Ch 7 Photosynthesis Using Light to Make Food

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BIOLOGY CONCEPTS & 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 PowerPoint® Lectures for Biology: Concepts & Connections

CHAPTER 7Photosynthesis:

Using Light to Make Food

Modules 7.1 ± 7.5




Light is central to the life of a plant

Photosynthesis is the most important chemicalprocess on Earth

± It provides food for virtually all organisms

Plant cells convert lightinto chemical signalsthat affect a plant¶slife cycle

Life in the Sun




Light can influence the architecture of a plant

± Plants that get adequate light are often bushy, with deep green leaves

± Without enough light, plants

become tall and spindly withsmall pale leaves

Too much sunlight can

damage a plant± Chloroplasts and carotenoids

help to prevent such damage




Photosynthesis is the process by whichautotrophic organisms use light energy to makesugar and oxygen gas from carbon dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS




Plants, some protists, and some bacteria arephotosynthetic autotrophs

± They are the ultimate producers of food

consumed by virtually all organisms

7.1 Autotrophs are the producers of the biosphere




On land, plants such as oak trees and cacti arethe predominant producers

Figure 7.1A Figure 7.1B




In aquatic environments, algae andphotosynthetic bacteria are the main foodproducers

Figure 7.1C Figure 7.1D




In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts

A chloroplast contains:

± stroma, a fluid

± grana, stacks of thylakoids

The thylakoids contain chlorophyll

± Chlorophyll is the green pigment that captureslight for photosynthesis

7.2 Photosynthesis occurs in chloroplasts




The location and structure of chloroplasts

Figure 7.2

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outer

membrane

Inner membrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana




Investigating Photosynthesis

y Investigations into photosynthesis began with the

following question:

³When a tiny seedling grows into

a tall tree with a mass of several tons,

where does the tree¶s increase in mass come from?´




1. ______________ Experiment (1643)

y Put soil in pot and took mass

y Took a seedling and took mass

y Put seed in soil...watered...waited five years...

the seedling became a tree.

y He concluded that

y He determined the

Van Helmont¶s

the mass came from water

the ³hydrate´ in the carbohydrate

portion of photosynthesis




2. ___________ Experiment (1771)

y Put a lit candle in a bell jar-

y Placed a mint plant in the jar with the candle-

y

Concluded

He determined

Priestly¶s

The flame died out.

Flame lasted longer

plants release a substance needed

for candle burning.

plants release oxygen




3. ________________ Experiment (1779)

y Put aquatic plants in light...

y Put aquatic plants in dark...

He determined:

4. _______________ (1948)

y He determines

y Known as the

Jan Ingenhousz

produced oxygen

No oxygen

Light is needed to produce oxygen

Melvin Calvin

carbon¶s path to make glucose

Calvin¶s cycle




The O2 liberated by photosynthesis is madefrom the oxygen in water

7.3 Plants produce O2 gas by splitting water

Figure 7.3A




Figure 7.3B

Figure 7.3C

Experiment 1

Experiment 2

Notlabeled

Labeled

Reactants:

Products:




Water molecules are split apart and electronsand H+ ions are removed, leaving O2 gas

± These electrons and H+ ions are transferred to

CO2, producing sugar

7.4 Photosynthesis is a redox process, as is cellularrespiration

Figure 7.4A

Figure 7.4B

Reduction

Oxidation

Oxidation

Reduction




The complete process of photosynthesisconsists of two linked sets of reactions:

± the light reactions and the Calvin cycle

The light reactions convert light energy tochemical energy and produce O2

The Calvin cycle assembles sugar moleculesfrom CO2 using the energy-carrying products of the light reactions

7.5 Overview: Photosynthesis occurs in two stageslinked by ATP and NADPH




A n overview of photosynthesis

Figure 7.5

Light

Chloroplast

LIGHTREACTIONS

(in grana)

CALVINCYCLE

(in stroma)

H2O

O2

CO2

NADP+

ADP

+ P

Sugar

ATP

NADPH




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




Figure 7.6B

Light

Chloroplast

Reflected

light

Absorbed

light

Transmitted

light




Each of the many light-harvestingphotosystems consists of:

± an ³antenna´ of chlorophyll and other pigment

molecules that absorb light

± a primary electron acceptor that receives excitedelectrons from the reaction-center chlorophyll

