1. 1 BMS208 Human Nutrition Topic 4: Photosynthesis Chris Blanchard

2. 2 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 C 3 and C 4 plants in terms of –biochemistry: metabolic modifications –botany: leaf structure

3. 3 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 3

4. 44 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

5. 55 Nelson 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

6. 6 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 CO 2 + 6 H 2 O  C 6 H 12 O 6 + 6 O 2 Light Chlorophyll

7. 7 Light and dark reactions –Light reactions photophosphorylation absorption of light => ATP, NADPH & O 2 –Dark reactions NADPH and ATP are used to make carbohydrates Calvin cycle Carbon dioxide fixation

8. 8 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

9. 9 Light reactions Photosystem I generates reducing power in the form of NADPH Photosystem II H 2 O split to produce H + and O 2 Synthesis of ATP Electron flow within/between each photosystem Similar to oxidative phosphorylation Summary reactions:

10. 10 Dark reactions (C 3 plants) Calvin cycle –Reduction of CO 2 in presence of ATP & NADPH Ribulose-1,5-bisphosphate + CO 2 Yields 2 mols of 3-phosphoglycerate (3-PG) –In summary C 5 → C 6 → C 3 + C 3 CO 2 H 2 O –Hexose sugar formation 3-PG => C 6 H 12 O 6 via gluconeogenesis

11. 11 Dark reactions (C 3 plants) Calvin Cycle or C 3 cycle Source: Mathews, van Holde & Ahern, 2000, Biochemistry 3 rd ed

12. 12 C 3 vs C 4 Plants The C 6 carbohydrate synthesised in plants can be achieved by two processes –Calvin Cycle (C 3 synthesis) –Hatch-Slack Pathway (C 4 synthesis)

13. 13 C 3 Plants and Photorespiration Most plants fix CO 2 by the C 3 pathway –Temperate climate conditions CO 2 fixation hampered by photorespiration –O 2 reacts with ribulose-1,5-bisphosphate in the place of CO 2 –Reaction makes photosynthesis 30-50% less efficient Photorespiration is stimulated by –Light –Heat Unfavourable for plants growing in hot climate

14. 14 C 4 Plants Alternative pathway for plants in hot (tropical) regions known as the C 4 pathway (or Hatch- Slack pathway) Utilise high light intensity even when CO 2 is low Plants grow rapidly Higher yield per unit area than C 3 plants. eg sugar cane and maize

15. 15 Hatch-Slack Pathway Photorespiration countered by CO 2 reaction with PEP => store of “fixed CO 2 ” not normally available Source: Stryer, 2002, Biochemistry, 5 th ed

16. 16 C 3 vs C 4 Plant Leaf Structure Source: Mathews, van Holde & Ahern, 2000, Biochemistry 3 rd ed Mesophyll and bundle sheath cell linked in C 4 plants