HPP22803 – Concepts in Environmental Plant Physiology
Photosynthesis
Lecture 1: Light reactions
1. Light absorption & charge separation
Light has properties of
- Particles (photons)
- Waves (wavelength per photon)
Energy of a photon depends on the frequency
c
E=h*f f=
λ
c
E=h* High wavelength = low energy
λ
Most light (~70-80%) in the green (500-600 nm) range is absorbed by leaves.
Photosynthetically active radiation (PAR) ranges from 350 – 750 nm, including UV-A, visible
and far-red light.
Light is absorbed by pigments organized in
supercomplexes.
- Antenna; collection of light
harvesting complexes (LHCs)
- Reaction center (RC)
Both contain pigments → antenna
increases the absorption cross-section of
the RC
Pigment molecule receives a
photon → electron jumps to a
higher orbital.
Photon falls back ground state
which passes the energy to its
surroundings → next pigment
molecule get excited; goes on
until excitation reaches the
reaction center.
Blue photons have more
energy than red photons. Still results in 1 excitation → excess energy is lost in the form of
heat.
Any photon, once absorbed by the leaf, results in the same number of excitations.
,In the supercomplex, excitations hop from one pigment to another until reaching the reaction
center. Hopping downhill energetically is strongly favored, and a small amount of energy can
be lost per hop.
Carotenoids can pass on excitation to Chlorophyll b, but not the other way around.
, Chloroplast contains grana stacks in stroma; each made up of thylakoid membrane which
encloses the lumen.
- PS2 (larger) is mostly located in the stacked region.
- PS1 (smaller) and ATPase are more often in the unstacked regions.
- Cyt b6f complex can be found everywhere.
- Mobile LHC2 can bind to both PS2 and PS1
LHC2 is everywhere, increases the absorption cross-section and the flexibility in the
movement of excitations between the two photosystems.
PS2 is larger → excitation is absorbed less quickly by the RC than in PS1.
- Excitation energy transfer in PS1
takes 20-50 ps, in PS2 100-300 ps.
- Quantum efficiency is ~0.99 is PS1,
~0.90 in PS2.
Many possible ways for excitation to reach
RC = increased flexibility.
Wooclap
1. Sort light color by energy per
photon (1 = highest, 4 = lowest)
1 = blue, 2 = green, 3 = red, 4 = far-
red
Stacked region contains mainly
LHC2 and PS2
Unstacked region contains mainly
LHC1 and PS1
2. PS1 has faster EET (20-50 ps) and
higher quantum efficiency (0.99)
PS2 has slower EET (100-300 ps) and lower quantum efficiency (0.90)
2. Electron and proton transport
PS2: energy from light is used to split water: H2O → O2 + H+
Electron transport: e- is transported through multiple protein complexes on the thylakoid
membrane (= linear electron transport) until it arrives in PS1. Light is used to transport e- to
stroma where NADPH is formed.
Proton transport: accumulation of H+ in lumen as a result of electron transport + buildup of H+
from H2O splitting. ATP synthase transports H+ from lumen to stroma which causes rotation
of ATP synthase, resulting in the synthetization of ATP from ADP and Pi.
Photosynthesis
Lecture 1: Light reactions
1. Light absorption & charge separation
Light has properties of
- Particles (photons)
- Waves (wavelength per photon)
Energy of a photon depends on the frequency
c
E=h*f f=
λ
c
E=h* High wavelength = low energy
λ
Most light (~70-80%) in the green (500-600 nm) range is absorbed by leaves.
Photosynthetically active radiation (PAR) ranges from 350 – 750 nm, including UV-A, visible
and far-red light.
Light is absorbed by pigments organized in
supercomplexes.
- Antenna; collection of light
harvesting complexes (LHCs)
- Reaction center (RC)
Both contain pigments → antenna
increases the absorption cross-section of
the RC
Pigment molecule receives a
photon → electron jumps to a
higher orbital.
Photon falls back ground state
which passes the energy to its
surroundings → next pigment
molecule get excited; goes on
until excitation reaches the
reaction center.
Blue photons have more
energy than red photons. Still results in 1 excitation → excess energy is lost in the form of
heat.
Any photon, once absorbed by the leaf, results in the same number of excitations.
,In the supercomplex, excitations hop from one pigment to another until reaching the reaction
center. Hopping downhill energetically is strongly favored, and a small amount of energy can
be lost per hop.
Carotenoids can pass on excitation to Chlorophyll b, but not the other way around.
, Chloroplast contains grana stacks in stroma; each made up of thylakoid membrane which
encloses the lumen.
- PS2 (larger) is mostly located in the stacked region.
- PS1 (smaller) and ATPase are more often in the unstacked regions.
- Cyt b6f complex can be found everywhere.
- Mobile LHC2 can bind to both PS2 and PS1
LHC2 is everywhere, increases the absorption cross-section and the flexibility in the
movement of excitations between the two photosystems.
PS2 is larger → excitation is absorbed less quickly by the RC than in PS1.
- Excitation energy transfer in PS1
takes 20-50 ps, in PS2 100-300 ps.
- Quantum efficiency is ~0.99 is PS1,
~0.90 in PS2.
Many possible ways for excitation to reach
RC = increased flexibility.
Wooclap
1. Sort light color by energy per
photon (1 = highest, 4 = lowest)
1 = blue, 2 = green, 3 = red, 4 = far-
red
Stacked region contains mainly
LHC2 and PS2
Unstacked region contains mainly
LHC1 and PS1
2. PS1 has faster EET (20-50 ps) and
higher quantum efficiency (0.99)
PS2 has slower EET (100-300 ps) and lower quantum efficiency (0.90)
2. Electron and proton transport
PS2: energy from light is used to split water: H2O → O2 + H+
Electron transport: e- is transported through multiple protein complexes on the thylakoid
membrane (= linear electron transport) until it arrives in PS1. Light is used to transport e- to
stroma where NADPH is formed.
Proton transport: accumulation of H+ in lumen as a result of electron transport + buildup of H+
from H2O splitting. ATP synthase transports H+ from lumen to stroma which causes rotation
of ATP synthase, resulting in the synthetization of ATP from ADP and Pi.
