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Chapter 10. Photosynthesis. Overview: The Process That Feeds the Biosphere. Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world.

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Chapter 10

Chapter 10

Photosynthesis


Overview the process that feeds the biosphere
Overview: The Process That Feeds the Biosphere

  • Photosynthesis is the process that converts solar energy into chemical energy

  • Directly or indirectly, photosynthesis nourishes almost the entire living world


Chapter 10

  • Autotrophs sustain themselves without eating anything derived from other organisms

  • Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

  • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide


Chapter 10


Le 10 2

LE 10-2 protists, and some prokaryotes

Plants

Unicellular protist

10 µm

Purple sulfur

bacteria

1.5 µm

Multicellular algae

Cyanobacteria

40 µm


Chapter 10


Concept 10 1 photosynthesis converts light energy to the chemical energy of food
Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

  • Chloroplasts are organelles that are responsible for feeding the vast majority of organisms

  • Chloroplasts are present in a variety of photosynthesizing organisms


Chloroplasts the sites of photosynthesis in plants
Chloroplasts: The Sites of Photosynthesis in Plants chemical energy of food

  • Leaves are the major locations of photosynthesis

  • Their green color is from chlorophyll, the green pigment within chloroplasts

  • Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

  • Through microscopic pores called stomata, CO2 enters the leaf and O2 exits


Chapter 10

  • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

  • A typical mesophyll cell has 30-40 chloroplasts

  • The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

  • Chloroplasts also contain stroma, a dense fluid


Le 10 3

LE 10-3 interior tissue of the leaf

Leaf cross section

Vein

Mesophyll

Stomata

O2

CO2

Mesophyll cell

Chloroplast

5 µm

Outer

membrane

Thylakoid

Intermembrane

space

Thylakoid

space

Stroma

Granum

Innermembrane

1 µm


Tracking atoms through photosynthesis scientific inquiry
Tracking Atoms Through Photosynthesis: interior tissue of the leafScientific Inquiry

  • Photosynthesis can be summarized as the following equation:

6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O


The splitting of water
The Splitting of Water interior tissue of the leaf

  • Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules


Le 10 4

LE 10-4 interior tissue of the leaf

12 H2O

6 CO2

Reactants:

6 O2

6 H2O

C6H12O6

Products:


Photosynthesis as a redox process
Photosynthesis as a Redox Process interior tissue of the leaf

  • Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced


The two stages of photosynthesis a preview
The Two Stages of Photosynthesis: interior tissue of the leafA Preview

  • Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

  • The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH

  • The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

  • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules


Le 10 5 1

LE 10-5_1 interior tissue of the leaf

H2O

Light

LIGHT

REACTIONS

Chloroplast


Le 10 5 2

LE 10-5_2 interior tissue of the leaf

H2O

Light

LIGHT

REACTIONS

ATP

NADPH

Chloroplast

O2


Le 10 5 3

LE 10-5_3 interior tissue of the leaf

H2O

CO2

Light

NADP+

ADP

+

P

i

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

Chloroplast

[CH2O]

(sugar)

O2


Concept 10 2 the light reactions convert solar energy to the chemical energy of atp and nadph
Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

  • Chloroplasts are solar-powered chemical factories

  • Their thylakoids transform light energy into the chemical energy of ATP and NADPH


The nature of sunlight
The Nature of Sunlight the chemical energy of ATP and NADPH

  • Light is a form of electromagnetic energy, also called electromagnetic radiation

  • Like other electromagnetic energy, light travels in rhythmic waves

  • Wavelength = distance between crests of waves

  • Wavelength determines the type of electromagnetic energy

  • Light also behaves as though it consists of discrete particles, called photons


Chapter 10


Le 10 6

LE 10-6 electromagnetic energy, or radiation

1 m

(109 nm)

10–3 nm

103 nm

106 nm

10–5 nm

103 m

1 nm

Gamma

rays

Micro-

waves

Radio

waves

X-rays

Infrared

UV

Visible light

650

750 nm

500

550

600

700

450

380

Shorter wavelength

Longer wavelength

Higher energy

Lower energy


Photosynthetic pigments the light receptors
Photosynthetic Pigments: The Light Receptors electromagnetic energy, or radiation

