BMS208%20Human%20Nutrition

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BMS208 Human Nutrition

• Topic 4 Photosynthesis
• Chris Blanchard

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Learning Objectives

• Outline the basic processes involved in
  photosynthesis
• conversion of energy into food
• Distinguish the two-part process
• light reactions (ATP and NADPH)
• dark reactions (Calvin cycle)
• Distinguish between C3 and C4 plants in terms of
• biochemistry metabolic modifications
• botany leaf structure

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The importance of photosynthesis

• Primary source of organic food and food energy
  (ATP) for all forms of life, either directly or
  indirectly.
• Helps to maintain balance of oxygen and carbon
  dioxide in the ecosystem.
• Oxygenic photosynthesis was responsible for
  converting the totally anaerobic condition on
  earth into the aerobic atmosphere present now.
• The fossil fuels (e.g. natural gas, coal,
  petroleum (oil), etc.) are all energy-rich
  materials of an organic origin. The energy stored
  in all these fuels is basically solar energy
  which was trapped and stored during
  photosynthesis in the geological past

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The location of photosynthesis

• The leaf is the primary site of photosynthesis in
  plants
• Carried out in organelles called chloroplasts
• Efficiency of energy conversion is about 90
  compared to 30 in a solar panel

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Chloroplasts

• Photosynthesis takes place in chloroplasts
• Structurally, they are similar to mitochondria
• Embedded in the thylakoid membranes are the
  photosynthetic pigments and the enzyme complexes
  which carry out the light reactions
• Chlorophylls harvest and concentrate the energy
  from sunlight

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Nelson and Cox Fig 19.38
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The Photosynthesis process

• Energy from the Sun (light photo) used in the
  synthesis of energy rich compounds (eg glucose)
• The process is summarised in the equation

Light
6 CO2 6 H2O ? C6H12O6 6 O2
Chlorophyll
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Light and dark reactions

• Light reactions
• photophosphorylation
• absorption of light gt ATP, NADPH O2
• Dark reactions
• NADPH and ATP are used to make carbohydrates
• Calvin cycle
• Carbon dioxide fixation

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Light reactions

• Chlorophylls (chlorophyll a and b)
• harvest and concentrate the energy from
  sunlight
• Membrane-bound protein-chlorophyll complexes form
  photosystems.
• In photosystem I P700 absorbs at l lt 700nm
• In photosystem II P680 absorbs at l lt 680nm

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Light reactions

• Photosystem I
• generates reducing power in the form of NADPH
• Photosystem II
• H2O split to produce H and O2
• Synthesis of ATP
• Electron flow within/between each photosystem
• Similar to oxidative phosphorylation
• Summary reactions

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Dark reactions (C3 plants)

• Calvin cycle
• Reduction of CO2 in presence of ATP NADPH
• Ribulose-1,5-bisphosphate CO2
• Yields 2 mols of 3-phosphoglycerate (3-PG)
• In summary
• C5 ? C6 ? C3 C3
• CO2 H2O
• Hexose sugar formation
• 3-PG gt C6H12O6 via gluconeogenesis

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Dark reactions (C3 plants)

• Calvin Cycle or C3 cycle

Source Mathews, van Holde Ahern, 2000,
Biochemistry 3rd ed
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C3 vs C4 Plants

• The C6 carbohydrate synthesised in plants can be
  achieved by two processes
• Calvin Cycle (C3 synthesis)
• Hatch-Slack Pathway (C4 synthesis)

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C3 Plants and Photorespiration

• Most plants fix CO2 by the C3 pathway
• Temperate climate conditions
• CO2 fixation hampered by photorespiration
• O2 reacts with ribulose-1,5-bisphosphate in the
  place of CO2
• Reaction makes photosynthesis 30-50 less
  efficient
• Photorespiration is stimulated by
• Light
• Heat
• Unfavourable for plants growing in hot climate

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C4 Plants

• Alternative pathway for plants in hot (tropical)
  regions known as the C4 pathway (or Hatch-Slack
  pathway)
• Utilise high light intensity even when CO2 is low
• Plants grow rapidly
• Higher yield per unit area than C3 plants.
• eg sugar cane and maize

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Hatch-Slack Pathway

• Photorespiration countered by CO2 reaction with
  PEP gt store of fixed CO2 not normally available

Source Stryer, 2002, Biochemistry, 5th ed
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C3 vs C4 Plant Leaf Structure

• Mesophyll and bundle sheath cell linked in C4
  plants

Source Mathews, van Holde Ahern, 2000,
Biochemistry 3rd ed