Abc. Photosynthesis Importance Where, Who? What is it? Limitations.

Photosynthesis

Importance

Where, Who?

What is it?

Limitations

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

• Photosynthesis (Phosyn) - conversion of light energy to chemical energy

Figure 7.1A Figure 7.1B


• Autotrophs = “self feeders”

• Includes plants, algae, some bacteria

Figure 7.1C Figure 7.1D


• Photosynthesis = light energy is used to make sugar and oxygen from CO2 and water

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS


• Aquatic plant shows production of O2

• Aerobic organisms dependent on this O2

Figure 7.3A


• The location and structure of chloroplasts

Figure 7.2

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana


• Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas

– These electrons and H+ ions are transferred to CO2, producing sugar

Photosynthesis is a redox process

Reduction

Oxidation

Oxidation

Reduction

Respiration

sunlight H2O CO2

NADPH

Calvincycle

ATP

ADP

NADP+

O2 sugar


Figure 7.6B

Light

Chloroplast

Reflectedlight

Absorbedlight

Transmittedlight

Certain wavelengths of visible light drive the light reactions of

photosynthesis


• Each light-harvesting photosystem consists of:

– an “antenna” of chlorophyll and other pigment molecules that absorb light

– a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

How do photosystems capture solar power?


Figure 7.7C

Primaryelectron acceptor

Photon

Reaction center

PHOTOSYSTEM

Pigmentmoleculesof antenna


Figure 7.7B

• Excitation of chlorophyll in a chloroplast


Othercompounds

Chlorophyllmolecule

Photon


• Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product

Figure 7.8



Electron transport chain

Electron transport

Photons

PHOTOSYSTEM I

PHOTOSYSTEM II

Energy forsynthesis of

by chemiosmosis


• H+ produced by photolysis of H2O

• electron transport chains pump H+ through the thylakoid membrane

– The flow of H+ back into the stroma is harnessed by ATP synthase to make ATP

– In the stroma, the H+ ions combine with NADP+ to form NADPH

Chemiosmosis powers ATP synthesis in the light reactions

chloroplast

sunlight

NADPH

PHOTOSYSTEM II PHOTOSYSTEM I

e-

2 H2O

thylakoid compartment thylakoid membrane stoma

O2 + 4 H+


• The Calvin cycle occurs in the chloroplast’s stroma,

• where carbon fixation takes place and sugar is made

ATP and NADPH power sugar synthesis in the Calvin cycle

INPUT

Figure 7.10A OUTPUT:

CALVINCYCLE

3molecules

CO2

1. carbonfixation

rubisco

2. energizingthe sugar

3. exit ofproduct

4. regenerationof RuBP

ATPATP

36

ADP6

ADP3

NADPH

NADP+

6 moleculesof G3P

1 moleculeof G3P

3 moleculesof G3P

6

6

3 moleculesof RuBP

3 molecules

6 moleculesof 3-PGA

6 moleculesof 3-PGA derivative

glucose andotherderivatives


• An overview of photosynthesis

Figure 7.5

Light

Chloroplast

LIGHTREACTIONS

(in grana)

CALVINCYCLE

(in stroma)

Electrons

H2O

O2

CO2

NADP+

ADP+ P

Sugar

ATP

NADPH


• Plants close their stomates to conserve water.

• Result: CO2 cannot reach the mesophyll cells.

Photorespiration occurs as O2 increases and CO2 decreases


• Photorespiration in a C3 plant

CALVIN CYCLE

2-C compound

Figure 7.12A


• Some plants have special adaptations that enable them to save water

CALVIN CYCLE

4-C compound

Figure 7.12B

– Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule

– This molecule can then donate CO2 to the Calvin cycle

3-C sugar


• CAM plants—pineapples, most cacti, and succulents—employ a different mechanism

CALVIN CYCLE

4-C compound

Figure 7.12C

– Stomates open at night; plants make a four-carbon compound

– Then use this as a CO2 source in the same cell during the day

3-C sugar

Night

Day


• Due to the increased burning of fossil fuels, atmospheric CO2 is increasing (+30% since 1900)

– CO2 warms Earth’s surface by trapping heat in the atmosphere = greenhouse effect

Is global warming really a threat to life?


Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat trapped by CO2 and other gases

Greenhouse gases trap solar energy in the atmosphere

- gases include CO2 and methane


Consequences predicted by models:

world temp may rise from 1 to 6 degrees C by 2100

Polar ice melts, sea levels rise

Drastic weather changes

Spread of tropical pests and diseases

Extinction of many species

What is the effect on phosyn?


• How can warming be stopped?

• 1. Plant more crops - phosyn removes CO2– Effects of deforestation?

2.Stop adding CO2, methane

- Change from fossil fuel to other energy

- Don’t eat hamburgers.


• The O2 in the atmosphere results from photosynthesis

– Solar radiation converts O2 high in the atmosphere to ozone (O3)

– Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation


• Industrial chemicals called CFCs speed up ozone breakdown, causing dangerous thinning of the ozone layer

Figure 7.14B

Sunlight

Southern tip of South America

• International restrictions on these chemicals are allowing recovery

Antarctica

Abc. Photosynthesis Importance Where, Who? What is it? Limitations.

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