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BIOLOGYCONCEPTS & CONNECTIONS

Fourth Edition

Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Neil A. Campbell Jane B. Reece Lawrence G. Mitchell Martha R. Taylor

From PowerPointLectures forBiology: Concepts & Connections

CHAPTER 7Photosynthesis:

Using Light to Make Food

Modules 7.67.14


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Certain wavelengths of visible light drive thelight reactions of photosynthesis

7.6 Visible radiation drives the light reactions

THE LIGHT REACTIONS: CONVERTINGSOLAR ENERGY TO CHEMICAL ENERGY

Gamma

raysX-rays UV Infrared

Micro-

waves

Radio

waves

Visible light

Wavelength (nm)Figure 7.6A


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Figure 7.6B

Light

Chloroplast

Reflected

light

Absorbed

light

Transmitted

light


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Each of the many light-harvestingphotosystems consists of:

an antenna of chlorophyll and other pigment

molecules that absorb light a primary electron acceptor that receives excited

electrons from the reaction-center chlorophyll

7.7 Photosystems capture solar power


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Figure 7.7C

Primary

electron acceptor

Photon

Reaction center

PHOTOSYSTEM

Pigment

molecules

of antenna


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Fluorescence of isolated chlorophyll in solution

Figure 7.7A

Heat

Photon(fluorescence)

PhotonChlorophyll

molecule


7/27Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Figure 7.7B

Excitation ofchlorophyll in

a chloroplast

Primary

electron acceptor

Other

compounds

Chlorophyll

molecule

Photon



Two connected photosystems collect photons oflight and transfer the energy to chlorophyllelectrons

The excited electrons are passed from theprimary electron acceptor to electron transportchains

Their energy ends up in ATP and NADPH

7.8 In the light reactions, electron transport chainsgenerate ATP, NADPH, and O2



Where do the electrons come from that keep

the light reactions running? In photosystem I, electrons from the bottom of

the cascade pass into its P700 chlorophyll



Photosystem II regains electrons by splittingwater, leaving O2gas as a by-product

Figure 7.8

Primaryelectron acceptor

Primaryelectron acceptor

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis



The electron transport chains are arranged withthe photosystems in the thylakoid membranesand pump H+through that membrane

The flow of H+back through the membrane isharnessed by ATP synthase to make ATP

In the stroma, the H+ions combine with NADP+

to form NADPH

7.9 Chemiosmosis powers ATP synthesis in thelight reactions



The production of ATP by chemiosmosis inphotosynthesis

Figure 7.9

Thylakoidcompartment(high H+)

Thylakoidmembrane

Stroma(low H+)

Light

Antennamolecules

Light

ELECTRON TRANSPORT

CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE



The Calvin cycle occurs

in the chloroplastsstroma

This is where carbon

fixation takes place andsugar is manufactured

7.10 ATP and NADPH power sugar synthesis in the

Calvin cycle

THE CALVIN CYCLE:CONVERTING CO2TO SUGARS

INPUT

Figure 7.10A OUTPUT:

CALVIN

CYCLE


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The Calvin cycle constructs G3P using

carbon from atmospheric CO2

electrons and H+from NADPH

energy from ATP

Energy-rich sugar is then converted intoglucose


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Figure 7.10B

Details of theCalvin cycle INPUT:

Step Carbon

fixation.

In a reaction catalyzed by

rubisco, 3 molecules of CO2

are fixed.

11

Step Energy

consumption and redox.

2

3 P P P6

6

2

ATP

6 ADP + P

6 NADPH

6 NADP+

6 P

G3P

Step Release of one

molecule of G3P.

3

CALVIN

CYCLE

3

OUTPUT: 1 PGlucoseand othercompounds

G3P

Step Regeneration

of RuBP.

4

G3P

4

3 ADP

3 ATP

3CO2

5 P

RuBP 3-PGA


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A summaryof the

chemicalprocessesof photo-

synthesis

7.11 Review: Photosynthesis uses light energy to

make food molecules

PHOTOSYNTHESIS REVIEWED ANDEXTENDED

Figure 7.11

Light

Chloroplast

Photosystem IIElectrontransport

chainsPhotosystem I

CALVINCYCLE Stroma

LIGHT REACTIONS CALVIN CYCLE

Cellularrespiration

Cellulose

Starch

Otherorganiccompounds


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Many plants make more sugar than they need

The excess is stored in roots, tuber, and fruits

These are a major source of food for animals


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Most plants are C3plants, which take CO2directly from the air and use it in the Calvincycle

In these types of plants, stomata on the leafsurface close when the weather is hot

This causes a drop in CO2and an increase in

O2in the leaf

Photorespiration may then occur

7.12 C4and CAM plants have special adaptationsthat save water


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Photorespiration in a C3plant

CALVIN

CYCLE

2-C compound

Figure 7.12A


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Some plants have special adaptations thatenable them to save water

CALVIN

CYCLE

4-C compound

Figure 7.12B

Special cells in C4plantscorn andsugarcaneincorporateCO

2

into a four-carbonmolecule

This molecule can thendonate CO2to the

Calvin cycle

3-C sugar


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The CAM plantspineapples, most cacti, andsucculentsemploy a different mechanism

CALVIN

CYCLE

4-C compound

Figure 7.12C

They open theirstomata at night andmake a four-carboncompound

It is used as a CO2source by the same cellduring the day

3-C sugar

Night

Day


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Due to the increased burning of fossil fuels,

atmospheric CO2is increasing CO2warms Earths surface by trapping heat in

the atmosphere

This is called the greenhouse effect

7.13 Human activity is causing global warming;photosynthesis moderates it

PHOTOSYNTHESIS, SOLAR RADIATION, ANDEARTHS ATMOSPHERE


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Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat

trapped by CO2

and other gases


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Because photosynthesis removes CO2from theatmosphere, it moderates the greenhouse

effect

Unfortunately, deforestation may cause adecline in global photosynthesis

7 14 T lki Ab S i M i M li lk


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Mario Molino received aNobel Prize in 1995 forhis work on the ozonelayer

His research focuses onhow certain pollutants(greenhouse gases)damage that layer

7.14 Talking About Science: Mario Molina talksabout Earths protective ozone layer

Figure 7.14A


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The O2in the atmosphere results fromphotosynthesis

Solar radiation converts O2high in theatmosphere to ozone (O3)

Ozone shields organisms on the Earths surfacefrom the damaging effects of UV radiation


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C i h 2003 P Ed i I bli hi B j i C i

Industrial chemicals called CFCs have hastenedozone breakdown, causing dangerous thinning

of the ozone layer

Figure 7.14B

Sunlight

Southern tip of

South America

International restrictions on these chemicalsare allowing recovery

Antarctica

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