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8 Announcements Chapter 7 Quiz – Deadline tonight. Don’t forget Examination 1. Check out sample exam on BIO 121 website. Likely Final Exam Date/Time: December.

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Presentation on theme: "8 Announcements Chapter 7 Quiz – Deadline tonight. Don’t forget Examination 1. Check out sample exam on BIO 121 website. Likely Final Exam Date/Time: December."— Presentation transcript:

1 8 Announcements Chapter 7 Quiz – Deadline tonight. Don’t forget Examination 1. Check out sample exam on BIO 121 website. Likely Final Exam Date/Time: December 15, 11:30-2:30 PM; do not schedule flights, etc. before then, as noted on the course syllabus.

2 8 Photosynthesis: Energy from the Sun

3 8 Identifying Photosynthetic Reactants and Products The Two Pathways of Photosynthesis: An Overview The Interactions of Light and Pigments The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Making Carbohydrate from CO 2 : The Calvin–Benson Cycle Photorespiration and Its Consequences Metabolic Pathways in Plants

4 8 Identifying Photosynthetic Reactants and Products Photosynthesis, the biochemical process by which plants capture energy from sunlight and store it in carbohydrates, is the very basis of life on Earth. 20rainforest.jpg Autotrophs & Heterotrophs 10,000,000,000 tons of carbon fixed (CO 2 to carbohydrate) per year 1/3 used by one species: Homo sapiens

5 8 Identifying Photosynthetic Reactants and Products By the 1800s, scientists had learned:  Three ingredients are needed for photosynthesis: water, CO 2, and light.  There are two products: carbohydrates and O 2.  The water, which comes primarily from the soil, is transported through the roots to the leaves.  The CO 2 is taken in from the air through stomata, or pores, in the leaves.

6 Figure 8.1 The Ingredients for Photosynthesis

7 8 Identifying Photosynthetic Reactants and Products By 1804, scientists had summarized the overall chemical reaction of photosynthesis: CO 2 + H 2 O + light energy  sugar + O 2 More recently, using H 2 O and CO 2 labeled with radioactive isotopes, it has been determined that the actual reaction is: 6 CO H 2 O  C 6 H 12 O O H 2 O

8 8 Overview of Photosynthesis “The process of photosynthesis can be neatly broken down into two steps. The first step is the conversion of energy from light to chemical bonds in reduced electron carriers and ATP. In the second step, these two sources of chemical energy are used to drive the synthesis of carbohydrates from carbon dioxide.” Purves et al. p.145

9 8 The Two Pathways of Photosynthesis: An Overview Photosynthesis occurs in the chloroplasts of green plant cells and consists of many reactions. Photosynthesis can be divided into two pathways:  The light reaction is driven by light energy captured by chlorophyll. 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.

10 8 Leaf Cross Section Chloroplasts?

11 8 Chloroplast

12 Figure 8.3 An Overview of Photosynthesis (Part 1)

13 Figure 8.3 An Overview of Photosynthesis (Part 2)

14 8 Visible Light (modified)

15 8 The Interactions of Light and Pigments Visible light is part of the electromagnetic radiation spectrum. Light behaves as if it were a wave. It also comes in discrete packets (particles) called photons. images/particlewavefigure5.jpg

16 8 Wavelength and Energy Level “The amount of energy in a single photon is inversely proportional to its wavelength: the shorter the wavelength, the greater the energy of the photons.” Purves et al. Visual interpretation of a photon. Art Gallery at Fermi National Accelerator Laboratory

17 Figure 8.5 The Electromagnetic Spectrum

18 8 Biological Aspects of Photons Two things are required for photons to be active in a biological process:  Photons must be absorbed by receptive molecules.  Photons must have sufficient energy (but not too much) to perform the chemical work required. Chlorophyll a lecture/chlorophyll.gif

19 8 The Interactions of Light and Pigments When a photon and a pigment molecule meet, one of three things happens: The photon may bounce off (reflected), pass through (transmitted), or be absorbed by the molecule. If absorbed, the energy of the photon is acquired by the molecule. The molecule is then raised from its ground state to an excited state of higher energy. %20Reflection%20and%20Absorption_engl.jpg

20 Figure 8.4 Exciting a Molecule

21 8 The Interactions of Light and Pigments Molecules that absorb wavelengths in the visible range are called 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. helter-skelter/pink-floyd-dark-side.jpg color/images/reflected_light.jpg

22 8 The Interactions of Light and Pigments A molecule can absorb radiant energy of only certain wavelengths. If we plot the absorption by the compound as a function of wavelength, the result is an absorption spectrum. If absorption results in an activity of some sort (e.g., photosynthesis), then a plot of the effectiveness of the light as a function of wavelength is called an action spectrum.

