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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. Plants and other autotrophs Are the producers of the biosphere. Figure 10.1. Plants are photoautotrophs

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chapter 10

Chapter 10

Photosynthesis

slide2
Overview: The Process That Feeds the Biosphere
  • Photosynthesis
    • Is the process that converts solar energy into chemical energy
slide3
Plants and other autotrophs
    • Are the producers of the biosphere
slide4

Figure 10.1

  • Plants are photoautotrophs
    • They use the energy of sunlight to make organic molecules from water and carbon dioxide
slide5

These organisms use light energy to drive the

synthesis of organic molecules from carbon dioxide

and (in most cases) water. They feed not only

themselves, but the entire living world. (a) On

land, plants are the predominant producers of

food. In aquatic environments, photosynthetic

organisms include (b) multicellular algae, such

as this kelp; (c) some unicellular protists, such

as Euglena; (d) the prokaryotes called

cyanobacteria; and (e) other photosynthetic

prokaryotes, such as these purple sulfur

bacteria, which produce sulfur (spherical

globules) (c, d, e: LMs).

(a) Plants

(c) Unicellular protist

10 m

(e) Pruple sulfur

bacteria

1.5 m

Figure 10.2

(d) Cyanobacteria

(b) Multicellular algae

40 m

  • Photosynthesis
    • Occurs in plants, algae, certain other protists, and some prokaryotes
slide6
Heterotrophs
    • Obtain their organic material from other organisms
    • Are the consumers of the biosphere
chloroplasts the sites of photosynthesis in plants

Leaf cross section

Vein

Mesophyll

CO2

O2

Stomata

Figure 10.3

Chloroplasts: The Sites of Photosynthesis in Plants
  • The leaves of plants
    • Are the major sites of photosynthesis
slide9

Mesophyll

Chloroplast

5 µm

Outer

membrane

Thylakoid

Intermembrane

space

Thylakoid

space

Granum

Stroma

Inner

membrane

1 µm

  • Chloroplasts
    • Are the organelles in which photosynthesis occurs
    • Contain thylakoids and grana
tracking atoms through photosynthesis scientific inquiry
Tracking Atoms Through Photosynthesis: Scientific Inquiry
  • Photosynthesis is summarized as

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

the splitting of water

Reactants:

12 H2O

6 CO2

6 H2O

6 O2

C6H12O6

Products:

Figure 10.4

The Splitting of Water
  • Chloroplasts split water into
    • Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
photosynthesis as a redox process
Photosynthesis as a Redox Process
  • Photosynthesis is a redox process
    • Water is oxidized, carbon dioxide is reduced
the two stages of photosynthesis a preview
The Two Stages of Photosynthesis: A Preview
  • Photosynthesis consists of two processes
    • The light reactions
    • The Calvin cycle
slide14
The light reactions
    • Occur in the grana
    • Split water, release oxygen, produce ATP, & form NADPH, which are used in the Calvin cycle, this is the primary function of the light reactions
slide15
The Calvin cycle
    • Occurs in the stroma of the chloroplast
    • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power
slide16

H2O

CO2

Light

NADP 

ADP

+ P

LIGHT REACTIONS

CALVIN

CYCLE

ATP

NADPH

Chloroplast

[CH2O]

(sugar)

O2

Figure 10.5

  • An overview of photosynthesis
the nature of sunlight
The Nature of Sunlight
  • Light
    • Is a form of electromagnetic energy, which travels in waves
slide19
Wavelength
    • Is the distance between the crests of waves
    • Determines the type of electromagnetic energy
slide20

