photosynthesis: using light to make food

© 2010 Pearson Education, Inc.

Lectures by Chris C. Romero, updated by Edward J. Zalisko

PowerPoint® Lectures forCampbell Essential Biology, Fourth Edition – Eric Simon, Jane Reece, and Jean DickeyCampbell Essential Biology with Physiology, Third Edition – Eric Simon, Jane Reece, and Jean Dickey

Chapter 7Chapter 70

Photosynthesis: Using Light to Make Food

Biology and Society: Green Energy• Wood has historically been the main fuel used to:

– Cook

– Warm homes

– Provide light at night


Figure 7.0


• Industrialized societies replaced wood with fossil fuels.

• To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources.


• Fast-growing trees, such as willows:

– Can be cut every three years

– Do not need to be replanted

– Are a renewable energy source

– Produce fewer sulfur compounds

– Reduce erosion

– Provide habitat for wildlife


THE BASICS OF PHOTOSYNTHESIS• Photosynthesis:

– Is used by plants, some protists, and some bacteria

– Transforms light energy into chemical energy

– Uses carbon dioxide and water as starting materials

• The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules.


• Organisms that use photosynthesis are:

– Photosynthetic autotrophs

– The producers for most ecosystems

Plants(mostly on land)

Photosynthetic Protists(aquatic)

PHOTOSYNTHETIC AUTOTROPHS

Photosynthetic Bacteria(aquatic)

Micrograph of cyanobacteriaKelp, a large algaForest plants

LM

Figure 7.1

Plants(mostly on land)

Forest plantsFigure 7.1a

Photosynthetic Protists(aquatic)

Kelp, a large algaFigure 7.1b

Photosynthetic Bacteria(aquatic)

Micrograph of cyanobacteria

LM

Figure 7.1c


Chloroplasts: Sites of Photosynthesis• Chloroplasts are:

– The site of photosynthesis

– Found mostly in the interior cells of leaves


• Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma.

• Thylakoids are concentrated in stacks called grana.

• The green color of chloroplasts is from chlorophyll, a light-absorbing pigment.


• Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits.

Leaf cross section

Stomata

Vein

CO2 O2

Figure 7.2-1

Leaf cross section

Stomata

Vein

CO2 O2

Interior cellLM

StromaGranum Thylakoid

Chloroplast Outermembrane

Innermembrane

TEM

Figure 7.2-2

Leaf cross section

Stomata

Vein

CO2 O2

Figure 7.2a

StromaGranum Thylakoid

Chloroplast Outermembrane

Innermembrane

TEM

Figure 7.2b


The Overall Equation for Photosynthesis• In the overall equation for photosynthesis, notice that:

– The reactants of photosynthesis are the waste products of cellular respiration.

Carbon dioxide

6 O26 CO2 6 H2O C6H12O6

Water GlucosePhoto-synthesis Oxygen gas

Light energy

Figure 7.UN1


• In photosynthesis:

– Sunlight provides the energy

– Electrons are boosted “uphill” and added to carbon dioxide

– Sugar is produced


• During photosynthesis, water is split into:

– Hydrogen

– Oxygen

• Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar.

• Oxygen escapes through stomata into the atmosphere.


A Photosynthesis Road Map• Photosynthesis occurs in two stages:

– The light reactions convert solar energy to chemical energy

– The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide

Blast Animation: Photosynthesis: Light-Independent Reaction

LightH2O

O2

Chloroplast

Lightreactions

NADPH

ATP

Figure 7.3-1

LightH2O

O2

Chloroplast

Lightreactions

NADPH

ATP

Calvincycle

CO2

NADP+

ADPP

Sugar(C6H12O6)

Figure 7.3-2

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY• Chloroplasts:

– Are chemical factories powered by the sun

– Convert solar energy into chemical energy



The Nature of Sunlight• Sunlight is a type of energy called radiation, or electromagnetic

energy.

• The full range of radiation is called the electromagnetic spectrum.

