Photosynthesis 1

2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2

Chloroplast Structure Thylakoid membrane – internal membrane –Contains chlorophyll and other photosynthetic pigments –Pigments clustered into photosystems Grana – stacks of flattened sacs of thylakoid membrane Stroma – semiliquid surrounding thylakoid membranes 3

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Vascular bundleStoma Cuticle Epidermis Mesophyll Chloroplast Inner membrane Outer membrane Cell wall 1.58 mm Vacuole Courtesy Dr. Kenneth Miller, Brown University

5 Stages Light-dependent reactions –Require light 1.Capture energy from sunlight 2.Make ATP and reduce NADP + to NADPH Carbon fixation reactions or light- independent reactions –Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO 2

6 O2O2 Stroma Photosystem Thylakoid NADP + ADP + P i CO 2 Sunlight Photosystem Light-Dependent Reactions Calvin Cycle Organic molecules O2O2 ATP NADPH H2OH2O Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7 Pigments & Light Pigments are molecules that absorb light energy in the visible range Photon – particle of light –Energy of photons vary with the wavelenth of the light. (inverse relationship) Photoelectric effect – removal of an electron from a molecule by light

8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 400 nm Visible light 430 nm500 nm560 nm600 nm650 nm740 nm 1 nm0.001 nm10 nm1000 nm Increasing wavelength Increasing energy 0.01 cm1 cm1 m Radio wavesInfraredX-raysGamma rays 100 m UV light

9 Absorption spectrum When a photon strikes a molecule, its energy is either –Lost as heat –Absorbed by the electrons of the molecule Boosts electrons into higher energy level Absorption spectrum – range of photons (by wavelength) a molecule is capable of absorbing

10 Wavelength (nm) Light Absorbtion low high carotenoids chlorophyll a chlorophyll b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Only two general types are used in green plant photosynthesis –Chlorophylls –Carotenoids 11 Pigments in Photosynthesis

Chlorophylls Chlorophyll a –Main pigment in plants –Absorbs violet-blue and red light Chlorophyll b –Secondary pigment –absorbs light wavelengths that chlorophyll a does not absorb 12

Structure of chlorophyll 13 H2CH2C CH CH 2 CH 3 H H H C O CH CCH 3 CHCH 3 CH 2 CHCH 3 CH 2 CHCH 3 CH 3 O CO 2 CH 3 O NN NN Mg H H Chlorophyll a: = CH 3 Chlorophyll b: = CHO R R R H Porphyrin head H3CH3C H3CH3C CH 3 CH 2 Hydrocarbon tail Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Action spectrum –Relative effectiveness of different wavelengths of light in promoting photosynthesis –Corresponds to the absorption spectrum for chlorophylls 14 Light Absorbtion low high Oxygen-seeking bacteria Filament of green algae Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Carotenoids –Absorb blue and violet wavelengths –Reflect red orange and yellow wavelengths 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oak leaf in summer Oak leaf in autumn © Eric Soder/pixsource.com

16 Photosystem Organization Antenna complex –Gather photons and feed the captured light energy to the reaction center Reaction center (membrane proteins) –1 or more chlorophyll a molecules –Passes excited electrons out of the photosystem

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. e–e– Photon Photosystem Thylakoid membrane Chlorophyll molecule Electron acceptor Reaction center chlorophyll Thylakoid membrane Electron donor e–e–

Reaction center Transmembrane proteins used When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level Light-energized electron can be transferred to the primary electron acceptor, reducing it 18

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Light e–e– – + – + Excited chlorophyll molecule Electron donor Electron acceptor Chlorophyll reduced Chlorophyll oxidized Donor oxidized Acceptor reduced e–e– e–e– e–e– e–e– e–e– e–e– e–e–

20 Light-Dependent Reactions 1.Light Capture –Photon of light is captured by a pigment molecule –electron excited 2.Charge separation –Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3.Electron transport –Electrons move through carriers to reduce NADP + 4.Chemiosmosis – diffusion of H+ ions across the membrane –Produces ATP using ATP synthase Capture of light energy

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Energy of electrons High Low e–e– Photon Photosystem Excited reaction center Electron acceptor Electron acceptor Reaction center (P 870 ) b-c 1 complex ATP e–e– e–e–

22 Chloroplasts have two connected photosystems Photosystem I (P 700 ) Photosystem II (P 680 ) –Membrane proteins –Working together, the two photosystems carry out a transfer of electrons that is used to generate both ATP and NADPH –Photosystems replenished with electrons obtained by splitting water Wavelength of light used (in nm)