1 m

106 nm

10–5 nm

106 nm

1 nm

10–3 nm

103 nm

103 m

Micro-

waves

Radio

waves

Gamma

rays

X-rays

UV

Infrared

Visible light

380

450

500

550

600

650

700

750 nm

Shorter wavelength

Longer wavelength

Lower energy

Higher energy

Figure 10.6

  • The electromagnetic spectrum
    • Is the entire range of electromagnetic energy, or radiation
slide21
The visible light spectrum
    • Includes the colors of light we can see
    • Includes the wavelengths that drive photosynthesis
photosynthetic pigments the light receptors
Photosynthetic Pigments: The Light Receptors
  • Pigments
    • Are substances that absorb visible light
    • Ex. Red & yellow leaves absorb other colors, but reflect red & yellow so we see these colors.
slide23

Light

Reflected

Light

Chloroplast

Absorbed

light

Granum

Transmitted

light

Figure 10.7

  • Reflect light, which include the colors we see
slide24
The spectrophotometer
    • Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength
slide25

Refracting

prism

Chlorophyll

solution

Photoelectric

tube

White

light

Galvanometer

2

3

1

0

100

4

Slit moves to

pass light

of selected

wavelength

Green

light

The high transmittance

(low absorption)

reading indicates that

chlorophyll absorbs

very little green light.

0

100

The low transmittance

(high absorption) reading

chlorophyll absorbs most blue light.

Blue

light

Figure 10.8

  • An absorption spectrum
    • Is a graph plotting light absorption versus wavelength
slide26
The absorption spectra of chloroplast pigments
    • Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis
    • Visible light excites electrons during photosynthesis.
slide27

Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.

EXPERIMENT

RESULTS

Chlorophyll a

Chlorophyll b

Absorption of light by

chloroplast pigments

Carotenoids

Wavelength of light (nm)

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Figure 10.9

  • The absorption spectra of three types of pigments in chloroplasts
most effective wavelength for photosynthesis 420nm

Rate of photosynthesis

(measured by O2 release)

(b)

Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.

Most effective wavelength for photosynthesis: 420nm
  • The action spectrum of a pigment
    • Profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis
slide29

Aerobic bacteria

Filament

of alga

500

600

700

400

(c)

Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.

Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.

Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. In other words, to see the relationship between wavelenths of light & the O2 released

CONCLUSION

  • The action spectrum for photosynthesis
    • Was first demonstrated by Theodor W. Engelmann
slide30

CH3

in chlorophyll a

in chlorophyll b

CHO

CH2

CH3

CH

H

C

C

C

Porphyrin ring:

Light-absorbing

“head” of molecule

note magnesium

atom at center

C

C

CH3

C

C

H3C

CH2

C

N

C

N

H

C

C

Mg

H

N

C

C

N

H3C

C

C

CH3

C

C

C

C

C

H

H

CH2

H

C

C

O

CH2

O

O

C

O

O

CH3

CH2

Hydrocarbon tail:

interacts with hydrophobic

regions of proteins inside

thylakoid membranes of

chloroplasts: H atoms not

shown

Figure 10.10

  • Chlorophyll a
    • Is the main photosynthetic pigment
  • Chlorophyll b
    • Is an accessory pigment
and there are others
AND there are others…
  • Other accessory pigments
    • Absorb different wavelengths of light and pass the energy to chlorophyll a
excitation of chlorophyll by light

Excited

state

e–

Heat

Energy of election

Photon

(fluorescence)

Ground

state

Chlorophyll

molecule

Photon

Figure 10.11 A

Excitation of Chlorophyll by Light
  • When a pigment absorbs light
    • It goes from a ground state to an excited state, which is unstable
slide33

Figure 10.11 B

  • If an isolated solution of chlorophyll is illuminated
    • It will fluoresce, giving off light and heat
slide34

Thylakoid

Photosystem

Photon

STROMA

Light-harvesting

complexes

Reaction

center

Primary election

acceptor

e–

Thylakoid membrane

Special

chlorophyll a

molecules

Transfer

of energy

Pigment

molecules

THYLAKOID SPACE

(INTERIOR OF THYLAKOID)

