthesis decreases
• As light intensity increases

  Rate of photosynthesis increases
• Above a certain light intensity

  Rate of photosynthesis stops increasing
• As CO2 concentration increases

  Rate of photosynthesis increases
• Above a certain CO2 concentration

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

  • Should increase rate of photosynthesis, leading to increased yield
  • Profit from extra yield should be greater than costs (money & environmental costs)
• Common mistakes in describing photosynthesis
• Paper chromatography to isolate leaf pigments

  1. Crush leaves with solvent to extract pigments

  2. Draw pencil line on filter paper, 1 cm above bottom

  3. Add drop of extract to line

  4. Stand paper in boiling tube of solvent below point of origin

  5. Add lid and leave to run (solvent moves up, carrying dissolved pigments)

  6. Remove before solvent reaches top and mark solvent front with pencil
• Rf value

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

  1. Extract chloroplasts from leaf sample

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

  3. Shine light on test tubes and time how long to it takes for DCPIP to turn from blue (oxidised) to colourless (reduced)

  4. Rate of dehydrogenase activity (s-1) = 1 / time taken

  
• Purpose of control 1 and control 2

  • Control 1 shows light is required for DCPIP to decolourise
  • Control 2 shows chloroplasts are required for DCPIP to decolourise
• Why DCPIP changes from blue to colourless

  DCPIP is a redox indicator / DCPIP gets reduced by electrons from photoionisation of chlorophyll
• Respiration

  Produces ATP to release energy for active transport, protein synthesis etc.
• Stages of aerobic and anaerobic respiration

  • Aerobic: Glycolysis, Link reaction, Krebs cycle, Oxidative phosphorylation
  • Anaerobic: Glycolysis, NAD regeneration
• Glycolysis

  1. Glucose phosphorylated to glucose phosphate

  2. Hydrolysed to 2 x triose phosphate

  3. Oxidised to 2 pyruvate
• What happens after glycolysis in anaerobic respiration

  • Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast)
  • Oxidising reduced NAD → NAD regenerated
• 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
• Anaerobic respiration after glycolysis

  • Pyruvate converted to lactate (animals & some bacteria) or ethanol (plants & yeast)
  • Oxidising reduced NAD → NAD regenerated
  • So glycolysis can continue (which needs NAD) allowing continued production of ATP
• Link reaction

  1. Pyruvate oxidised (and decarboxylated) to acetate

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

  1. Acetyl coenzyme A (2C) reacts with a 4C molecule

  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)

  3. Energy released by electrons used in the production of ATP from ADP + Pi (chemiosmotic theory)

  4. In matrix at end of ETC, oxygen is final electron acceptor (electrons can't pass along otherwise)
• Other respiratory substrates

  • Fatty acids from hydrolysis of lipids → converted to Acetyl Coenzyme A
  • Amino acids from hydrolysis of proteins → converted to intermediates in Krebs cycle
• Common mistakes in respiration
• Using a respirometer to measure aerobic respiration

  Add a set mass of single-celled organism, 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

  
• Why the respirometer apparatus is left open for 10 minutes
• Why the respirometer apparatus must be airtight
• More accurate way to measure volume of gas

  Use a gas syringe
• Calculating rate of respiration from a respirometer

  Calculate volume of O2 / CO2 consumed / released, Divide by mass of organism and time taken
• Using a respirometer to measure anaerobic respiration

  Repeat experiment as above but remove chemical that absorbs CO2, Make conditions anaerobic

  
• Why the liquid moves in an anaerobic respirometer

  Yeast anaerobically respire → release CO2, So volume of gas and pressure in container increase, So fluid in capillary tube moves down a pressure gradient away from organism
• Why the anaerobic respirometer apparatus is left for an hour after the culture has reached a constant temperature