Photosynthesis Overview

Transcription

1 Photosynthesis 1

2 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 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants chloroplasts

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

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

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

7 7 Pigments Molecules that absorb light energy in the visible range Light is a form of energy Photon particle of light Acts as a discrete bundle of energy Energy content of a photon is inversely proportional to the wavelength of the light Photoelectric effect removal of an electron from a molecule by light

8 8 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Increasing energy nm 1 nm 10 nm 1000 nm Increasing wavelength 0.01 cm 1 cm 1 m 100 m Gamma rays X-rays UV light Infrared Radio waves Visible light 400 nm 430 nm 500 nm 560 nm 600 nm 650 nm 740 nm

9 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 and efficiency of photons a molecule is capable of absorbing

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

11 11 Pigments in Photosynthesis Organisms have evolved a variety of different pigments Only two general types are used in green plant photosynthesis Chlorophylls Carotenoids In some organisms, other molecules also absorb light energy

12 12 Chlorophylls Chlorophyll a Main pigment in plants and cyanobacteria Only pigment that can act directly to convert light energy to chemical energy Absorbs violet-blue and red light Chlorophyll b Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

13 13 Photosystem Organization Antenna complex Hundreds of accessory pigment molecules Gather photons and feed the captured light energy to the reaction center Reaction center 1 or more chlorophyll a molecules Passes excited electrons out of the photosystem

14 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 14 Photon Chlorophyll molecule Photosystem e Electron donor e Electron acceptor Reaction center chlorophyll Thylakoid membrane

15 15 Reaction center Transmembrane protein pigment complex 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 Oxidized chlorophyll then fills its electron hole by oxidizing a donor molecule

16 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16 Light Electron donor Excited chlorophyll molecule Electron acceptor e e e e Chlorophyll reduced Donor oxidized Chlorophyll oxidized Acceptor reduced + + e e e e

17 Capture of light energy 17 Light-Dependent Reactions 1. Primary photoevent Photon of light is captured by a pigment molecule 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 Produces ATP

18 18 Cyclic photophosphorylation In sulfur bacteria, only one photosystem is used Generates ATP via electron transport Anoxygenic photosynthesis Excited electron passed to electron transport chain Generates a proton gradient for ATP synthesis

19 Energy of electrons Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 19 High Excited reaction center e Electron acceptor e b-c 1 complex ATP Photon Low Reaction center (P 870 ) e Electron acceptor Photosystem

20 20 Chloroplasts have two connected photosystems Oxygenic photosynthesis Photosystem I (P 700 ) Functions like sulfur bacteria Photosystem II (P 680 ) Can generate an oxidation potential high enough to oxidize water Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

21 21 Photosystem I transfers electrons ultimately to NADP +, producing NADPH Electrons lost from photosystem I are replaced by electrons from photosystem II Photosystem II oxidizes water to replace the electrons transferred to photosystem I 2 photosystems connected by cytochrome/ b 6 -f complex

22 22 Noncyclic photophosphorylation Plants use photosystems II and I in series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with electrons obtained by splitting water Z diagram

23 Energy of electrons Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 23 Excited reaction center 2 e 2 e 2. The electrons pass through the b 6 -f complex, which uses the energy released to pump protons across the thylakoid membrane. The proton gradient is used to produce ATP by chemiosmosis. Plastoquinone PQ b 6 -f complex H + Plastocyanin PC Excited reaction center 2 e 2 e Reaction center Ferredoxin Fd NADP + + H + Photon NADP reductase NADPH Photon Reaction center 2 e Proton gradient formed for ATP synthesis H 2 O 2H / 2 O 2 Photosystem I 3. A pair of chlorophylls in the reaction center absorb two photons. This excites two electrons that are passed to NADP +, reducing it to NADPH. Electron transport from photosystem II replaces these electrons. Photosystem II 1. A pair of chlorophylls in the reaction center absorb two photons of light. This excites two electrons that are transferred to plastoquinone (PQ). Loss of electrons from the reaction center produces an oxidation potential capable of oxidizing water.

24 24 Photosystem II Core of 10 transmembrane protein subunits with electron transfer components and two P 680 chlorophyll molecules Reaction center contains four manganese atoms Essential for the oxidation of water b 6 -f complex Proton pump embedded in thylakoid membrane

25 25 Photosystem I Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P 700 chlorophyll molecules Photosystem I accepts an electron from plastocyanin into the hole created by the exit of a light-energized electron Passes electrons to NADP + to form NADPH

26 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP + P i Light-Dependent Reactions ATP NADP 26 NADPH Photon Photon NADPH ADP H + ATP Calvin Cycle Thylakoid membrane Antenna complex H + + NADP + Fd 2 e Stroma PQ e e 2 e 2 2 H 2 O Water-splitting enzyme Thylakoid space 1 / 2 O 2 2H + Photosystem II Plastoquinone H + b 6 -f complex PC Plastocyanin Photosystem I Ferredoxin NADP reductase Proton gradient H + H + H + ATP synthase 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.

27 27 Chemiosmosis Electrochemical 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

28 28 Production of additional ATP Noncyclic photophosphorylation generates NADPH ATP Building organic molecules takes more energy than that alone Cyclic photophosphorylation used to produce additional ATP Short-circuit photosystem I to make a larger proton gradient to make more ATP

29 29 Carbon Fixation Calvin Cycle To build carbohydrates cells use Energy ATP from light-dependent reactions Cyclic and noncyclic photophosphorylation Drives endergonic reaction Reduction potential NADPH from photosystem I Source of protons and energetic electrons

30 30 Calvin cycle Named after Melvin Calvin ( ) 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

31 31 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

32 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP+ P i Light-Dependent Reactions NADP ATP + 32 NADPH 6 molecules of Carbon dioxide (CO 2 ) Stroma of chloroplast Calvin Cycle 6 molecules of Ribulose 1,5-bisphosphate (5C) (RuBP) Rubisco 12 molecules of 3-phosphoglycerate (3C) (PGA) 12 ATP 6 ADP 6 ATP Calvin Cycle 12 ADP 12 molecules of 1,3-bisphosphoglycerate (3C) 12 NADPH 4 P i 12 NADP + 10 molecules of Glyceraldehyde 3-phosphate (3C) 12 P i 12 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) 2 molecules of Glyceraldehyde 3-phosphate (3C) (G3P) Glucose and other sugars

33 33 Output of Calvin cycle Glucose is not a direct product of the Calvin cycle G3P is a 3 carbon sugar Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose Used to make starch Insoluble glucose polymer Stored for later use