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Chapter 8: Photosynthesis: Energy from the Sun Photosynthesis: Energy from the Sun CHAPTER 8.

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Presentation on theme: "Chapter 8: Photosynthesis: Energy from the Sun Photosynthesis: Energy from the Sun CHAPTER 8."— Presentation transcript:

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2 Chapter 8: Photosynthesis: Energy from the Sun Photosynthesis: Energy from the Sun CHAPTER 8

3 Chapter 8: Photosynthesis: Energy from the Sun Photosynthesis  Biochemical process in which light energy is converted to chemical energy  Photos = light synthesis = to put together  In plants, photosynthesis takes place in chloroplasts.  It involves many enzyme controlled steps

4 Chapter 8: Photosynthesis: Energy from the Sun Photosynthetic Reactants and Products 6 CO H 2 O + light  C 6 H 12 O O H 2 O

5 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.1 figure jpg

6 Chapter 8: Photosynthesis: Energy from the Sun Photosynthesis  Photosynthesis can be divided into two pathways: –The light reaction - driven by light energy captured by chlorophyll. Consists of Photosystem I and Photosystem II. It produces ATP and NADPH + H +. –The Calvin–Benson cycle - does not use light directly. It uses ATP, NADPH + H +, and CO 2 to produce sugars.

7 Chapter 8: Photosynthesis: Energy from the Sun Properties of Light and Pigments  Light is the source of energy that drives photosynthesis  Molecules that absorb light energy in the visible range are called pigments.

8 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.5 figure jpg

9 Chapter 8: Photosynthesis: Energy from the Sun Properties of Light and Pigments When light and a pigment molecule meet, one of 3 things happen  Reflection – the light bounces off the molecule  Transmission – the light passes through the molecule  Excitation – the light is absorbed by the molecule. If absorbed, the molecule goes from its ground state to and excited state of higher energy  An electron is boosted to another orbital

10 Chapter 8: Photosynthesis: Energy from the Sun Pigments  When a beam of white light shines on an object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red.  In the case of chlorophyll, plants look green because they absorb green light less effectively than the other colors found in sunlight and reflect the green light not absorb

11 Chapter 8: Photosynthesis: Energy from the Sun Properties of Light and Pigments  Different pigment molecules absorb different wavelengths of light  The particular set of wavelengths that a pigment absorbs is called its absorption spectrum  Review Figures

12 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.7 figure jpg

13 Chapter 8: Photosynthesis: Energy from the Sun Properties of Light and Pigments  Chlorophylls are the most important pigments in photosynthesis  Chlorophyll a is the primary pigment in photosynthesis.  Chlorophylls and accessory pigments trap light and transfer energy to a reaction center

14 Chapter 8: Photosynthesis: Energy from the Sun Chlorophyll

15 Chapter 8: Photosynthesis: Energy from the Sun An excited pigment molecule may  lose its energy by emitting light of longer wavelength or  transfer the absorbed energy to another pigment molecule as a redox reaction.

16 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.8 Energy Transfer and Electron Transport

17 Chapter 8: Photosynthesis: Energy from the Sun  There are two different systems for transport of electrons in photosynthesis. 1. Noncyclic electron transport produces NADPH + H + and ATP and O Cyclic electron transport produces only ATP.

18 Chapter 8: Photosynthesis: Energy from the Sun Noncyclic  In noncyclic electron transport, two photosystems are required.  Photosystems consist of many chlorophyll molecules and accessory pigments bound to proteins.

19 Chapter 8: Photosynthesis: Energy from the Sun Photosystem I  Photosystem I uses light energy to reduce NADP + to NADPH + H +.  The reaction center contains a chlorophyll a molecule called P 700 because it best absorbs light at a wavelength of 700 nm.

20 Chapter 8: Photosynthesis: Energy from the Sun Photosystem II  Photosystem II uses light energy to split water, producing electrons, protons, and O 2.  The reaction center contains a chlorophyll a molecule called P 680 because it best absorbs light at a wavelength of 680 nm.  To keep noncyclic electron transport going, both photosystems must constantly be absorbing light.

21 Chapter 8: Photosynthesis: Energy from the Sun After absorbing light energy:  an energized electron leaves the Chl* in the reaction center and participates in a series of redox reactions.  the electron flows through a series of carriers in the thylakoid membrane.  producing ATP

22 Chapter 8: Photosynthesis: Energy from the Sun Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

23 Chapter 8: Photosynthesis: Energy from the Sun Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)

24 Chapter 8: Photosynthesis: Energy from the Sun  Cyclic electron transport produces only ATP.  The electron passes from an excited P 700 molecule and cycles back to the same P 700 molecule.  No O 2 is released.  In cyclic electron flow, photosystem I acts on its own. Cyclic Electron Transfer

25 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP

26 Chapter 8: Photosynthesis: Energy from the Sun “Z” Scheme  Photosystem I & II (P680 & P700) work together to generate ATP and NADPH.  This pathway is called the “Z” scheme.  Noncyclic

27 Chapter 8: Photosynthesis: Energy from the Sun Noncyclic Electron Flow or Z Scheme  In Photosystem II chlorophyll a absorbs light energy to become energized chloropyll a  2 electrons are released and caught by the primary electron acceptor.  H 2 0  ½ O e - + 2H +

28 Chapter 8: Photosynthesis: Energy from the Sun  The electrons pass through a redox chain for chemiosmotic ATP production.  The electron transport chain pumps protons across the membrane into the thylakoid space.  The protons accumulate establishing a proton concentration gradient  ATP synthases open and the protons diffuse to generate ATP from ADP.

