LEAF Describe the structure of A dicotyledonous leaf A Palisade cell Chloroplast 2

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1 Photosynthesis By: K. Foster-McFarlane L.M.S.N.; MSc.; BSc. 602 Biology 1

2 LEAF Describe the structure of A dicotyledonous leaf A Palisade cell Chloroplast 2

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7 Keisha FosterFoster-McFarlane 7

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12 Stages involved in Photosynthesis 12

13 Objectives Study the general equation for photosynthesis and be able to indicate in which Stage/process each reactant is used and each product is produced. List the two major processes of photosynthesis and state what occurs in those sets of reactions. Distinguish between organisms known as autotrophs and those known as heterotrophs as pertains to their modes of nutrition. 13

14 Autotrophs & Heterotrophs As we learnt earlier: Almost all organisms depend on photosynthesis for energy and food. AUTOTROPHS = photo-autotrophs and chemo-autotrophs All other organisms are Heterotrophs Heterotrophs: rely on the products of autotrophs for food. 14

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16 4 th form Light Stage ( Light dependent stage) Dark Stage 2H 2 O 4H + O 2 + CO 2 + 4H (CH 2 O) n + H 2 O CO 2 + H 2 O (CH 2 O) n +O 2 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 6CO H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O 16

17 The Equation The reaction is however over simplified! SUNLIGHT 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 CHLOROPHYLL 17

18 Photosynthesis Photosynthesis takes place in the chloroplasts of plant cells. Leaf cell may contain > 60 chloroplasts 6CO H 2 O = C 6 H 12 O 6 + 6O 2 + 6H 2 O But not a simple reaction; rather 20 chemical reactions, some linear, some cyclic. Grouped in Light Reactions and Dark or Synthesis Reactions. 18

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21 Light Dependent Reactions The light-dependent reactions require light. These reactions occur in the thylakoid membrane. They produce ATP and NADPH, which are needed to produce glucose in the light- independent reactions 21

22 Light Dependent Reactions Suns energy captured by light absorbing pigments. Each pigment absorbs only certain wavelengths of light. Many pigment types = absorb wide range This range known as PAR Photosynthetically Active Radiation. 22

23 Light-Independent Reactions Light-independent independent reactions occur in stroma of the chloroplast in light or dark conditions. They function to reduce CO 2 to glucose. 23

24 The Electromagnetic Spectrum Pigments 24

25 Describe the nature of light and how it is associated with the release of electrons from a photosystem. Describe how the pigments found on thylakoid membranes are organized into photosystems and how they relate to photon light energy. Describe the role that chlorophylls and the other pigments found in chloroplasts play to initiate the light-dependent reactions. 25

26 Electromagnetic Spectrum 26

27 Visible Light The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more RED the color. Likewise the SHORTER wavelengths are towards the VIOLET SIDE OF THE SPECTRUM. Wavelengths longer than red are referred to as INFRARED, while those SHORTER than Violet ARE ULTRAVIOLET. 27

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30 Pigments Pigments are coloured because they absorb some wavelength of light and transmit others. Light capturing pigments classified into two (in higher plants): i. i. Chlorophyll pigments ii. ii. Carotenoid pigments 30

31 Pigments hlorophyll a (Two Primary pigment- Chlorophyll a types P680 and P700) Absorbs energy in wavelength 680nm & 700nm respectively. Accessory Pigments- the other chlorophylls and carotenoids. They can absorb wavelengths that chlorophyll a cannot absorb; hence can make that energy accessible ( pass energy on)to chlorophyll a! 31

32 i. Chlorophyll pigments Types: a, b,c & d Effectively absorb: nm (violet & blue) : nm (orange & red) These reflect green hence plants look green. 32

33 ii. Carotenoid pigments Absorb in the range nm. Yellow orange & red are reflected by carotenoids, giving carrots, tomatoes and fallen leaves their colour. 33

34 PHOTOPHOSPHORYLATION 34

35 Objectives Explain the significance of the ATP/ADP cycle. Describe the function of electron transport systems in the thylakoid membrane. 35

36 Photosystems I & II Intricate organisation of pigments in thylakoid membranes called PHOTOSYSTEMS. A single chloroplast contains several thousand photosystems. ( where light dependent rxn takes place) Two kinds of reaction centre chlorophyll: Chlorophyll activated by 700 nm wavelength light = P700 reaction centre or PHOTOSYSTEM I. Chlorophyll activated by 680 nm = P680 reaction centre or PHOTOSYSTEM II. 36

37 Cyclic Photophosphorylation Cyclic = electron returns to reaction centre. Photo = light energy used to energise electrons. Phosphorylation = formation of ATP Only P700 operates in cyclic PPN. Cyclic PPN. does not produce enough ATP for cellular metabolism of a multicellular organism but in combination Cyclic & Noncyclic PPN enough ATP is obtained. 37

