PHOTOSYNTHESISCHAPTER 7

Where It Starts - Photosynthesis

IMPACTS, ISSUES: SUNLIGHT AND SURVIVAL

Plants are autotrophs, or self-nourishing organisms

The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O3) layer

The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out of the ocean

ELECTROMAGNETIC SPECTRUM

Shortest Gamma rayswavelength X-rays

UV radiationVisible lightInfrared

radiationMicrowaves

Longest Radio waveswavelength

PHOTONS

Packets of light energy

Each type of photon has fixed amount of energy

Photons having most energy travel as shortest wavelength (blue-violet light)

VISIBLE LIGHT

Wavelengths humans perceive as different colors

Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy

Figure 7-2Page 108

shortest wavelengths

(most energetic)

range of most radiation

reaching Earth’s surfacegamm

a rays

range of heat escaping

from Earth’s surface

longest wavelengths

(lowest energy)x

raysultraviol

etradiation

near-infraredradiation

infraredradiation microwave

s

radiowaves

VISIBLE LIGHT

Wavelengths of light (nanometers)

Fig. 7-2, p.108

VISIBLE LIGHT

400 450 500 550 600 650 700

PIGMENTS

Color you see is the wavelengths not absorbed Light-catching part of molecule often has

alternating single and double bonds These bonds contain electrons that are capable

of being moved to higher energy levels by absorbing light

VARIETY OF PIGMENTS

Chlorophylls a and b

Carotenoids

Anthocyanins

Phycobilins

CHLOROPHYLLSMain pigments in most photoautotrophs

Wavele

ng

th a

bsorp

tion

(%

)

Wavelength (nanometers)

chlorophyll b

chlorophyll a

ACCESSORY PIGMENTSp

erc

en

t of

wavele

ng

ths a

bsorb

ed

wavelengths (nanometers)

beta-carotenephycoeryth

rin (a phycobilin)

Carotenoids, Phycobilins, Anthocyanins

PIGMENTS IN PHOTOSYNTHESIS

Bacteria Pigments in plasma membranes

Plants Pigments and proteins organized into

photosystems that are embedded in thylakoid membrane system

T.E. ENGLEMANN’S EXPERIMENT

Background Certain bacterial cells will move

toward places where oxygen concentration is high

Photosynthesis produces oxygen

T.E. ENGLEMANN’S EXPERIMENT

Fig. 7-4c, p.110

T.E. Englemann’s Experiment

LINKED PROCESSES

Photosynthesis

Energy-storing pathway

Releases oxygen

Requires carbon dioxide

Aerobic Respiration

Energy-releasing pathway

Requires oxygen

Releases carbon dioxide

CHLOROPLAST STRUCTURE

two outer membranes

inner membrane system(thylakoids connected by channels)

stroma

Fig. 7-6, p.111

PHOTOSYNTHESIS EQUATION

12H2O + 6CO2 6O2 + C2H12O6 + 6H2OWater Carbon

Dioxide

Oxygen Glucose Water

LIGHT ENERGY

In-text figurePage 111

Fig. 7-6a, p.111

Photosynthesis

CO2H2O

SUNLIGHT

Fig. 7-6c, p.111

O2

light-dependa

nt reaction

s

light-independa

nt reactions

sugars

CHLOROPLAST

NADPH, ATP

NADP+, ADP

Photosynthesis

WHERE ATOMS END UP

Products 6O2 C6H12O

6

6H2

O

Reactants 12H2

O6CO2

TWO STAGES OF PHOTOSYNTHESIS

sunlight water uptake carbon dioxide uptake

ATP

ADP + Pi

NADPH

NADP+

glucoseP

oxygen release

LIGHT-INDEPENDENT

REACTIONS

LIGHT-DEPENDENT REACTIONS

new water

ARRANGEMENT OF PHOTOSYSTEMS

water-splitting complex thylakoidcompartment

H2O 2H + 1/2O2

P680

acceptor

P700

acceptor

pool of electron carriers stromaPHOTOSYSTEM II

PHOTOSYSTEM I

LIGHT-DEPENDENT REACTIONS

Pigments absorb light energy, give up e-, which enter electron transfer chains

Water molecules split, ATP and NADH form, and oxygen is released

Pigments that gave up electrons get replacements

Fig. 7-7, p.112

photon

Light-Harvesting Complex

Photosystem

LIGHT-DEPENDENT REACTIONS

NADPH

NADP + + H+

thylakoidcompartment

thylakoidmembrane

stroma

ATPADP + Pi

H+

H+

H+H+

H+

H+

H+

H+ H+

H+

H+

PHOTOSYSTEM IsunlightPHOTOSYSTEM II

LIGHT-HARVESTIN

GCOMPLEX

Fig. 7-8, p.113

H+

e- e-e- e-e- e-

H+

e-

O2

H2O

cross-section through a disk-

shaped fold in the thylakoid

membrane

PIGMENTS IN A PHOTOSYSTEM

reaction center

PHOTOSYSTEM FUNCTION: REACTION CENTER Energy is reduced to level that can be

captured by molecule of chlorophyll a

This molecule (P700 or P680) is the reaction center of a photosystem

Reaction center accepts energy and donates electron to acceptor molecule

ELECTRON TRANSFER CHAIN Adjacent to photosystem Acceptor molecule donates electrons from

