Chapter 7 Capturing Solar Energy: Photosynthesis 7.1 What is photosynthesis? 7.2 Light-dependent reactions: How is light energy converted to chemical energy?

Chapter 7Capturing Solar Energy:

Photosynthesis• 7.1 What is photosynthesis?• 7.2 Light-dependent reactions: How is light

energy converted to chemical energy?• 7.3 Light-independent reactions: How is

chemical energy stored in glucose molecules?

• 7.4 What is the relationship between light-dependent and light-independent reactions?

• 7.5 Water, CO2, and the C4 pathway

7.1 What Is Photosynthesis?• ~ 2 billion years ago, some cells acquired the

ability to do photosynthesis (through chance genetic mutation)

• Oxygen levels in the atmosphere increased greatly

• Free oxygen (O2) is corrosive

• But as time went on (more mutation), some organisms used oxygen to break down glucose in cellular respiration

cellularrespiration

photosynthesis

(chloroplast)

H2O CO2 sugarATP O2

(mitochondrion)

Relationships

• Photosynthesis:6 CO2 + 6 H20 + light energy -> C6H12O6 + 6 O2

• Cellular Respiration:C6H12O6 + 6 O2 -> 6 CO2 + 6 H20 + chemical

and heat energy

7.1 What Is Photosynthesis?

• Leaves and chloroplasts are adaptations for photosynthesis

• A leaf is just a few cells thick so the sun can penetrate

• Stomata (stoma is singular) adjust to let more or less CO2 in

cuticle

Internal leaf structure

upperepidermis

mesophyllcells

lower epidermis

chloroplastsbundle sheath

vascular bundle(vein)

stoma

outer membraneinner membrane

thylakoidstroma

granum(stack ofthylakoids)

channel interconnecting thylakoids

Chloroplast in mesophyll cell

Leaves

cuticle

Internal leaf structure

upperepidermis

mesophyllcells

lowerepidermis

chloroplasts

bundle sheathvascular bundle(vein)

stoma

Figure 7-3 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Figure 7-2c Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

Mesophyll cell containing chloroplasts

outer membrane

inner membrane

thylakoid

stroma

granum(stack ofthylakoids

channelinterconnectingthylakoids

Chloroplast in mesophyll cell

7.1 What Is Photosynthesis?

• Light-dependent reactions– Chlorophyll captures sunlight energy and

transfer to energy carrier molecules (ATP & NADPH)

– Uses H20 and releases O2

• Light-independent reactions– Enzymes use ATP & NADPH to drive

synthesis of glucose

– Uses CO2 and H20 and releases glucose

LIGHT-DEPENDENTREACTIONS(thylakoids)

LIGHT-INDEPENDENTREACTIONS

(stroma)

depletedcarriers

(ADP, NADP+)

H2O O2

CO2 + H2O glucose

energizedcarriers

(ATP, NADPH)

How Is Light Energy Converted to Chemical Energy?

• Sun emits energy in a broad spectrum of electromagnetic radiation

• A packet of energy is called a photon, and the level of energy corresponds to a wavelength

• Light can be absorbed, reflected or transmitted

Visible light ("rainbow colors")

Wavelength (nanometers)

Visible light

400 450 500 550 600 650 700 750

Gamma rays X-rays UV InfraredMicro-waves

Radiowaves

Sun emits energy in a broad spectrum of electromagnetic radiation

How Is Light Energy Converted to Chemical Energy?

• During photosynthesis, light is first captured by pigments in chloroplasts

• Chloroplasts contain different pigments that can absorb certain wavelengths of photons

chlorophyll b

carotenoids

chlorophyll a

Absorbance of photosynthetic pigments

20

40

60

80

100

400 500 600 700

wavelength (nanometers)

0

light

abs

orpt

ion

(per

cent

)

Chloroplast Pigments

• Chlorophyll a and b strongly absorb violet, blue and red light

• Carotenoids absorb blue and red light• So what colors do you see?

• Carotenoids are also vitamin A and forms the visual pigment in our eyes (captures light energy so we can see!)

Figure 7-6 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

The Light-Dependent Reactions Occur Within the Thylakoid Membranes

• Thylakoid membranes contain photosystems I and II

• These are highly organized assemblies of proteins, chlorophyll and accessory pigment molecules

• The pigment molecules absorb light energy (photon)

Copyright © 2005 Pearson Prentice Hall, Inc.

