Photosynthesis 1. 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from...

Photosynthesis

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Photosynthesis Overview

• Energy for all life on Earth ultimately comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Chloroplast Structure

• Thylakoid membrane – internal membrane– Contains chlorophyll and other photosynthetic

pigments– Pigments clustered into photosystems

• Grana – stacks of flattened sacs of thylakoid membrane

• Stroma – semiliquid surrounding thylakoid membranes

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Vascular bundle Stoma

Cuticle

Epidermis

Mesophyll

Chloroplast

Inner membraneOuter membrane

Cell wall

1.58 mm

Vacuole

Courtesy Dr. Kenneth Miller, Brown University

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Stages

• Light-dependent reactions– Require light

1.Capture energy from sunlight

2.Make ATP and reduce NADP+ to NADPH• Carbon fixation reactions or light-

independent reactions– Does not require light

3.Use ATP and NADPH to synthesize organic molecules from CO2

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O2

Stroma

Photosystem

Thylakoid

NADP+ADP + Pi

CO2

Sunlight

PhotosystemPhotosystem

Light-DependentReactions

CalvinCycle

Organicmolecules

O2

ATP NADPH

H2O


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Pigments & Light

• Pigments are molecules that absorb light energy in the visible range

• Photon – particle of light– Energy of photons vary with the wavelenth of

the light. (inverse relationship)• Photoelectric effect – removal of an

electron from a molecule by light

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400 nm

Visible light

430 nm 500 nm 560 nm 600 nm 650 nm 740 nm

1 nm0.001 nm 10 nm 1000 nm

Increasing wavelength

Increasing energy

0.01 cm 1 cm 1 m

Radio wavesInfraredX-raysGamma rays

100 m

UVlight

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Absorption spectrum

• When a photon strikes a molecule, its energy is either – Lost as heat– Absorbed by the electrons of the molecule

• Boosts electrons into higher energy level

• Absorption spectrum – range of photons (by wavelength) a molecule is capable of absorbing

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Wavelength (nm)400 450 500 550 600 650 700

Lig

ht

Ab

so

rbti

on

low

highcarotenoidschlorophyll achlorophyll b


• Only two general types are used in green plant photosynthesis– Chlorophylls– Carotenoids

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Pigments in Photosynthesis

Chlorophylls

• Chlorophyll a– Main pigment in plants– Absorbs violet-blue and red light

• Chlorophyll b– Secondary pigment – absorbs light wavelengths that chlorophyll a

does not absorb

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• Structure of chlorophyll

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H2C CH

CH2CH3

H

H

H

CO

CHCCH3

CHCH3

CH2

CH2

CH2

CHCH3

CH2

CH2

CH2

CHCH3

CH3

O

CO2CH3

O

N N

N N

Mg

H

HChlorophyll a: = CH3

Chlorophyll b: = CHO

R

R

R

H

Porphyrinhead

H3C

H3CCH3

CH2

CH2

CH2

CH2

CH2

CH2

Hydrocarbontail


• Action spectrum– Relative effectiveness of different

wavelengths of light in promoting photosynthesis

– Corresponds to the absorption spectrum for chlorophylls

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Lig

ht

Ab

so

rbti

on

low

high Oxygen-seeking bacteria

Filament of green algae


• Carotenoids– Absorb blue and

violet wavelengths– Reflect red orange

and yellow wavelengths

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Oak leafin summer

Oak leafin autumn

© Eric Soder/pixsource.com

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Photosystem Organization

• Antenna complex– Gather photons and feed the captured light

energy to the reaction center• Reaction center (membrane proteins)

– 1 or more chlorophyll a molecules– Passes excited electrons out of the

photosystem

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e–Photon

Photosystem

Thylakoid membrane

Chlorophyllmolecule

Electronacceptor

Reaction centerchlorophyll

Thylakoid membrane

Electrondonor e–

Reaction center

• Transmembrane proteins used• When a chlorophyll in the reaction center

absorbs a photon of light, an electron is excited to a higher energy level

• Light-energized electron can be transferred to the primary electron acceptor, reducing it

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Light

e–

–+–+

Excitedchlorophyllmolecule

Electrondonor

Electronacceptor

Chlorophyllreduced

Chlorophylloxidized

Donoroxidized

Acceptorreduced

e–

e– e–

e–

e–

e–

e–

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Light-Dependent Reactions

1. Light Capture– Photon of light is captured by a pigment molecule– electron excited

2. Charge separation – Energy is transferred to the reaction center; an

excited electron is transferred to an acceptor molecule

3. Electron transport– Electrons move through carriers to reduce NADP+

4. Chemiosmosis – diffusion of H+ ions across the membrane– Produces ATP using ATP synthase

Cap

ture

of

light

ene

rgy

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En

erg

y o

f el

ectr

on

s

High

Low

e–

Photon

Photosystem

Excited reaction center

Electronacceptor

Electronacceptor

Reactioncenter (P870)

b-c1complex ATPe–

e–

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Chloroplasts have two connected photosystems

• Photosystem I (P700)

• Photosystem II (P680)– Membrane proteins– Working together, the two photosystems carry out a

transfer of electrons that is used to generate both ATP and NADPH

– Photosystems replenished with electrons obtained by splitting water

Wavelength of light used (in nm)

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Photosystem II Photosystem Ib6-f complex

Stroma

PlastoquinoneProton

gradientPlastocyanin Ferredoxin

H+

H+H+

H+

NADPH

ATPADP

+ NADP+

NADPHNADPATPADP + Pi

CalvinCycle

PhotonPhoton

H2O

e–e–

e–

Fd

PC

PQ

1. Photosystem II absorbs photons, exciting electrons that are passed to plastoquinone (PQ). Electrons lost from photosystem II are replaced by the oxidation of water, producing O2

2. The b6-f complex receives electrons from PQ and passes them to plastocyanin (PC). This provides energy for the b6-f complex to pump protons into the thylakoid.

