photosynthesis: capturing energy photosynthesis: capturing energy chapter 9

Photosynthesis:Photosynthesis:Capturing EnergyCapturing Energy

Chapter 9

Learning Objective 1Learning Objective 1

• What are the physical properties of light?

• What is the relationship between a wavelength of light and its energy?

Electromagnetic SpectrumElectromagnetic Spectrum

Fig. 9-1, p. 192

One wavelength

Longer wavelength

TV and radio waves Red 700 nmMicro- waves

OrangeInfrared Color

spectrum of visible light

600 nm

YellowVisible

X-rays500 nm

Green

BlueGamma

raysViolet 400 nm

Electromagnetic spectrum Shorter wavelength

760 nm

380 nm

UV

LightLight

• Consists of particles (photons) that move as waves

• Photons with shorter wavelengths have more energy than those with longer wavelengths

SunlightSunlight

Fig. 9-2, p. 192

Sun

Sunlight is a mixture of many wavelengths

Light and EnergyLight and Energy

Fig. 9-3, p. 193

Photon Photon is absorbed by an excitable electron that moves into a higher energy level.

Low energy level

ElectronHigh energy level

Either

Electron acceptor molecule

The electron may return to ground level by emitting a less energetic photon.

The electron may be accepted by an electron acceptor molecule.

Or

KEY CONCEPTSKEY CONCEPTS

• Light energy powers photosynthesis, which is essential to plants and most life on Earth

Learning Objective 2Learning Objective 2

• What is the internal structure of a chloroplast?

• How do its components interact and facilitate the process of photosynthesis?

StructuresStructures

• Photosynthesis in plants• occurs in chloroplasts• located in mesophyll cells• inside the leaf

Leaf StructureLeaf Structure

Fig. 9-4a, p. 194

(a) This leaf cross section reveals that the mesophyll is the photosynthetic tissue. CO2 enters the leaf through tiny pores or stomata, and H2O is carried to the mesophyll in veins.

Palisade mesophyll

Vein

Air space

Stoma

Spongy mesophyll

Fig. 9-4b, p. 194

(b) Notice the numerous chloroplasts in this LM of plant cells.

Mesophyll cell

10 μm

Fig. 9-4c, p. 194

Outer membrane

Inner membrane

Stroma

1 μm

Intermembrane space

Thylakoid membrane

(c) In the chloroplast, pigments necessary for the light-capturing reactions of photosynthesis are part of thylakoid membranes, whereas the enzymes for the synthesis of carbohydrate molecules are in the stroma.

Granum (stack of thylakoids)

Thylakoid lumen

ChloroplastsChloroplasts

• Enclosed by a double membrane • inner membrane encloses stroma and

thylakoids

• Thylakoids • enclose thylakoid lumen• arranged in stacks (grana)

Photosynthetic PigmentsPhotosynthetic Pigments

• In thylakoid membranes• chlorophyll a• chlorophyll b • Carotenoids

Fig. 9-5, p. 195

Hydrocarbonside chain

Porphyrin ring(absorbs light)

in chlorophyll bin chlorophyll a

Learning Objective 3Learning Objective 3

• What happens to an electron in a biological molecule such as chlorophyll when a photon of light energy is absorbed?

Energizing ElectronsEnergizing Electrons

• Photons excite photosynthetic pigments• chlorophyll

• Energized electrons• move to electron acceptor compounds

Active WavelengthsActive Wavelengths

• Combined absorption spectra of chlorophylls a and b • action spectrum for photosynthesis

Absorption SpectraAbsorption Spectra

Fig. 9-6a, p. 195

Chlorophyll b

Chlorophyll a

Est

imat

ed a

bso

rpti

on

(%

)

Wavelength (nm)

(a) Chlorophylls a and b absorb light mainly in the blue (422 to 492 nm) and red (647 to 760 nm) regions.

Fig. 9-6b, p. 195

Rel

ativ

e ra

te o

f p

ho

tosy

nth

esis

Wavelength (nm)

(b) The action spectrum of photosynthesis indicates the effectiveness of various wavelengths of light in powering photosynthesis. Many plant species have action spectra for photosynthesis that resemble the generalized action spectrum shown here.

Action SpectrumAction Spectrum

Fig. 9-7a, p. 196

100 μm(a)

Fig. 9-7b, p. 196

Wavelength of light (nm)

Learning Objective 4Learning Objective 4

• Describe photosynthesis as a redox process

PhotosynthesisPhotosynthesis

• Light energy• is converted to chemical energy

(carbohydrates)

• Hydrogens from water• reduce carbon

• Oxygen from water• is oxidized, forming molecular oxygen

Learning Objective 5Learning Objective 5

• What is the difference between light-dependent reactions and carbon fixation reactions of photosynthesis?