Figure 10.12

  • A photosystem
    • Is composed of a reaction center surrounded by a number of light-harvesting complexes
slide35
The light-harvesting complexes
    • Consist of pigment molecules bound to particular proteins
    • Funnel the energy of photons of light to the reaction center
slide36
When a reaction-center chlorophyll molecule absorbs energy
    • One of its electrons gets bumped up to a primary electron acceptor
slide37
The thylakoid membrane
    • Is populated by two types of photosystems, I and II, these are the electron transport chains in plants
noncyclic electron flow
Noncyclic Electron Flow
  • Noncyclic electron flow
    • Is the primary pathway of energy transformation in the light reactions
    • Cooperation of both photosystems is required for the reduction of NADP+
noncyclic electron flow1

H2O

CO2

Light

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTIONS

ATP

NADPH

Electron

Transport

chain

O2

[CH2O] (sugar)

Primary

acceptor

7

Primary

acceptor

4

Fd

Electron transport chain

Pq

2

e

8

e–

e

H2O

NADP+

+ 2 H+

Cytochrome

complex

2 H+

NADP+

reductase

+

3

NADPH

O2

PC

e–

+ H+

P700

e–

5

Light

P680

Light

1

6

ATP

Photosystem-I

(PS I)

Photosystem II

(PS II)

Figure 10.13

NONCYCLIC electron flow:

Produces NADPH, ATP, and oxygen

slide40

e–

ATP

e–

e–

NADPH

e–

e–

e–

Mill

makes

ATP

Photon

e–

Photon

Photosystem I

Photosystem II

Figure 10.14 

  • A mechanical analogy for the light reactions
cyclic electron flow
Cyclic Electron Flow
  • Under certain conditions
    • Photoexcited electrons take an alternative path
slide42

Primary

acceptor

Primary

acceptor

Fd

Fd

NADP+

Pq

NADP+

reductase

Cytochrome

complex

NADPH

Pc

Photosystem I

ATP

Photosystem II

Figure 10.15

  • In cyclic electron flow
    • Only photosystem I is used
    • Only ATP is produced
a comparison of chemiosmosis in chloroplasts and mitochondria
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
  • Chloroplasts and mitochondria
    • Generate ATP by the same basic mechanism: chemiosmosis, thus both have ATP synthase complexes
    • But use different sources of energy to accomplish this
slide44

Key

Higher [H+]

Lower [H+]

Chloroplast

Mitochondrion

CHLOROPLAST

STRUCTURE

MITOCHONDRION

STRUCTURE

H+

Diffusion

Thylakoid

space

Intermembrance

space

Electron

transport

chain

Membrance

ATP

Synthase

Stroma

Matrix

ADP+

P

ATP

H+

Figure 10.16

  • The spatial organization of chemiosmosis
    • Differs in chloroplasts and mitochondria
slide45
In both organelles, by chemiosmosis
    • Redox reactions of electron transport chains generate a H+ gradient across a membrane
  • ATP synthase
    • Uses this proton-motive force to make ATP
notice nadp is reduced to nadph

H2O

CO2

LIGHT

NADP+

ADP

CALVIN

CYCLE

LIGHT

REACTOR

ATP

NADPH

STROMA

(Low H+ concentration)

O2

[CH2O] (sugar)

Cytochrome

complex

Photosystem II

Photosystem I

NADP+

reductase

Light

2 H+

3

NADP+ + 2H+

Fd

NADPH

+ H+

Pq

Pc

2

H2O

1⁄2

O2

THYLAKOID SPACE

(High H+ concentration)

1

2 H+

+2 H+

To

Calvin

cycle

ATP

synthase

Thylakoid

membrane

STROMA

(Low H+ concentration)

ADP

ATP

P

H+

Figure 10.17

Notice: NADP+ is reduced to NADPH
  • The light reactions and chemiosmosis: the organization of the thylakoid membrane
slide47
Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar, which is then used by respiration to release energy.
  • The Calvin cycle
    • Is similar to the citric acid cycle
    • Occurs in the stroma
slide48
The Calvin cycle has three phases
    • Carbon fixation: addition of carbon dioxide to RuBP
    • Reduction (NADP+ to NADPH)
    • Regeneration of the CO2 acceptor
slide49

