5.1 Photosynthesis

• What are the stages of photosynthesis (and where do they occur)?

  1. Light dependent reaction
  2. Thylakoid membrane of chloroplast
  3. Light independent reaction
  4. Stroma of chloroplast
• Describe photoionisation in the light-dependent reaction (LDR)

  • ● Chlorophyll absorbs light energy which excites its electrons (higher energy level)
  • ● So electrons are released from chlorophyll (chlorophyll becomes positively charged)
• Describe what happens after photoionisation in the LDR

  Some energy from electrons released in photoionisation is conserved in the production of ATP / reduced NADP (chemiosmotic theory):

  1. Electrons move along electron transfer chain (electron carriers), releasing energy
  2. This energy is used to actively pump protons from stroma into thylakoid
  3. Protons move by facilitated diffusion down electrochemical gradient into stroma via ATP synthase
  4. Energy used to join ADP and Pi to form ATP (photophosphorylation)
  5. NADP accepts a proton and an electron to become reduced NADP
• Describe photolysis of water in the LDR

  • ● Water splits to produce protons, electrons and oxygen (H2O → 1⁄2 O2 + 2e- + 2H+)
  • ○ Electrons replace those lost from chlorophyll
• Describe the light-independent reaction of photosynthesis (Calvin cycle)

  1. CO2 reacts with ribulose bisphosphate (RuBP)
  2. ○ Catalysed by the enzyme rubisco
  3. Forming 2 glycerate 3-phosphate (GP) molecules
  4. GP reduced to triose phosphate (TP)
  5. ○ Using products from light-dependent reaction - reduced NADP and energy from ATP
  6. Some TP converted to useful organic substances (eg. glucose)
  7. Some TP used to regenerate RuBP in the Calvin cycle (using energy from ATP)

  
• Describe and explain how temperature affects rate of photosynthesis

  1. As temperature increases, rate increases
  2. ○ Enzymes eg. rubisco gain kinetic energy
  3. ○ So more enzyme-substrate complexes form
  4. Above an optimum temperature, rate decreases
  5. ○ Enzymes denature as H bonds in tertiary structure break
  6. ○ So fewer enzyme-substrate complexes form

  
• Describe and explain how light intensity affects rate of photosynthesis

  1. As light intensity increases, rate increases
  2. ○ Light-dependent reaction increases (eg. more photoionisation of chlorophyll) so more ATP and reduced NADP produced
  3. ○ So light-independent reaction increases as more GP reduced to TP and more TP regenerates RuBP
  4. Above a certain light intensity, rate stops increasing
  5. ○ Another factor is limiting eg. temperature / CO2 concentration

  
• Describe and explain how CO2 concentration affects rate of photosynthesis

  1. As CO2 concentration increases, rate increases
  2. ○ Light-independent reaction increases
  3. ○ As more CO2 combines with RuBP to form GP
  4. ○ So more GP reduced to TP
  5. ○ So more TP converted to organic substances and more RuBP regenerated
  6. Above a certain CO2 concentration, rate stops increasing
  7. ○ Another factor is limiting eg. temperature / light intensity

  
• Explain the key consideration when evaluating data relating to agricultural practices used to overcome the effect of limiting factors

  • ● Agricultural practice should increase rate of photosynthesis, leading to increased yield
  • ○ As more glucose produced for faster respiration
  • ○ So more ATP to release energy for growth eg. cell division, protein synthesis
  • ● But profit from extra yield should be greater than costs (money & environmental costs)
• RP7: Describe how pigments from a leaf of a plant can be isolated with paper chromatography

  1. Crush leaves with solvent to extract pigments
  2. Draw a pencil line on filter / chromatography paper, 1 cm above bottom
  3. Add a drop of extract to line (point of origin)
  4. Stand paper in boiling tube of (organic) solvent below point of origin
  5. Add lid and leave to run (solvent moves up, carrying dissolved pigments)
  6. Remove before solvent reaches top and mark solvent front with pencil
• RP7: Explain why the origin should be drawn in pencil rather than ink. (2)

  ● Ink is soluble in solvent

  ● So ink would mix with pigments / line would move
• RP7: Explain why the point of origin should be above the level of the solvent. (2)

  ● Pigments are soluble in solvent

  ● So would run off paper / spots dissolve into solvent
• RP7: Explain why a pigment may not move up the chromatography paper in one solvent. (1)

  May be soluble in one solvent but insoluble in another
• RP7: Describe how pigments can be identified

  ● Rf value = distance moved by spot / distance moved by solvent front

  ● Compare Rf value to published value
• RP7: Explain why the solvent front should be marked quickly once chromatography paper is removed. (1)

  Once solvent evaporates, solvent front not visible
• RP7: Explain why the centre of each pigment spot should be measured. (1)

  ● Standardises readings as pigment is spread out

  ● So allows comparisons to be made
• RP7: Explain why the obtained Rf values were similar, but not identical, to the published values. (1)

  ● Different solvent / paper / running conditions may

  affect Rf value
• RP7: Explain why Rf values are used and not the distances moved by pigment spots. (2)

  • ● Solvent / pigment moves different distances
  • ● Rf value is constant for same pigment / can be compared