Chapter 7 · •The reaction center of the photosystem consists of chlorophyll a molecules that sit...

© 2013 Pearson Education, Inc.Lectures by Edward J. Zalisko

PowerPoint® Lectures forCampbell Essential Biology, Fifth Edition, and

Campbell Essential Biology with Physiology,

Fourth Edition

– Eric J. Simon, Jean L. Dickey, and Jane B. Reece

Chapter 7Photosynthesis: Using Light

to Make Food

20_02SeaLion_SV.mpg

Biology and Society: Biofuels

• Wood has historically been the main fuel used to

produce

– heat and

– light.

© 2013 Pearson Education, Inc.

Figure 7.0

• Industrialized societies replaced wood with fossil

fuels including

– coal,

– gas, and

– oil.

• To limit the damaging effects of fossil fuels,

researchers are investigating the use of biomass

(living material) as efficient and renewable energy

sources.



• There are several types of biofuels.

– Bioethanol is a type of alcohol produced by the

fermentation of glucose made from starches in

crops such as grains, sugar beets, and sugar cane.

– Bioethanol may be used

– directly as a fuel source in specially designed

vehicles or

– as a gasoline additive.



– Cellulosic ethanol is a type of bioethanol made

from cellulose in nonedible plant material such as

wood or grass.

– Biodiesel is made from plant oils or recycled frying

oil.



THE BASICS OF PHOTOSYNTHESIS

• Photosynthesis

– is used by plants, algae (protists), and some

bacteria,

– transforms light energy into chemical energy, and

– 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 and

– the producers for most ecosystems.

THE BASICS OF PHOTOSYNTHESIS


Figure 7.1

Plants(mostly on land)

Photosynthetic Protists(aquatic)

PHOTOSYNTHETIC AUTOTROPHS

Photosynthetic Bacteria(aquatic)

Micrograph of cyanobacteriaKelp, a large, multicellular algaForest plants

LM

Figure 7.1a

Plants(mostly on land)

Forest plants

Figure 7.1b

Photosynthetic Protists(aquatic)

Kelp, a large, multicellular alga

Figure 7.1c

Photosynthetic Bacteria(aquatic)

Micrograph of cyanobacteria

LM

Chloroplasts: Sites of Photosynthesis

• Chloroplasts are

– the site of photosynthesis and

– found mostly in the interior cells of leaves.

• Inside chloroplasts are interconnected,

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 that plays a

central role in converting solar energy to chemical

energy.

• Stomata are tiny pores in leaves where

– carbon dioxide enters and

– oxygen exits.

Chloroplasts: Sites of Photosynthesis


Figure 7.2-3

Interior cell

LM

StromaGranum

Thylakoidspace

ChloroplastInner and outermembranes

Co

lori

zed

TE

MLeaf cross section

Stomata

Vein

CO2O2

Photosyntheticcells

The Simplified Equation for Photosynthesis

• In the overall equation for photosynthesis, notice

that the reactants of photosynthesis are the waste

products of cellular respiration.


Figure 7-UN01

Carbon

dioxide

6 O26 CO2 6 H2O C6H12O6

Water GlucosePhoto-

synthesis Oxygen gas

Light energy

• In photosynthesis,

– sunlight provides the energy,

– electrons are boosted “uphill” and added to carbon

dioxide, and

– sugar is produced.



• During photosynthesis, water is split into

– hydrogen and

– 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 multistep stages:

1. the light reactions convert solar energy to

chemical energy and

2. the Calvin cycle uses the products of the light

reactions to make sugar from carbon dioxide.


A Photosynthesis Road Map

• The initial incorporation of carbon from the

atmosphere into organic compounds is called

carbon fixation.

– This lowers the amount of carbon in the air.

– Deforestation reduces the ability of the biosphere

to absorb carbon by reducing the amount of

photosynthetic plant life.


Figure 7.3-2

Calvincycle

CO2

NADP+

ADPP

Sugar

Light

H2O

O2

Chloroplast

Lightreactions

NADPH

ATP

– –

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

• Chloroplasts

– are chemical factories powered by the sun and

– convert sunlight into chemical energy.


Figure 7-UN02

Light reactions

CO2

O2

H2O

NADPH

Light

Sugar

ATP

ADP

P

NADP

Calvin cycle

– –

+

The Nature of Sunlight

• Sunlight is a type of energy called radiation, or

electromagnetic energy.

• The distance between the crests of two adjacent

waves is called a wavelength.

• The full range of radiation is called the

electromagnetic spectrum.


Figure 7.4

Visible light

Wavelength (nm)

Radiowaves

400 500 600 750700

Wavelength

580

nm

Micro-waves

Gamma

raysInfraredUVX-rays

10–5 nm 1 nm 1 m

Increasing wavelength

380

10–3 nm 103 nm 106 nm 103 m

Figure 7.5

Light

Chloroplast

Absorbed

light

Transmittedlight (detectedby your eye)

Reflectedlight

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, and

– 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 and

– rarely moved to areas of green light.

• Conclusion: Chloroplasts absorb light mainly in

the blue-violet and red-orange part of the

spectrum.



