Photosynthesis

• Photosynthesis is a two stage reaction:

  • Light dependent reaction ( Thylakoid membranes / grana )
  • Light independent reaction ( stroma )
• These reactions both occur within chloroplast of plant cell but in a different location.

  LDR first stage of photosynthesis, requiring light.

  Light energy and water used to create ATP and reduced NADP. needed for light independent stage.
• Light dependent stage

  1. Photoionisation of chlorophyll
  2. Production of ATP and reduced NADP
  3. Chemiosmosis
  4. Photolysis
• Photolysis of water

  Light energy is absorbed by chlorophyll and splits water into oxygen, H+ and e- .

  H+ picked up by NADP to form NADPH to be used in LIR.

  E- passed along chain of electron carrier proteins

  Oxygen wither used for respiration or diffuses out of the leaf via stomata.
• Photoionisation of chlorophyll

  Light energy is absorbed by chlorophyll and the light energy picked up by electrons becoming excited and raising up an energy level to leave chlorophyll. 

  Therefore chlorophyll has been ionised by light.

  Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis.
• Chemiosmosis step 1 and two

  1. The electrons that gained energy and left the chlorophyll move along a series of proteins embedded within the thylakoid membrane, passing along an electron transfer chain.
  2. As they move along the proteins, they release energy at each movement, this energy from electrons used to pump the protons (actively transport) across chloroplast membranes into the thylakoid lumen
• Chemiosmosis step 3 and 4

  1. That results in a high concentration of protons in thylakoid lumen. Electrochemical gradient created. The protons pass through enzyme ATP synthase resulting in production of ATP.
  2. Protons combine with coenzyme NADP = NADPH. Due to protons move from high to low concentration gradient. This is chemiosmosis
• Light independent reaction (Calvin cycle)

  • Calvin cycle occurs in the stroma contains enzyme RuBisCo which catalyses this reaction.
  • This stage is temperature sensitive as involves enzymes.
  • Calvin cycles uses carbon dioxide, reduced NADP and ATP to form a hexose sugar.
  • The ATP is hydrolysed to release energy for this reaction and reduced NADP donated the hydrogen to reduce molecules which picks it up (GP).
• Calvin cycle

  1. Carbon dioxide reacts with RuBP to form two molecules of GP a 3C compound. Catalysed by the enzyme Rubisco.
  2. GP is reduced to triose phosphate (TP) using energy from ATP and accepting a hydrogen from reduced NADP.
  3. Some of the carbon from TP leaves the cycle to be converted into useful organic substances.
  4. Now losing one carbon, remaining 5C is used to regenerate RuBP with energy from cycle
• Use of glucose product

  • Can join to form disaccharides such as sucrose.
  • Form polysaccharides such as ce;;ulose and starch
  • Can convert to glycerol and combine with fatty acids to make lipids for the plants.
• Photosynthesis limiting factors 

  • Temperature affect LIR due to enzyme controlled reaction
  • CO2 concentration affects LIR, one of reactants entering the calvin cycle.
  • Light intensity: affects LDR for photolysis and photoionisation of chlorophyll
• Limiting factors ( agricultural practices )

  • Artificial lighting
  • Heating greenhouse
  • Burning fuel (CO2)