Visible light

Wavelength (nm)

Radiowaves

380 400 500 600 750700

Wavelength = 580 nm

Micro-waves

Gammarays

InfraredUVX-rays

10–5 nm 10–3 nm 103 nm1 nm 106 nm 1 m 103 m

Increasing wavelength

Figure 7.4

The Process of Science: What Colors of Light Drive Photosynthesis?• Observation: In 1883, German biologist Theodor Engelmann

saw that certain bacteria tend to cluster in areas with higher oxygen concentrations.

• Question: Could this information determine which wavelengths of light work best for photosynthesis?

• Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis.



• Experiment: Engelmann:

– Laid a string of freshwater algal cells in a drop of water on a microscope slide

– Added oxygen-sensitive bacteria to the drop

– Used a prism to create a spectrum of light shining on the slide


• Results: Bacteria:

– Mostly congregated around algae exposed to red-orange and blue-violet light

– Rarely moved to areas of green light

• Conclusion: Chloroplasts absorb light mainly in the blue-violet and red-orange part of the spectrum.

Animation: Light and Pigments

Bacterium

Wavelength of light (nm)400 500 600 700

Light

Num

ber o

f bac

teria

Algal cells

Prism

Microscope slide

Figure 7.5

Light

Chloroplast

Reflectedlight

Absorbedlight Transmitted

light

Figure 7.6

Light

Chloroplast

Reflectedlight

Absorbedlight

Transmittedlight

Figure 7.6a

Figure 7.6b


Chloroplast Pigments• Chloroplasts contain several pigments:

– Chlorophyll a:

– Absorbs mostly blue-violet and red light

– Participates directly in the light reactions

– Chlorophyll b:

– Absorbs mostly blue and orange light

– Participates indirectly in the light reactions


– Carotenoids:

– Absorb mainly blue-green light

– Participate indirectly in the light reactions

– Absorb and dissipate excessive light energy that might damage chlorophyll

– The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids.

Figure 7.7


How Photosystems Harvest Light Energy• Light behaves as photons, discrete packets of energy.


• Chlorophyll molecules absorb photons.

– Electrons in the pigment gain energy.

– As the electrons fall back to their ground state, energy is released as heat or light.

Light

(b) Fluorescenceof a glow stick

Photon

HeatLight (fluorescence)

Ground stateChlorophyllmolecule

Excited statee–

(a) Absorptionof a photon

Figure 7.8

Light

Photon

Heat

Light (fluorescence)

Ground stateChlorophyllmolecule

Excited state

e–

(a) Absorption of a photonFigure 7.8a

(b) Fluorescence of a glow stickFigure 7.8b


• A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna.

Chloroplast

Figure 7.9-1

Chloroplast

Thylakoidmembrane

Pigment molecules

Figure 7.9-2

Chloroplast

Thylakoidmembrane

Pigment molecules

Antenna pigmentmolecules

PrimaryelectronacceptorReaction-centerchlorophyll a

Photon

Electrontransfer

Reactioncenter

PhotosystemTransfer of energy

Figure 7.9-3


How the Light Reactions Generate ATP and NADPH

• Two types of photosystems cooperate in the light reactions:

– The water-splitting photosystem

– The NADPH-producing photosystem

Primaryelectronacceptor

Water-splittingphotosystem

Light

H2O

2 H

Reaction-centerchlorophyll

2e–

2e–

O2++ 12

Figure 7.10-1



Light

H2O

2 H


2e–

2e–

O2++

Electron transport chain

Energyto make ATP

12

Figure 7.10-2



Light

H2O

2 H


2e–

2e–

O2++


Energyto make ATP


2e–

Light

NADPH-producingphotosystem


2e–

NADPH

NADP+

12

Figure 7.10-3


• The light reactions are located in the thylakoid membrane.


