Photosynthesis

• Interrelationship between photosynthesis and respiration

  The products of one reaction are the reactants for the other
• Components of a chloroplast

  Outer membrane, lamellae, grana, thylakoid, stroma, DNA
• Where are the pigments in a chloroplast?

  Embedded in the thylakoid membranes
• Examples of different pigments

  Chloropyll a, chlorophyll b, xanthophyll, carotenoids
• Two main stages of photosynthesis

  Light-dependent, light-independent
• Where do the light-dependent reactions occur?

  Thylakoid membranes
• Where do the light-independent reactions occur?

  Stroma
• Structures involved in the light-dependent reactions

  Light harvesting systems, reaction centres, photosystems
• Components of a photosystem

  Light harvesting system, reaction centre
• Components of light harvesting systems

  Chlorophyll b, xanthophylls, carotenoids, proteins
• Role of light harvesting systems


  Absorb light energy of different wavelengths, transfer it to the reaction centre
• Component of reaction centres


  Chlorophyll a
• Role of reaction centres


  Where the reactions of photosynthesis take place
• Summary of the light-dependent stage

  Light energy is absorbed and used to form ATP, NADP is reduced
• Summary of the light-independent stage

  Glucose made from hydrogen and carbon dioxide
• Reactions in the light-dependent stage

  Non-cyclic photophosphorylation, photolysis, cyclic photophosphorylation
• How non-cyclic photophosphorylation occurs

  Photosystem 2 absorbs light, electrons in reaction centre excited, electrons passed to electron transport chain, used to make ATP by chemiosmosis, lost electron replaced by those produced in photolysis, electrons passes to photosystem 1, passed to different electron transport chain, ATP synthesised by chemiosmosis, electrons leaving the electron transport chain are accepted with a hydrogen ion by NADP to form NADPH
• Wavelength of light absorbed by photosystem 2

  680nm
• Wavelength of light absorbed by photosystem 1

  700nm
• How photolysis works

  Oxygen-evolving complex catalyses the breakdown of water into protons and electrons and oxygen molecules, protons released into lumen of the thylakoids to increase proton concentration across the membrane, used in chemiosmosis
• Equation for photolysis


  H2O -> 2H+ + 2e- + 1/2O2
• Role of water in the light-dependent stage


  Can be broken down to provide electrons to replace those lost by PS2
• Which molecules produced by the light-dependent reactions can be used as a source of energy for other metabolic processes?

  ATP, NADPH
• What can ATP and NADPH be used for?

  Source of energy for other metabolic processes
• How cyclic photophosphorylation works

  Electrons that leave the electron transport chain after photosystem 1 are returned to photosystem 1, no NADPH formed, ATP produced
• How are the products of the light-dependent stage used in the Calvin Cycle?


  Energy from ATP and hydrogen atom from NADPH used to convert glycerate 3-phosphate to triose phosphate
• Reactions in the light-independent stage

  Calvin Cycle, regeneration of ribulose bisphosphate
• How the Calvin Cycle work


  Carbon dioxide combines with ribulose bisphosphate which fixes the carbon dioxide, catalysed by RuBisCo, unstable six carbon intermediate formed, breaks down into 2 molecules of glycerate 3-phosphate, GP converted into triose phosphate, TP used to generate complex biological molecules or regenerate RuBp
• Fixing

  Incorporating a carbon into an organic molecules
• Summary of the Calvin Cycle

  Fixation, reduction, regeneration
• Uses of triose phosphate

  Starting material for synthesis of carbohydrates and lipids and proteins and nucleic acids, recycled to regenerate RuBP
• Limiting factors in photosynthesis

  Carbon dioxide concentration, light intensity, temperature
• Limiting factor


  A factor if, in short supply, it reduces the rate of reaction
• Effect on photosynthesis of increased light intensity

  ATP and NADPH produced at higher rate
• Effect on photosynthesis of increased carbon dioxide concentration

  Increased rate of carbon fixation, increased rate of TP production
• Effect of temperature on photosynthesis


  Increase up to a point increases rate because enzymes, decreases after the point because denaturation
• Implications of water stress on photosynthesis

  Stomata close, diffusion of CO2 stops, rate of light-independent reaction decreases, photosynthesis stops
• Why is water not considered a limiting factor?


  If water potential becomes low enough to affect the rate of photosynthesis, the plant will close its stomata and stop photosynthesis
• Law of limiting factors


  The rate of a physiological process will be limited by the factor which is in shortest supply
• Effect of decreasing light intensity on levels of GP, RuBP and TP


  Quantity of ATP and NADPH produced decreases, ATP and NADPH needed to convert GP to TP, concentration of GP increases, concentration of TP decreases, concentration of RuBP decreases as less TP to regenerate it