Photosynthesis

1. Photosynthesis Unit 1 Communication, Homeostasis and Energy

2. Think about it!!! 10 minutes Which process evolved first on Earth – aerobic respiration or photosynthesis? Give reasons for your answer You are going to present your answer with your reasons to the rest of the class!!

3. Answer Plants and animals rely on aerobic respiration Requires oxygen Oxygen is a by-product of photosynthesis Until photosynthesis evolved there was no free oxygen in the atmosphere Photosynthesis evolved first!!

4. Test yourself Where in plants does photosynthesis take place? What are the raw materials needed for photosynthesis? What is the energy source for photosynthesis? Draw a flow diagram showing how energy from sunlight is used to produce muscle contractions in your arm.

5. Learning outcomes • Define the terms autotroph and heterotroph. • State that light energy is used during photosynthesis to produce complex organic molecules. • Explain how respiration in plants and animals depends upon the products of photosynthesis. • State that, in plants, photosynthesis is a two-stage process taking place in chloroplasts.

6. The importance of photosynthesis Photosynthesis transfers light energy into the chemical potential energy of organic molecules. Photosynthesis releases oxygen from water, so all aerobes depend on photosynthesis for their respiration.

7. Heterotroph and Autotroph Autotroph an organism that uses an external energy source and inorganic molecules to make complex organic molecules. Chemoautotroph Photoautotroph Heterotroph Organism that needs to take in complex organic molecules which act as a source of energy and as usable carbon compounds.

8. Energy in Living Organisms • In order to maintain life, organisms need a source of energy. • In most organisms this is provided by the oxidation of organic molecules. • Autotrophic nutrition • Synthesise organic materials from inorganic sources e.g. photosynthesis • Heterotrophic nutrition • Obtained in organic form

9. Photosynthesis: an outline Photosynthesis (p/s) is the fixation of carbon dioxide and its reduction to carbohydrate, using hydrogen from water

10. Photosynthesis Equations Word equation for photosynthesis Light energy Carbon dioxide + watercarbohydrate + oxygen chlorophyll

11. Photosynthesis Equations • Overall Chemical Equation Light energy nCO2 + nH20  (CH2O)n + nO2 chlorophyll • Balanced Equation for hexose sugars Light energy 6CO2 + 6H2O  C6H12O6 + 6O2 chlorophyll

12. Photosynthesis experiments Testing a leaf for starch What are the requirements for photosynthesis Light Chlorophyll carbon dioxide

13. Factors affecting the rate of photosynthesis Factors limiting photosynthesis chlorophyll (enzymes) carbon dioxide Light Water Temperature

14. Photosynthesis Photosynthesis is a 2 stage process Light dependent reactions thylakoid membranes Light independent reactions Stroma

15. Learning outcome Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions.

16. Chloroplast

17. Chloroplast – electro micrograph x y z

18. Chloroplast Structure 3 – 10μm diameter Envelope of 2 phospholipid membranes Stroma = fluid interior Thylakoids are series of flattened sacs, which form stacks (grana) in places

19. Chloroplast function Grana Provides a LSA to hold pigments, electron carriers, and enzymes for light dependent reactions. Photosystems arranged in funnel like structure in thylakoid Membrane of grana holds ATPsynthase (chemiosmosis)

20. Chloroplast Function Stroma Site of light independent reactions (carbon fixation) Contains sugars, organic acids and enzymes for Calvin cycle Store starch grains Loop DNA – codes for chloroplast proteins

21. Chloroplast Function Lamellae Do not contain chlorophyll Form a network between the grana

22. Learning Outcomes Define the term photosynthetic pigment. Explain the importance of photosynthetic pigments in photosynthesis. State that the light-dependent stage takes place in thylakoid membranes and that the light-independent stage takes place in the stroma.

23. Trapping Light energy The fate of light which strikes the leaf

24. Trapping the Light Energy The fate of light which strikes the leaf  Light shining on leaf (100%) 12% light reflected 83% light absorbed, but only 4% of this is used in photosynthesis 5% of light transmitted

25. These values will be affected by the amount of chloroplasts in the leaf how shiny the leaf is how thick the leaf is Features of light that make it important spectral quality (colour) intensity (brightness) duration (time) Visible light has a wavelength between 400nm and 700nm

26. Absorption of Light Leaves contain a variety of photosynthetic pigments, of which chlorophyll is the most obvious. It is these pigments which absorb light energy. There are two different groups of pigments chlorophylls – chlorophyll a, chlorophyll b Carotenoids – xanthophyll, carotene Different photosynthetic pigments absorb different wavelengths.