Energy for ReactionsEnergy for Reactions

• Light-dependent reactions• light energizes electrons that generate ATP

and NADPH

• Carbon fixation reactions• use energy of ATP and NADPH to form

carbohydrate

PhotosynthesisPhotosynthesis

Fig. 9-8, p. 197

Light-dependent reactions (in thylakoids)

Carbon fixation reactions (in stroma)

Chloroplast

ATP

Light reactions

ADPCalvin cycle

NADPH

NADP+

H2O O2 CO2Carbohydrates

Stepped Art

Fig. 9-8, p. 197

Light-dependent reactions (in thylakoids)

Chloroplast

Carbon fixation reactions (in stroma)

Light reactions

ATP

ADP Calvin cycleNADPH

NADP+

H2O O2CO2 Carbohydrates

Learning Objective 6Learning Objective 6

• How do electrons flow through photosystems I and II in the noncyclic electron transport pathway?

• What products are produced?

• Contrast this with cyclic electron transport

The PhotosystemsThe Photosystems

• Photosystems I and II • photosynthetic units• include chlorophyll, accessory pigments• organized with pigment-binding proteins

into antenna complexes

A PhotosystemA Photosystem

Fig. 9-10, p. 198

Primary electron acceptor

e-

Chloroplast

Photon

Thylakoid membrane

Photosystem

Reaction CentersReaction Centers

• Reaction center of antenna complex• special pair of chlorophyll a molecules • release energized electrons to acceptor

• P700• reaction center for photosystem I

• P680• reaction center for photosystem II

Noncyclic Electron TransportNoncyclic Electron Transport

• Light-dependent reactions• form ATP and NADPH

Noncyclic SystemsNoncyclic Systems

• Electrons in photosystem I• energized by light• pass through electron transport chain • convert NADP+ to NADPH

• Redox reactions • pass energized electrons along ETC• from photosystem II to photosystem I

Noncyclic SystemsNoncyclic Systems

• Electrons given up by P700 (photosystem I) • replaced by electrons from P680

(photosystem II)

• Electrons given up by P680 (photosystem II)• replaced by electrons from photolysis of

H2O (releasing oxygen)

Cyclic Electron TransportCyclic Electron Transport

• Electrons from photosystem I• return to photosystem I

• ATP produced by chemiosmosis

• No NADPH or oxygen generated

Table 9-1, p. 200

KEY CONCEPTSKEY CONCEPTS

• Photosynthesis, which occurs in chloroplasts, is a redox process

Learning Objective 7Learning Objective 7

• Explain how a proton (H+) gradient is established across the thylakoid membrane and how this gradient functions in ATP synthesis

Proton GradientProton Gradient

Fig. 9-12, p. 200

Stroma

Thylakoid lumen

Thylakoid membrane Protons (H+)

PhotophosphorylationPhotophosphorylation

• Photophosphorylation • synthesis of ATP coupled to transport of

electrons energized by photons

• Electron energy pumps protons across thylakoid membrane • energy gradient generates ATP by

chemiosmosis

ATP SynthesisATP Synthesis

• ATP synthase• enzyme complex in thylakoid membrane• protons diffuse through enzyme• phosphorylate ADP to ATP

Electron Transport and Electron Transport and ChemiosmosisChemiosmosis

Fig. 9-13, p. 201

Thylakoid lumen

Thylakoid membrane

PhotosystemII

Photon

Thylakoidmembrane

Plastocyanin

Plastoquinone

Ferredoxin

Cytochromecomplex

Photosystem I

Photon

Ferredoxin-NADP+

reductase

NADPHNADP+

ADP Pi

ATP

ATPsynthase

KEY CONCEPTSKEY CONCEPTS

• Light-dependent reactions convert light energy to the chemical energy of NADPH and ATP

Learning Objective 8Learning Objective 8

• Summarize the three phases of the Calvin cycle, and the roles of ATP and NADPH

Calvin Cycle (CCalvin Cycle (C33 pathway) pathway)

• Carbon fixation reactions

• 3 phases• CO2 uptake phase• Carbon reduction phase• RuBP regeneration phase

COCO22 Uptake Phase Uptake Phase

• Enzyme rubisco • (ribulose bisphosphate carboxylase/

oxygenase)• combines CO2 with ribulose bisphosphate

(RuBP), a five-carbon sugar• forms 3-carbon phosphoglycerate (PGA)

