Photosynthesis

• Coenzymes

  • Molecules which transfer chemical groups in reactions and are continually recycled

  • NADP transfers hydrogen ions and electrons in photosynthesis

  • In respiration, NAD and FAD transfer hydrogen atoms, and coenzyme A transfers acetyl groups
• Photosynthesis

  • An endothermic reaction transferring light energy to chemical energy (stored in glucose)
• Light-dependent reaction

  • Happens across the thylakoid membranes of the chloroplast

  • Thylakoid membranes contain photosystems

  → made up of protein and photosynthetic pigments e.g. chlorophyll

  → two different photosystems (photosystem 1 and photosystem 2)

  • Photophosphorylation can be cyclic or non-cyclic

  → non-cyclic requires water, NADP, light energy, photosystem 1 and photosystem 2

  → cyclic only requires photosystem 1 and light energy

  → only non-cyclic produces oxygen and reduced NADP, but both produce ATP
• Non-cyclic photophosphorylation


  1. Photolysis of water

  2. Photoionisation of chlorophyll in photosystem 2

  3. Electrons are transferred down the electron transport chain

  4. Photoionisation of chlorophyll in photosystem 1

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


  Light energy enables water to split into protons, electrons and oxygen
• Photoionisation of chlorophyll in photosystem 2

  Chlorophyll in photosystem 2 absorbs light energy which excites electrons in chlorophyll, the electrons are lost from chlorophyll and are passed to the electron transport chain, electrons are replaced by the ones released from photolysis of water
• Electron transport chain

  Electrons are passed down a series of carrier proteins in the thylakoid membrane, energy from the electrons is used to actively transport protons (H+) across the thylakoid membrane into the thylakoid from the stroma, which creates a proton gradient
• Photoionisation of chlorophyll in photosystem 1

  Chlorophyll in photosystem 1 absorbs light energy which excites electrons in chlorophyll, the electrons are lost from chlorophyll and are accepted by NADP, electrons are replaced by the ones from the electron transport chain
• • Chemiosmosis

  → protons diffuse back across the thylakoid membrane (down the proton gradient) into the stroma through ATP synthase

  → ATP synthase catalyses the reaction of ADP with Pi to produce ATP
• NADP

  The final electron acceptor, accepts a proton and an electron to become reduced NADP
• Photosynthetic pigments

  Pigments in leaves that absorb different wavelengths of light
• Thin layer chromatography of photosynthetic pigments

  1. Grind up leaves in organic solvent

  2. Spot pigments onto TLC plate

  3. Transfer TLC plate into organic solvent

  4. Compare Rf values to databases
• Photoionisation of chlorophyll in photosystem 1


  1. Chlorophyll in photosystem 1 absorbs light energy which excites electrons in chlorophyll

  2. The electrons are lost from chlorophyll and are passed to the electron transport chain

  3. Electrons are replaced by the ones from the electron transport chain (a cyclic process)
• Electron transfer down the electron transport chain

  1. Electrons are passed down a series of carrier proteins in the thylakoid membrane

  2. Energy from the electrons is used to actively transport protons (H+) across the thylakoid membrane into the thylakoid from the stroma, which creates a proton gradient
• Chromatography

  • Separates pigments by relative solubility in mobile phase and relative adsorption to stationary phase
• Non-polar molecules

  • Have higher solubility in non-polar solvents
• Polar molecules

  • Have higher solubility in polar solvents
• Handling TLC plate

  1. Use gloves to avoid skin oils

  2. Avoid damage to plate
• Photosynthetic pigments


  Found in photosystems in the thylakoid membranes
• Transferring TLC plate

  1. Plate should not touch walls of beaker

  2. Plate must be level

  3. Work quickly to prevent solvent evaporating
• Photosynthetic pigments absorb light energy


  Causes electrons to be excited (raised to a higher energy level)
• Rf values


  Used to work out which pigments the leaves contain
• Primary pigments and accessory pigments

  • Work together to form light harvesting systems
• Primary pigments

  • Absorb light energy
  • Lose electrons which are passed to the electron transport chain
  • e.g. chlorophyll a
• Accessory pigments

  • Absorb light energy
  • Transfer energy to the primary pigments
  • e.g. chlorophyll b and carotenoids
• Different pigments absorb different wavelengths of visible light
• The other wavelengths are reflected
• Chlorophyll mainly absorbs blue and red light, but reflects green light
• Limiting factor

  Limits the rate of photosynthesis when the level is sub-optimal
• Temperature

  • Photosynthesis requires many enzymes e.g. RuBisCo
  • Enzyme activity increases with increasing temperature (due to increased kinetic energy) until they become denatured
  • Stomata close to conserve water at high temperatures (so less CO2 enters leaves)
• Wavelength of light

  • Different coloured pigments (e.g. chlorophyll, carotenoids) absorb different wavelengths of light
• Light intensity

  • A higher intensity provides more energy for the light-dependent reaction
• Carbon dioxide concentration

  • A higher CO2 concentration means there is more substrate for RuBisCo in the light-independent reaction
• Water availability

  • Water stress (limited water) can limit photosynthesis because stomata will close to conserve water (so less CO2 enters leaves)
• Mineral availability


  • Magnesium is needed to make chlorophyll
• Effects of limiting factors on the Calvin cycle

  → low CO2 concentration = less CO2 to react with RuBP = decreased GP, but increased RuBP until GP and TP run out

  → low temperature = less enzyme activity = all enzyme catalysed reactions happen more slowly = RuBP, TP and GP will all decrease

  → too high temperature = enzymes denature = all enzyme catalysed reactions stop = RuBP, TP and GP will all decrease

  → low light intensity = the light dependent reaction slows down = less ATP and reduced NADP available = decreased TP and RuBP, but increased GP until RuBP runs out
• Investigating limiting factors

  • Pondweed can be used to investigate the rate of photosynthesis under different conditions

  → rate of oxygen production is proportional to the rate of photosynthesis

  → measure rate of oxygen production using a gas syringe or oxygen sensor, and a timer

  • Effect of temperature → use a water bath to have the pondweed at different temperatures

  • Effect of light intensity → have a light at different distances from the pondweed

  • Effect of CO2 concentration → use sodium hydrogencarbonate dissolved in the water to vary the CO2 concentration
• • Possible control variables

  → wavelength of light

  → CO2 concentration (have sodium hydrogencarbonate in excess if not investigating this)

  → temperature

  → light intensity

  → pH of the water

  → age and type of pondweed

  → mass of pondweed
• • Redox indicators (e.g. DCPIP) can be used to measure the rate of the light-dependent reaction

  → the indicator changes colour when it accepts electrons from photoionisation of chlorophyll

  → a colorimeter could be used to measure the rate of colour change
• Light-independent reaction (the Calvin cycle)


  Happens in the stroma of the chloroplast