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Photosynthesis




Photosynthesis as an energy
     transfer process
                              ALBIO9700/2006JK
• Photosynthesis transfers light energy into
  chemical potential energy of organic
  molecules
• This energy can then be released for work
  in respiration
• Photoautotrophs – green plants, the
  photosynthetic prokaryotes and both
  single-celled and many-celled protoctists
  (including the green, red and brown algae)
• Chemoautotrophs – nitrifying bacteria
  (obtain their energy from oxidising
  ammonia to nitrite, or nitrite to nitrate)

                                        ALBIO9700/2006JK
ALBIO9700/2006JK
Outline of the process
• Photosynthesis is the trapping (fixation) of CO2 and its
  subsequent reduction to carbohydrate, using H from H2O
• Overall equation for photosynthesis in green plants is:
                light energy
  nCO2 + nH2O                   (CH2O)n + nO2
                chlorophyll


• Hexose sugars and starch are commonly formed:
                 light energy
   6CO2 + 6H2O                  C6H12O6 + 6O2
                 chlorophyll

                                                     ALBIO9700/2006JK
• 2 sets of reactions involved:
  – Light-dependent reactions (light energy
    necessary)
     • Only takes place in the presence of suitable
       pigments which absorb certain wavelengths of light
     • Light energy is necessary:
        – for the splitting of water into hydrogen and oxygen
        – to provide chemical energy (ATP) for the reduction of
          CO2 to carbohydrate in the light-independent reactions

  – Light-independent reactions (light energy
    not needed)


                                                            ALBIO9700/2006JK
The light-dependent reactions
• Include the synthesis of ATP in
  photophosphorylation and the splitting of
  water by photolysis to give H+
• H+ + NADP     NADPH
• ATP and NADPH - passed from the light-
  dependent to the light-independent
  reactions
• Photophosphorylation of ADP to ATP:
  – Cyclic
  – Non-cyclic
                                        ALBIO9700/2006JK
• Cyclic photophosphorylation
  – Only photosystem I
  – Light absorbed by photosystem I and
    passed to chlorophyll a (P700)
  – An e- in the chlorophyll a molecule is
    excited and emitted
  – Captured by an e- acceptor and passed
    back to a chlorophyll a (P700) molecule
    via a chain of electron carriers
  – Synthesis of ATP
  – ATP passes to light-independent
    reactions
                                       ALBIO9700/2006JK
• Non-cyclic photophosphorylation
  – ‘Z scheme’
  – Light absorbed by both photosystem and
    excited e- emitted from the primary pigments
    of both reaction centres (P680 and P700)
  – e- absorbed by e- acceptors and pass along
    chains of e- carriers leaving the photosystems
    positively charged
  – The P700 of photosystem I absorbs electrons
    from photosystem II
  – P680 receives replacement e- from the
    splitting (photolysis) of water
  – ATP synthesised
                                              ALBIO9700/2006JK
• Photolysis of water:
  – Photosystem II includes a water-splitting
    enzymes which catalyses the breakdown of
    water:
     H2O → 2H+ + 2e- + ½O2
  – H+ combine with e- from photosystem I and the
    carrier molecule NADP to give reduced NADP
     2H+ + 2e- + NADP → reduced NADP
  – This passes to the light-independent reactions
    and is used in the synthesis of carbohydrate


                                             ALBIO9700/2006JK
ALBIO9700/2006JK
Light-independent reactions
• The fixation of CO2
• CO2 combines with a 5C sugar {ribulose
  biphosphate (RuBP)} 2 molecules of a 3C
  compound {glycerate-3-phosphate (GP/PGA)}
• GP is reduced to triose phosphate (3C sugar) in
  the presence of ATP and NADPH
• Some condense to form hexose phosphates,
  sucrose, starch and cellulose or are converted to
  acetyl CoA to make amino acids and lipids
• Others regenerate RuBP
• The enzyme ribulose biphosphate carboxylase
  (rubisco), catalyses the combination of CO 2 and
  RuBP
                                              ALBIO9700/2006JK
Calvin cycle




