Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 6Where It Starts – Photosynthesis

(Sections 6.5 - 6.8)

6.5 Light-Dependent Reactions

• The light-dependent reactions of the first stage of photosynthesis convert the energy of light to the energy of chemical bonds

• There are two different sets of light-dependent reactions : a noncyclic pathway and a cyclic pathway

Capturing Light for Photosynthesis

• Light-harvesting complexes in the thylakoid membrane absorb photons and pass the energy to photosystems, which then release electrons

• photosystem • A cluster of hundreds of chlorophylls, accessory pigments,

and other molecules that converts light energy to chemical energy in photosynthesis

The Thylakoid Membrane

• Some components of the thylakoid membrane as seen from the stroma

Fig. 6.6, p. 98photosystemlight-harvesting complex

The Thylakoid Membrane

The Noncyclic Pathway

• Electrons released from photosystem II flow through an electron transfer chain, then to photosystem I

• Photon energy causes photosystem I to release electrons, which end up in NADPH

Replacing Lost Electrons

• Photosystem II replaces lost electrons by pulling them from water, which then dissociates into H+ and O2 (photolysis)

• photolysis • Process by which light energy breaks down a

molecule

Harvesting Electron Energy

• The process by which the flow of electrons through electron transfer chains drives ATP formation is called electron transfer phosphorylation

• electron transfer phosphorylation • Electron flow through electron transfer chains sets

up a hydrogen ion gradient that drives ATP formation

Steps in Noncyclic Reactions

1. Light energy ejects electrons from photosystem II

2. Photosystem II pulls replacement electrons from water molecules, which break apart into oxygen and hydrogen ions;the oxygen leaves the cell as O2

3. Electrons enter transfer chains in the thylakoid membrane

4. Energy from electrons in the transfer chain pumps hydrogen ions from the stroma into the thylakoid compartment; a hydrogen ion gradient forms across the thylakoid membrane

Steps in Noncyclic Reactions (cont.)

5. Light energy ejects electrons from photosystem I; replacement electrons come from an electron transfer chain

6. The electrons move through a second electron transfer chain, then combine with NADP+ and H+ to form NADPH

7. Hydrogen ions in the thylakoid compartment diffuse through the interior of ATP synthases and across the thylakoid membrane; hydrogen ion flow causes ATP synthases to attach phosphate to ADP, forming ATP in the stroma

Noncyclic Light-Dependent Reactions

Fig. 6.7, p. 99

The Light-Dependent Reactions of Photosynthesis

to light-independent reactions

light energy light energy

1

2

3

4 5

6

7

Noncyclic Light-Dependent Reactions

ANIMATION: Sites of photosynthesis

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The Cyclic Pathway

• Electrons released from photosystem I enter an electron transfer chain, then cycle back to photosystem I

• NADPH does not form – ATP forms by electron transfer phosphorylation

• Electrons flowing through electron transfer chains cause H+ to accumulate in the thylakoid compartment

• H+ follows its gradient back across the membrane through ATP synthases, driving ATP synthesis

3D ANIMATION: Photophosphorylation

Animation: Noncyclic pathway of electron flow

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ANIMATION: Harvesting photo energy

ANIMATION: Photosynthesis - Light system

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ANIMATION: Light-dependent reactions

6.6 Energy Flow in Photosynthesis

• Energy flow in light-dependent reactions is an example of how organisms use energy harvested from the environment to drive cellular processes

• The simpler cyclic pathway evolved first, and operates in nearly all photosynthesizers

• Some organisms became modified to add photosystem II, beginning a sequence of reactions that removes electrons from water molecules, releasing hydrogen ions and oxygen

Making ATP and NADPH

• Having alternate pathways is efficient because cells can produce NADPH and ATP, or produce ATP alone

• NADPH accumulates when it is not being used, which backs up the noncyclic pathway, so the cyclic pathway predominates – the cell makes ATP, but not NADPH

• When sugar production is high, NADPH is used quickly, and does not accumulate – the noncyclic pathway predominates

The Cyclic Pathway

Fig. 6.8b, p. 100

B In the cyclic pathway, electrons ejected from photosystem I are returned to it. As long as electrons continue to pass through its electron transfer chain, H+ continues to be carried across the thylakoid membrane, and ATP continues to form. Light provides the energy boost that keeps the cycle going.

Energy flow in the cyclic reactions of photosynthesis

P700 (photosystem I) light energy

Excited P700

ener

gy

The Cyclic Pathway

Fig. 6.8b, p. 100

Energy flow in the cyclic reactions of photosynthesis

P700 (photosystem I) light energy

Excited P700

ener

gy

Stepped Art

The Cyclic Pathway

The Noncyclic Pathway

Fig. 6.8a, p. 100

A The noncyclic pathway is a one-way flow of electrons from water, to photosystem II, to photosystem I, to NADPH. As long as electrons continue to flow through the two electron transfer chains, H+ continues to be carried across the thylakoid membrane, and ATP and NADPH keep forming. Light provides the energy boosts that keep the pathway going.

