Photosynthesis

1. Photosynthesis

2. This symbol in the corner of a slide indicates a picture, diagram or table taken from your textbook

3. Introduction • Almost all the energy transferred to all the ATP molecules in living organisms originally comes from the energy in sunlight

4. Introduction • Green plants, some protoctista and some bacteria are able to transfer sunlight energy into energy trapped in the molecular structure of carbohydrates.

5. Introduction • This is the process called photosynthesis. • Once carbohydrates such as glucose have been made, plants can convert some of them to other organic substances such as oils, nucleic acids and proteins

6. Introduction • Animals cannot make organic molecules from inorganic one and so rely entirely on plants for their supply of organic molecules.

8. Photosynthesis – a summary • Photosynthesis can be summarised by the equation: nCO2 + nH2O (CH2O)n + nO2 • This shows that photoautotrophs synthesise carbohydrate using carbon dioxide, water and light energy. light carbohydrate • Is photosynthesis a reduction or an oxidation? • A. CO2 is reduced; water is oxidised. This simple summary hides the fact that photosynthesis is a series of reactions controlled by specific enzymes.

9. The light dependent stage (LDS). In these reactions, ATP and a reduced coenzyme (NADPH) are made. Oxygen is a waste product of this stage. The light independent stage (LIS). In these reactions, the products of the light dependent reactions are used to reduce carbon dioxide to carbohydrate. Stages of photosynthesis The reactions of photosynthesis can be divided into two distinct stages.

10. An overview carbon dioxide (CO2) oxygen (O2) water (H2O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage ATP reduced NADP carbohydrates ADP inorganic phosphate oxidised NADP Note: the light-independent stage is also known as the Calvin cycle.

11. outer membrane chloroplast envelope inner membrane Granum. A stack of thylakoid membranes The chloroplast Ribosomes. Smaller than cytoplasmic ribosomes. Starch grain. Produced from sugars made in photosynthesis Stroma. The site of the Calvin cycle Lipid droplet. Made from the sugars made in photosynthesis Lamella. A pair of membranes containing chlorophyll Small circular DNA coding for some chloroplast proteins Thylakoid. A membranous sac Thylakoid space. Space between lamellae.

12. Internal compartmentalisation. The two stages of photosynthesis are effectively separated, thus allowing rate-determining factors such as pH and enzyme concentrations to be optimized DNA and ribosomes mean chloroplast can code for and produce its own proteins such as RuBPC Double membrane provides control of substances entering/leaving the organelle Thylakoid membranes provide a large surface area for light absorption Structure to function: chloroplast

13. Trapping light energy • Light energy is trapped by photosynthetic pigments. • Different pigments absorb different wavelengths of light. • The photosynthetic pigments of higher plants form two groups: the chlorophylls and the carotenoids. • Chlorophylls absorb mainly in the red and blue-violet regions of the light spectrum. They reflect green light which is why plants look green. • The carotenoids absorb mainly in the blue-violet region of the spectrum.

14. chlorophyll a carotenoid

15. Thylakoid membranesthe possible arrangement of chlorophyll and associated molecules within the thylakoid membranes based on studies of isolated grana chlorophyll combined with protein electron carriers stalked particles containing the enzymes for catalysing the synthesis of ATP

17. Trapping light energy • The photosynthetic pigments fall into two categories: primary pigments and accessory pigments • The primary pigments are two forms of chlorophyll a with slightly different absorption peaks. • The accessory pigments include other forms of chlorophyll a, chlorophyll b and the carotenoids. The pigments are arranged in light-harvesting clusters called photosystems. • In a photosystem, several hundred accessory pigment molecules surround a primary pigment molecule and the energy of the light absorbed by the different pigments is passed to the primary pigment. • The primary pigments are said to act as reaction centres.

18. Absorption spectra: • An absorption spectrum is a graph of the absorbance of different wavelengths of light by a pigment. Action spectra: • An action spectrum is a graph of the rate of photosynthesis at different wavelengths of light.

19. chlorophyll a chlorophyll b carotenoids Absorption spectra: absorbance 400 450 500 550 600 650 700 Wavelength of light (nm)

20. Action spectrum: Rate of photosynthesis 400 450 500 550 600 650 700 Wavelength of light (nm)

21. Photosystem I This is arranged around a molecule of chlorophyll a with a peak absorption at 700nm The reaction centre of photosystem I is therefore known as P700 Photosystem II This is arranged around a molecule of chlorophyll a with a peak absorption at 680nm The reaction centre of photosystem II is therefore known as P680 Photosystems

22. chlorophyll a Found in PS1 Found in PS2 absorbance 400 450 500 550 600 650 700 Wavelength of light (nm)

23. A photosystem: light thylakoid membrane photosystem accessory pigments primary pigment reaction centre P700 or P680

25. An overview carbon dioxide (CO2) oxygen (O2) water (H2O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage ATP reduced NADP carbohydrates ADP inorganic phosphate oxidised NADP Note: the light-independent stage is also known as the Calvin cycle.