               ALBIO9700/2006JK
Leaf structure and function
• Has a broad, thin lamina, a midrib and a network of
  veins, leaf stalk (petiole)
• To perform its function the leaf must:
   – Contain chlorophyll and other photosynthetic pigments arranged
     in such a way that they can absorb light
   – Absorb CO2 and dispose of the waste product O2
   – Have a water supply and be able to export manufactured
     carbohydrate to the rest of the plant
• Large surface area of lamina makes it easier to absorb
  light and thinness minimises diffusion pathway for
  gaseous exchange
• Upper epidermis is made of thin, flat, transparent cells
  which allow light through to the cells of the mesophyll ,
  where photosynthesis takes place
• A waxy transparent cuticle provides a watertight layer
• Cuticle and epidermis together form a protective layer
                                                             ALBIO9700/2006JK
• Stomata are pores in the epidermis
  through which diffusion of gases occurs
• Each stoma is bounded by 2 sausage-
  shaped guard cells
• Changes in turgidity cause them to
  change shape so that they open and close
  the pore
• Guard cells gain and loss water by
  osmosis

                                      ALBIO9700/2006JK
ALBIO9700/2006JK
• The palisade mesophyll is the main site of photosynthesis (many
  chloroplasts per cell than in the spongy mesophyll)
• Adaptations for light absorption:
    – Long cylinders arranged at right-angles to the upper epidermis (reduces
      number of light-absorbing cross walls in the upper part of the leaf so that
      as much light as possible can reach the chloroplasts)
    – Large vacuole with a thin layer of cytoplasm (restricts chloroplasts to a
      layer near the outside of the cell where light can reach them most
      easily)
    – Chloroplasts can be moved within cells (to absorb the most light or to
      protect it from excessive light intensities)
• Adaptations for gaseous exchange:
    – Cylindrical cells pack together with long, narrow air spaces between
      them (large surface area of contact between cell and air)
    – Cell walls are thin (gases can diffuse through them easily)
• Spongy mesophyll is adapted as a surface for the exchange of
  CO2 and O2
    – Smaller number of chloroplasts
    – Photosynthesis only at high light intensities
    – Irregular packing and large air spaces produced provide a large surface
      area of moist cell wall for gaseous exchange
• Veins in leaf help to support large surface area of leaf
    – contains xylem and phloem

                                                                         ALBIO9700/2006JK
ALBIO9700/2006JK
Investigation of limiting factors
• External factors:
   – Light intensity
   – Temperature
   – CO2 concentration
• Light intensity
   – Rate initially increases as the light intensity increases
   – Rate reaches a plateau at higher light intensities
• Temperature
   – At high light intensities the rate of photosynthesis increases
     as the temperature is increased over a limited range
   – At low light intensities, increasing the temperature has little
     effect on the rate of photosynthesis
• Photochemical reactions are not generally affected by
  temperature
• Since temperature affects rate, there must be 2 sets of reactions
   – Light-dependent photochemical stage
   – Light-independent, temperature-dependent stage
• Limiting factor
                                                              ALBIO9700/2006JK
ALBIO9700/2006JK

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Photosynthesis: Energy Transfer Through Light and Dark Reactions

