Photosynthesis

• How are photosynthesis and respiration related?


  CO2 and H2O are raw materials for photosynthesis and products of respiration.
• What are the raw materials for respiration?


  O2 and glucose are the raw materials for respiration.
• Describe the structure of a chloroplast.


  • Usually disc-shaped
  • Double membrane (envelope)
  • Thylakoids: flattened discs stack to form grana
  • Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
  • Stroma: fluid-filled matrix with high enzyme & substrate concentration & own loop of DNA
• Where do the light-dependent reactions occur in plants?


  In the thylakoids of chloroplasts.
• Where do the light-independent reactions occur in plants?


  In the stroma of chloroplasts.
• What is the role of photosynthetic pigments and name the two main groups?


  • Embedded within thylakoid membrane
  • Absorb different wavelengths of light to maximize the rate of photosynthesis
  • Primary pigment: chlorophyll (chlorophyll a & b)
  • Accessory pigments: carotenoids (carotene & xanthophylls)
• Name the processes in the light-dependent reaction.

  • Photoionisation
  • Electron transfer chain
  • Chemiosmosis
  • Non-cyclic only: reduction of NADP, photolysis of water
• What is the role of light in photoionisation?


  Chlorophyll absorbs energy from photons, exciting electrons to a higher energy level.
• What happens in the electron transfer chain (ETC)?


  Electrons move down carrier proteins in the thylakoid membrane, undergoing redox reactions that release energy.
• How does chemiosmosis produce ATP in the light-dependent stage?


  Energy from the ETC is used to transport H+ ions, which flow back into the stroma through ATP synthase to produce ATP.
• Describe non-cyclic photophosphorylation.


  • Uses Photosystems I & II
  • Excited electrons enter ETC to produce ATP
  • NADP acts as final electron acceptor and is reduced
  • Water is photolysed to replace lost electrons from PS II
  • Purpose: produce ATP & reduced NADP for the Calvin cycle
• Describe cyclic photophosphorylation.


  • Uses only Photosystem I
  • Excited electrons enter ETC to produce ATP and return to photosystem
  • No reduction of NADP and no water needed
  • Purpose: produce additional ATP for energy demands
• What happens in photolysis of water?


  Light energy splits water molecules into 4H+, 4e-, and O2.
• What happens to the products of the photolysis of water?


  H+ ions reduce NADP, electrons replace lost chlorophyll electrons, and O2 is released as waste.
• How and where is reduced NADP produced in the light-dependent reaction?


  In the stroma, NADP combines with 2H+ and 2e- to form reduced NADP, catalysed by dehydrogenase enzymes.
• Name the three main stages in the light-independent reaction.


  1. Carbon fixation
  2. Reduction
  3. Regeneration
• What happens during carbon fixation?


  CO2 reacts with ribulose bisphosphate (RuBP) to form an unstable 6C intermediate that breaks down into 2 x glycerate 3-phosphate (GP).
• What happens during reduction in the light-independent reaction?


  2 x GP are reduced to 2 x triose phosphate (TP) using 2 x reduced NADP and 2 x ATP.
• Outline the roles of TP from the light-independent reaction.


  • Raw material: 1C leaves to produce monosaccharides, amino acids, etc.
  • Involved in regeneration of RuBP: RuP forms and is converted into RuBP using 1x ATP.
• Outline the sequence of events in the light-independent reaction.


  1. Carbon fixation
  2. Reduction
  3. Regeneration
• State the number of carbon atoms in RuBP, GP, and GALP.


  RuBP: 5, GP: 3, GALP: 3.
• Define ‘limiting factor’.


  A limiting factor is one that determines the maximum rate of a reaction, even if other factors become more favorable.
• Name four environmental factors that can limit the rate of photosynthesis.


  • Light intensity
  • CO2 levels
  • Temperature
  • Mineral/magnesium levels
• How does light intensity affect the rate of photosynthesis?


  Low light intensity slows the light-dependent reaction, reducing ATP and NADPH production, causing GP levels to rise and TP levels to fall.
• Describe the implications of water stress.


  1. Abscisic acid binds to receptors on guard cell membrane, opening Ca2+ ion channels.
  2. Other ion channels open, causing K+ to diffuse out of guard cells.
  3. Water potential of guard cells becomes more positive, leading to water loss via osmosis.
  4. Guard cells become flaccid, closing stomata.
• State the purpose and principle of thin layer chromatography (TLC).


  TLC separates molecules based on their relative attraction to the mobile phase versus the stationary phase.
• Outline a method for extracting photosynthetic pigments.


  • Grind a leaf with an extraction solvent (e.g., propanone) using a pestle and mortar.
• Outline how TLC can be used to separate photosynthetic pigments.


  1. Spot pigment extract onto pencil ‘start line’ on TLC plate.
  2. Place chromatography paper in solvent (origin above solvent level).
  3. Allow solvent to run until it almost touches the other end of the paper.