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Chapter 10 notes Photosynthesis. Photosynthesis In Nature All life acquires organic compounds for energy and carbon skeletons by one of two ways - Autotrophs:

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Presentation on theme: "Chapter 10 notes Photosynthesis. Photosynthesis In Nature All life acquires organic compounds for energy and carbon skeletons by one of two ways - Autotrophs:"— Presentation transcript:

1 Chapter 10 notes Photosynthesis

2 Photosynthesis In Nature All life acquires organic compounds for energy and carbon skeletons by one of two ways - Autotrophs: (autos = self, trophos = feed) - they sustain themselves w/out eating other organisms

3 Photosynthesis In Nature - plants are photoautotrophs because they use light as a source of energy - Heterotrophs: (hetero = other, different) - live on compounds produced by other organisms Heterotrophs are dependent on photoautotrophs for food and oxygen

4 Photosynthesis In Nature Chloroplasts are the site of photosynthesis in plants All green parts of plants have chloroplasts - color is from chlorophyll, the green pigment in the chloroplasts - chloroplasts are mainly found in the mesophyll, interior tissue of the leaf

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6 Photosynthesis In Nature - CO 2 enters/O 2 leaves the leaf through pores called stomata - water is absorbed through the roots 6CO 2 +6H 2 O+light  C 6 H 12 O 6 +6O 2

7 Pathways of Photosynthesis The oxygen produced from photosynthesis comes from the spliting of water, not CO 2 - use of oxygen-18 to trace pathway Hydrogen is extracted from water to make sugar - O 2 is a waste product

8 Pathways of Photosynthesis

9 Photosynthesis is made of two processes, each containing several steps 1) Light reaction (AKA light harvesting reaction, light dependent reaction) 2) Calvin cycle (AKA dark reaction, carbon fixation reaction)

10 Pathways of Photosynthesis The light rxn. converts solar energy to chemical energy; occurs in the thylokoid membrane - light absorbed drives a transfer of e- and hydrogen to an acceptor, NADP + - NADP + is reduced to NADPH - ATP is generated from ADP through photophosphorylation

11 Pathways of Photosynthesis The Calvin cycle stores the chemical energy in the form of sugar; occurs in the stroma - CO 2 is incorporated w/ other molecules in the chloroplast through carbon fixation - fixed carbon is reduced to carbohydrates

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16 Pathways of Photosynthesis Light is a form of energy known as electromagnetic energy; travels in waves - wavelength is the distance between crests of electromagnetic waves - electromagnetic spectrum: the entire range of radiation

17 Pathways of Photosynthesis Visible light is the band that ranges from 380 to 750 nm in wavelength - detected as colors by the human eye The amount of energy is inversely related to the wavelength - shorter the wavelength, greater the energy

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19 Pathways of Photosynthesis In the chloroplasts light is absorbed mainly by chlorophyll a. - blue and red light is primarily absorbed - green is the least absorbent color Chlorophyll b absorbs light at slightly different wavelengths

20 Pathways of Photosynthesis

21 Chloroplasts also contain accessory pigments, carotenoids - may broaden the spectrum of colors that can drive photosynthesis - act in photoprotection: they absorb excessive light that might damage chlorophyll

22 Pathways of Photosynthesis Light Dependent Reaction Inside the thylakoid, chlorophyll is organized w/ proteins into photosystems - light gathering “antenna complex” Two types of photosystems - PS II, PS I

23 Pathways of Photosynthesis PS II = P680 - best at absorbing light w/ a wavelength of 680 nm PS I = P700 - best at absorbing light w/ a wavelength of 700 nm

24 Pathways of Photosynthesis Light drives the synthesis of ATP and NADPH by energizing e- in the photosystems Two possible routes for e- flow - cyclic - noncyclic

25 Pathways of Photosynthesis Noncyclic electron flow: - predominant route 1) P680 absorbs light, and splits a water molecule 2) e- absorb the energy and then cascade down an ETC from PS II to PS I 3) ATP is formed by noncyclic photophosphorylaton

26 Pathways of Photosynthesis 4) P700 absorbs more light to reenergize the e- so it can pass down another ETC 5) NADP + is the final e- accepter as it is reduced to NADPH (through redox rxns.)

