Chapter 10 Photosynthesis.

Chapter 10 Photosynthesis

Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these cells allows for the chemical reactions of photosynthesis

I. Chloroplasts: Photosynthesis in Plants
Leaves = major locations of photo Chlorophyll = green pigment w/in chloroplasts, absorbs light energy to make organic molecules in chloroplast CO2 enters and O2 exits leaf through microscopic pores = stomata

Found mainly in cells of the mesophyll (interior tissue of leaf)
Typical mesophyll cell has 30–40 chloroplasts Chlorophyll is in membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Stroma - dense fluid inside chloroplasts

Fig. 10-3a Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 10.3 Zooming in on the location of photosynthesis in a plant 5 µm

Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum
Fig. 10-3b Chloroplast Outer membrane Thylakoid Intermembrane space Stroma Granum Thylakoid space Inner membrane Figure 10.3 Zooming in on the location of photosynthesis in a plant 1 µm

II. Tracking Atoms Through Photosynthesis
Photosynthesis summarized as: 6 CO H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O

A. The Splitting of Water
Chloroplasts split H2O into H and O, incorporating the electrons of H into sugar molecules Reactants: 6 CO2 Products: 12 H2O 6 O2 6 H2O C6H12O6 Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

III. Two Stages of Photosynthesis: A Preview
Photosynthesis = light reactions (photo part) and Calvin cycle (synthesis part) Light reactions (in the thylakoids): Split H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation

Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH. Begins with carbon fixation, incorporating CO2 into organic molecules

i Light NADP+ ADP Light Reactions Chloroplast H2O + P Fig. 10-5-1
Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle Chloroplast

i Light NADP+ ADP Light Reactions ATP NADPH Chloroplast O2 H2O + P
Fig H2O Light NADP+ ADP + P i Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast O2

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH
Fig H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast O2

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH
Fig H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast [CH2O] (sugar) O2

Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
Chloroplasts are solar-powered chemical factories Thylakoids transform light energy into chemical energy of ATP and NADPH

IV. Nature of Sunlight Light = form of electromagnetic energy, also called electromagnetic radiation Light travels in waves Wavelength = dist between crests of waves Wavelength determines the type of electromagnetic energy

Electromagnetic spectrum = range of electromagnetic energy (radiation)
Visible light = wavelengths (inc. those for photosynthesis) that produce colors we can see Light also acts as though it consists of small particles (photons)

1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Micro- waves
Fig. 10-6 1 m (109 nm) 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 103 m Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Figure 10.6 The electromagnetic spectrum 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy

V. Photosynthetic Pigments
Absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green b/c chlorophyll reflects and transmits green light

Light Reflected light Chloroplast Absorbed Granum light Transmitted
Fig. 10-7 Light Reflected light Chloroplast Figure 10.7 Why leaves are green: interaction of light with chloroplasts Absorbed light Granum Transmitted light

Spectrophotometer = measures a pigment’s ability to absorb various wavelengths
Galvanometer Slit moves to pass light of selected wavelength White light Green Blue The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. The high transmittance (low absorption) very little green light. Refracting prism Photoelectric tube Chlorophyll solution 1 2 3 4

Absorption spectrum = graph plotting a pigment’s light absorption vs wavelength
Absorption spectrum of chlorophyll a suggests violet-blue and red light work best Action spectrum profiles the effectiveness of different wavelengths of radiation in a process For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.

RESULTS Fig. 10-9 Chloro- Chlorophyll b Absorption of light by
phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) (measured by O2 release) Rate of photosynthesis Figure 10.9 Which wavelengths of light are most effective in driving photosynthesis? (b) Action spectrum Aerobic bacteria Filament of alga (c) Engelmann’s experiment 400 500 600 700

Chlorophyll a = main photosynthetic pigment Accessory pigments
chlorophyll b = broaden spectrum used for photosynthesis Carotenoids = absorb excessive light that would damage chlorophyll Overview Photosynthesis Structures

interacts with hydrophobic regions of proteins inside
Fig CH3 in chlorophyll a CHO in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Figure Structure of chlorophyll molecules in chloroplasts of plants Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

VI. Excitation of Chlorophyll by Light
Pigment absorbs light, it goes from a ground state to an excited state (unstable) Excited e-s fall back to ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence
Fig Excited state e– Heat Energy of electron Photon (fluorescence) Photon Figure Excitation of isolated chlorophyll by light Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

VII. Photosystem Photosystem = a reaction-center complex (protein) surrounded by light-harvesting complexes Light-harvesting complexes (pigment molecules bound to proteins) funnel energy of photons to reaction center

Primary electron acceptor in reaction center accepts an excited e- from chlorophyll a
This solar-powered transfer of an e- from chlorophyll a to PEA is first step of light reactions

(INTERIOR OF THYLAKOID)
Fig Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Figure How a photosystem harvests light Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Two types of photosystems in thylakoid membrane
Photosystem II (PS II) functions first (# reflects order of discovery) and is best at absorbing a wavelength of 680 nm The reaction-center chlorophyll a of PS II is called P680 Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 Photosynthesis Part 1 Bleier (15 min)

VIII. Linear Electron Flow
During light reactions, there are two possible routes for e- flow: cyclic and linear Linear electron flow (primary pathway) involves both photosystems and produces ATP and NADPH using light energy

Photon hits pigment and its energy is passed among pigment molecules until it excites P680
An excited e- from P680 is transferred to the primary electron acceptor Pigment molecules Light P680 2 1 Photosystem II (PS II) Primary acceptor

P680+ (P680 missing an e-) is a very strong oxidizing agent
H2O is split by enzymes, and e-s are transferred from H atoms to P680+, reducing it to P680 O2 is released as a by-product

Fig. 10-13-2 Primary acceptor
H2O e– 2 H+ + 3 1/2 O2 e– e– P680 1 Light Figure How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem II (PS II)

Each e- “falls” down an electron transport chain from primary electron acceptor of PS II to PS I
Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane Diffusion of H+ (protons) across the membrane drives ATP synthesis (sound familiar???)