29 Chapter 8: Photosynthesis: Energy from the Sun Z Scheme Cont’d  The electrons are passed to P 700 chlorophyll  P700 loses electrons to Ferredoxin (Fd)  NADP combines with H to form NADPH.  NADPH is the source of H used to make C 6 H 12 O 6

30 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.11 Chloroplasts Form ATP Chemiosmotically

31 Chapter 8: Photosynthesis: Energy from the Sun The Calvin–Benson Cycle  The Calvin–Benson cycle makes sugar from CO 2  ATP and NADPH provide the needed energy  This pathway was elucidated through use of radioactive tracers

32 Chapter 8: Photosynthesis: Energy from the Sun The Calvin–Benson Cycle Three phases:  1. Carbon Fixation – RuBP + CO 2  carbon sugar  PG (first stable product)  The reaction is catalyzed by rubisco (ribulose bisphosphate carboxylase).  2. Series of reactions to produce G3P  3. Regeneration of RuBP (7 enzymatic steps)  RuBP (ribulose biphosphate) is the initial CO 2 acceptor

33 Chapter 8: Photosynthesis: Energy from the Sun The Calvin–Benson Cycle  The end product of the cycle is glyceraldehyde 3-phosphate, G3P.  There are two fates for the G3P: –One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose. –Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.

34 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.13 The Calvin-Benson Cycle

35 Chapter 8: Photosynthesis: Energy from the Sun Rubisco  Rubisco is a carboxylase, adding CO 2 to RuBP. It can also be an oxygenase, adding O 2 to RuBP.  These two reactions compete with each other.  When RuBP reacts with O 2, it cannot react with CO 2, which reduces the rate of CO 2 fixation.

36 Chapter 8: Photosynthesis: Energy from the Sun Photorespiration  A specialized metabolic pathway in which rubisco reacts with O 2 instead of CO 2  Occurs under stress conditions of hot, dry, bright days when the internal leaf concentration of O 2 is greater than CO 2 concentration.  Glucose production is reduced thereby limiting plant growth

37 Chapter 8: Photosynthesis: Energy from the Sun C3 Plants  Most common type of plants on earth.  Grow best in temperate zones  Includes rice, wheat, soybeans, bluegrass  On hot days the stomata close, O 2 builds up and photorespiration occurs.  The first product is the 3-C molecule of 3PG  CO 2 + RuBP  3 phosophoglycerate (3 C compound)

38 Chapter 8: Photosynthesis: Energy from the Sun Figure 8.16 Leaf Anatomy of C 3 and C 4 Plants

39 Chapter 8: Photosynthesis: Energy from the Sun C4 Plants  C4 plants have 2 enzymes (PEP carboxylase & rubisco) for CO 2 fixation in 2 different parts of the leaf.  PEP carboxylase does not have an affinity for O 2 and fixes CO 2 even at very low CO 2 levels.  What is the significance of this fact?  C4 plants include sugarcane, corn and other plants that grow in hot, dry climates.

40 Chapter 8: Photosynthesis: Energy from the Sun C4 Plants Cont’d  CO 2 + PEP carboxylase  Oxaloacetate (4 C compound).  Occurs in cells near top of leaf  Oxaloacetate diffuses into bundle sheath cells in the interior of the cells.  Here oxaloacetate loses a C forming CO 2  CO 2 enters the Calvin-Benson Cycle

41 Chapter 8: Photosynthesis: Energy from the Sun Crassulacean Acid Metabolism (CAM)  CAM plants are succulents or water storing plants.  Include cacti and pineapples  CAM plants open their stomata only at night  CO 2 enters and forms malic acid which is stored as an acid in the vacuoles until morning  In daylight, the CO 2 is released from the acid and enters the Calvin Benson Cycle.

42 Chapter 8: Photosynthesis: Energy from the Sun Stomates  Stomates close when weather is hot & dry.  O 2 concentration increases, CO 2 concentration decreases.  Why?  Ribulose requires high concentrations of CO 2  If sufficient CO 2 is unavailable, photorespiration occurs.

43 Chapter 8: Photosynthesis: Energy from the Sun Metabolic Pathways in Plants  Both photosynthesis and respiration occurs in plants.  Compare photosynthesis and respiration.


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