3. Photosystem I absorbs photons, exciting electrons that are passed through a carrier to reduce NADP+ to NADPH. These electrons are replaced by electron transport from photosystem II.

4. ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi

enzyme acts as a channel for protons to diffuse back into the stroma using this energy to drive the synthesis of ATP.

NADPreductase

ATPsynthase

1/2O2 2H+

Water-splittingenzyme

Thylakoidspace

AntennacomplexThylakoid

membrane


H+

H+

e–22 22

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Chemiosmosis

• Proton, (H+), gradient can be used to synthesize ATP

• Chloroplast has ATP synthase enzymes in the thylakoid membrane– Allows protons back into stroma

• Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

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Carbon Fixation – Calvin Cycle

• To build carbohydrates cells use• Energy

– ATP from light-dependent reactions– Drives endergonic reaction

• Reduction potential– NADPH from photosystem I– Source of protons and energetic electrons

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Calvin cycle

• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis

• Key step is attachment of CO2 to RuBP to form PGA

• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

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4 Pi

12 NADP+

12

12 ADP

NADPHNADP+ADP+ Pi


CalvinCycle

6 molecules of12 molecules of

12 molecules of

1,3-bisphosphoglycerate (3C)

12 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

10 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

Stroma of chloroplast6 molecules of

Carbondioxide (CO2)

12 ATP

6 ADP

6 ATP

Rubisco

Calvin Cycle

Pi

Ribulose 1,5-bisphosphate (5C) (RuBP)3-phosphoglycerate (3C) (PGA)

Glyceraldehyde 3-phosphate (3C)

2 molecules of

Glucose andother sugars

12 NADPH

ATP

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3 phases

1. Carbon fixation– RuBP + CO2 → PGA

2. Reduction– PGA is reduced to G3P

3. Regeneration of RuBP– PGA is used to regenerate RuBP

• 3 turns incorporate enough carbon to produce a new G3P

• 6 turns incorporate enough carbon for 1 glucose

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Output of Calvin cycle

• Glucose is not a direct product of the Calvin cycle

• Glyceraldehyde 3-phosphate is produced– G3P is a 3 carbon sugar– Used to form glucose and sucrose

• Major transport sugar in plants• Disaccharide made of fructose and glucose

– Used to make starch• Insoluble glucose polymer• Stored for later use

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Photorespiration

• Rubisco has 2 enzymatic activities– Carboxylation

• Addition of CO2 to RuBP

• Favored under normal conditions

– Photorespiration• Oxidation of RuBP by the addition of O2

• Favored when stoma are closed in hot conditions• Creates low-CO2 and high-O2

• CO2 and O2 compete for the active site on RuBP

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Heat

Stomata

O2O2

CO2 CO2

Under hot, arid conditions, leaves lose water byevaporation through openings in the leavescalled stomata.

The stomata close to conserve water but as aresult, O2 builds up inside the leaves, and CO2

cannot enter the leaves.

Leafepidermis

H2OH2O

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Types of photosynthesis

• C3

– Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

• C4 and CAM– Add CO2 to PEP to form 4 carbon molecule

– Use PEP carboxylase– Greater affinity for CO2, no oxidase activity

– C4 – spatial solution

– CAM – temporal solution

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CO2

RuBP

3PG(C3)

a. C4 pathway

Bundle-sheath cellMesophyll cell

Stoma Vein

G3P

b. C4 pathwayStoma Vein

Mesophyll cell

G3P

CO2

CO2

C4

Bundle-sheath cell

Mesophyllcell

Bundle-sheathcell

CalvinCycle

Mesophyllcell

CalvinCycle

a: © John Shaw/Photo Researchers, Inc. b: © Joseph Nettis/National Audubon Society Collection/Photo Researchers, Inc.

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C4 plants

• Corn, sugarcane, sorghum, and a number of other grasses (monocots)

• Initially fix carbon using PEP carboxylase in mesophyll cells

• Produces oxaloacetate, converted to malate, transported to bundle-sheath cells

• Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2

• Carbon fixation then by rubisco and the Calvin cycle

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Oxaloacetate

Pyruvate Malate

Glucose

MalatePyruvate

+ Pi

Mesophyllcell

Phosphoenolpyruvate (PEP)

Bundle-sheathcell

CalvinCycle

AMP +PPi

ATP

CO2

CO2

• C4 pathway, although it overcomes the problems of photorespiration, does have a cost

• To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone

• C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

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CAM plants

• Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups

• Stomata open during the night and close during the day– Reverse of that in most plants

• Fix CO2 using PEP carboxylase during the night and store in vacuole

• When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2

• High levels of CO2 drive the Calvin cycle and minimize photorespiration

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Compare C4 and CAM

• Both use both C3 and C4 pathways

• C4 – two pathways occur in different cells

• CAM – C4 pathway at night and the C3 pathway during the day

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Photosynthesis 1. 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from...

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Transcript of Photosynthesis 1. 2 Photosynthesis Overview Energy for all life on Earth ultimately comes from...