  • Pigments are substances that absorb visible light

  • Different pigments absorb different wavelengths

  • Wavelengths that are not absorbed are reflected or transmitted

  • Leaves appear green because chlorophyll reflects and transmits green light

Animation: Light and Pigments


Le 10 7

LE 10-7 electromagnetic energy, or radiation

Light

Reflected

light

Chloroplast

Absorbed

light

Granum

Transmitted

light


Chapter 10


Le 10 8a

LE 10-8a various wavelengths

Refracting

prism

White

light

Photoelectric

tube

Chlorophyll

solution

Galvanometer

0

100

The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.

Green

light

Slit moves to pass light

of selected wavelength


Le 10 8b

LE 10-8b various wavelengths

White

light

Chlorophyll

solution

Photoelectric

tube

Refracting

prism

0

100

The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.

Slit moves to pass light

of selected wavelength

Blue

light


Chapter 10

  • An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

  • The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

  • An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process


Le 10 9a

LE 10-9a light absorption versus wavelength

Chlorophyll a

Chlorophyll b

Carotenoids

Absorption of light by

chloroplast pigments

400

700

500

600

Wavelength of light (nm)

Absorption spectra


Le 10 9b

LE 10-9b light absorption versus wavelength

Rate of photo-

synthesis (measured

by O2 release)

Action spectrum


Chapter 10

  • The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann

  • In his experiment, he exposed different segments of a filamentous alga to different wavelengths

  • Areas receiving wavelengths favorable to photosynthesis produced excess O2

  • He used aerobic bacteria clustered along the alga as a measure of O2 production


Le 10 9c

LE 10-9c in 1883 by Thomas Engelmann

Aerobic bacteria

Filament

of algae

600

500

700

400

Engelmann’s experiment


Chapter 10

  • Chlorophyll in 1883 by Thomas Engelmanna is the main photosynthetic pigment

  • Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

  • Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll


Le 10 10

LE 10-10 in 1883 by Thomas Engelmann

in chlorophyll a

CH3

in chlorophyll b

CHO

Porphyrin ring:

light-absorbing

“head” of molecule; note magnesium atom at center

Hydrocarbon tail:

interacts with hydrophobic

regions of proteins inside

thylakoid membranes of chloroplasts; H atoms not shown


Excitation of chlorophyll by light
Excitation of Chlorophyll by Light in 1883 by Thomas Engelmann

  • When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

  • When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

  • If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat


Le 10 11

LE 10-11 in 1883 by Thomas Engelmann

Excited

state

e–

Heat

Energy of electron

Photon

(fluorescence)

Photon

Ground

state

Chlorophyll

molecule

Fluorescence

Excitation of isolated chlorophyll molecule


A photosystem a reaction center associated with light harvesting complexes
A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

  • A photosystem consists of a reaction center surrounded by light-harvesting complexes

  • The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center


Chapter 10


Le 10 12

LE 10-12 an excited electron from chlorophyll

Thylakoid

Photosystem

STROMA

Photon

Light-harvesting

complexes

Reaction

center

Primary electron

acceptor

e–

Thylakoid membrane

Special

chlorophyll a

molecules

Pigment

molecules

Transfer

of energy

THYLAKOID SPACE

(INTERIOR OF THYLAKOID)


Chapter 10

  • There are two types of photosystems in the thylakoid membrane

  • Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

  • Photosystem I is best at absorbing a wavelength of 700 nm

  • The two photosystems work together to use light energy to generate ATP and NADPH


Noncyclic electron flow
Noncyclic Electron Flow membrane

  • During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic

  • Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH


Le 10 13 1

LE 10-13_1 membrane

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

Primary

acceptor

e–

Energy of electrons

Light

P680

Photosystem II

(PS II)


