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Plants and Photosynthesis

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Plants and Photosynthesis

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1 Plants and Photosynthesis




5 Photosynthesis Organisms Autotrophs: “Self Feeders” Photo-: Light
Chemo-: Oxidize inorganics (Ex: Sulfur, Ammonia), unique to bacteria Heterotrophs: “Other Feeders”

6 History Jean-Baptiste van Helmont (1600’s) grew willow tree
Weighed soil before and after Added only water Tree gained 75 kg No change in mass of soil Concluded: mass in plants comes from water

7 Site of Photosynthesis
Upper Epidermis Mesophyll Cells Lower Epidermis Vein Stoma

8 Site of Photosynthesis
Thylakoids Stroma Granum Inner & Outer Membranes

9 Photosynthesis Conversion of Light E into Chem E Light E
Travels in waves (photons) Wavelength (): crest to crest (measured in nm)  inversely related to frequency Higher frequency = more E Different  = different properties

10 Wavelength (nanometers)
Nature of Light Visible spectrum is ~380–750 nm Gamma Rays X-Rays UV Infrared Micro- waves Radio Waves Visible Light 400 450 500 550 600 650 700 750 Wavelength (nanometers)

11 Nature of Light Pigments absorb certain  and reflect or transmit others

12 Nature of Light Spectrophotometers measure amount of Light pigments absorb or reflect

13 Nature of Light Pigments Absorb and reflect light
Specific pigment = specific light Chlorophylls a and b – both absorb blues and reds a is 1 pigment for photosynthesis – focuses solar E onto a pair of e-s

14 Nature of Light Accessory pigments – funnel the E they collect to a central Chlorophyll A Carotenoids Carotenes – reflect oranges Xanthophylls – reflect yellows Phycocyanins – reflect blues Some accessory pigments provide photoprotection against excess light Carotenoids in human eyes serve same function

15 Absorption/Action Spectra
Visible Light 400 450 500 550 600 650 700 750 20 40 60 80 100 % Light Absorption Collectively Chlorophyll Carotenoids Phycocyanin 400 450 500 550 600 650 700 750 Wavelength (nanometers)

16 Engelmann’s Experiment
Simple experiment in 1883 Compare to action spectrum

17 Photosynthesis Can be divided into Light-dependent rxn
Makes E storing compounds NADPH and ATP to fuel L-i rxn Occurs in thylakoids Light-independent rxn Uses NADPH and ATP to produce glucose, a more stable form of E Occurs in stroma

18 Photosynthesis

19 Light-dependent rxn

20 Light-dependent rxn Light is absorbed in photosystem II, an “antenna complex” of hundreds of pigments that funnel E to a reaction center Rxn Center: central chlorophyll a molecule next to a protein, the 1° e- acceptor


22 Light-dependent rxn Chemi osmosis

23 Photosynthesis

24 Light-dependent rxn



27 Light-dependent rxn The e-s from the broken bonds slide down the ETC, slowly losing E The e-s are recharged by sunlight in photosystem I and are passed along more carrier proteins to NADP+, reducing it to NADPH

28 sun O2 Light-dependent H+ H+ H20 H+

29 Light-dependent sun sun O2 ADP ATP H+ H+ H20 H+

30 Light-dependent rxn summary
H2O is broken up by sunlight O2 is released as waste e-s flow down ETC, pump H+ ions, and finally make NADPH H+ ions diffuse across thylakoid membrane and help form ATP ATP and NADPH move on to the light-independent rxn

31 Photosynthesis


33 L-i rxn – C fixation

34 L-i rxn – Reduction 12 ATPs phosphorylate the 12 3PGs to form 12 1,3 bisphosphoglycerates A pair of e-s from NADPH reduces each 1,3 bisphosphoglycerate to glyceraldehyde-3-phosphate (G3P) The electrons reduce a carboxyl group to a carbonyl group

35 L-i rxn – Reduction

36 L-i rxn – Reduction Two G3Ps can now be removed from the cycle to make glucose or be used for as any other carb the plant cell needs


38 Light-independent rxn summary
Carbon Fixation CO2 needed to begin the process Synthesis of G3P (Glyceraldehyde 3 phosphate) ATP and NADPH are used Regeneration of 5C compound Need more ATP to reset the cylce

39 Photorespiration Stomata not only allow gas exchange, but transpiration also Hot, dry day – stomata close Problem: CO2 , O2  Rubisco can bind either CO2 OR O2 to RuBP When O2 binds, no useful cellular E is produced

40 Photorespiration When rubisco adds O2 to RuBP, RuBP splits into a 3-C piece and a 2-C piece The 2-C fragment is exported from the chloroplast and degraded to CO2 by mitochondria and peroxisomes Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle Up to 50% of the C fixed by Calvin cycle can be drained away on a hot, dry day

41 C4 Plants Mesophyll cells use PEP carboxylase to fix CO2 to phosphoenolpyruvate, forming oxaloacetate (4C) PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot - on hot, dry days with the stomata closed

42 C4 Plants Oxaloacetate then dumps the extra CO2 into the Calvin cycle in bundle-sheath cells Rubisco can then work with a high concentration of CO2, thus minimizing photorespiration C4 plants thrive in hot regions with intense sunlight Examples: sugar, corn

43 C4 Plants

44 CAM Plants Crassulacean Acid Metabolism
CO2 is fixed at night, but NO photosynthesis takes place at night During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids

45 CAM Plants Allows plants to keep their stomata closed during the hot, dry hours of day and open in the cooler hours of night Less water is lost in the process Less photorespiration occurs Ex: succulent plants, cacti, pineapples, and several other plant families

46 CAM Plants

47 Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle
In C4 plants, carbon fixation and the Calvin cycle are spatially separated In CAM plants, carbon fixation and the Calvin cycle are temporally separated Both eventually use the Calvin cycle to incorporate light energy into the production of sugar

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