Figure 7.6

Light

Microscope slide

Prism

Bacteria

Algal cells

Bacteria

Wavelength of light (nm)

400 500 600 700

Nu

mb

er

of

bacte

ria

Chloroplast Pigments

• Chloroplasts contain several pigments:

1. Chlorophyll a

– absorbs mainly blue-violet and red light and

– participates directly in the light reactions.

2. Chlorophyll b

– absorbs mainly blue and orange light and

– participates indirectly in the light reactions.


• Carotenoids

– absorb mainly blue-green light,

– participate indirectly in the light reactions, and

– 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.

Chloroplast Pigments


Figure 7.7

How Photosystems Harvest Light Energy

• Light behaves as photons, a fixed quantity of light

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.


Figure 7.8

Light

(b) Fluorescence of a glow stick

Photon

Heat

Light (fluorescence)

Ground stateChlorophyll

molecule

Excited state

e–

(a) Absorption of a photon

The electronfalls to itsground state.

Absorption of a photon excitesan electron.

• In the thylakoid membrane, chlorophyll molecules

are organized with other molecules into

photosystems.

• A photosystem is a cluster of a few hundred

pigment molecules that function as a light-

gathering antenna.



• The reaction center of the photosystem consists of chlorophyll a molecules that sit next to another molecule called a primary electron acceptor, which traps the light-excited electron from chlorophyll a.

• Another team of molecules built into the thylakoid membrane then uses that trapped energy to make

– ATP and

– NADPH.



Figure 7.9

Chloroplast

Thylakoid membrane

Cluster of pigment

molecules

Pigment

molecules

Primary

electron

acceptor

Reaction-

center

chlorophyll a

Electron

transfer Reaction

center

Photosystem

Transfer

of energy

Photon

e–

How the Light Reactions Generate ATP and NADPH

• Two types of photosystems cooperate in the light

reactions:

1. the water-splitting photosystem and

2. the NADPH-producing photosystem.


Figure 7.10-3

Primaryelectronacceptor

Water-splittingphotosystem

Light

H2O

2 H O2

Energyto make ATP

Primaryelectronacceptor

2e–

Light

NADPH-producingphotosystem

Reaction-centerchlorophyll

2e–

NADPH

NADP

1

2

2e–

2e–

1

2

3

– –

Reaction-centerchlorophyll

• The light reactions are located in the thylakoid

membrane.

• An electron transport chain

– connects the two photosystems and

– releases energy that the chloroplast uses to make

ATP.

How the Light Reactions Generate ATP and NADPH


Figure 7.11

Light

H2O

Thylakoid

membrane

2e–

O2

ATP

NADP

Light

Stroma

Inside thylakoid

Photosystem Photosystem

Electron transport chain

NADPH

ADP P

H

ATP

synthase

To Calvin cycle

H

Electron flow

HH

H

H

H

1

2

Thylakoid

membrane

– –

Figure 7.12

Water-splittingphotosystem

ATP

NADPH

e–

NADPH-producingphotosystem

– –e–

e–

e–

e–

e–

e–

Figure 7-UN06

acceptor

Water-splitting

photosystem

Photon

H2O

2 H

Chlorophyll

2e–

O2+

ATP

NADPH-producing

photosystem

Chlorophyll

NADPH

NADP

e–

2e–

2e–e–

acceptorADP

Photon

21

– –

2e–

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

• The Calvin cycle

– functions like a sugar factory within a chloroplast

and

– regenerates the starting material with each turn.


Figure 7-UN03

Light reactions

CO2

O2

H2O

NADPH

Light

Sugar

ATP

ADP

P

NADP

Calvin cycle

– –

+

Figure 7-UN07

NADPH

Calvin

cycle

ADP P

NADP

P

ATP

G3P

CO2

Glucose andother compounds(such as celluloseand starch)

– –

Figure 7.13-4

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)

– –

23

1

4

ATP

ADP P

Figure 7-UN05

Light H2O

O2

Chloroplast

Light

reactions

NADPH

ATP

Calvin

cycle

CO2

NADP

ADP

P

Sugar

Stack of

thylakoids Stroma

– –

Sugar used for

• cellular respiration

• cellulose

• starch

• other organic compounds

Evolution Connection:Solar-Driven Evolution

• C3 plants

– use CO2 directly from the air and

– are very common and widely distributed.


• C4 plants

– close their stomata to save water during hot and

dry weather and

– can still carry out photosynthesis.

• CAM plants

– are adapted to very dry climates and

– open their stomata only at night to conserve water.

Evolution Connection:Solar-Driven Evolution


Figure 7.14

Sugar

C4 Pathway

(example: sugarcane)

C4 plant CAM plant

Sugar

Calvincycle

Calvin

cycle

Day

Celltype 1

Four-carboncompound

Night

Four-carboncompound

Celltype 2

CAM Pathway

(example: pineapple)

ALTERNATIVE PHOTOSYNTHETIC PATHWAYS

CO2 CO2

CO2CO2

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Chapter 7 · •The reaction center of the photosystem consists of chlorophyll a molecules that sit...

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