5.2.1 - photosynthesis

• Photosynthesis and respiration are related because CO2 and H2O are the raw materials for photosynthesis and the products of respiration. Oxygen and glucose are the raw materials for photosynthesis and the products of respiration.
• A photon is a particle of light, each photon contains a quantum of energy
• Plants only photosynthesise during daylight but respire all the time. When photosynthesis and respiration occur at the same rate the plant is at its compensation point.
• Granum is the inner part of chloroplasts made of stacks of thylakoid membranes.

  Where the light dependent stage of photosynthesis takes place
• Photosynthetic pigments are pigments that absorb specific wavelengths of light and trap energy associated with light.

  Eg, chlorophyll , carotene
• A photosystem is a system of photosynthetic pigments found in thylakoids that trap photons and pass energy to reaction centre
• stroma is the fluid filed matrix of chloroplast where light independent stage takes place
• thylakoid is the flattened membrane bound sac inside chloroplasts that contains photosystems and is the site of the light dependent stage
• Chloroplasts are disc shaped and have a double membrane.
• Light dependent stage occurs in the thylakoids
• light independent stage occurs in the stroma
• Photosynthetic pigments are embedded within the thylakoid membrane to absorb different wavelengths of light to maximise rate of photosynthesis
• the two main groups of photosynthetic pigments are

  • Primary pigments: chlorophyll found in photosystems
  • accessory pigments: carotenoids found in light harvesting systems
• Processes in the light dependent reaction

  • photoionisation
  • electron transfer chain
  • chemiosmosis
  • reduction of NADP
  • photolysis of water
• clorophyll molecules absorb energy from photons of light. this excites 2 electrons causing them to be released from the chlorophyll.
• In the electron transfer chain, electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of oxidation reduction reactions which releases energy
• Chemiosmosis produces ATP in the light dependent stage
• Energy released from the ETC is used for active transport of H+ ions from the stroma into the thylakoid space

  H+ ions then move down a proton gradient from the thylakoid space into the stroma via ATP synthase.

  ATP synthase catalyses reaction: ADP + Pi ——> ATP
• Non cyclic photophosphoryation uses photosystems I and II
• In non cyclic photophosphorylation:

  Light absorbed by PSII, excited electrons leave chlorophyll and enter electron transport chain via PSI to produce ATP. NADP acts as the final electron acceptor and is reduced.

  Water is photolysed to release electrons to replace those lost from PSII.
• purpose of non cyclic Photophosphorylation is to produce atp and NADPH for the Calvin cycle.
• Cyclic photophosphorylation uses photosystem I
• Light absorbed by PSI, excited electrons enter electron transport chain to produce ATP then return directly to PSI.
• purpose of cyclic photophosphorylation is to produce additional ATP to meet energy demands of the cell
• photolysis of water is when light splits molecules of water into protons, electrons and oxygen.

  2H2O ——> 4H+ + 4e- + O2
• The products of photolysis each have uses:

  H+ ions used to maintain proton gradient in the thylakoid space

  e- replace lost electrons from chlorophyll

  O2 is used for respiration or diffuses out of the leaf
• NADPH is produced in light dependent reaction when H+ ions in the stroma and the 2 e- from the end of the electron transport chain react with NADP.

  NADP + 2H+ + 2e- ——> NADPH
• The three stages in the light independent reaction are

  1. Carbon fixatin
  2. reduction
  3. regeneration
• during carbon fixation of the Calvin cycle, CO2 reacts with Ribulose bisphosphate (RuBP) catalysed by RuBisCO.

  forms unstable 6C intermediate that breaks down into 2 lots of Glycerate 3-phosphate (GP)
• During reduction of the Calvin cycle, 2 lots of GP are reduced to 2 lots of triose phosphate (TP). This reaction required NADPH being converted back into NADP as well as ATP breaking down into ADP and Pi.
• ADP , Pi and NADP from reduction in the Calvin cycle are returned back to the thylakoid membrane to form new molecules of NADPH and ATP
• TP in the light dependent reaction is used in many ways.

  6 turns of the cycle is needed to make important molecules. From each cycle 1C leaves the cycle to produce monosaccharides, amino acids and other biological molecules. The other 5C are involved in the regeneration of RuBP Where RuP forms and is converted into RuBP using ATP
• A limiting factor is a factor that determines the maximum rate of a reaction. Even if other factors change to become more favourable.
• 4 environmental factors that can limit the rate of photosynthesis are:

  1. light intensity
  2. co2 levels
  3. temperature
  4. mineral ion levels
• Light intensity affects the rate of photosynthesis as;

  low light intensity = slower light dependent reaction = less ATP and NADPH produced to convert GP into TP in light independent reaction.

  So GP levels rise and TP levels fall so RuBP levels also fall
• CO2 diffuses through the stomata into the leaf
• water is taken in by the plant from the roots
• As light intensity increases, there is a higher rate of photosynthesis than respiration.
• Chloroplasts have 70s ribosomes
• chloroplasts have starch granules used for the storage of glucose.