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: n CO 2 + nH 2 O (CH 2 O) n + nO 2  This shows that photoautotrophs synthesise carbohydrate using carbon dioxide, water and light energy. light carbohydrate Q.Is photosynthesis a reduction or an oxidation? A. CO 2 is reduced; water is oxidised. This simple summary hides the fact that photosynthesis is a series of reactions controlled by specific enzymes.

9. Stages of photosynthesis 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. The reactions of photosynthesis can be divided into two distinct stages.

10. An overview water (H 2 O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage oxygen (O 2 ) carbon dioxide (CO 2 ) carbohydrates ADP inorganic phosphate oxidised NADP ATP reduced NADP Note: the light-independent stage is also known as the Calvin cycle.

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

12. Structure to function: chloroplast  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

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. PigmentColour Chlorophylls:chlorophyll a Yellow-green chlorophyll b Blue-green Carotenoids:ß carotene Orange xanthophyll Yellow

14. carotenoid chlorophyll a

15. Thylakoid membranes the possible arrangement of chlorophyll and associated molecules within the thylakoid membranes based on studies of isolated grana electron carriers chlorophyll combined with protein 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. Absorption spectra: 400 450 500 550 600 650 700 Wavelength of light (nm) absorbance chlorophyll a chlorophyll b carotenoids

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

21. Photosystems 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

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

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

25. An overview water (H 2 O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage oxygen (O 2 ) carbon dioxide (CO 2 ) carbohydrates ADP inorganic phosphate oxidised NADP ATP reduced NADP Note: the light-independent stage is also known as the Calvin cycle.

26. The light-dependent reaction  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

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

31. PSI 2e - electron carrier ADP + P i ATP light PSII light 2e - H 2 O  O 2 + 2e - + 2H + NADP  NADPH 2e - waste product to LIS

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

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

40. 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. Non-cyclic photophosphorylation

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

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. 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 H 2 O O 2

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

49. An overview water (H 2 O) light energy ADP Pi oxidised NADP light-dependent stage light-independent stage oxygen (O 2 ) carbon dioxide (CO 2 ) carbohydrates ADP inorganic phosphate oxidised NADP ATP reduced 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

51. The light-independent reaction  Also known as the Calvin cycle. This is the stage that fixes carbon dioxide from the atmosphere  Carbon dioxide combines with a 5-carbon sugar called ribulose bisphosphate (RuBP). The reaction is catalysed by the enzyme RuBPC. [ ribulose bisphosphate carboxylase]  The 6-carbon compound formed is unstable and immediately splits to give two molecules of a 3-carbon compound called glycerate-3-phosphate (GP).  GP is reduced to 3-carbon triose phosphate (TP) in the presence of ATP and reduced NADP [ from the light-dependent stage ]  The triose phosphate is then either used to regenerate RuBP or to make carbohydrates and other metabolites for the plant

52. unstable intermediate (6C) 2 x glycerate 3-phosphate (3C) 2 x triose phosphate (3C) ATP ADP + P i reduced NADP CO 2 (1C) RuBP (5C) ATP ADP The Calvin cycle Glucose (6C), amino acids and lipids

53. The Calvin cycle

54. Fate of triose phosphate  Some is used to regenerate RuBP  Some molecules condense to form hexose phosphates, sucrose, starch and cellulose  Some are converted to acetyl coenzyme A to make amino acids and lipids

58. The light-independent reaction  This cycle of events was worked out by Calvin, Benson and Bassham between 1946 and 1953  The cycle is usually called the Calvin cycle  The enzyme ribulose bisphosphate carboxylase (ribisco or RuBPC for short), which catalyses the combination of carbon dioxide and RuBPC is the most common enzyme in the world!

59. Photosynthesis – a summary 6O 2 24 electrons 6CO 2 + 12H 2 O chlorophyll 12H 2 O Light energy ADP + P i ATP 24H + + 6H 2 O + C 6 H 12 O 6 24H + 1 2 3 4 5 6 Light-dependent stage (thylakoid membranes)Light-independent stage (stroma)

60. Structure to function:the leaf  Thin, therefore rapid light penetration  Waxy cuticle reduces water loss  Upper epidermis transparent to light  Palisade mesophyll arranged at 90 o to surface thus minimising amount of light absorbed by cell walls  Chloroplasts in mesophyll can be moved to maximise absorption  Spongy mesophyll has many air sps spaces for rapid gas diffusion

61. Light and shade A variety of environmental factors can affect the size and thickness of leaves. In many species, leaves grown under high light intensity (sun leaves) are smaller and thicker than those grown under low light intensity (shade leaves). Increased thickness of sun leaves is due to greater development of palisade parenchyma. Acer : maple Shade leaf Sun leaf