23 Figure 8.6 Absorption and Action Spectra SubmergedPlants/Anacharis.jpg

24 8 The Interactions of Light and Pigments Plants have two predominant chlorophylls: chlorophyll a and chlorophyll b. These chlorophylls absorb blue and red wavelengths, which are near the ends of the visible spectrum. Other accessory pigments absorb photons between the red and blue wavelengths and then transfer a portion of that energy to chlorophylls. Examples of accessory pigments are the carotenoids, such as  -carotene.

25 Figure 8.7 The Molecular Structure of Chlorophyll

26 8 The Interactions of Light and Pigments A pigment molecule enters an excited state when it absorbs a photon. The excited state is unstable, and the molecule may return to the ground state. When this happens, some of the absorbed energy is given off as heat and the rest is given off as light energy, or fluorescence. If fluorescence does not occur, the pigment molecule may pass some of the absorbed energy to other pigment molecules.

27 8 The Interactions of Light and Pigments Pigments in photosynthetic organisms are arranged into antenna systems. In these systems, pigments are packed together and attached to thylakoid membrane proteins to enable the transfer of energy. The excitation energy is passed to the reaction center of the antenna complex. In plants, the pigment molecule in the center is always a molecule of chlorophyll a.

28 8 Chlorophyll Molecules and Thylakoids

29 Figure 8.8 Energy Transfer and Electron Transport

30 8 The Interactions of Light and Pigments Excited chlorophyll (Chl*) in the reaction center acts as a reducing agent. The electrons of an excited molecule are less tightly held by the nucleus, and more likely to be passed on in a redox reaction to an oxidizing agent. Chl* can react with an oxidizing agent in a reaction such as:  Chl* + A  Chl + + A –.

31 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation The energized electron that leaves the Chl* in the reaction center immediately participates in a series of redox reactions. The electron flows through a series of carriers in the thylakoid membrane, a process termed electron transport. Sound familiar? Two energy rich products of the light reactions, NADPH + H + and ATP, are the result. Chemiosmotic synthesis of ATP in the thylakoid membrane is called photophosphorylation.

32 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation There are two different systems for transport of electrons in photosynthesis.  Noncyclic electron transport produces NADPH + H + and ATP and O 2.  Cyclic electron transport produces only ATP. … and then we need to look at how the plant uses the ATP and NADPH.

33 8 A Preview A preview of what’s to come.

34 Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

35 Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)

36 Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP

37 Figure 8.11 Chloroplasts Form ATP Chemiosmotically

38 Figure 8.13 The Calvin-Benson Cycle

39 8 Until next time…

40 8 Announcements Examination 1. Next class! Come early; exams will be distributed so class can begin taking exam at 11:00 AM. No extended time at end. [50 multiple choice + 1 essay] Recommendation: Don’t change an answer unless you are absolutely sure you have a better one. Pre-Med Orientation Workshop (This Evening; 5:30-6:30 PM; TCC Alcove A)

41 Figure 8.6 Absorption and Action Spectra SubmergedPlants/Anacharis.jpg

42 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation There are two different systems for transport of electrons in photosynthesis.  Noncyclic electron transport produces NADPH + H + and ATP and O 2.  Cyclic electron transport produces only ATP. In noncyclic electron transport, two photosystems are required. Photosystems are light-driven molecular units that consist of many chlorophyll molecules and accessory pigments bound to proteins in separate energy- absorbing antenna systems.

