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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.


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 cellL

M

StromaGranum Thylakoid

ChloroplastOutermembrane

Innermembrane

TE

M

Figure 7.2-2

Leaf cross section

Stomata

Vein

CO2 O2

Figure 7.2a

StromaGranum Thylakoid

Chloroplast Outermembrane

Innermembrane

TE

M

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

Light

H2O

O2

Chloroplast

Lightreactions

NADPH

ATP

Figure 7.3-1

Light

H2O

O2

Chloroplast

Lightreactions

NADPH

ATP

Calvincycle

CO2

NADP+

ADPP

Sugar

(C6H12O6)

Figure 7.3-2


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

Photon

Heat

Light (fluorescence)

Ground stateChlorophyllmolecule

Excited state

e–

(a) Absorption of a photonFigure 7.8a


• 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

Primaryelectronacceptor

Reaction-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


Water-splittingphotosystem

Light

H2O

2 H

Reaction-centerchlorophyll

2e–

2e–

O2++ 1

2

Figure 7.10-1



Light

H2O

2 H


2e–

2e–

O2++

Electron transport chain

Energyto make

ATP

1

2

Figure 7.10-2



Light

H2O

2 H


2e–

2e–

O2++


Energyto make

ATP


2e–

Light

NADPH-producingphotosystem


2e–

NADPH

NADP

1

2

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

1

2

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

1

2

HH

H

H H

H

Figure 7.11a


e–

Ph

oto

n

ATP

NADPH

e–

e–

e–

e–

e–

Ph

oto

n

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

Calvincycle

Celltype 1

Four-carboncompound

Celltype 2

CO2

CO2

Figure 7.14a

CAM plant

Sugar

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

ADP

P

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

07 lecture presentation0

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