  • 1. Photosynthesis Photosynthesis as an energy transfer process ALBIO9700/2006JK
  • 2. • Photosynthesis transfers light energy into chemical potential energy of organic molecules • This energy can then be released for work in respiration • Photoautotrophs – green plants, the photosynthetic prokaryotes and both single-celled and many-celled protoctists (including the green, red and brown algae) • Chemoautotrophs – nitrifying bacteria (obtain their energy from oxidising ammonia to nitrite, or nitrite to nitrate) ALBIO9700/2006JK
  • 4. Outline of the process • Photosynthesis is the trapping (fixation) of CO2 and its subsequent reduction to carbohydrate, using H from H2O • Overall equation for photosynthesis in green plants is: light energy nCO2 + nH2O (CH2O)n + nO2 chlorophyll • Hexose sugars and starch are commonly formed: light energy 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll ALBIO9700/2006JK
  • 5. • 2 sets of reactions involved: – Light-dependent reactions (light energy necessary) • Only takes place in the presence of suitable pigments which absorb certain wavelengths of light • Light energy is necessary: – for the splitting of water into hydrogen and oxygen – to provide chemical energy (ATP) for the reduction of CO2 to carbohydrate in the light-independent reactions – Light-independent reactions (light energy not needed) ALBIO9700/2006JK
  • 6. The light-dependent reactions • Include the synthesis of ATP in photophosphorylation and the splitting of water by photolysis to give H+ • H+ + NADP NADPH • ATP and NADPH - passed from the light- dependent to the light-independent reactions • Photophosphorylation of ADP to ATP: – Cyclic – Non-cyclic ALBIO9700/2006JK
  • 7. • Cyclic photophosphorylation – Only photosystem I – Light absorbed by photosystem I and passed to chlorophyll a (P700) – An e- in the chlorophyll a molecule is excited and emitted – Captured by an e- acceptor and passed back to a chlorophyll a (P700) molecule via a chain of electron carriers – Synthesis of ATP – ATP passes to light-independent reactions ALBIO9700/2006JK
  • 8. • Non-cyclic photophosphorylation – ‘Z scheme’ – Light absorbed by both photosystem and excited e- emitted from the primary pigments of both reaction centres (P680 and P700) – e- absorbed by e- acceptors and pass along chains of e- carriers leaving the photosystems positively charged – The P700 of photosystem I absorbs electrons from photosystem II – P680 receives replacement e- from the splitting (photolysis) of water – ATP synthesised ALBIO9700/2006JK
  • 9. • Photolysis of water: – Photosystem II includes a water-splitting enzymes which catalyses the breakdown of water: H2O → 2H+ + 2e- + ½O2 – H+ combine with e- from photosystem I and the carrier molecule NADP to give reduced NADP 2H+ + 2e- + NADP → reduced NADP – This passes to the light-independent reactions and is used in the synthesis of carbohydrate ALBIO9700/2006JK
  • 11. Light-independent reactions • The fixation of CO2 • CO2 combines with a 5C sugar {ribulose biphosphate (RuBP)} 2 molecules of a 3C compound {glycerate-3-phosphate (GP/PGA)} • GP is reduced to triose phosphate (3C sugar) in the presence of ATP and NADPH • Some condense to form hexose phosphates, sucrose, starch and cellulose or are converted to acetyl CoA to make amino acids and lipids • Others regenerate RuBP • The enzyme ribulose biphosphate carboxylase (rubisco), catalyses the combination of CO 2 and RuBP ALBIO9700/2006JK
  • 12. Calvin cycle ALBIO9700/2006JK
  • 13. Leaf structure and function • Has a broad, thin lamina, a midrib and a network of veins, leaf stalk (petiole) • To perform its function the leaf must: – Contain chlorophyll and other photosynthetic pigments arranged in such a way that they can absorb light – Absorb CO2 and dispose of the waste product O2 – Have a water supply and be able to export manufactured carbohydrate to the rest of the plant • Large surface area of lamina makes it easier to absorb light and thinness minimises diffusion pathway for gaseous exchange • Upper epidermis is made of thin, flat, transparent cells which allow light through to the cells of the mesophyll , where photosynthesis takes place • A waxy transparent cuticle provides a watertight layer • Cuticle and epidermis together form a protective layer ALBIO9700/2006JK
  • 14. • Stomata are pores in the epidermis through which diffusion of gases occurs • Each stoma is bounded by 2 sausage- shaped guard cells • Changes in turgidity cause them to change shape so that they open and close the pore • Guard cells gain and loss water by osmosis ALBIO9700/2006JK
  • 16. • The palisade mesophyll is the main site of photosynthesis (many chloroplasts per cell than in the spongy mesophyll) • Adaptations for light absorption: – Long cylinders arranged at right-angles to the upper epidermis (reduces number of light-absorbing cross walls in the upper part of the leaf so that as much light as possible can reach the chloroplasts) – Large vacuole with a thin layer of cytoplasm (restricts chloroplasts to a layer near the outside of the cell where light can reach them most easily) – Chloroplasts can be moved within cells (to absorb the most light or to protect it from excessive light intensities) • Adaptations for gaseous exchange: – Cylindrical cells pack together with long, narrow air spaces between them (large surface area of contact between cell and air) – Cell walls are thin (gases can diffuse through them easily) • Spongy mesophyll is adapted as a surface for the exchange of CO2 and O2 – Smaller number of chloroplasts – Photosynthesis only at high light intensities – Irregular packing and large air spaces produced provide a large surface area of moist cell wall for gaseous exchange • Veins in leaf help to support large surface area of leaf – contains xylem and phloem ALBIO9700/2006JK
  • 18. Investigation of limiting factors • External factors: – Light intensity – Temperature – CO2 concentration • Light intensity – Rate initially increases as the light intensity increases – Rate reaches a plateau at higher light intensities • Temperature – At high light intensities the rate of photosynthesis increases as the temperature is increased over a limited range – At low light intensities, increasing the temperature has little effect on the rate of photosynthesis • Photochemical reactions are not generally affected by temperature • Since temperature affects rate, there must be 2 sets of reactions – Light-dependent photochemical stage – Light-independent, temperature-dependent stage • Limiting factor ALBIO9700/2006JK