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32 Cyclic electron flow: - uses PS I, but not PS II - makes ATP, but not NADPH or oxygen - cyclic photophosphorylation - because the Calvin cycle uses more ATP than NADPH, cyclic e- flow makes up the difference

33 Pathways of Photosynthesis

34 Chloroplasts and mitochondria both generate ATP through chemiosmosis - both ETC’s pump protons across the membrane during the redox rxns. (transforms redox energy into proton motive force) - both use ATP synthase to couple diffusion to phosphorylation

35 Pathways of Photosynthesis Differences between oxidative phosphorylation and photophosphorylaton - in mitochondria, high energy e- are from food - in chlorplasts, photosystems capture light energy to drive e- transfer

36 Pathways of Photosynthesis Difference in the spatial organization - mitochondria: the inner membrane pumps protons from the matrix to the innermembrane space - chloroplasts: thylakoid pumps protons to the thylakoid space from the stroma

37 Pathways of Photosynthesis

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39 Calvin Cycle - uses ATP and NADPH to incorporate CO 2 into sugar (does not make glucose but a 3 carbon sugar called G3P) - 3 steps to the Calvin cycle

40 Pathways of Photosynthesis Step 1: Carbon fixation - CO 2 is attached to RuBP (rxn. is catalyzed by rubisco) - a 6-carbon intermediate is created  splits into 2 3-carbon molecules Step 2: Reduction - ATP and NADPH are used to transform the molecule into G3P

41 Pathways of Photosynthesis - for every three molecules of CO 2, there are 6 molecules of G3P; only 1 is a net gain Step 3: Regeneration of RuBP - the remaining 5 molecule of G3P are reconverted back into RuBP to continue the Calvin cycle

42 Pathways of Photosynthesis In hot, dry environments, plants have adapted to maximize photosynthesis, while limiting water loss (transpiration) Most plants will close the stomata to prevent water loss, but this limits CO 2 intake

43 Pathways of Photosynthesis

44 In most plants, carbon fixation leads to a 3-carbon compound; these plants are called C 3 plants - ex. rice, wheat, soy - in hot weather, the stomata close which deprives the plant of CO 2 ; rubisco can accept O 2 instead of CO 2

45 Pathways of Photosynthesis - when oxygen binds to rubisco, photorespiration occurs, not photosynthesis - process break the resulting compound down into CO 2 - photorespiration does not make ATP (like cellular respiration) nor food (like photosynthesis)

46 Pathways of Photosynthesis Plants have adapted by developing C 4 photosynthesis and CAM C 4 plants preface the Calvin cycle w/ an alternate mode of carbon fixation - ex. Sugarcane, corn, grasses - form a 4-carbon compound as its first product

47 Pathways of Photosynthesis In C 4 plants there are 2 types of photosynthetic cells - Bundle-sheath cells are arranged into tightly packed sheaths around the veins; mesophyll cells are more loosely arranged

48 Pathways of Photosynthesis Step 1: In the mesophyll cells, PEP carboxylase adds CO 2 to PEP (phosphoenolpyruvate); has a high affinity for CO 2 ; forms the 4-carbon product Step 2: 4-carbon compounds move from mesophyll cells to the bundle-sheath

49 Pathways of Photosynthesis Step 3: Within the bundle-sheath, the 4- carbon compound releases CO 2 which then is combined via rubisco The mesophyll cells pump CO 2 into the bundle-sheath so the [CO 2 ] is high enough for rubisco to accept CO 2 and not oxygen

50 Pathways of Photosynthesis

51 CAM plants open their stomata at night, but keep them closed during the day - found in succulent plants - at night the CO 2 is incorporated into other organic acids; this mode of carbon fixation is called crassulacean acid metabolism (CAM)

52 Pathways of Photosynthesis The mesophyll cells of CAM plants store the acids in their vacuoles until they are needed the following day C 4 and cam are similar in that CO 2 is first incorporated into an intermediate before entering the Calvin cycle However, C 4 plants have a spatial separation of steps, while CAM is temporal

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