• An electron transport chain:

– Connects the two photosystems

– Releases energy that the chloroplast uses to make ATP

Light

H2O

Thylakoidmembrane

2e–

O2

ATP

NADP+

Light

Stroma

Inside thylakoid

Photosystem PhotosystemElectron transport chain

NADPH

ADP + P

H+

ATPsynthase

To Calvin cycle

H+

Electron flow

H+H+

H+

H+

H+

12

Figure 7.11

Light

H2O

Thylakoidmembrane

2e–

O2

ATP

NADP+

Light

Stroma

Inside thylakoid

Photosystem PhotosystemElectron transport chain

NADPH

ADP + P

ATPsynthase

To Calvin cycle

Electron flow

H+

12

H+H+

H+

H+ H+

H+

Figure 7.11a


e–

Phot

on

ATP

NADPH

e–

e–

e–

e–e–

Phot

on

e–


Figure 7.12

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

• The Calvin cycle:

– Functions like a sugar factory within the stroma of a chloroplast

– Regenerates the starting material with each turn


Calvincycle

P P

P

Three-carbon moleculeRuBP sugar

CO2 (from air)

Figure 7.13-1

ATP

NADPH

Calvincycle

ADP + P

NADP+

P

P P

P

G3P sugar

Three-carbon moleculeRuBP sugar

CO2 (from air)

Figure 7.13-2

ATP

NADPH

ADP + P

NADP+

P

P

P

P P

P

G3P sugar

Three-carbon molecule

G3P sugar

G3P sugar

RuBP sugar

CO2 (from air)

Calvincycle

Glucose (and other organic compounds)

Figure 7.13-3

ATP

NADPH

Calvincycle

ADP + P

NADP+

P

P

P

P P

P

ADP + P

ATP

G3P sugar

Three-carbon molecule

G3P sugar

G3P sugar

RuBP sugar

CO2 (from air)

Glucose (and other organic compounds)

Figure 7.13-4

Evolution Connection:Solar-Driven Evolution• C3 plants:

– Use CO2 directly from the air

– Are very common and widely distributed



• C4 plants:

– Close their stomata to save water during hot and dry weather

– Can still carry out photosynthesis

• CAM plants:

– Are adapted to very dry climates

– Open their stomata only at night to conserve water

Sugar

C4 Pathway(example: sugarcane)

C4 plant CAM plantSugar

Calvincycle

Calvincycle

Day

Celltype 1

Four-carboncompound

Night

Four-carboncompound

Celltype 2

CAM Pathway(example: pineapple)

ALTERNATIVE PHOTOSYNTHETIC PATHWAYS

CO2 CO2

CO2 CO2

Figure 7.14

Sugar

C4 Pathway(example: sugarcane)

C4 plant

Calvincycle

Celltype 1

Four-carboncompound

Celltype 2

CO2

CO2

Figure 7.14a

CAM plantSugar

Calvincycle

Day

Four-carboncompound

Night

CAM Pathway(example: pineapple)

CO2

CO2

Figure 7.14b

Carbon dioxide

6 O26 CO2 6 H2O C6H12O6

Water GlucosePhoto-synthesis Oxygen gas

Light energy

Figure 7.UN1

Light reactions

CO2

O2

H2O

(C6H12O6)

NADPH

Light

Sugar

ATP

ADPP

NADP+

Calvin cycle

Figure 7.UN2

Light reactions

CO2

O2

H2O

(C6H12O6)

NADPH

Light

Sugar

ATP

ADPP

NADP+

Calvin cycle

Figure 7.UN3

Carbon dioxide

6 O26 CO2 6 H2O C6H12O6

Water GlucosePhotosynthesis

Oxygen gas

Light energy

Figure 7.UN4

Light H2O

O2

Chloroplast

Lightreactions

NADPH

ATP

Calvincycle

CO2

NADP+

ADPP

Sugar

(C6H12O6)

Stack ofthylakoids Stroma

Figure 7.UN5

acceptor


Photon

H2O

2 H

Chlorophyll

2e–

O2++


ATP


Chlorophyll

NADPH

NADP+

e–

2e–

2e–

2e–

e–acceptorADP

Photon

21

Figure 7.UN6

NADPH

Calvincycle

ADP P

NADP+

P

ATP

G3P

CO2

Glucose andother compounds

Figure 7.UN7

photosynthesis: using light to make food

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