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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

55Nelson and Cox Fig 19.38

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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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

6 CO2 + 6 H2O C6H12O6 + 6 O2

Light

ChlorophyllChlorophyll

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Light and dark reactions

–Light reactions•photophosphorylation•absorption of light => 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 < 700nm– In photosystem II – P680 absorbs at < 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:

H2O + NADP+ NADPH + H+ + 1/2 O2

ADP + Pi ATP

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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 => 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 => store of “fixed CO2” not normally available

Source: Stryer, 2002, Biochemistry, 5th ed

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C3 vs C4 Plant Leaf Structure

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

• Mesophyll and bundle sheath cell linked in C4 plants