Chapter 14
Energy conversion
From Foods:
Mitochondria
Energy conversion
& Free energy
Works in hydroelectricity
Heat
Potential E
Kinetic E
Heat
Electrical E
FREE ENERGY : Energy available for this work (conversion)
Designs for ATP synthesis
Chemiosmosis
Chemiosmosis is the name given to the generation of ATP from a proton
gradient. It occurs in all living things:
Energy
A Design for ATP synthesis:
Oxidative phosphorylation
Pyruvate  CO2 + NADH
NADH + O2 ATP + H2O
Another Design for ATP synthesis:
Light-induced phosphorylation
Cyclic phosphorylation
PS-I only
Non-cyclic phosphorylation
PS-I and II
Cyclic Photophosphorylation
• Process for ATP generation associated with some
Photosynthetic Bacteria
• Reaction Center => 700 nm
Noncyclic Photophosphorylation
Photosystem II regains electrons by splitting water, leaving O2
gas as a by-product
Primary
electron acceptor
Primary
electron acceptor
Photons
Energy for
synthesis of
PHOTOSYSTEM I
PHOTOSYSTEM II
by chemiosmosis
Chapter 14
Energy conversion From Light:
Chloroplast
Green plants in Ecosystem
THE SUN: MAIN SOURCE OF
ENERGY FOR LIFE ON EARTH
Autotrophs (self + nutrition in Greek) : an organism that produces
complex organic compounds from simple inorganic molecules using
energy from light (by photoynthesis) or inorganic chemical reactions.
The proximate cause…
Why
are green?
green?
Why plants
plants are
Why plants are green?
Gamma
rays
X-rays
UV
Infrared &
Microwaves
Visible light
Wavelength (nm)
Radio waves
THE COLOR OF LIGHT SEEN IS THE
COLOR NOT ABSORBED
The thylakoid membrane of
the chloroplast is impregnated
with photosynthetic pigments
Light
Reflected
light
Transmitted
light
Chloroplast
Absorbed
light
(i.e., chlorophylls, carotenoids).
• The location and structure of chloroplasts
Chloroplast
LEAF CROSS SECTION
MESOPHYLL CELL
LEAF
Mesophyll
CHLOROPLAST
Intermembrane space
Outer
membrane
Granum
Grana
Stroma
Inner
membrane
Stroma
Thylakoid
Thylakoid
compartment
The location and structure of chloroplasts
A chloroplast contains:
-stroma, a fluid
-grana, stacks of thylakoids
The thylakoids contain chlorophyll
-Chlorophyll is the green pigment that
captures light for photosynthesis
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
outer membrane
intermembrane space
inner membrane (1+2+3: envelope)
stroma (aqueous fluid)
thylakoid lumen (inside of thylakoid)
thylakoid membrane
granum (stack of thylakoids)
thylakoid (lamella)
starch
ribosome
plastidial DNA
plastoglobule (drop of lipids)
Absorbance of pigments in
chloroplasts
– Chlorophyll-a
– Chlorophyll-b
Carotenoids
Chlorophyll a & b
•Chl a has a methyl
group
•Chl b has a carbonyl
group
Porphyrin ring
delocalized e-
Phytol tail
Chlorophyll-a
(type-a in green plants and algae)
Beta-carotene
(Mostly in algae)
Chlorophyll-b
(type-b in green plants and algae)
Chloroplast
Dark reactions
for Photosynthesis
PHOTOSYNTHESIS
Photosynthesis is the process by which autotrophic
organisms use light energy
to make sugar and oxygen gas from carbon dioxide
and water
Carbon
dioxide
Water
Glucose
PHOTOSYNTHESIS
Oxygen
gas
Photosynthesis
Light and Dark reactions
Light reactions:
12H2O + 12NADP + 18ADP → 6O2 + 12NADPH + 18ATP
Dark reactions:
6CO2 + 12NADPH + 18ATP → C6H12O6 + 12NADP + 18ADP + 6H2O
Two distinct stages, the light reactions, which convert light energy to
ATP and NADPH; and the dark reactions, which convert CO2 to
carbohydrate using ATP and NADPH. Both occur in the chloroplasts.