38 Cyclic Photophosphorylation 38

39 Non-cyclic Photophosphorylation Both P700 & P680 take part in non-cyclic photophosphorylation and both are activated simultaneously by light energy. Non-cyclic flow of electrons begins with light energy at P680 boosting 2 electrons to a higher energy level and to Q acceptor. (nb. In order to get to this rxn centre accessory pigments were involved in absorbing photons of light!) P680 loses 2 electrons thus has a high affinity for electrons which it pulls from Hydrogen atoms of water molecules (1 e from each Hydrogen) Photolysis; Photo = light & Lysis = splitting 39

40 Photosystem 1 & 2 40

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42 Non-Cyclic Photophosphorylation P680 now satisfied, H + liberated remains in thylakoid Oxygen combines with another to from O 2. (stable) 2H+ : 1/2 O 2 : 2e - = water Energised electrons from P680 - Q acceptor shuttled down electron transport system from carrier to carrier until it reaches P700, This process of moving from a high energy level to a lower energy level produces ATP* (photoppn)-lata 42

43 Non-Cyclic Ppn cont d Due to simultaneous light energy at P680 & P700, P700 also lost electrons to its electron acceptor = Ferredoxin; leaving a vacancy for electrons from cytochrome f to fill. Ferredoxin passes the energised electrons again along a transport system to NADP to from NADPH using 2H+ from water splitting. NADP = Nicotinamide Adenine Dinucleotide Phosphate 43

44 Why? Why not just split water and give the H+ to the NADPH why go through cyclic and/or noncyclic PPN? 44

45 What happens in Thylakoid membrane 45

46 Photophosphorylation As e - pass from carrier to carrier the energy they lose drives the active transport of H + from the stroma to the thylakoid space. The protons accumulate inside the thylakoids and establish a ph difference= H + gradient b/w inside and outside of Thyl. membrane The protons diffuse back down the stroma through ATP synthase (chemiosmosis) This movement triggers PPN of ADP to ATP 46

47 FUNCTION OF LIGHT REACTIONS THUS a. Light splitting of water to provide H atoms to reduce NADP + to form NADPH b. Producing ATP which in turn provides energy for dark reactions. 47

48 Summary 48

49 CALVIN CYCLE 49

50 Objectives Explain the role of the two energy-carrying molecules produced in the light-dependent reactions (ATP and NADPH) in the light- independent reactions. Describe the Calvin-Benson cycle in terms of its reactants and products. 50

51 Calvin Cycle/Dark Reactions Both ATP and NADPH are forms of chemical energy. Why bother to make high energy compounds if these already exist? Neither ATP nor NADPH can be stored or translocated from one part of plant to another but glucose can. Glucose more flexible, it can form other cpds, ATP & NADPH cannot. 51

52 Steps in Calvin Cycle 1. CO 2 fixation by RUBP 2. CO 2 reduction 2 3. Regeneration of CO 2 acceptor molecule RUBP 52

53 CO 2 Fixation Begins with CO 2 fixation onto a 5 carbon sugar RuBP (Ribulose Biphosphate) by RuBP carboxylase = 6 carbon molecule which is so unstable that it immediately breaks into two 3 carbon molecules = PGA (Phosphoglyceric acid) which is stable. This 6CO 2 addition to 6RuBP to form 12PGA needs no energy 53

54 CO 2 Reduction G3P/ PGA Glycerate phosphate/ phosphoglyceric acid is reduced by NADPH using the energy from some ATP = glyceraldehyde 3P/PGAL(1 st product of psis) Some PGAL goes to regenerate RUBP while some combines with itself and other elements in the cell to form glucose/proteins and fats and other organic molecules. 54

55 Overview of the Calvin Cycle. 55

56 Summation 56

57 The end oops not yet! Concept checks! 1. What are the products of the light reactions of photosynthesis? 2. What conditions must exist for the light dependent reactions to proceed? 3. What events capture and transfer photon energy into photosystem II reaction centre? 4. How are electrons passed from PSII to PSI? 5. How are the boosted electrons from PSII replaced? 6. How is ATP produced in Non cyclic PPN 7. What happens in PSI? 8. How do two photosystems increase the efficiency of Photosynthesis? 57

58 Concept check! Why are almost all living things dependent on the process of photosynthesis and solar energy? Which light rays are most important for photosynthesis? Describe the anatomy of the chloroplast and associate the absorption of solar energy and the reduction of CO2 with a particular portion of the organelle. 58

59 Continued. What are the two sets of reactions that occur during photosynthesis and how are the pathways related? Trace the cyclic and non-cyclic electron pahways. Explain what is meant by the chemiosmotic A.T.P. synthesis, and relate this process to the electron transport system present in the thylakoid. 59

60 Continued Describe the three stages of the Calvin cycle. Mention which stage utilizes the A.T..P. and NADPH from the light dependent reactions. Why is it beneficial to place aquatic plants in an aquarium, even though fish are not expected to feed on them? The extinction of the dinosaurs may have been due to a meteorite that struck the earth and filled the air with dust blocking out the rays of the sun. why would this have caused dinosaurs to slowly become extinct? 60