Photosynthesis: The ultimate source of energy for biofuel production.

How does it work?

Daniel BushDepartment of BiologyColorado Sate University

CO2 + H2 OLo we r e ne rg y mo le c ule

Sugar + O2Hig h e ne rg y mo le c ule

Ene rg y

Re s pira t io n( oxidat ion)

Ene rg y

Pho t o s ynt he s is(reduct ion)

On a global scale, life revolves around these two reactions!!!

Light fromthe sun photoautotrophic

heterotrophic

Photosynthesis:Have to understand the basic reactions to understand the impact it has on the physiology of the plant (or algae).

WHY?

The challenges for developing “new energy”crops

• Identify and improve new bioenergy crops that:– generate the maximum biomass per m2 *– maximize water and nutrient use efficiency *– are tolerant of sub-optimal soils and/or environments

(temperature & H2O) *– have value added traits that enhance their suitability for

biofuel production (cell wall & designer chemicals) *– are amenable to genetic and transgenic modification

• Sustainable production system– economic: makes sense to growers and industry– Environment (reduced water, pesticide, fertilizer use)

Leaf cross sectionVein

Mesophyll

Stomata CO2 O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i CALVIN

CYCLE

[CH2O](sugar)

Energy Transduction Carbon Assimilation

Why are leaves green• When light hits

something, it may be reflected, transmitted, or absorbed– A leaf is green

because chlorophyll absorbs red and blue light, so light reflected and/or transmitted through the leaf is enriched in green light.

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Abs

orpt

ion

of li

ght b

ych

loro

plas

t pig

men

ts

400 500 600 700

CH3

CHO

in chlorophyll ain chlorophyll b

Porphyrin ring:light-absorbing“head” of molecule; note magnesium atom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes of chloroplasts; H atoms not shown

SO….. What happens when chlorophyll absorbs light and HOW isthat related to energy??

heat

A high energy electron captured, first step in transforming light into ATP and NADPH!!!

lightlight

Three fates of excited electron

Early experiment illuminating filamentous algae with light spectrum demonstrated not all wave lengths of light are able to drive photosynthesis. Englemann (1883) used aerobic bacteria as indirect reporter of oxygen evolution.

In the energy transduction reactions of photosynthesis, two photons of light energy are absorbed in series to excite an electron to a higher energy state. This electron is captured in NADPH. The electron is first removed from chlorophyll in photosystem II, then passed to photosystem I, and finally onto NADP+ as the final acceptor. The electron removed from PS II is replaced by one extracted from H2O. For every 4 electrons removed from two waters, one oxygen (O2) is released.

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Thyl

akoi

d m

embr

ane

In photosystems of PS, the excited electron is efficiently captured by the electron acceptor.The key next step is to move that electron along via the electron transport chain, before it falls back to the special chlorophylls.

Antenna pigments include: carotenoids and other chlorophylls. The antenna systems are organized in large systems called Light Harvesting Complexes.

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

Ener

gy o

f ele

ctro

nsO2

e–

e–

+2 H+

H2O

O21/2

Replace the missing e- by another e- from water!

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

Ener

gy o

f ele

ctro

nsO2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

Grab the excited e-and move it into the electron transport chain.

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

Ener

gy o

f ele

ctro

ns

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

Light

Pass the excited e- to PS1, and “pump it up” again. Thus, two photons of light are used to energize every electron!!

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2En

ergy

of e

lect

rons

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

e–e–

ElectronTransportchain

NADP+

reductase

Fd

NADP+

NADPH+ H+

+ 2 H+

Light

The excited e- is finally captured in a stable chemical form in NADPH.

chlorophyll

chlorophyll

Every time light is absorbed by a photosystem, an electron is pushed to a higher energy state. By the time it is stabilized in NADPH, it is at a higher energy state than where it was in H2O (difference between two red lines).

There are a lot of intermediate electron transfer steps that make the energy transduction reactions VERY efficient.

ATP is made by the ATP-synthase in the thylakoid membrane. Its uses proton motive force generated as electrons move through the electron transport chain. WHAT is PROTON MOTIVE FORCE?

Proton Motive Force (PMF) is the chemical energy available when unequal concentrations of protons (H+) are separated across a membrane. Moreover, if protons are being moved across the membrane by any energy source, there may also be an electrical component to the PMF. ∆H

+ (mV) = -2.3 RT ∆pH + F ∆Ψ. At room temperature = -60 ∆pH + F ∆Ψ

H+H+

H+ H+ H+

H+H+

H+

H+

H+

H+

H+

H+

H+ H+

H+

H+

H+

H+

H+

( - )This side is negative relative to the other side

(a proton-pump can use light or ATP for energy)membrane

Direction of electrochemical potential

Proton pump

The ATP-synthase in the thylakoid membrane uses the energy in the proton motive force to synthesize ATP.

STROMA(Low H+ concentration)

Light

Photosystem II Cytochromecomplex

2 H+

LightPhotosystem I

NADP+

reductaseFd

PcPq

H2O O2

+2 H+

1/22 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATPADP

+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i CALVIN

CYCLE

[CH2O](sugar)

Energy Transduction Carbon Assimilation

Excitedstate

Heat

Photon(fluorescence)

GroundstateChlorophyll

molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

Ener

gy o

f ele

ctro

n

e–

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

Ener

gy o

f ele

ctro

nsO2

Absorb light, pass e- to the primary acceptor

Chlorophyll and other pigments are part of an antenna systemthat feeds electrons to PS II and PSI. The antenna make light absorption very efficient.

Some herbicides target electron transport in photosynthesis