1. Chapter 11: Photosynthesis Leaving Certificate Biology Higher Level

2. Photosynthesis Photosynthesis is the process of using sunlight energy and chlorophyll to produce glucose from carbon dioxide and water Carbon dioxide + water Chlorophyll Sunlight glucose + oxygen 6CO 2 + 6H 2 O Chlorophyll Sunlight C 6 H 12 O 6 + 6O 2

3. Photosynthesis Light: from the sun Carbon dioxide comes from either –Atmosphere (animals breathe out carbon dioxide and burning of fuels creates carbon dioxide) –Respiration within the leaf Water: from the ground through the roots

4. Photosynthesis General process: –Sunlight is absorbed by chlorophyll (found in chloroplasts) –This absorbed energy is used to split water into three different components: Hydrogen ions (also called protons) go into the pool of protons of the chloroplast Oxygen passes out of plant cell and leaf into the atmosphere where animals breathe it in Electrons are given to chlorophyll –Chlorophyll that has energy because of sunlight transfers that energy to its electrons These electrons get combined with protons and CO 2 to make glucose

5. Structure of chloroplasts Chlorophyll absorbs all colours in white light except green – that is why plants appear green

6. Photosynthesis occurs in two stages: 1.Light stage: Light-dependent – requires light Occurs in thylakoid membranes of the chloroplast Photosynthesis Granum (stacks of thylakoids) Chlorophyll molecules Thylakoid membrane Stroma

7. Photosynthesis occurs in two stages: 2.Dark stage: Light-independent – does not require light to occur Occurs in the stroma of the chloroplast Photosynthesis Granum (stacks of thylakoids) Chlorophyll molecules Thylakoid membrane Stroma

8. Light Stage Sunlight photon strikes a cluster of chlorophyll molecules called a photosystem The chlorophyll molecules transfer the energy to a reaction center chlorophyll (RCC)

9. Light Stage The energy is absorbed by an electron which travels to a higher orbit as a result The RCC is now unstable and ordinarily the electron would give out its excess energy in the form of fluorescence However, nature has evolved a way of slowing this process down so that the electron’s energy can be released slowly to make carbohydrate The energised electron is picked up by an electron acceptor and can take one of two paths: –Cyclic pathway (pathway 1) –Non-cyclic pathway (pathway 2)

10. Cyclic Pathway

11. The energised electron from RCC is transferred along the electron transport chain It gives off it excess energy slowly to power the production of ATP When the electron has given off all of its energy it returns to the RCC

12. Non-Cyclic Pathway Occurs in the thylakoid membrane

13. Non-Cyclic Pathway Two energised electrons are captured by NADP + to become NADP – NADP + + 2e – → NADP – NADP – attracts protons because of its negative charge NADP – + H + → NADPH NADPH is the electron and proton carrier and transfers them to the dark stage (Calvin Cycle) The electrons are those from the RCC

14. Non-Cyclic Pathway Due to the NADPH carrying the electrons lost from RCC to the dark stage, the RCC is therefore electron-deficient (positively- charged: RCC + ) The RCC + splits two water molecules producing 4 electrons (4e – ), 4 protons (4H + ions) and oxygen (O 2 ) RCC + + 2H 2 O → RCC + + 4e – + 4H + + O 2

15. Non-Cyclic Pathway The electron from water has too much energy to go back to the RCC and so is passed along the electron transport chain producing ATP in the process When the electron has lost all its energy in the electron transport chain it then goes back to the RCC The protons go into the proton pool within the chloroplast The oxygen does one of two things: –Leaves the leaf and diffuses out into the atmosphere –Is used by the cells of the leaf or other regions of the plant for respiration Fates of the Products of the Splitting of Water

16. Light Stage Products The products of the light stage which go onto the dark stage are: –ATP –NADPH The by-product of the light stage which are either released into atmosphere or used in respiration is: –O 2

17. Dark Stage (Calvin Cycle) NADPH ATP CO 2 Carbohydrate (Glucose) ADP & P i NADP + Back to light stage STROMA

18. Dark Stage (Calvin Cycle) Carbon dioxide enters leaf through the stomata and enters leaf cells (ground tissue) and then enters the chloroplasts (specifically the stroma) NADPH and ATP leave the thylakoid membrane and enter the stroma Using energy supplied by ATP, carbon dioxide is reduced by the H + ions and electrons from the NADPH The product of the dark stage is carbohydrate (glucose) which is then changed into starch and stored within the cell (vacuole) or transported through the phloem to another area of the plant where it may be needed (e.g. at the meristems) or stored (e.g. root tubers)

19. SUMMARY