23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Photosystem IIPhotosystem Ib 6 -f complex Stroma Plastoquinone Proton gradient PlastocyaninFerredoxin H+H+ H+H+ H+H+ H+H+ NADPH ATP ADP +NADP + NADPH NADP ATP ADP + P i Calvin Cycle Photon H2OH2O e–e– e–e– e–e– Fd PC PQ 1. Photosystem II absorbs photons, exciting electrons that are passed to plastoquinone (PQ). Electrons lost from photosystem II are replaced by the oxidation of water, producing O 2 2. The b 6 -f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b 6 -f complex to pump protons into the thylakoid. 3. Photosystem I absorbs photons, exciting electrons that are passed through a carrier to reduce NADP + to NADPH. These electrons are replaced by electron transport from photosystem II. 4. ATP synthase uses the proton gradient to synthesize ATP from ADP and P i enzyme acts as a channel for protons to diffuse back into the stroma using this energy to drive the synthesis of ATP. NADP reductase ATP synthase 1/2O21/2O2 2H + Water-splitting enzyme Thylakoid space Antenna complex Thylakoid membrane Light-Dependent Reactions H+H+ H+H+ e–e–

Chemiosmosis Proton, (H+), gradient can be used to synthesize ATP Chloroplast has ATP synthase enzymes in the thylakoid membrane –Allows protons back into stroma Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions 24

25 Carbon Fixation – Calvin Cycle To build carbohydrates cells use Energy –ATP from light-dependent reactions –Drives endergonic reaction Reduction potential –NADPH from photosystem I –Source of protons and energetic electrons

26 Calvin cycle Named after Melvin Calvin (1911–1997) Also called C 3 photosynthesis Key step is attachment of CO 2 to RuBP to form PGA Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 4 PiPi 12 NADP ADP NADPH NADP + ADP+PiPi Light-Dependent Reactions Calvin Cycle 6 molecules of 12 molecules of 1,3-bisphosphoglycerate (3C) 12 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) 10 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) Stroma of chloroplast 6 molecules of Carbon dioxide (CO 2 ) 12 ATP 6 ADP 6 ATP Rubisco Calvin Cycle PiPi Ribulose 1,5-bisphosphate (5C) (RuBP) 3-phosphoglycerate (3C) (PGA) Glyceraldehyde 3-phosphate (3C) 2 molecules of Glucose and other sugars 12 NADPH ATP

28 3 phases 1.Carbon fixation –RuBP + CO 2 → PGA 2.Reduction –PGA is reduced to G3P 3.Regeneration of RuBP –PGA is used to regenerate RuBP 3 turns incorporate enough carbon to produce a new G3P 6 turns incorporate enough carbon for 1 glucose

29 Output of Calvin cycle Glucose is not a direct product of the Calvin cycle Glyceraldehyde 3-phosphate is produced –G3P is a 3 carbon sugar –Used to form glucose and sucrose Major transport sugar in plants Disaccharide made of fructose and glucose –Used to make starch Insoluble glucose polymer Stored for later use

30 Photorespiration Rubisco has 2 enzymatic activities –Carboxylation Addition of CO 2 to RuBP Favored under normal conditions –Photorespiration Oxidation of RuBP by the addition of O 2 Favored when stoma are closed in hot conditions Creates low-CO 2 and high-O 2 CO 2 and O 2 compete for the active site on RuBP

31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Heat Stomata O2O2 O2O2 CO 2 Under hot, arid conditions, leaves lose water by evaporation through openings in the leaves called stomata. The stomata close to conserve water but as a result, O 2 builds up inside the leaves, and CO 2 cannot enter the leaves. Leaf epidermis H2OH2OH2OH2O

32 Types of photosynthesis C 3 –Plants that fix carbon using only C 3 photosynthesis (the Calvin cycle) C 4 and CAM –Add CO 2 to PEP to form 4 carbon molecule –Use PEP carboxylase –Greater affinity for CO 2, no oxidase activity –C 4 – spatial solution –CAM – temporal solution

33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CO 2 RuBP 3PG (C 3 ) a. C 4 pathway Bundle-sheath cellMesophyll cell Stoma Vein G3P b. C 4 pathway Stoma Vein Mesophyll cell G3P CO 2 C4C4 Bundle- sheath cell Mesophyll cell Bundle- sheath cell Calvin Cycle Mesophyll cell Calvin Cycle a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.

34 C 4 plants Corn, sugarcane, sorghum, and a number of other grasses (monocots) Initially fix carbon using PEP carboxylase in mesophyll cells Produces oxaloacetate, converted to malate, transported to bundle-sheath cells Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO 2 Carbon fixation then by rubisco and the Calvin cycle

35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oxaloacetate PyruvateMalate Glucose Malate Pyruvate + P i Mesophyll cell Phosphoenolpyruvate (PEP) Bundle-sheath cell Calvin Cycle AMP + PP i ATP CO 2

C 4 pathway, although it overcomes the problems of photorespiration, does have a cost To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone C 4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C 3 pathway alone 36

37 CAM plants Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups Stomata open during the night and close during the day –Reverse of that in most plants Fix CO 2 using PEP carboxylase during the night and store in vacuole

When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO 2 High levels of CO 2 drive the Calvin cycle and minimize photorespiration 38

39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. night day CO 2 C4C4 G3P Calvin Cycle (inset): © 2011 Jessica Solomatenko/Getty Images RF

Compare C 4 and CAM Both use both C 3 and C 4 pathways C 4 – two pathways occur in different cells CAM – C 4 pathway at night and the C 3 pathway during the day 40