Energy flow in the noncyclic reactions of photosynthesis

light energy

Excited P680

light energy

ener

gy

Excited P700

P680 (photosystem II)

P700 (photosystem I)

The Noncyclic Pathway

light energy

Excited P680

ener

gy

P680(photosystem II)

P700(photosystem I)

Fig. 6.8a, p. 100

Stepped Art

Energy flow in the noncyclic reactions of photosynthesis

light energy

Excited P700

The Noncyclic Pathway

Key Concepts

• Making ATP and NADPH • ATP forms in the first stage of photosynthesis,

which is light-dependent because the reactions run on the energy of light

• The coenzyme NADPH forms in a noncyclic pathway that also releases oxygen

• ATP also forms in a cyclic pathway that does not release oxygen

ANIMATION: Energy Changes in Photosynthesis

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6.7 Light-Independent Reactions: The Sugar Factory

• The cyclic, light-independent reactions of the Calvin–Benson cycle are the “synthesis” part of photosynthesis

• Carbon fixation occurs, and sugars are synthesized

• Inside the stroma, the enzyme rubisco attaches a carbon from CO2 to RuBP to start the Calvin–Benson cycle

Key Terms

• Calvin–Benson cycle • Light-independent reactions of photosynthesis;

cyclic carbon-fixing pathway that forms sugars from CO2

• carbon fixation • Process by which carbon from an inorganic source

such as CO2 gets incorporated into an organic molecule

• rubisco (ribulose bisphosphate carboxylase)• Carbon-fixing enzyme of the Calvin–Benson cycle

Energy for Sugar Synthesis

• Photo:• ATP and NADPH are produced by the light-dependent

reactions using light energy

• Synthesis:• Light-independent reactions use energy from ATP, and

hydrogen and electrons from NADPH, to synthesize sugars from CO2

Steps of the Calvin–Benson Cycle

1. 6 CO2 enter a chloroplast; rubisco attaches each to a RuBP molecule – resulting intermediates split –12 PGA form

2. Each PGA gets a phosphate group from ATP, plus hydrogen and electrons from NADPH – 12 PGAL form

3. 2 PGAL combine to form 1 glucose molecule

4. Remaining 10 PGAL receive phosphate groups from ATP –endergonic reactions regenerate 6 RuBP

Steps of the Calvin–Benson Cycle

Fig. 6.9, p. 101

glucoseother molecules

Calvin– Benson Cycle

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4

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Steps of the Calvin–Benson Cycle

Fig. 6.9, p. 101

1

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Stepped Art

Calvin– Benson Cycle

glucose3 other molecules

Steps of the Calvin–Benson Cycle

ANIMATION: Calvin-Benson cycle

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Key Concepts

• Making Sugars • The second stage is the “synthesis” part of

photosynthesis • Sugars are assembled with carbon and oxygen

atoms from CO2

• The reactions run on the chemical bond energy of ATP, and electrons donated by NADPH—molecules that formed in the first stage of photosynthesis

6.8 Adaptations: Carbon-Fixing Pathways

• When environments differ, so do details of light-independent reactions

• Three pathways of sugar synthesis:• C3 plants• C4 plants• CAM plants

Key Terms

• C3 plant • Type of plant that uses only the Calvin–Benson

cycle to fix carbon

• C4 plant • Type of plant that minimizes photorespiration by

fixing carbon twice, in two cell types

• CAM plant • Type of C4 plant that conserves water by fixing

carbon twice, at different times of day

Photorespiration

• On dry days, plants conserve water by closing their stomata

• When stomata are closed, O2 from photosynthesis can’t escape, and CO2 for photosynthesis can’t enter

• In C3 plants, high O2 levels cause rubisco to attach O2 to RuBP instead of CO2

• This pathway (photorespiration) reduces the efficiency of sugar production on dry days

Key Terms

• stomata • Openings through plant surfaces• Allow water vapor and gases to diffuse across the

epidermis (through the cuticle)

• photorespiration • Reaction in which rubisco attaches oxygen instead

of carbon dioxide to ribulose bisphosphate

Photorespiration

Fig. 6.10b, p. 102

sugars

Calvin– Benson Cycle

RuBP

O2

PGA

NADPH

glycolate

CO2

ATP

ANIMATION: C3-C4 comparison

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C3 Plants

• C3 plants use only the Calvin–Benson cycle

• Most plants, including basswood (Tilia americana), are C3 plants

C4 Plants• In C4 plants, carbon

fixation occurs twice

• The first reactions release CO2 near rubisco, which limits photorespiration when stomata are closed

• Example: corn (Zea mays)

CAM Plants

• CAM plants minimize photorespiration by opening stomata and fixing carbon at night

• Example: Jade plants (Crassula argentea)

Key Concepts

• Alternate Pathways • Details of light-independent reactions that vary

among organisms are evolutionary adaptations to different environmental conditions

Green Energy (revisited)

• Photosynthesis removes carbon dioxide from the atmosphere, and locks its carbon atoms inside organic compounds

• When aerobic organisms break down the organic compounds for energy, carbon atoms are released in the form of CO2

• Since photosynthesis evolved, these two processes have constituted a balanced cycle of the biosphere

• Burning fossil fuels for energy has put Earth’s atmospheric cycle of carbon dioxide out of balance

Fossil Fuel Emissions• The sky over New York City on a sunny day

ANIMATION: Photosynthesis - Carbon Fixing

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