Le 10 13 2

LE 10-13_2 membrane

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

Primary

acceptor

e–

H2O

2 H+

+

O2

1/2

e–

e–

Energy of electrons

Light

P680

Photosystem II

(PS II)


Le 10 13 3

LE 10-13_3 membrane

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

Primary

acceptor

Electron transport chain

Pq

e–

H2O

Cytochrome

complex

2 H+

+

O2

1/2

Pc

e–

e–

Energy of electrons

Light

P680

ATP

Photosystem II

(PS II)


Le 10 13 4

LE 10-13_4 membrane

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

Primary

acceptor

Primary

acceptor

Electron transport chain

e–

Pq

e–

H2O

Cytochrome

complex

2 H+

+

O2

1/2

Pc

e–

P700

e–

Energy of electrons

Light

P680

Light

ATP

Photosystem I

(PS I)

Photosystem II

(PS II)


Le 10 13 5

LE 10-13_5 membrane

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

Electron

Transport

chain

O2

[CH2O] (sugar)

Primary

acceptor

Primary

acceptor

Electron transport chain

Fd

e–

Pq

e–

e–

e–

NADP+

H2O

Cytochrome

complex

2 H+

+ 2 H+

NADP+

reductase

+

NADPH

O2

1/2

Pc

e–

+ H+

P700

Energy of electrons

e–

Light

P680

Light

ATP

Photosystem I

(PS I)

Photosystem II

(PS II)


Le 10 14

LE 10-14 membrane

e–

ATP

e–

e–

NADPH

e–

e–

e–

Mill

makes

ATP

Photon

e–

Photon

Photosystem II

Photosystem I


Cyclic electron flow
Cyclic Electron Flow membrane

  • Cyclic electron flow uses only photosystem I and produces only ATP

  • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle


Le 10 15

LE 10-15 membrane

Primary

acceptor

Primary

acceptor

Fd

Fd

NADP+

Pq

NADP+

reductase

Cytochrome

complex

NADPH

Pc

Photosystem I

ATP

Photosystem II


A comparison of chemiosmosis in chloroplasts and mitochondria
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

  • Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

  • Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

  • The spatial organization of chemiosmosis differs in chloroplasts and mitochondria


Le 10 16

LE 10-16 Mitochondria

Mitochondrion

Chloroplast

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

Diffusion

H+

Thylakoid

space

Intermembrane

space

Electron

transport

chain

Membrane

ATP

synthase

Key

Stroma

Matrix

Higher [H+]

Lower [H+]

ADP +

P

i

ATP

H+


Chapter 10

  • The current model for the thylakoid membrane is based on studies in several laboratories

  • Water is split by photosystem II on the side of the membrane facing the thylakoid space

  • The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase

  • ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

Animation: Calvin Cycle


Le 10 17

LE 10-17 studies in several laboratories

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

O2

[CH2O] (sugar)

STROMA

(Low H+ concentration)

Cytochrome

complex

Photosystem I

Photosystem II

Light

NADP+

reductase

Light

2 H+

NADP+ + 2H+

Fd

NADPH

+ H+

Pq

Pc

H2O

O2

1/2

THYLAKOID SPACE

(High H+ concentration)

2 H+

+2 H+

To

Calvin

cycle

Thylakoid

membrane

ATP

synthase

STROMA

(Low H+ concentration)

ADP

+

ATP

P

i

H+


Concept 10 3 the calvin cycle uses atp and nadph to convert co 2 to sugar
Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

  • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

  • The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

  • Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

  • For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2


Chapter 10

Play


Le 10 18 1

LE 10-18_1 CO

H2O

CO2

Input

Light

(Entering one

at a time)

3

NADP+

CO2

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

Phase 1: Carbon fixation

NADPH

Rubisco

O2

[CH2O] (sugar)

3

P

P

Short-lived

intermediate

P

6

3

P

P

3-Phosphoglycerate

Ribulose bisphosphate

(RuBP)

6

ATP

6 ADP

CALVIN

CYCLE


Le 10 18 2

LE 10-18_2 CO

H2O

CO2

Input

Light

(Entering one

at a time)