reaction center

As electrons pass along chain, energy they release is used to produce ATP

CYCLIC ELECTRON FLOW

Electrons are donated by P700 in photosystem I to

acceptor molecule flow through electron transfer chain and back to

P700

Electron flow drives ATP formation No NADPH is formed

CYCLIC ELECTRON FLOW

electron acceptor

electron transfer chain

e–

e–

e–

e–

ATP

Electron flow through transfer

chain sets up conditions for ATP formation at other membrane sites.

NONCYCLIC ELECTRON FLOW

Two-step pathway for light absorption and electron excitation

Uses two photosystems: type I and type II

Produces ATP and NADPH

Involves photolysis - splitting of water

MACHINERY OF NONCYCLIC ELECTRON FLOW

photolysis

H2O

NADP+ NADPH

e–

ATP

ATP SYNTHASE

PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi

e–

first electron transfer chain

second electron

transfer chain

ENERGY CHANGESP

ote

nti

al to

tra

nsfe

r en

erg

y (

volt

s)

H2O 1/2O2 + 2H+

(Photosystem II)

(Photosystem I)

e– e–

e–e–

secondtransfer

chain

NADPHfirst

transfer chain

PHOTOSYSTEM I

p700*

photon e-

H+

Cyclic Pathway of ATP Formation

Hig

her

en

erg

y

p700

Fig. 7-9a, p.114

PHOTOSYSTEM I

e-

Noncyclic Pathway of ATP and NADPH Formation

2H2O

NADPH

NADH+

p700*

p700photon

PHOTOSYSTEM II

p680*

p680

4H+ + O2

Fig. 7-9b, p.114

CHEMIOSMOTIC MODEL OF ATP FORMATION Electrical and H+ concentration gradients are

created between thylakoid compartment and stroma

H+ flows down gradients into stroma through ATP synthesis

Flow of ions drives formation of ATP

CHEMIOSMOTIC MODEL FOR ATP FORMATION

ADP + Pi

ATP SYNTHASE

Gradients propel H+ through ATP synthases;ATP forms by phosphate-group transfer

ATP

H+ is shunted across membrane by some components of the first electron transfer chain

PHOTOSYSTEM II

H2Oe–

acceptor

Photolysis in the thylakoid compartment splits water

PHOTOSYNTHESIS QUIZ 1 1. Give the equation for photosynthesis. 2. Give the 2 major reactions occurring in photosynthesis. 3. What reactant goes into the light reactions? What product? 4. What reactant goes into the Calvin cycle? What product? 5. Within the chloroplast, where do the light reactions occur? 6. Within the chloroplast, where does the Calvin cycle occur? 7. What does noncyclic electron flow produce (besides ATP) that

that cyclic electron flow does not produce? 8. Why doesn’t the overall action spectrum of photosynthesis

exactly match the absorption spectrum for chlorophyll? 9. Which photosystem acts first in noncyclic electron flow? ****BONUS**** 1. What are Pq, Pc, and Fd?

2. EXPLAIN WHAT CHEMIOSMOSIS IS.

LIGHT-INDEPENDENT REACTIONS

Synthesis part of photosynthesis

Can proceed in the dark

Take place in the stroma

Calvin-Benson cycle

CALVIN-BENSON CYCLE

Overall reactantsCarbon dioxide

ATP

NADPH

Overall productsGlucose

ADP

NADP+

Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

CALVIN- BENSON CYCLE

CARBON FIXATION

6 CO2 (from the air)

6 6RuBP

PGA

unstable intermediate

6 ADP

6

12

12ATP

ATP

NADPH

10

12PGAL

glucoseP

PGAL2

Pi

12 ADP12 Pi

12 NADP+

12

4 Pi

PGAL

Fig. 7-10a, p.115

THESE REACTIONS PROCEED IN THE CHLOROPLAST’S

STROMA

Calvin- Benson Cycle

ATP

6 RuBP

phosphorylated glucose

10 PGAL

1 Pi

12 PGA

Calvin-Benson cycle

Fig. 7-10b, p.115

6 ADP

ATP

12 ADP +12 Pi

6CO2

NADPH

12 NADP+

12 PGAL

4 Pi

1

12

12

Calvin- Benson

Cycle

THE C3 PATHWAY

In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA

Because the first intermediate has three carbons, the pathway is called the C3 pathway

PHOTORESPIRATION IN C3 PLANTS

On hot, dry days stomata close Inside leaf

Oxygen levels rise Carbon dioxide levels drop

Rubisco attaches RuBP to oxygen instead of carbon dioxide

Only one PGAL forms instead of two

Fig. 7-11a1, p.116

C3 Plants

upperepidermis

palisademesophyll

spongymesophyll

lowerepidermis

stoma leaf vein air space

Basswood leaf, cross-section.