43 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Photosystem II comes before Photosystem I; they are named this way because Photosystem I existed on Earth prior to the arrival of Photosystem II. Photosystem II uses light energy to oxidize water molecules, 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.

44 Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

45 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation 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.

46 Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)

47 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Cyclic electron transport produces only ATP by using a modified version of photosystem I. The process is called cyclic because the electron passed from an excited P 700 molecule cycles back to the same P 700 molecule. Water does not enter into the cyclic electron flow reactions, and no O 2 is released. The Calvin-Benson cycle uses more ATP than NADPH; noncyclic electron transport makes equal amounts of ATP and NADPH. Therefore, if the ratio of NADPH to NADP+ is high, cyclic electron transport kicks in to crank out only ATP.

48 Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP

49 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation ATP is produced by a chemiosmotic mechanism similar to that of mitochondria, called photophosphorylation. High-energy electrons move through a series of redox reactions, releasing energy that is used to transport protons across the membrane. Active proton transport results in the proton- motive force: a difference in pH and electric charge across the membrane.

50 8 The Light Reactions: Electron Transport, Reductions, and Photophosphorylation The electron carriers in the thylakoid membrane are oriented so as to move protons into the interior of the thylakoid, and the inside becomes acidic with respect to the outside. This difference in pH leads to the diffusion of H+ out of the thylakoid through specific protein channels, ATP synthases. The ATP synthases couple the formation of ATP to proton diffusion back across the thylakoid membrane.

51 8 Chemiosmosis Layered Figure:  Figure 8.11 Chloroplasts Form ATP Chemiosmotically

52 Figure 8.11 Chloroplasts Form ATP Chemiosmotically

53 8 Making Carbohydrate from CO2: The Calvin–Benson Cycle TIME TO MAKE SOME CARBOHYDRATES The reactions of the Calvin-Benson cycle take place in the stroma of the chloroplasts. This cycle does not use sunlight directly; but it requires the ATP and NADPH + H + produced in the light reactions, and these can not be “stockpiled”. Thus the Calvin-Benson reactions require light indirectly and take place only in the presence of light.

54 Figure 8.13 The Calvin-Benson Cycle

55 8 Making Carbohydrate from CO2: The Calvin–Benson Cycle Just to summarize the graphic… The Calvin–Benson cycle consists of three processes:  Fixation of CO 2, by combination with RuBP (catalyzed by rubisco)  Conversion of fixed CO 2 into carbohydrate (G3P) (this step uses ATP and NADPH)  Regeneration of the CO 2 acceptor RuBP by ATP

56 8 Making Carbohydrate from CO2: The Calvin–Benson Cycle The initial reaction of the Calvin–Benson cycle fixes one CO 2 into a 5-carbon compound, ribulose 1,5-bisphosphate (RuBP). An intermediate 6-carbon compound forms, which is unstable and breaks down to form two 3-carbon molecules of 3PG. The enzyme that catalyzes the fixation of CO 2 is ribulose bisphosphate carboxylase/oxygenase, called rubisco. Rubisco is the most abundant protein in the world.

57 8 Rubisco

58 Figure 8.14 RuBP Is the Carbon Dioxide Acceptor

59 8 Making Carbohydrate from CO2: 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.

60 8 Making Carbohydrate from CO2: The Calvin–Benson Cycle The products of the Calvin–Benson cycle are vitally important to the biosphere as they are the total energy yield from sunlight conversion by green plants. Most of the stored energy is released by glycolysis and cellular respiration by the plant itself. Some of the carbon of glucose becomes part of amino acids, lipids, and nucleic acids. Some of the stored energy is consumed by heterotrophs, where glycolysis and respiration release the stored energy.

61 8 Metabolic Pathways in Plants Green plants are autotrophs and can synthesize all their molecules from three simple starting materials: CO 2, H 2 O, and NH 4. To satisfy their need for ATP, plants, like all other organisms, carry out cellular respiration. Both aerobic respiration and fermentation can occur in plants, although respiration is more common. Cellular respiration takes place both in the dark and in the light.

62 Figure 8.18 Metabolic Interactions in a Plant Cell (Part 1)

63 Figure 8.18 Metabolic Interactions in a Plant Cell (Part 2)


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