3

NADP+

CO2

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

Phase 1: Carbon fixation

NADPH

Rubisco

O2

[CH2O] (sugar)

3

P

P

Short-lived

intermediate

P

6

3

P

P

3-Phosphoglycerate

Ribulose bisphosphate

(RuBP)

6

ATP

6 ADP

CALVIN

CYCLE

6

P

P

1,3-Bisphosphoglycerate

6

NADPH

6 NADP+

6

P

i

P

6

Glyceraldehyde-3-phosphate

(G3P)

Phase 2:

Reduction

1

P

G3P

(a sugar)

Glucose and

other organic

compounds

Output


Le 10 18 3

LE 10-18_3 CO

H2O

CO2

Input

Light

(Entering one

at a time)

3

NADP+

CO2

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

Phase 1: Carbon fixation

NADPH

Rubisco

O2

[CH2O] (sugar)

3

P

P

Short-lived

intermediate

P

6

3

P

P

3-Phosphoglycerate

Ribulose bisphosphate

(RuBP)

6

ATP

6 ADP

3 ADP

CALVIN

CYCLE

6

P

P

3

ATP

1,3-Bisphosphoglycerate

6

NADPH

Phase 3:

Regeneration of

the CO2 acceptor

(RuBP)

6 NADP+

6

P

i

P

5

G3P

P

6

Glyceraldehyde-3-phosphate

(G3P)

Phase 2:

Reduction

1

P

G3P

(a sugar)

Glucose and

other organic

compounds

Output


Concept 10 4 alternative mechanisms of carbon fixation have evolved in hot arid climates
Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

  • Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis

  • On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis

  • The closing of stomata reduces access to CO2 and causes O2 to build up

  • These conditions favor a seemingly wasteful process called photorespiration


Photorespiration an evolutionary relic
Photorespiration: An Evolutionary Relic? evolved in hot, arid climates

  • In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

  • In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2

  • Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar


Chapter 10

  • Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

  • In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle


C 4 plants
C rubisco first evolved at a time when the atmosphere had far less O4 Plants

  • C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

  • These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle


Le 10 19

LE 10-19 rubisco first evolved at a time when the atmosphere had far less O

Mesophyll

cell

Mesophyll cell

CO2

PEP carboxylase

Photosynthetic

cells of C4 plant

leaf

Bundle-

sheath

cell

The C4 pathway

Oxaloacetate (4 C)

PEP (3 C)

Vein

(vascular tissue)

ADP

Malate (4 C)

ATP

C4 leaf anatomy

Pyruvate (3 C)

Bundle-sheath

cell

CO2

Stoma

CALVIN

CYCLE

Sugar

Vascular

tissue


Cam plants
CAM Plants rubisco first evolved at a time when the atmosphere had far less O

  • CAM plants open their stomata at night, incorporating CO2 into organic acids

  • Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle


Le 10 20

LE 10-20 rubisco first evolved at a time when the atmosphere had far less O

Sugarcane

Pineapple

CAM

C4

CO2

CO2

Night

Mesophyll

cell

CO2 incorporated

into four-carbon

organic acids

(carbon fixation)

Organic acid

Organic acid

Bundle-

sheath

cell

Day

CO2

CO2

Organic acids

release CO2 to

Calvin cycle

CALVIN

CYCLE

CALVIN

CYCLE

Sugar

Sugar

Spatial separation of steps

Temporal separation of steps


The importance of photosynthesis a review
The Importance of Photosynthesis: rubisco first evolved at a time when the atmosphere had far less OA Review

  • The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

  • Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

  • In addition to food production, photosynthesis produces the oxygen in our atmosphere


Le 10 21

LE 10-21 rubisco first evolved at a time when the atmosphere had far less O

Light reactions

Calvin cycle

H2O

CO2

Light

NADP+

ADP

+

P

i

RuBP

3-Phosphoglycerate

Photosystem II

Electron transport

chain

Photosystem I

ATP

G3P

Starch

(storage)

NADPH

Amino acids

Fatty acids

Chloroplast

O2

Sucrose (export)