A mechanical analogy for the light reactions

MillmakesATP

ATP

e–

e–e–

e–

e–

Pho

ton

Photosystem II Photosystem I

e–

e–

NADPH

Pho

ton

Figure 10.14 

sunlight

ener

gy le

vel o

f el

ectr

ons

+ H+NADP+

1/2 O2 + 2 H+ H2O

2e–

NADPH2e–

photosystemII

reactioncenter

within thylakoid m

embrane

2e–

synthesisenergy to driveATP

photosystem I

electron transport chain

2e–

NADPH

1/2 O2 + 2 H+

H2O

2e–

photosystemII

photosystem I

within thylakoid m

embrane

synthesis

energy to driveATP

+ H+

2e–

2e–

2e–

NADP+

sunlight

ener

gy le

vel o

f el

ectr

ons

reactioncenter

electron transport chain

How Is Light Energy Converted to Chemical Energy?

• Photosystem II generates ATP

• Photosystem I generates NADPH

• Splitting water maintains the flow of electrons through the photosystems

• Chemiosmosis: creating the hydrogen ion gradient

• Chemiosmosis: ATP synthesis

• Oxygen is a by-product of photosynthesis

Figure 7-8 (part 1) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

chloroplast

thylakoid

Figure 7-8 (part 2) Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

C3

cycle

PSII PSIETCstroma

ETC

thylakoid space

Energy fromenergizedelectrons powersNADPH synthesis.

Flow of H+ downconcentration gradientpowers ATP synthesis.

Energy from energizedelectrons powers activetransport of H+ by ETC.

High H+ concentrationgenerated by activetransport.

H+ channel coupledto ATP-synthesizingenzyme.

Energy-carriermolecules powerthe C3 cycle.

Figure E7-1 Biology: Life on Earth 8/e ©2008 Pearson Prentice Hall, Inc.

1 Energy is released aswater flows downhill.

2 Energy isharnessed torotate turbine.

3 Energy of rotatingturbine is used togenerate electricity.

H+ H+

(stroma)

2e–

photosystem II

thylakoidmembrane

(thylakoidinterior)

H+ H+

ADP ATPH+

P

H+

H+

H+

H+H+

H+

(high H+ concentrationin thylakoid)

(low H+ concentrationin stroma)

Light-Independent Reactions: How Is Chemical Energy Stored in Glucose Molecules?

• The C3 cycle captures carbon dioxide

• The C3 (3 carbons) cycle of carbon fixation

• It is also called the Calvin-Benson cycle

• Carbon is fixed during the C3 cycle is used to synthesize glucose

• You do not need to know the details of this cycle, nor Figure 7-10 (ed. 7 Figure 7-6)

6

6

glucose(or other organic

compounds)

6 H2O6 CO2

G3P

12

12

12

12

12

12

PGAC3

cycle

RuBP

6

ADP12

ATP

ATP

12

NADPH12

NADP+12G3P

12 C C C

6 H2O

12

PGAC3

cycle

RuBP

ADP6

6

6 CO2

6

C C C C CC

glucose(or other organic

compounds)

C C C C C

C

C CC

O2H2O

H20

ATP

ADP

NADPH

glucose

NADP+

CO2

Light-dependentreactions occurin thylakoids.

energy fromsunlight

Light-independentreactions(C3 cycle) occurin stroma.

chloroplast

cuticle

upperepidermis

mesophyllcells

lowerepidermis

chloroplasts

bundle sheathvascular bundle(vein)

stoma

Water, CO2, C3 and C4 Pathways

7.5 Water, CO2, and the C4 Pathway

• When stomata are closed to conserve water, CO2 cannot enter, and O2 cannot leave– A wasteful process called photorespiration

occurs when it is too hot for C3 plants

– Glucose is not made, and seedlings may die.

– Some plants have a C4 pathway

– Compare pathways of C3 and C4 plants

CO2

O2

PGA

C3

CYCLE

CO2

G3P

CO2

CO2

O2

PGA

C3

CYCLE RuBP

within chloroplast in mesophyll cell

within chloroplast in mesophyll cell

within chloroplast in bundle-sheath cell

PEP

4-carbonmolecule

AMP

ATP

pyruvate

bundle-sheathcells

bundle-sheath cells

G3P

CO2

C4

Pathway

C3 plants use the C3 pathway

RuBP

glucose

C4 plants use the C4 pathway

glucose

7.5 Water, CO2, and the C4 Pathway

• C4 plants reduce photorespiration by means of a two-stage carbon-fixation process

• C3 and C4 plants are each adapted to different environmental conditions