7.7 Photosystems capture solar power




Figure 7.7C

Primary

electron acceptor

Photon

Reaction center

PHOTOSYSTEM

Pigment

molecules

of antenna




Fluorescence of isolated chlorophyll in solution

Figure 7.7A

Heat

Photon(fluorescence)

PhotonChlorophyll

molecule




Figure 7.7B

Excitation of

chlorophyll ina chloroplast

Primary

electron acceptor

Other

compounds

Chlorophyll

molecule

Photon




Two connected photosystems collect photons of light and transfer the energy to chlorophyllelectrons

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

± Their energy ends up in A TP and N ADPH

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 splitting water, leaving O2 gas as a by-product

Figure 7.8

Primaryelectron acceptor

Primaryelectron acceptor

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy for synthesis 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 A TP synthase to make A TP

± In the stroma, the H+ ions combine with N ADP+

to form N ADPH

7.9 Chemiosmosis powers ATP synthesis in thelight reactions




The production of A TP 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 chloroplast¶sstroma

± This is where carbon

fixation takes place andsugar is manufactured

7.10 ATP and NADPH power sugar synthesis in theCalvin cycle

THE CALVIN CYCLE:CONVERTING CO2 TO SUGARS

INPUT

Figure 7.10A OUTPUT:

CALVIN

CYCLE




The Calvin cycle constructs G3P using

± carbon from atmospheric CO2

± electrons and H+ from N ADPH

± energy from A TP

Energy-rich sugar is then converted intoglucose




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 Energyconsumption 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 other compounds

G3P

Step Regeneration

of RuBP.

4

G3P

4

3 ADP

3 ATP

3CO2

5 P

RuBP 3-PGA




A summary of thechemicalprocessesof photo-

synthesis

7.11 Review: Photosynthesis uses light energy tomake food molecules

PHOTOSYNTHESIS REVIEWED ANDEXTENDED

Figure 7.11

Light

Chloroplast

Photosystem IIElectrontransport

chainsPhotosystem I

CALVINCYCLE Stroma

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration

Cellulose

Starch

Other organiccompounds




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




Most plants are C3 plants, which take CO2

directly from the air and use it in the Calvincycle

± In these types of plants, stomata on the leaf surface close when the weather is hot

± This causes a drop in CO2 and an increase in

O2 in the leaf

± Photorespiration may then occur

7.12 C4 and CAM plants have special adaptationsthat save water




Photorespiration in a C3 plant

CALVINCYCLE

2-C compound

Figure 7.12A

EXAMPLES: wheat, barley, potatoes and sugar beet.




Some plants have special adaptations thatenable them to save water

CALVIN

CYCLE

4-C compound

Figure 7.12B

± Special cells in C4

plants²corn, crabgrassand sugarcane²

incorporate CO2 into afour-carbon molecule

± This molecule can then

donate CO2 to theCalvin cycle

3-C sugar







In C4 plants, the bundle sheath cells contain chloroplasts;

carbon is fixed in mesophyll cells, then transported to bundle

sheath cells where Calvin Cycle reactions occur in the absence

of oxygen.




The C AM plants²pineapples, most cacti, andsucculents²employ a different mechanism

CALVIN

CYCLE

4-C compound

Figure 7.12C

± They open theirstomata at night andmake a four-carbon

compound± It is used as a CO2

source by the same cellduring the day

3-C sugar

Night

Day




Due to the increased burning of fossil fuels,

atmospheric CO2 is increasing

± CO2 warms Earth¶s surface by trapping heat inthe atmosphere

± This is called the greenhouse effect

7.13 Human activity is causing global warming;photosynthesis moderates it

PHOTOSYNTHESIS, SOLAR RADIATION, ANDEARTH¶S ATMOSPHERE




Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat

trapped by CO2

and other gases




Because photosynthesis removes CO2 from theatmosphere, it moderates the greenhouseeffect

± Unfortunately, deforestation may cause adecline in global photosynthesis




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 Earth¶s protective ozone layer

Figure 7.14A




The O2 in the atmosphere results from

photosynthesis

± Solar radiation converts O2 high in theatmosphere to ozone (O3)

± Ozone shields organisms on the Earth¶s surfacefrom the damaging effects of UV radiation




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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Ch 7 Photosynthesis Using Light to Make Food