26. Occurs in the thylakoids Results in photophosphorylation This can be either cyclic photophosphorylation (CPP) or non-cyclic photophosphorylation (NCP) CPP produces ATP Hydrogen ions NCP produces Oxygen Reduced NADP ATP The light-dependent reaction

27. Once the light energy is passed to the reaction centre, electrons are energised to a level where they are emitted from the chlorophyll. • These are used in the light dependent stage to: • Produce reduced NADP [NADPH] • Transfer light energy to ATP by the process of photophosphorylation

28. Depending on the route taken by the released electrons this can be either • non-cyclic photophosphorylation or • cyclic photophosphorylation. • Non cyclic photophosphorylation produces oxygen, NADPH and ATP • While cyclic photophosphorylation produces just ATP and hydrogen ions

29. At the same time water molecules are split to produce electrons • These replace the electrons ejected from the chlorophyll and hydrogen ions. • Oxygen is given off as a waste product

30. ADP + Pi 2e- ATP light Cyclic photophosphorylation Electrons are cycled (PSI  carriers  PSI  carriers etc.) electron carrier electron carrier electron carrier electron carrier PSI

31. electron carrier electron carrier electron carrier electron carrier 2e- PSI ADP + Pi 2e- ATP 2e- H2O  O2 + 2e-+ 2H+ light light to LIS NADP  NADPH PSII waste product

32. NON-CYCLIC PHOTOPHOSPHORYLATION

33. The production of ATP in non-cyclic photophosphorylation When light energy is passed to the reaction centre in PSII, electrons are energised to a level where they are emitted from the chlorophyll

34. The production of ATP in non-cyclic photophosphorylation As the result of the flow of electrons from PSI to PSII and the breakdown of water there is a build up of hydrogen ions.

35. The production of ATP in non-cyclic photophosphorylation These accumulate within the thylakoid space and create a concentration gradient.

36. The production of ATP in non-cyclic photophosphorylation The consequent passage of H+ across the thylakoid membranes provides the energy for the production of ATP in the presence of ATPase. (chemiosmosis).

37. The production of NADPH in non-cyclic photophosphorylation When light is absorbed by the chlorophyll in photosystem I (PSI), an electron is ejected and taken up by an electron acceptor (ferredoxin).

38. The production of NADPH in non-cyclic photophosphorylation This in turn passes the electron to a molecule of NADP, which is thus reduced to NADPH.

40. Non-cyclic photophosphorylation This process would eventually stop if the released electrons were not replaced in PSI. This happens as a result of light energy displacing electrons from PSII.

41. Non-cyclic photophosphorylation Electrons from PSII are passed along a series of electron carriers (cytochromes) and eventually replace the lost electrons in PSI

42. If there is sufficient NADPH then the plant will automatically switch to CYCLIC PHOTOPHOSPHORYLATION

43. In this, the electrons follow a different route; PSI is both the donator and acceptor of the electrons; i.e. they follow a cyclical route.

44. As light enters PSI electrons are lost from the chlorophyll and pass along a chain of electron carrier molecules before re-entering PSI.

45. The accumulation of H+ still occurs in the thylakoid space, with the consequent synthesis of ATP, but no NADPH is formed

47. H2O O2 The Hill Reaction • The photolysis of water was first shown by Robert Hill in 1939 • Working on isolated chloroplast he showed that they had ‘reducing power’ • In the presence of an oxidising agent, oxygen was liberated from water • He used a substance that changed colour on reduction • This can be demonstrated with a variety of substances but is usually shown using the blue dye DCPIP (dichlorophenolindophenol) • This is substituting for the plant’s NADP. • DCPIP becomes colourless when reduced oxidised DCPIP reduced DCPIP

48. The Hill Reaction Chloroplasts in the light Chloroplasts in the dark for 5 minutes and then in the light blue Colorimeter readings colourless time

49. An overview carbon dioxide (CO2) oxygen (O2) water (H2O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage ATP reduced NADP carbohydrates ADP inorganic phosphate oxidised NADP Note: the light-independent stage is also known as the Calvin cycle.

50. The light-independent reaction • Occurs in the stroma of the chloroplast • The product is sugar which can be converted into fats, amino acids etc