Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ch. 10 Photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Ch. 10

• Photosynthesis


Photosynthesis

• Energy flows into ecosystem as sunlight, • Feeds the Biosphere• Converts solar E into chemical E

Light energy

ECOSYSTEM

CO2 + H2O

Photosynthesisin chloroplasts

Cellular respirationin mitochondria

Organicmolecules

+ O2

ATP

powers most cellular work

HeatenergyFigure 9.2


Energy Transformations

• Photoautotrophs (producers)– Use E of sunlight to make organic molecules

from water and CO2

(a) Plants

(b) Multicellular algae

(c) Unicellular protist 10 m

40 m(d) Cyanobacteria

1.5 m(e) Pruple sulfurbacteria

Figure 10.2


• Photosynthesis converts light E to the chemical E of food


Heterotrophs

• Obtain organic material f/ other organisms• Consumers of the biosphere


Chloroplasts: Site of Photosynthesis (plants)

• Leaf

Vein

Leaf cross section

Figure 10.3

Mesophyll

CO2 O2Stomata


Leaf Anatomy


Chloroplasts

• Chloroplast Structure– Contain grana which consisting of thylakoid stacks

Chloroplast

Mesophyll

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm


Chloroplasts


Photosynthesis summary reaction

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


Chloroplasts split water into

• H2 and O2, incorporating the e- of H2 into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O6

6 H2O

6 O2

Figure 10.4


Photosynthesis as a Redox Process

• Water is oxidized, CO2 is reduced

• Protons and Electron are taken from water and added to CO2


The Two Stages of Photosynthesis: A Preview

• Light Reactions– Occurs on thylakoid membranes– Converts solar E to chemical E

• Dark Reaction (Calvin Cycle)– Occurs in the stroma– Forms sugar from carbon

dioxide, using ATP for energy and NADPH for reducing power


Overview of photosynthesis

H2O CO2

Light

LIGHT REACTIONS

CALVINCYCLE

Chloroplast

[CH2O](sugar)

NADPH

NADP

ADP

+ P

O2Figure 10.5

ATP


Lets Talk about Light

• Form of electromagnetic E, travels in waves


Wavelength ()

• Distance between the crests of waves

• Determines the type of electromagnetic E

More

Power

ful


Electromagnetic spectrum

Entire range of electromagnetic E, or radiation

Gammarays X-rays UV Infrared

Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

106 nm 103 m

380 450 500 550 600 650 700 750 nm

Visible light

Shorter wavelength

Higher energy

Longer wavelength

Lower energy


• Visible light spectrum

– Colors of light we can see

– ’s that drive photosynthesis


Photosynthetic Pigments: The Light Receptors

• Substances that absorb visible light


Pigments

Reflect light, which include the colors we see

Light

ReflectedLight

Chloroplast

Absorbedlight

Granum

Transmittedlight

Figure 10.7


Transmitted vs. Absorbed Light

Figure 10.8

Whitelight

Refractingprism

Chlorophyllsolution

Photoelectrictube

Galvanometer

Slit moves topass lightof selectedwavelength

Greenlight The high transmittance

(low absorption)reading indicates thatchlorophyll absorbsvery little green light.

The low transmittance(high absorption) readingchlorophyll absorbs most blue light.

Bluelight

1

2 3

4

0 100

0 100


Absorption spectra of 3 types of pigments

Ab

sorp

tion

of

ligh

t b

ych

loro

pla

st p

igm

en

tsChlorophyll a

Wavelength of light (nm)

Chlorophyll b

Carotenoids


Action spectrum of a pigment

• Effectiveness of different of radiation in driving photosynthesis

Rat

e o

f ph

otos

ynth

esis

(mea

sure

d by

O2 r

elea

se)

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.

(b)


First demonstrated by Theodor W. Engelmann

400 500 600 700

Aerobic bacteria

Filamentof alga

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.

(c)

Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.

CONCLUSION



Chlorophylls: Photosynthetic Pigments

• Chlorophyll a

– Main photosynthetic pigment

• Chlorophyll b

– Accessory pigment

C

CH

CH2

CC

CC

C

CNNC

H3C

C

C

C

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

H

CH3C

HH

CH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

Figure 10.10


Assesory proteins

• Other accessory pigments– Absorb different s of light and pass the E to

chlorophyll a


Excitation of Chlorophyll by Light

• When a pigment absorbs light electrons go from their ground state to an excited state (unstable)

