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Modern Biology Chapter 6: Photosynthesis. Plant cell.

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Presentation on theme: "Modern Biology Chapter 6: Photosynthesis. Plant cell."— Presentation transcript:

1 Modern Biology Chapter 6: Photosynthesis

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3 Plant cell

4 6-1: Capturing the Energy in Light

5 Energy for life processes photosynthesis: process by which green plants convert solar energy into chemical energy –produces carbohydrates –produces oxygen

6 Energy for life processes chloroplast structure –double membrane surrounds entire organelle –thylakoids: flattened sacs inside double membrane –grana: stacks of thylakoids –stroma: fluid surrounding thylakoids inside double membrane

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8 Energy for life processes sunlight –provides heat and energy to earth white light from sun contains mixture of colors of light –wavelength of light determines its color –only a small portion of sun-light is visible to humans

9 The sun emits all visible wavelengths of light Green plants absorb red, orange, blue and violet They reflect yellow and green

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11 O2 To air C6H12O 6 To plant CO2 From Air H2O From Soil Light energy From Sun Overview of photosynthesis

12 Energy for life processes pigment: colored substance that reflects or absorbs light

13 Energy for life processes chlorophyll –type of pigment in thylakoid membranes two types of chlorophyll chlorophyll a absorbs light in red end of spectrum chlorophyll b absorbs light in blue end of the spectrum (accessory pigment) green light is not absorbed, but reflected giving the leaves the appearance of being green –by absorbing light pigments also absorb energy

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15 Energy for life processes Cartenoids: other accessory pigments –absorb different colors depending on chemical structure –become apparent when chlorophylls fade (fall colors)

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17 THE LIGHT REACTIONS

18 The light reactions consist of three basic components Photosystem 2 Photosystem 1 ATP synthase (chemiosmosis)

19 Photosystem 2 water-plastoquinone oxidoreductase Uses the energy from sunlight to split the water molecule into three parts 2H 2 O  4 H e - + O 2

20 Photosystem 1 plastocyanin: ferredoxin oxidoreductase Uses the energy from sunlight to move the electrons onto NADP + for transport to the next phase of the process

21 ATP synthase Synthesizes ATP using a concentration gradient created by photosystem II

22 Light reactions Light and the energy associated with it are absorbed into photosystem I and photosystem II

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24 Light-dependent reactions a.k.a. light reactions Electron transport occurs within membranes

25 Light-dependent reactions a.k.a. light reactions photosystem II (PSII) –accessory pigments absorb light and acquire energy (E) (step 1) –energy is passed along membrane pigments until it reaches a specific pair of chlorophyll a molecules

26 Light-dependent reactions a.k.a. light reactions photosystem II (PSII) –electron transport E forces e - to increase E level (e - are said to be “excited”) excited e - leave chlorophyll a chlorophyll a is oxidized PEA donates e - e- reduces primary e - acceptor (PEA) (step 2) e- transported down ETC (step 3) each transfer, the e- loses some E E is used to move p+ into thylakoid

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28 Light-dependent reactions a.k.a. light reactions photosystem I (PSI) –light absorbed by PSI (step 1b) –e - move from chlorophyll a to PEA (step 4) –e - lost are replaced by e - from PSII –PEA of PSI donates e - to NADP + (step 5) brings e - to edge of thylakoid membrane by stroma e - combine with p + and NADP + NADP + reduced to NADPH

29 Light-dependent reactions a.k.a. light reactions replacing e - (step 6) –recall e - from chlorophyll in PSII replace e - that leave chlorophyll on PSI –e - from PSII need to be replaced or both ETCs cease

30 Light-dependent reactions a.k.a. light reactions replacing e - (step 6) –replacement e - come from water enzyme in thylakoid splits water molecule 2H 2 O  4 H e - + O 2 p + (H + ) remain in thylakoid O 2 diffuses out and leaves the plant replace e - lost by chlorophyll in PSII

31 Summary of Light Reactions Summary: what is produced during the light reactions –p + concentration gradient –NADPH

32 Summary of Light Reactions Summary: what is produced during the light reactions –p + concentration gradient –NADPH

33 Chemiosmosis potential E from gradient is harnessed by ATP Synthase in thylakoid membrane –ATP Synthase serves two functions –Catalyzes ADP + (P)  ATP –Acts as carrier protein for p + as H + ions pass through ATP Synthase, ATP is produced

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35 Section 6.2: The Calvin cycle

36 Stomata Open Closed

37 Section 6.2: The Calvin cycle Light-independent reactions Many names –Calvin (or Calvin-Benson) cycle (men who first described cycle) –Dark reactions (does not directly require light) –Carbon fixation (incorporation of C into organic substances)

38 Section 6.2: The Calvin cycle sugars are long term energy storage (much more stable than ATP of NADP + ) requires carbon dioxide (CO 2 ) and water (H 2 O) –CO 2 enters plants through stomata (little tiny pores controlled by the plant) –H 2 O enters plant through osmosis, capillarity or stomata

39 Step 1 after diffusing into the stroma from the cytosol, CO 2 joins with a 5-C sugar (RuBP) to produce 2 3-C molecules of PGA (process is known as carbon-fixation)

40 Step 2 PGA is converted into PGAL –2PGA + 2ATP + 2NADPH  2PGAL + 2ADP + 2NADP + +2 phosphate

41 Step 3 and 3B Most PGAL converted back into RuBP –2PGAL + ATP  RuBP + ADP + phosphate + some fixed C Some PGAL leave Calvin cycle as fixed C (3B)

42 Balance Sheet Each turn of Calvin cycle results in fixation of 1 CO 2 Three times around Calvin cycle results in 1 PGAL –each turn requires 3 ATP and 2 NADPH 2 ATP from step 2 1 ATP from step 3 –3 turns requires 9 ATP and 6 NADPH

43 … PGAL and other organic molecules like carbohydrates are formed and then used all over the cell for a variety of functions. 6CO 2 + 6H 2 O + energy  C 6 H 12 O 6 + 6O 2 nt/chp08/ html

44 Alternative Pathways C 4 pathway –when CO 2 is low, enables plants to continue fixing carbon –grasses, corn –uses less water, but moves much more slowly

45 Alternative Pathways CAM pathway –when very hot and dry, enables plant to continue to fix carbon –cacti, pineapples –stomata open at night instead of during the day –uses less water, but moves much more slowly

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