Photosynthesis
- conversion of light energy to chemical energy
- consists of 3 major steps:
1) absorption of light – requires a pigment system
2) transfer of electrons away from an excited
pigment molecule which results in:
a) ATP synthesis (chemical energy)
b) NADPH formation (reducing power)
c) O2 evolution
3) atmospheric CO2 is converted to carbohydrate
using ATP and NADPH
Figure 8.1 The light and carbon reactions of photosynthesis in chloroplasts of land plants
Site of Photosynthesis is the Chloroplast
Chloroplasts are cellular organelles:
Thylakoid membrane separates 2 compartments
inside the chloroplast:
• lumen - inside the thylakoid membrane
• stroma - outside the thylakoid membrane
Outer membrane
Thylakoid membrane
lumen
stroma
Chloroplast Membranes
Inner membrane
Figure 7.15 Transmission electron micrograph of a chloroplast from pea (Pisum sativum)
Figure 7.16 Schematic picture of the overall organization of the membranes in the chloroplast
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Light Reactions of Photosynthesis
- requires pigment – examine chlorophyll structure
- chlorophyll molecule consists of:
1) head group – Porphyrin Ring
N atoms with Mg atom – light absorption
2) Hydrocarbon tail – anchors the molecule into
thylakoid membrane
- two types of chlorophyll (‘a’ & ‘b’) that differ
in a functional group on the porphyrin ring
Figure 7.6 Molecular structure of some photosynthetic pigments (A)
Light Absorption Spectrum of Chlorophyll
- chlorophyll absorbs light in the visible region
400–700 nm (photosynthetically active radiation)
- there are slightly different absorption spectrums
for chlorophyll ‘a’ and ‘b’
- both molecules strongly absorb blue & red light
Carotenoids – are other important pigments involved
in light absorption and funneling
light energy to the reaction center
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Photosystems
- chlorophyll and other pigments are associated with
proteins in the thylakoid membrane
- they form higher-order functional systems called
Photosystem I and Photosystem II
- Photosystems consist of:
1) Antenna Complex – which serves to absorb
and pass on solar energy to reaction the center
2) Reaction Center – complex group of proteins and
chlorophyll that initiate electron transfer reactions
7.20 Two-dimensional view of the structure of the LHCII antenna complex from higher plants
Simplified two-dimensional view of the LHCII Antenna complex
Photosystems consist of:
Figure 7.20 Structure of the LHCII Antenna complex – contains chlorophyll, carotenoid and protein
Figure 7.25 Structure of the photosystem II reaction center (A)
1) Antenna Complex – chlorophyll and carotenoids
attached to a protein structure
2) Reaction Center – protein component consists
of a complex arrangement of several gene products
Photosystem I – contains a specialized pair of
chlorophyll ‘a’ molecules at center
Photosystem II – contains a single modified
chlorophyll ‘a’ molecule at center
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Simplified Photosystem Structure
Figure 7.10 Basic concept of energy transfer in a Photosystem during photosynthesis
The two Photosystems have slightly different
structure, orientation and pigment content –
as a result their absorption spectrums differ:
Photosystem I – P700 – peak absorption at 700 nm
Photosystem II – P680 – peak absorption at 680 nm
Light Energy and Reaction Center Molecules
Antenna Complex
- physical transfer of solar energy
Figure 7.19 Funneling of excitation from the antenna system toward the reaction center
Reaction Center
- chlorophyll molecules become chemically excited
- the excited state is chemically unstable
- the excitation energy is released by one of:
1) Fluorescence – re-emit light (different wavelength)
2) Energy released as heat
3) Electron transport – eject an electron
(chemical change)
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Electron Transport – drives chemical work
• a series of oxidation/reduction reactions that
result in the production of ATP and NADPH
• electron transport can occur in two modes:
1) cyclic – around PSI (P700)
2) non-cyclic – from PSII to PSI
also known as Z-scheme transfer
• involves 4 major electron transport components:
1) Plastoquinone (PQ)
2) Cytochrome Complex
3) Plastocyanin (PC)
4) Ferrodoxin (Fd)
Figure 7.14 Z scheme of photosynthesis
Figure 7.18 Four major protein complexes of the thylakoid membrane (B)
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Figure 7.22 Transfer of electrons and protons in the thylakoid membrane
Figure 7.31 Summary of the experiment carried out by Jagendorf and co-workers
Electron Transport – drives chemical work
• major electron transport is non-cyclic,
from PSII to PSI (Z-scheme transfer)
- results in ATP, NADPH and O2 release
• cyclic electron transport can occur to balance
energy distribution between PS II and PSI
- only results in ATP formation
Water Splitting Reaction - Photolysis
• In non-cyclic electron transport – the electron released from the PSII reaction
center needs to be replaced. An electron is donated from the water splitting
reaction:
2 H2O > 4 H+ + O2 + 4 e• The water splitting reaction is catalyzed inside the lumen of the thylakoid
membrane by parts of the PSII complex
• Protons (H+) released from the reaction help to develop the proton gradient
required for ATP synthesis (H+ also comes from oxidation of plastoquinone pool)
• The electrons replace the electrons released from the PSII reaction center
• Oxygen gas is released to the atmosphere – this reaction is the source of all the
oxygen gas in our atmosphere
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Water Splitting Reaction and the Hill Reaction
• Robert Hill (professor at Cambridge University) showed in 1937 that isolated
chloroplast suspensions could release oxygen gas when illuminated – if the
chloroplast suspensions were provided with suitable electron acceptors (oxidants)
Plant Life's Boxy Heart (2011 Science 334: 1630)
Improved crystal structure of PSII
that sheds light on the water-splitting
reaction and oxygen (O2) release
• such electron acceptors could be iron (Fe) salts such as potassium ferrioxalate or
potassium ferricyanide – the reactions were analogous to water splitting:
4 Fe+3 + 2 H2O > 4 Fe+2 + O2 + 4 H+
• this reaction has become known as the Hill reaction
• Hill’s chloroplast suspensions failed to reduce (or take up) carbon dioxide (CO2)
• the Hill reaction represents a light-driven transfer of electrons from water to nonphysiological oxidants (Hill reagents) against a chemical potential gradient, the
significance was the demonstration that O2 evolution was separate from CO2
reduction in photosynthesis
• 18O tracer was later used to confirm that O2 was released from water
The crystal structure revealed that these core atoms form a cube with a short tail hanging off one end.
That shape, it turns out, is critical for holding pairs of oxygen atoms (from water) close enough
together to be “knitted” into O2.
4 x Mg – purple, 5 x O – red, 1 x Ca - yellow
Figure 7.30 Chemical structure and mechanism of action of two important herbicides
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