Photosynthesis: The ultimate source of energy for biofuel production.
How does it work?
Daniel Bush
Department of Biology Colorado Sate University
CO2 + H2 O
Lo w e r e ne rg y m o le c ule
Sug ar + O2
Hig h e ne rg y m o le c ule
Ene rg y
Re s p ira t io n ( o xidat io n)
En e rg y
Pho t o s y nt he s is ( re duct io n)
On a global scale, life revolves around these two reactions!!!
Light from the 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 section Vein
Mesophyll
Stomata CO2O2
Mesophyll cell Chloroplast
5 µm
Outer membrane Intermembrane space
Inner membrane Thylakoid
space Thylakoid Granum
Stroma
1 µm
H2O
LIGHT REACTIONS
Chloroplast Light
ATP NADPH
O2
NADP+
CO2
ADP + Pi
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
Absorption of light by chloroplast pigments
400 500 600 700
CH3 CHO
in chlorophyll a in chlorophyll b
Porphyrin ring:
light-absorbing
“head” of
molecule; note magnesium atom at center
Hydrocarbon tail:
interacts with hydrophobic
regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown
SO….. What happens when chlorophyll absorbs light and HOW is that related to energy??
heat
A high energy electron captured, first step in transforming light into ATP and NADPH!!!
light
light
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-harvesting complexes
Photosystem Reaction center
STROMA
Primary electron acceptor
e–
Transfer of energy
Special
chlorophyll a molecules
Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)
Thylakoid membrane
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.
Light
P680 e–
Photosystem II (PS II) Primary acceptor
[CH2O] (sugar) NADPH
ATP ADP
CALVIN CYCLE LIGHT
REACTIONS NADP+ Light
H2O CO2
Energy of electrons
O2
e– e– +
2 H+
H2O
O2
1/2
Replace the missing e- by another e- from water!
Light
P680 e–
Photosystem II (PS II) Primary acceptor
[CH2O] (sugar) NADPH
ATP ADP
CALVIN CYCLE LIGHT
REACTIONS NADP+ Light
H2O CO2
Energy of electrons
O2
e– e– +
2 H+
H2O
O2
1/2
Pq
Cytochrome complex Electron tran
sport chain
Pc
ATP
Grab the excited e- and move it into the electron transport chain.
Light
P680 e–
Photosystem II (PS II) Primary acceptor
[CH2O] (sugar) NADPH
ATP ADP
CALVIN CYCLE LIGHT
REACTIONS NADP+ Light
H2O CO2
Energy of electrons
O2
e– e– +
2 H+
H2O
O2
1/2
Pq
Cytochrome complex Electron tran
sport chain
Pc
ATP
P700 e– Primary acceptor
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!!
Light
P680 e–
Photosystem II (PS II) Primary acceptor
[CH2O] (sugar) NADPH
ATP
ADP
CALVIN CYCLE LIGHT
REACTIONS NADP+ Light
H2O CO2
Energy of electrons
O2
e– e– +
2 H+
H2O
O2
1/2
Pq
Cytochrome complex
Electron tran
sport chain
Pc
ATP
P700 e– Primary acceptor
Photosystem I (PS I)
e– e–
Electron Trans
port chain
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 Cytochrome complex 2 H+
Light
Photosystem I
NADP+ reductase Fd
Pq Pc H2O
O2 +2 H+
1/2
2 H+
NADP++ 2H+ + H+
NADPH
To Calvin
cycle THYLAKOID SPACE
(High H+concentration)
STROMA
(Low H+concentration)
Thylakoid
membrane ATP
synthase
ATP ADP
+ P
H+
i [CH2O] (sugar)
O2
NADPH ATP ADP NADP+
CO2 H2O
LIGHT REACTIONS
CALVIN CYCLE Light
H2O
LIGHT REACTIONS
Chloroplast Light
ATP NADPH
O2
NADP+
CO2
ADP + Pi
CALVIN CYCLE
[CH2O]
(sugar)
Energy Transduction Carbon Assimilation
Excited state
Heat
Photon
(fluorescence) Ground
state Chlorophyll
molecule Photon
Excitation of isolated chlorophyll molecule Fluorescence
Energy of electron
e–
Light
P680 e–
Photosystem II (PS II) Primary acceptor
[CH2O] (sugar) NADPH
ATP ADP
CALVIN CYCLE LIGHT
REACTIONS NADP+ Light
H2O CO2
Energy of electrons
O2
Absorb light, pass e- to the primary acceptor
Chlorophyll and other pigments are part of an antenna system that feeds electrons to PS II and PSI. The antenna make light absorption very efficient.
Some herbicides target electron transport in photosynthesis
