All biological energy ultimately comes from solar or geothermal energy harnessed by autotrophic...

All biological energy ultimately comes from solar or geothermal energy harnessed by

autotrophic organisms

Chemoautotrophs

Photoautotrophs

Photosynthesis occurs in 5 of the 9 phylogenetic divisions of eubacteria

Bacteriorhodopsin, the simplest form of phototrophy

All trans retinal 13-cis retinal

CO2 + 2H2S ---> (CH2O) + 2S + H2O

Photosynthetic green sulfur bacteria

Hypothesis

CO2 + 2H2A ---> (CH2O) + 2A + H2A

h

h

A = oxygen in cyanobacteria and plants and anoxic sulfur in green and purple

sulfur bacteria

CO2 + 2H2A ---> (CH2O) + 2A + H2A

2 half reactions

2H2A ---> 2A + 4[H•]

4[H•] + CO2 ---> (CH2O) + H2O

Light reactions

h

Dark reactions

Electron micrograph of a section through the purple photosynthetic bacterium Rhodobacter

sphaeroides.

Amount needed to make ATP = 30-30 kJ

E = h = hc/

The nature of light

h = 6.626 x 10-34 J•s

c = 2.998 x 108 m•s-1

Amount of light absorbed is a funtion of the physical properties of the absorbing medium

A = log I0/I = cl

= molar extinction coefficient M-1cm-1)c = concentration (M)l = pathlength (cm)

for chlorophylls are among the highest for organic molecules ≈ 105 M-1cm-1

Absorbed energy can be dissipated in several ways

Internal conversion: very fast < 10-11 s

Electronic energy is converted to kinetic energy

Fluorescence: very fast ≈ 108 sAbsorbed energy is re-emitted

generally at a lower energy/longer wavelength

Absorbed energy can be dissipated in several ways

Excition transfer: slowEnergy is passed from molecule to molecule

Photoxidation: slow

Energy is transferred to a photosynthetic reaction system

The amount of O2 evolved by Chlorella algae versus the intensity of light flashes.

300 chlorophyll/Reaction center

Since there is an excess of chlorophyll it is unlikely that all of them function as reaction

centers

Most are act as antennae to harvest light from a variety of wavelengths

and transfer it to a single reaction center

What is the nature of the antennae

Take the example of purple non-sulfur bacteria

-proteobacteria

Light harvesting complex 2

Green = bacteriochlorophyll aYellow = lycopene

Light harvesting complex 1 surrounds the reaction center

Its chlorophyll is slightly lower in energy to facilitate exciton transfer

Ultimately all the photons harvested make their way to the reaction center

Cyanobacteria

PE = phycoerythrinPC = phycocyaninAP = allophyocyanin

Light harvesting complexes in plants are much more complex and have a wide array of pigment molecules

-Carotene

HN

O

R

HN

R

HN

R

HN

O

R

Phycocyanobilin

LH2 from pea

Green = chlorophyll aRed = chlorophyll bYellow = lutein

Alpha proteobacteria

(Chl)2 + 1 exciton ---> (Chl)*2

(Chl)*2 + Pheo ---> •(Chl)+2 + •Pheo-

2 •Pheo- + 2H+ + QB ---> 2Pheo + QBH2

∆E’º = +0.95V !!!!

Coenzyme Q UbiquinoneCoQQ

Redox loops pumps out four protons!

Photosynthetic electron-transport system of purple photosynthetic bacteria.

Related to complex III

Oxidation of sulfur

Electrons taken from reaction center to reduce NAD+ are replaced by the

oxidation of H2S to S0 and SO4

2-

Plants and cyanobacteria

FeS type

Pheo type

H3CO CH3

O

O

(CH2H3CO CH

C

CH3

CH2)nH

H3CO CH3

OH

OH

(CH2H3CO CH

C

CH3

CH2)nH

H3C H

O

O

(CH2H3C CH

C

CH3

CH2)nH

H3C H

OH

OH

(CH2H3C CH

C

CH3

CH2)nH

Ubiquinone Plastoquinone

Net reaction

2H2O + 2NADP+ + 8 photons ---> O2 + 2NADPH + 2H+

4P680 + 4H+ + 2PQB + 4photons ---> 4P680+ + 2PQBH2

Oxygen evolvingComplex

In cyanobacteria plastocyanin is be replaced by a small cytochrome c like protein

Cyt c6 can perform both roles in this bacterium

Photosystem I is related to bacterial FeS type photosystem

2Fdred + 2H+ + NADP+ ---> 2Fdox + NADPH + H+

During Cu deficiency plastocyanin can be replaced with a cytochrome c like molecule

About 3ATP are made per O2 produced

2H2O + 8 photons + 2NADP+ + 3ADP + 3Pi --->O2 + 3ATP + 2NADPH

Cyclic pathway does not generate NADPH

Photosystem I and II are spatially separated to prevent exciton transfer and loss of

proton gradient

Photosystem I in unstacked stroma lamellae

Photosystem II in closely stacked grana

The Calvin cycle.

3CO2 -----> GAP

9 ATP and 6 NADPH

3C53C1

6C3

6C3

1C3

C6

C3+C3

C3+C4

C6+C3

C5

C4

C7+C3

C7

C5

C3 + C3 ---> C6

C3 + C6 ---> C4 + C5

C3 + C4 ---> C7

C3 + C7 ---> C5 + C5

Overal reaction = 5C3 ---> 3C5

1 GAP molecule is made from 3CO2

3CO2 + 9ATP + 6NADPH ---> GAP + 9ADP + 8Pi + 6NADP+

GAP is converted to glucose by gluconeogenesis

aldolasetransketolase

aldolasetransketolase

3C5 + 3C1 ---> 6C3

Photorespiration

Dissipates ATP and NADPH

What is the purpose?

To protect from photo oxidation in the absence

of CO2?

On a hot bright day CO2 may be depleted and O2 may accumulate

Under these conditions photorespiration may take over

This may prevent the photooxidation of reaction centers

By decreasing photorespiration plants save water because they do not have to

have their pores open to acquire CO2

C4 plants (such as grasses) reduce photorespiration by

physically separating CO2 and O2 acquisition from rubisco

These plants assimilateCO2 in mesophyll cells as

malate and transporting this to the site of rubisco in

bundle-sheath cells

C4 plants outgrow C3 plants on hot days

It uses more ATP to make sugars

Another type of plants called CAM plants use a variant of the C4 cycle

In this case CO2 acquisition is temporally separated from rubisco

At night when the air is cool and moist CAM plants open their pores and let CO2 in. The CO2 is

incorporated into malate and stored in the vacuole.

During the day the CO2 is released from malate and there is a steady supply of CO2 to prevent

photorespiration.

Control of the Calvin Cycle

PhosphoribulokinaseRubiscoPhosphoglycerate kinase/GAPDHFructose bisphosphataseSedoheptulose bisphosphatase

Regulation of enzymes by light

PhosphoribulokinaseGlyceraldehyde-3-phosphate dehydrogenase

Fructose bisphosphataseSedoheptulose bisphosphatase

What about Rubisco?

Responds to light dependent factors

pH of stroma increases by 1 unit when photosynthesis is on. Rubisco has a pH optimum at pH 8.0

Rubisco is activated by Mg2+, light induced influx of H+ into lumen is accompanied by Mg2+ efflux into

stroma