H2O

Input

CO2

Light

3

(Entering one

at a time)

NADP+

CO2

ADP

CALVINCYCLE

LIGHTREACTION

ATP

NADPH

Rubisco

O2

[CH2O] (sugar)

3 P

P

Short-livedintermediate

P

6

3 P

P

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

6 ATP

6 ADP

CALVIN

CYCLE

3 ADP

6 P

P

3 ATP

1,3-Bisphoglycerate

6 NADPH

6 NADPH+

6 P

P

5

(G3P)

6

P

Glyceraldehyde-3-phosphate

(G3P)

P

1

Glucose andother organiccompounds

G3P(a sugar)Output

Figure 10.18

  • The Calvin cycle

Phase 1: Carbon fixation

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

Phase 2:Reduction

slide51
On hot, dry days, plants close their stomata
    • Conserving water but limiting access to CO2
    • Causing oxygen to build up
photorespiration an evolutionary relic
Photorespiration: An Evolutionary Relic?
  • In photorespiration
    • O2 substitutes for CO2 in the active site of the enzyme rubisco
    • The photosynthetic rate is reduced by preventing the formation of
    • 3-phosphoglycerate (3-PGA)
c 4 plants
C4 Plants
  • C4 plants minimize the cost of photorespiration
    • By incorporating CO2 into four carbon compounds in mesophyll cells where the Calvin cycle occurs.
slide54
These four carbon compounds
    • Are exported to bundle sheath cells, where they release CO2 used in the Calvin cycle
    • C4 plants use PEP carboxylase to initially fix CO2
slide55

Mesophyll

cell

Mesophyll cell

Photosynthetic

cells of C4 plant

leaf

PEP carboxylase

Bundle-

sheath

cell

CO2

CO2

PEP (3 C)

Oxaloacetate (4 C)

ADP

Vein

(vascular tissue)

Malate (4 C)

ATP

C4 leaf anatomy

Pyruate (3 C)

Bundle-

Sheath

cell

CO2

Stoma

CALVIN

CYCLE

Sugar

Vascular

tissue

Figure 10.19

  • C4 leaf anatomy and the C4 pathway
cam plants
CAM Plants
  • CAM plants
    • Open their stomata at night, incorporating CO2 into organic acids
in cam plants
In CAM plants:
  • During the day, the stomata close
    • And the CO2 is released from the organic acids for use in the Calvin cycle
slide58

2

1

Pineapple

Sugarcane

C4

CAM

CO2

CO2

Mesophyll Cell

Night

CO2 incorporated

into four-carbon

organic acids

(carbon fixation)

Organic acid

Organic acid

Bundle-

sheath cell

(b)

Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.

Day

(a)

Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different

types of cells.

Organic acids

release CO2 to

Calvin cycle

CALVINCYCLE

CALVINCYCLE

Sugar

Sugar

Figure 10.20

  • The CAM pathway is similar to the C4 pathway
the importance of photosynthesis a review

Light reaction

Calvin cycle

H2O

CO2

Light

NADP+

ADP

+ P1

RuBP

3-Phosphoglycerate

Photosystem II

Electron transport chain

Photosystem I

ATP

G3P

Starch

(storage)

NADPH

Amino acids

Fatty acids

Chloroplast

O2

Sucrose (export)

Calvin cycle reactions:

• Take place in the stroma

• Use ATP and NADPH to convert

CO2 to the sugar G3P

• Return ADP, inorganic phosphate, and NADP+ to the light reactions

Light reactions:

• Are carried out by molecules in the

thylakoid membranes

• Convert light energy to the chemical

energy of ATP and NADPH

• Split H2O and release O2 to the

atmosphere

Figure 10.21

The Importance of Photosynthesis: A Review
  • A review of photosynthesis
slide60
Organic compounds produced by photosynthesis
    • Provide the energy and building material for ecosystems