Electron transport chain
Fig Primary acceptor 4 Electron transport chain Pq 2 H2O e– Cytochrome complex 2 H+ + O2 3 1/2 Pc e– e– 5 P680 1 Light ATP Figure How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem II (PS II)

PS I (like PS II), transferred light energy excites P700, which loses an e- to an electron acceptor
P700+ (P700 missing an e-) accepts an electron passed down from PS II via the electron transport chain

Fig Primary acceptor Primary acceptor 4 Electron transport chain Pq e– 2 H2O e– Cytochrome complex 2 H+ + 3 1/2 O2 Pc e– e– P700 5 P680 Light 1 Light 6 ATP Figure How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) e-s are then transferred to NADP+ and reduce it to NADPH e-s of NADPH are available for the Calvin cycle Photo ETC and ATP Production

Fig Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 3 1/2 O2 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Figure How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

ATP NADPH Mill makes ATP Photosystem II Photosystem I e– e– e– e– e–
Fig e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon Figure A mechanical analogy for the light reactions e– Photon Photosystem II Photosystem I

IX. Cyclic Electron Flow
Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH Generates surplus ATP, satisfying the higher demand in the Calvin cycle

Primary acceptor Primary Fd acceptor Fd NADP+ Pq + H+ NADP+ reductase
Fig Primary acceptor Primary acceptor Fd Fd NADP+ + H+ Pq NADP+ reductase Cytochrome complex NADPH Pc Figure Cyclic electron flow Photosystem I Photosystem II ATP

Some organisms (purple sulfur bacteria) have PS I but not PS II
Cyclic electron flow is thought to have evolved before linear electron flow and may protect cells from light-induced damage Cyclic vs Linear E- Flow

X. Comparison of Chloroplasts and Mitochondria
Both generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into chemical energy of ATP Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

In mitochondria, protons are pumped OUT to intermembrane space and drive ATP synthesis as they diffuse back INTO mitochondrial matrix In chloroplasts, protons are pumped INTO thylakoid space and drive ATP synthesis as they diffuse back OUT to the stroma

H+ Diffusion Electron transport chain ADP + P
Fig Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

ATP and NADPH are produced on the side facing the stroma, where Calvin cycle takes place
In summary, light reactions generate ATP and increase the potential energy of e-s by moving them from H2O to NADPH

Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex
Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

XI. Calvin Cycle The Calvin cycle, like citric acid cycle, regenerates its starting material after molecules enter and leave the cycle Builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Calvin Cycle

The Calvin cycle has three phases:
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2 The Calvin cycle has three phases: Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP)

Phase 1: Carbon fixation Ribulose bisphosphate
Fig Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Figure The Calvin cycle

Figure 10.18 The Calvin cycle
Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i Figure The Calvin cycle 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

Figure 10.18 The Calvin cycle
Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 3 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i Figure The Calvin cycle 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

Photosynthesis Part 2 Bleier (15 min)

XII. Alternative carbon fixation
Dehydration is a problem for plants Hot, dry days, plants close stomata, which conserves H2O but limits photosynthesis This reduces access to CO2 and causes O2 to build up Favor a wasteful process called photorespiration

XIII. Photorespiration
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (thus the C3) In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle Photoresp consumes O2 and organic fuel and releases CO2 w/o producing ATP or sugar

May be an evolutionary relic because rubisco first evolved at a time when atmosphere had far less O2 and more CO2 Limits damaging products of light reactions that build up in the absence of the Calvin cycle Photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

XIV. C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells Requires enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2
Fig C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell Oxaloacetate (4C) PEP (3C) Vein (vascular tissue) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell Stoma CO2 Calvin Cycle Figure C4 leaf anatomy and the C4 pathway Sugar Vascular tissue

CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

(a) Spatial separation of steps (b) Temporal separation of steps
Fig Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell 1 CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Figure C4 and CAM photosynthesis compared Bundle- sheath cell CO2 CO2 Day 2 Organic acids release CO2 to Calvin cycle Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps

XV. Photosynthesis: A Review
Sunlight gets stored as chemical energy in organic compounds Sugar supplies chemical energy and carbon skeletons to synthesize molecules of cells Plants store excess sugar as starch in roots, tubers, seeds, and fruits Produces the O2 in our atmosphere Overview - Anderson (13 min)

Fig H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)

Electron transport chain Electron transport chain NADP+ + H+
Fig. 10-UN1 H2O CO2 Primary acceptor Electron transport chain Primary acceptor Electron transport chain Fd NADP+ + H+ H2O Pq NADP+ reductase O2 Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II O2

3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C
Fig. 10-UN2 3 CO2 Carbon fixation 3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Reduction 1 G3P (3C)

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