Photosynthesis

• chloroplasts:

  • range if pigments - chlorophyll
  • accessory pigments
  • light harvesting clusters - range of pigments
• photosynthesis Rate factors:

  • light intensity - initially increases but becomes limited by another factor
  • CO2 concentration - initially increases but becomes limited by another factor
  • Temperature - initially increases but decreases due to denatured enzymes
• overcome factors being limited:

  • use artificial light
  • green houses at specific temperatures
  • carbon dioxide waste
• Photosynthesis

  1. Light dependent reaction

  2. Light independent reaction
• Photolysis

  1. Light that is absorbed splits water - ionising it

  2. H20 → 2H+ + 2e- + 1/2O2

  3. H2O → H+ + O2

  4. Electrons produced are used to replace the electrons in the chlorophyll

  5. Protons are used for ATP synthesis - maintain the electrochemical gradient

  6. Oxygen is either diffused out or used for respiration
• Adaptations of chloroplasts

  • Thylakoid membrane has a large surface area for molecules involved in the light-dependent reaction
  • Proteins in the grana hold the chlorophyll to maximum amount of light can be absorbed at one time
  • Thylakoid membranes contain ATP synthase for efficient ATP production & membrane is selectively permeable to establish a electrochemical gradient
  • Chloroplasts have ribosomes and DNA which allows for proteins to easily be produced for light dependant reaction
• Photosystems

  • Light are absorbed by photosystems
  • System 1 absorbs shorter wavelengths
  • System 2 absorbs longer wavelengths of light
  • PSII = NADPH and ATP
  • PSI = ATP only
• Light independent reaction

  1. Happens in the stroma

  2. Organic substances are made using CO2

  3. Calvin cycle

  4. Needs ATP and reduced NADP (NAPDH)

  5. [5C] Ribulose Bisphosphate (RuBP) + CO2 ⇒ 2 Glycerate-3-phosphate (GP)

  6. Carbon fixation

  7. CO2 is fixed - removed from environment and fixed into GP

  8. Catalysed by Rubisco - lower Ea for reaction

  9. 2 Glycerate-3-phosphate ⇒ Triose phosphate (TP)

  10. GP is reduced ⇒ O- is replaced with H+

  11. NADPH → donated H+ to form NADP and TP

  12. ATP is used for this reaction → ADP and Pi are returned back to thylakoid space to be reused - loop

  13. ATP and NADPH are used then formed again and are re-used = cycle

  14. Triose phosphate ⇒ ribulose bisphosphate

  15. Around 80% are converted back to allow for the cycle using ATP

  16. The remaining produce glucose → to allow for respiration

  17. Glucose + oxygen → co2 + water

  18. Produce amino acids → proteins

  19. Produce lipids → with glycerol and fatty acids forms triglyceride
• Limiting factors

  • Light intensity
  • Carbon dioxide
  • Temperature
• Light intensity

  • Proportional to change
  • Used for photoionisation and photolysis
  • Increases the rate for the two reactions and ETC has a greater rate
  • Electrons move faster and NADPH and ATP are produced more
  • Light dependant reaction happens much greater
• Carbon dioxide

  • Proportional to change
  • CO2 used in the light-independent reaction
  • Increases the rate of ribulose bisphosphate forms into Glycerate-3-phosphate
  • In general increases the rate of the Calvin cycle
• Temperature

  • Enzyme rate increases
  • More molecules are produced
  • Too high = enzymes denatured which decreases the rate
  • Same applied to then thylakoid membrane enzymes and protein complexes
  • Causes reactions to stop working at optimum and decreases the rate
  • Affects both light reactions
• Light dependent reaction


  1. Light hits the leaf and hits chlorophyll

  2. Light energy is transferred and excited a pair electrons

  3. Pair of electron are transferred out the chlorophyll into protein complexes

  4. Water is split into protons, oxygen and electrons to replace the electrons and provide for a electrochemical gradient

  5. Electrons go through consecutive oxidation and reduction reactions with complexes and proteins down the electron transfer chain - which releases energy

  6. Protein complexes such as protein channels are given energy for the movement of protons against the electrochemical gradient

  7. Electrons at the end of the electron transfer chain react with NADP along with some protons to form reduced NADP (NADPH)

  8. Proton gradient provides energy for the ATP synthase to catalyse the photophosphorylation of ADP and Pi into ATP
• • electron transfer chain - ETC
  • electrons are then consecutively transferred along proteins within the membrane
  • successive oxidation and reduction (REDOX) reaction - electrons
  • proteins channels that are oxidised and reduced allow the movement of H+ back into the thylakoid space - active transport
  • at the end of the ETC - NADP (coenzyme) reacts with the electrons and H+ ⇒ becomes reduced NADP (NADPH)
  • ATP is also formed from this consecutively
• phosphorylation:

  • PS2(II) - follows standard process as above
  • non-cyclic photophosphorylation
  • PS1(I) - absorbs light and transfers to single electron → only ATP is produced
  • moves down the ETC
  • electron transfers to another ETC to provide energy to other reactions
  • provides energy to protein complexes to actively transport protons
  • Then returns back to Photosystem 1 and does not cause photolysis
  • cyclic photophosphorylation
  • absorbs different wavelength → helpful to provide more ATP when NADPH is saturated
• light independent reaction:

  • happens in the stroma
  • organic substances are made using CO2
• • Calvin cycle
  • needs ATP and reduced NADP (NAPDH)
  • Ribulose Bisphosphate (RuBP) + CO2 ⇒ 2 Glycerate-3-phosphate (GP)
  • 2 Glycerate-3-phosphate ⇒ Triose phosphate (TP)
  • triose phosphate ⇒ ribulose bisphosphate
• Calvin cycle


  Needs ATP and reduced NADP (NAPDH)
• Calvin cycle

  1. [5C] Ribulose Bisphosphate (RuBP) + CO2 ⇒ 2 Glycerate-3-phosphate (GP)

  2. Carbon fixation

  3. CO2 is fixed - removed from environment and fixed into GP

  4. Catalysed by Rubisco - lower Ea for reaction

  5. 2 Glycerate-3-phosphate ⇒ Triose phosphate (TP)

  6. GP is reduced ⇒ O- is replaced with H+

  7. NADPH → donated H+ to form NADP and TP

  8. ATP is used for this reaction → ADP and Pi are returned back to thylakoid space to be reused - loop

  9. ATP and NADPH are used then formed again and are re-used = cycle

  10. triose phosphate ⇒ ribulose bisphosphate

  11. around 80% are converted back to allow for the cycle using ATP

  12. The remaining produce glucose → to allow for respiration
• glucose + oxygen → co2 + water
• produce amino acids → proteins
• produce lipids → with glycerol and fatty acids forms triglyceride
• • light intensity
  • proportional to change
  • used for photoionisation and photolysis
  • increases the rate for the two reactions and ETC has a greater rate
  • electrons move faster and NADPH and ATP are produced more
  • Light dependant reaction happens much greater
• • Carbon dioxide
  • proportional to change
  • CO2 used in the light-independent reaction
  • increases the rate of ribulose bisphosphate forms into Glycerate-3-phosphate
  • in general increases the rate of the Calvin cycle
• • Temperature
  • enzyme rate increases
  • more molecules are produced
  • too high = enzymes denatured which decreases the rate
  • same applied to then thylakoid membrane enzymes and protein complexes
  • causes reactions to stop working at optimum and decreases the rate
  • affects both light reactions