27. Absorption and Action Spectra Absorption Spectrum A graph of absorbance of different wavelengths of light by a pigment Action Spectrum A graph of the rate of photosynthesis at different wavelengths of light. Chlorophylls absorb red and blue violet regions of light, and reflect green Carotenoids absorb the blue-violet region of the spectrum.

28. Absorption Spectrum

29. Action Spectrum

30. Absorption and Action Spectra

31. The Chemistry of photosynthesis

32. Learning Outcomes Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the light-dependent stage. Explain the role of water in the light-dependent stage.

33. Photosynthesis is a two-stage process Evidence for this comes from experiments with isotopes of oxygen. Plants provided with C18O2 combine the atoms into carbohydrates Plants provided with H218O release the 18O atoms as oxygen gas All the oxygen released by photosynthesis comes from water.

34. Harvesting Light In p/s the light energy absorbed by the p/s pigments is converted to chemical energy. The absorbed light energy excites electrons in pigment molecules. In functioning photosystems this is the energy which drives the process of photosynthesis.

35. There are two categories of p/s pigment Primary pigments chlorophyll a Accessory pigments chlorophyll a, chlorophyll b and carotenoids

36. Light-dependent reactions Water is split in a reaction called photolysis, These reactions provide the energy to: Synthesis ATP from ADP and Pi (photophosphorylation) Transfer H+ and e- to NADP to form reduced NADP

37. Photophosphorylation Photophosphorylation can be cyclic or non cyclic depending on the pattern of electron flow in one or both photo systems Cyclic photophosphorylation PSI Non cyclic photophosphorylation PSII & PSI

38. Harvesting LightPhotosystems Pigments are arranged into light harvesting clusters called photosystems light energy absorbed by pigments is passed to the primary pigment, which acts as a reaction centre.

39. Photosystems Photosystem I Arranged around chlorophyll a molecule with an absorption peak at 700 nm. Reaction centre P700 Photosystem II Chlorophyll a molecule with absorption peak at 680nm Reaction centre P680

40. Light harvesting system

41. Cyclic Photophosphorylation ADP + Pi ATP Electron acceptor Electron carriers 2e- Light energy absorbed by Chlorophyll a PSI

42. Cyclic Photophosphorylation involves only photosystem I, which has a chlorophyll a with a reaction centre P 700. An electron from the molecule is excited to a higher energy level. It is captured by an electron acceptor, and then is passed back to one of the chlorophyll a P700 molecules. This happens due to a chain of electron carriers .

43. Cyclic photophosphorylation The whole process releases energy to make ATP from ADP and inorganic phosphate. This ATP will then be used in the light – independent reaction.

44. Non cyclic photophosphorylation ADP + Pi ATP PSII PSI Electron Acceptor B Electron Acceptor A NADP Electron carrier Electron carrier 2e- 2e- NADPH+ H+ Light energy Light energy H2O ½O2 + 2e- + 2H+

45. Non-cyclic photophosphorylation involves both photosystems. Both absorb light and the electrons which are excited leave the reaction centres of P680 and P700 of the chlorophyll a molecules. Electron acceptors pass the electrons along chains of electron carriers. The P700 of the photosystem I absorbs electrons from photosystem II. Replacement electrons from the photolysis of water go to photosystem II.

46. Non-cyclic photophosphorylation The electrons lose energy passing along the electron chain and this goes towards synthesising ATP. The photolysis of water releases two protons/H+s H+ combine with electrons from photosystem I and NADP to give reduced NADP (NADPH + H+)

47. The Photolysis of water H2O  2H+ + 2e- + ½O2 Oxygen is released as a waste product The H+ and e- are transferred to NADP to give reduced NADP 2H+ + 2e- + NADP  reduced NADP The reduced NADP then passes onto the light independent reactions

48. Pupil Activity Complete the diagram of Photophosphorylation

49. Prep Question – 10 marks Describe the structure of a chloroplast and then give an account of the role played by chlorophyll in photosynthesis. Refer to action and absorption spectra in your answer. Write in bullet points and include a diagram.

50. Jan 03 Question 1 (a) blue and red light used in photosynthesis; (light of) wavelength 420 – 450 nm, gives high rate / AW; (light of) wavelength 650 – 690 nm, gives high rate / AW; (light of) wavelength of 500 – 650 nm / green light, less effective / reflected; sharp / AW, drop after 680 – 690 nm;