Carbon Reduction PhaseCarbon Reduction Phase

• Energy of ATP and NADPH • convert PGA molecules to glyceraldehyde-

3-phosphate (G3P)

• For each 6 CO2 fixed• 12 G3P are produced• 2 G3P leave cycle to produce 1 glucose

RuBP Regeneration PhaseRuBP Regeneration Phase

• Remaining G3P molecules are modified to regenerate RuBP

The Calvin CycleThe Calvin Cycle

KEY CONCEPTSKEY CONCEPTS

• Carbon fixation reactions incorporate CO2 into organic molecules

Learning Objective 9Learning Objective 9

• How does photorespiration reduce photosynthetic efficiency?

PhotorespirationPhotorespiration

• C3 plants use O2 and generate CO2 • by degrading Calvin cycle intermediates• but do not produce ATP

• On bright, hot, dry days• plants close stomata, conserving water• prevents passage of CO2 into leaf

Learning Objective 10Learning Objective 10

• Compare the C4 and CAM pathways

CC44 Pathway Pathway

• Takes place in mesophyll cells

• Enzyme PEP carboxylase binds CO2

• CO2 fixed in oxaloacetate• converted to malate

• Malate moves into bundle sheath cell• CO2 is removed

• Released CO2 enters Calvin cycle

CC44 and C and C33 Plants Plants

Fig. 9-15a, p. 205

Upper epidermis

Palisade mesophyll

Bundle sheath cells of veins

Spongy mesophyll

Chloroplasts

(a) In C3 plants, the Calvin cycle takes place in the mesophyll cells and the bundle sheath cells are nonphotosynthetic.

Fig. 9-15b, p. 205

Upper epidermis

Bundle sheath cells of veins

Mesophyll

Chloroplasts

(b) In C4 plants, reactions that fix CO2 into four-carbon compounds take place in the mesophyll cells. The four-carbon compounds are transferred from the mesophyll cells to the photosynthetic bundle sheath cells, where the Calvin cycle takes place.

CC4 4 PathwayPathway

Fig. 9-16, p. 206

CO2

Mesophyll cell

Phosphoenol- pyruvate

Oxaloacetate

ADP NADPH

ATP NADP+

Pyruvate

Pyruvate Bundle sheath cellNADP+

CO2

Glucose NADPH

Vein

(4C)(3C)

(3C)Malate (4C)

Malate(3C) (4C)

(CAM) Pathway(CAM) Pathway

• Crassulacean acid metabolism (CAM)• similar to C4 pathway

• PEP carboxylase fixes carbon at night• in mesophyll cells

• Calvin cycle occurs during the day

A CAM PlantA CAM Plant

Learning Objective 11Learning Objective 11

• How do photoautotrophs and chemoheterotrophs differ with respect to their energy and carbon sources?

Energy SourcesEnergy Sources

• Photoautotrophs • use light as energy source• incorporate atmospheric CO2 into pre-

existing carbon skeletons

• Chemoheterotrophs • obtain energy by oxidizing chemicals• obtain carbon from other organisms

KEY CONCEPTSKEY CONCEPTS

• Most photosynthetic organisms are photoautotrophs

Learning Objective 12Learning Objective 12

• What is the importance of photosynthesis to plants and other organisms?

PhotosynthesisPhotosynthesis

• Ultimate source of all chemical energy and organic molecules• available to plants and other organisms

• Replenishes oxygen in the atmosphere• vital to all aerobic organisms

Summary ReactionsSummary Reactions

• Light-dependent reactions (noncyclic electron transport)

12 H2O + 12 NADP+ + 18 ADP + 18 Pi

→ (light energy, chlorophyll) →

6 O2 + 12 NADPH + 18 ATP

Summary ReactionsSummary Reactions

• Carbon fixation reactions (Calvin cycle)

12 NADPH + 18 ATP + 6 CO2 →

C6H12O6 + 12 NADP+ + 18 ADP

+ 18 Pi + 6 H2O

Summary ReactionsSummary Reactions

• Overall equation for photosynthesis

6 CO2 +12 H2O

→ (light energy, chlorophyll) →

C6H12O6 + 6 O2 + 6 H2O

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Calvin-Benson CycleCalvin-Benson Cycle

Sites of PhotosynthesisSites of Photosynthesis

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Wavelengths of LightWavelengths of Light

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Photosynthesis OverviewPhotosynthesis Overview

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