Overview: The Process That Feeds the Biosphere
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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 Plants are photoautotrophs

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Overview the process that feeds the biosphere photosynthesis

  • Overview: The Process That Feeds the Biosphere

  • Photosynthesis

    • Is the process that converts solar energy into chemical energy


Overview the process that feeds the biosphere photosynthesis

  • Plants and other autotrophs

    • Are the producers of the biosphere

  • Plants are photoautotrophs

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


Overview the process that feeds the biosphere photosynthesis

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


Overview the process that feeds the biosphere photosynthesis

  • Heterotrophs

    • Obtain their organic material from other organisms

    • Are the consumers of the biosphere


Overview the process that feeds the biosphere photosynthesis

  • Concept 10.1: Photosynthesis converts light energy to the chemical energy of food


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


Overview the process that feeds the biosphere photosynthesis

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


Photosynthesis as a redox process

Photosynthesis as a Redox Process

  • Photosynthesis is a redox process

    • Water is oxidized, carbon dioxide is reduced

    • This is the reverse flow of electrons as in cellular respiration


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


Overview the process that feeds the biosphere photosynthesis

  • The light reactions

    • Occur in the grana

    • Split water, release oxygen, produce ATP, and form NADPH

      • The ATP is produced by photophosphorylation – generating ATP using chemiosmosis to power the addition of phosphate to ADP

        • The third way of generating ATP we’ve seen


Overview the process that feeds the biosphere photosynthesis

  • The Calvin cycle

    • Occurs in the stroma

    • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power


Overview the process that feeds the biosphere photosynthesis

H2O

CO2

Light

NADP 

ADP

+ P

LIGHT REACTIONS

CALVIN

CYCLE

ATP

NADPH

Chloroplast

[CH2O]

(sugar)

O2

Figure 10.5

  • An overview of photosynthesis


Overview the process that feeds the biosphere photosynthesis

  • Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH


Overview the process that feeds the biosphere photosynthesis

  • The absorption spectra of chloroplast pigments

    • Provide clues to the relative effectiveness of different wavelengths for driving photosynthesis


Overview the process that feeds the biosphere photosynthesis

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


Overview the process that feeds the biosphere photosynthesis

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


Overview the process that feeds the biosphere photosynthesis

  • 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


A photosystem a reaction center associated with light harvesting complexes

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: A Reaction Center Associated with Light-Harvesting Complexes

  • A photosystem

    • Is composed of a reaction center surrounded by a number of light-harvesting complexes


Overview the process that feeds the biosphere photosynthesis

  • The light-harvesting complexes

    • Consist of pigment molecules bound to particular proteins

    • Act as an antenna for the reaction center

    • Funnel the energy of photons of light to the reaction center


Overview the process that feeds the biosphere photosynthesis

  • Reaction center –

    • Is a protein complex, including two special chlorophyll molecules and the primary electron acceptor


First step of light reactions

First step of light reactions

  • When a reaction-center chlorophyll molecule absorbs energy

    • One of its electrons gets bumped up to a primary electron acceptor

    • This is a redox rxn


Overview the process that feeds the biosphere photosynthesis

  • The thylakoid membrane

    • Is populated by two types of photosystems, I and II


Noncyclic electron flow

Noncyclic Electron Flow

  • Noncyclic electron flow

    • Is the primary pathway of energy transformation in the light reactions

    • Produces ATP and NADPH which will provide chemical energy and reducing power to the sugar making Calvin cycle


Overview the process that feeds the biosphere photosynthesis

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

  • Produces NADPH, ATP, and oxygen


Cyclic electron flow

Cyclic Electron Flow

  • Under certain conditions

    • Photoexcited electrons take an alternative path

    • This is because the Calvin cycle needs more ATP than NADPH.


Overview the process that feeds the biosphere photosynthesis

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

    • But use different sources of energy to accomplish this


Overview the process that feeds the biosphere photosynthesis

  • In both organelles

    • Redox reactions of electron transport chains generate a H+ gradient across a membrane

  • ATP synthase

    • Uses this proton-motive force to make ATP


Overview the process that feeds the biosphere photosynthesis

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

  • The light reactions and chemiosmosis: the organization of the thylakoid membrane


Overview the process that feeds the biosphere photosynthesis

  • Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar

  • The Calvin cycle

    • Is similar to the citric acid cycle

    • Occurs in the stroma


Overview the process that feeds the biosphere photosynthesis

  • The Calvin cycle has three phases

    • Carbon fixation

    • Reduction

    • Regeneration of the CO2 acceptor


Overview the process that feeds the biosphere photosynthesis

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


Overview the process that feeds the biosphere photosynthesis

  • Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates


Overview the process that feeds the biosphere photosynthesis

  • 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

    • No sugar is made, ATP is used, not made

    • Hold over from earlier time


C 4 plants

C4 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 used in the Calvin cycle


Overview the process that feeds the biosphere photosynthesis

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

    • During the day, the stomata close

    • And the CO2 is released from the organic acids for use in the Calvin cycle


Overview the process that feeds the biosphere photosynthesis

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


Overview the process that feeds the biosphere photosynthesis

  • Organic compounds produced by photosynthesis

    • Provide the energy and building material for ecosystems


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