Fig. 7-11a2, p.116

C3 Plants

6 PGA + 6 glycolate

6 PGAL

1 PGAL

Twelve turns of the cycle, not just six, to make one 6-carbon sugar

RuBP

Calvin-Benson Cycle

CO2

+ wate

r

5 PGAL

Stomata closed: CO2 can’t get in; O2 can’t get out

Rubisco fixes oxygen, not carbon, in mesophyll cells in leaf

Fig. 7-11a3, p.117

C3 Plants

C4 PLANTS

Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form

four-carbon oxaloacetate

Oxaloacetate is transferred to bundle-sheath

cells

Carbon dioxide is released and fixed again in

Calvin-Benson cycle

Fig. 7-11b1, p.117

C4 Plants

upperepidermis

mesophyllcell

bundle-sheath cell

lowerepidermis

Basswood leaf, cross-section.

Fig. 7-11b2, p.117

C4 Plants

oxaloacetate

malateC4

cycle

pyruvate

CO2

12 PGAL

10 PGAL

2 PGAL

1 sugar

RuBPCalvin-Benson

Cycle

Carbon fixed in the mesophyll cell, malate diffuses into adjacent bundle-sheath cell

In bundle-sheath cell, malate gets converted to pyruvate with release of CO2, which enters Calvin-Benson cycle

12 PGAL

PEP

Stomata closed: CO2 can’t get in; O2 can’t get out

Fig. 7-11b3, p.117

C4 Plants

CAM PLANTS

Carbon is fixed twice (in same cells) Night

Carbon dioxide is fixed to form organic acids

Day Carbon dioxide is released and fixed in Calvin-

Benson cycle

Fig. 7-11c1, p.117

CAM Plants

Fig. 7-11c2, p.117

epidermis with thick cuticle

mesophyll cell

air space

stoma

CAM Plants

Stomata stay closed during day, open for CO2 uptake at

night only.

C4 CYCLE

Calvin-Benson

Cycle

C4 cycle operates at night when CO2 from aerobic respiration fixed

1 sugar

CO2 that accumulated overnight used in C3 cycle during the day

Fig. 7-11c3, p.117

CAM Plants

SUMMARY OF PHOTOSYNTHESIS

Figure 7-14Page 120

light6O2

12H2O

CALVIN-BENSON CYCLE

C6H12O6

(phosphorylated glucose)

NADPHNADP+ATPADP + Pi

PGA PGAL

RuBP

P

6CO2

end product (e.g., sucrose, starch, cellulose)

LIGHT-DEPENDENT REACTIONS

6H2O

LIGHT-INDEPENDENT REACTIONS

CARBON AND ENERGY SOURCES

Photoautotrophs

Carbon source is carbon dioxide

Energy source is sunlight

Heterotrophs

Get carbon and energy by eating

autotrophs or one another

Fig. 7-12, p.118

sunlight

energy

Photosynthesis1. H2O is split by light energy.

Its oxygen diffuses away; its electrons, hydrogen enter transfer chains with roles in ATP formation. Coenzymes pick up the electrons and hydrogen2. ATP energy drives the synthesis of glucose from hydrogen and electrons (delivered by coenzymes), plus carbon and oxygen (from carbon dioxide).

glucose

(stored energy, buildin

g blocks)

carbon dioxide,water

oxygen

Aerobic Respiration1. Glucose is broken down completely to carbon dioxide and water. Coenzymes pick up the electrons, hydrogens.2. The coenzymes give up the electrons and hydrogen atoms to oxygen-requiring transfer chains that have roles in forming many ATP molecules.

ATP available to drive nearly all cellular

tasks

Carbon and Energy Sources

PHOTOAUTOTROPHS

Capture sunlight energy and use it to carry out photosynthesis

Plants

Some bacteria

Many protistans

SATELLITE IMAGES SHOW PHOTOSYNTHESIS

 Photosynthetic activity in spring

Atlantic Ocean

Figure 7-13Page 119

sunlight

Calvin-Benson cycle

Light-DependentReactions

end products (e.g., sucrose, starch, cellulose)

ATP

12 PGAL

Light-Independent

Reactionsphosphorylated glucose

6H2O

6 RuBP

12H2O 6O2

NADPH NADP+

6CO2

ADP + Pi

Fig. 7-14, p.120

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