Excitedstate

Ene

rgy

of e

lect

ion

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

e–

Figure 10.11 A


Photosystems I and II

MillmakesATP

ATP

e–

e–e–

e–

e–

Pho

ton

Photosystem II Photosystem I

e–

e–

NADPH

Pho

ton


Starter Question

• Compare and contrast the electron transport chain in cellular respiration with the light reactions in photosynthesis. Be sure to indicate similarities and differences


Photosystem II and I: Site of Photophosphorylation

• Proton Motive Force? Non Cyclic Flow to Calvin Cycle


Chemiosmosis in Chloroplasts v. Mitochondria

• Spatial organization of chemiosmosisKey

Higher [H+]Lower [H+]

Mitochondrion Chloroplast

MITOCHONDRIONSTRUCTURE

Intermembrancespace

Membrance

Matrix

Electrontransport

chain

H+ DiffusionThylakoidspace

Stroma

ATPH+

PADP+

ATPSynthase

CHLOROPLASTSTRUCTURE

Figure 10.16


Light reactions and chemiosmosis: Cyclic Flow

LIGHTREACTOR

NADP+

ADP

ATP

NADPH

CALVINCYCLE

[CH2O] (sugar)STROMA(Low H+ concentration)

Photosystem II

LIGHT

H2O CO2

Cytochromecomplex

O2

H2OO2

1

1⁄2

2

Photosystem ILight

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane

ATPsynthase

PqPc

Fd

NADP+

reductase

NADPH + H+

NADP+ + 2H+

ToCalvincycle

ADP

PATP

3

H+

2 H++2 H+

2 H+

NADPH and O2 Not produce. Does not go toCalvin Cycle



Calvin cycle

• Uses ATP and NADPH to convert CO2 to sugar

• Similar to the citric acid cycle

• Occurs in the stroma


Calvin Cycle Happens in the Stroma


The “Other” Calvin Cycle

• Calvin cycle

(G3P)

Input(Entering one

at a time)CO2

3

Rubisco

Short-livedintermediate

3 P P

3 P P

Ribulose bisphosphate(RuBP)

P

3-Phosphoglycerate

P6 P

6

1,3-Bisphoglycerate

6 NADPH

6 NADPH+

6 P

P6

Glyceraldehyde-3-phosphate(G3P)

6 ATP

3 ATP

3 ADP CALVINCYCLE

P5

P1

G3P(a sugar)Output

LightH2O CO2

LIGHTREACTION

ATP

NADPH

NADP+

ADP

[CH2O] (sugar)

CALVINCYCLE

O2

6 ADP

Glucose andother organiccompounds

Phase 1: Carbon fixation

Phase 2:Reduction

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

From Light

Reactions

From Light

Reactions

Glyceraldehyde-3-PCan go to sugars, amino acids,fatty acids


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


• On hot, dry days, plants close their stomata

– Conserving water but limiting access to CO2

– Causing O2 to build up photorespiration


Photorespiration: An Evolutionary Relic?

Photosynthetic rate is reduced


C4 Plants (e.g. corn)

• Minimize photorespiration

– Incorporate CO2 into four carbon compounds in mesophyll cells


• 4 carbon compounds in bundle sheath cells

release CO2 CO2 Calvin cycle


• C4 leaf anatomy and the C4 pathway

CO2

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Photosyntheticcells of C4 plantleaf

Stoma

Mesophyllcell

C4 leaf anatomy

PEP carboxylase

Oxaloacetate (4 C) PEP (3 C)

Malate (4 C)

ADP

ATP

Bundle-Sheathcell CO2

Pyruate (3 C)

CALVINCYCLE

Sugar

Vasculartissue

Figure 10.19

CO2


CAM Plants (e.g. pineapple)

• Open their stomata at night, CO2 organic acids


• During the day, stomata close

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


• CAM pathway is similar to the C4 pathway

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

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

(b)

PineappleSugarcane

Bundle-sheath cell

Mesophyll Cell

Organic acid

CALVINCYCLE

Sugar

CO2 CO2

Organic acid

CALVINCYCLE

Sugar

C4 CAM

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Night

Day

1

2 Organic acidsrelease CO2 toCalvin cycle

Figure 10.20


• Review

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

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

O2

CO2H2O

Light

Light reaction Calvin cycle

NADP+

ADP

ATP

NADPH

+ P 1

RuBP 3-Phosphoglycerate

Amino acidsFatty acids

Starch(storage)

Sucrose (export)

G3P

Photosystem IIElectron transport chain

Photosystem I

Chloroplast

Figure 10.21


• Organic compounds produced by photosynthesis

– Provide the E and building material for ecosystems


Extra stuff: Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released.


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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ch. 10 Photosynthesis