Photosynthesis
Light and Dark reactions
Light
NADP
Light
reactions
Calvin
cycle
Photosynthesis
Two types of photosystems
in the light reactions
By theory, it appears to takes just four
electrons (and four protons) to reduce CO2 to
carbohydrate. However, we find we need
eight photons per CO2, implying that two
photochemical reactions are needed per
electron, and that there are two kinds of
photosystem operating in series, each
physically separate in its own kind of particle.
ATP
mill
Water-splitting
Photosystem II
NADPH-producing
Photosystem I
PSII: Plants produce O2 gas by
splitting H2O
The O2 liberated by photosynthesis is made from the
oxygen in water (H+ and e-)
Photosystem II
PSII
Chemiosmosis for ATP production
Thylakoid
compartment
(high H+)
Light
Light
Thylakoid
membrane
Antenna
molecules
Stroma
(low H+)
Reaction
center
ELECTRON TRANSPORT
CHAIN
PHOTOSYSTEM II
PHOTOSYSTEM I
ATP SYNTHASE
PSII
(PSII+ LHC)
Resonance transfer of electrons to PSII
PSII
(water oxidase system)
Antenna a complex of
the pigment molecules are arranged in
blocks of about 50
The reaction centre, where the photochemical reaction occurs. The
excited chlorophyll-a ejects an electron, becoming an extremely strong oxidising
agent, capable of pulling electrons out of water. The antenna plus the reaction centre
taken together are termed a photosystem.
P680, Chlorophyll-b mainly absorb 60 nm light
LHC-II
(Light-harvesting complexes)
1) Resonance transfer of photons
From LHC to PSII
2) Preventing ‘Back-up’ of electrons
This is important because if photosystem I receives too little energy
compared to PSII, electrons will 'back up' the transport chain, and
prevent excited electrons from escaping photosystem II.
2) Preventing ‘Photoinhibition’
Excited chlorophyll molecules in photosystem II will not be
quenched by electrons from water, and will cause oxidative damage
to the reaction centre. This causes the destruction of photosystem II,
LHC-II phosphorylated
 Association with PSII
Green, Chlorophyll-a/b; Yellow, Cartenoids
Too much light
 LHC-II de-phosphorylated
 Dissociation with PSII
An Energy spacer?
Light pumping to P680
When a chlorophyll molecule within the
LHC contacts a photon of light, resonance
energy is produced.
This resonance energy is transferred
through several more chlorophyll
molecules until it reaches the P680
chlorophyll molecules at the heart of the
Photosystem II reaction center.
The resonance energy causes the loss of
an electron from the P680 molecules. This
electron is then transferred to a pheophytin
molecule, then to Qa and finally to Qb.
P680 is then reduced by the splitting of a
water molecule which replaces the electron
lost in this process.
Why photons in LHC
are channeled to PSII?
An Energy spacer?
Resonance transfer of photons
LHC
P680 or P700
LHC
P680
E=hc⁄λ
h, Planck's constant, 6.6 × 10−34 J s.
c, speed of light, 3 × 108 m s−1.
Blue light is more energetic
The ultimate cause…
Why plants
green?
Plants
must are
be green?
LHC
P680, 700
Transfer of electrons
Primary
electron
acceptor
Primary
electron
acceptor
Energy
to make
NADP
3
2
Light
Light
Primary
electron
acceptor
1
Reactioncenter
chlorophyll
Water-splitting
photosystem
2 H + 1/2
P680 become hungry
for e- and take it from
water
NADPH-producing
photosystem
PSI
(ferredoxin reductase system)
Chemiosmosis for ATP production
Thylakoid
compartment
(high H+)
Light
Light
Thylakoid
membrane
Antenna
molecules
Stroma
(low H+)
Reaction
center
ELECTRON TRANSPORT
CHAIN
PHOTOSYSTEM II
PHOTOSYSTEM I
ATP SYNTHASE
Chemiosmosis
in PSII + electron transport chains
• A Photosynthesis Road Map
Chloroplast
Light
Stroma
NADP
Stack of
thylakoids
ADP
+P
Light
reactions
Calvin
cycle
Sugar used for
 Cellular respiration
 Cellulose
 Starch
 Other organic compounds
The Calvin Cycle
3 CO2 + 6 NADPH + 5 H2O + 9 ATP → C3H5O3-PO32- + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi
RUBISCO
From
Photosynthesis
Photosynthesis uses light energy
to make food molecules
Chloroplast
Light
Photosystem II
Electron
transport
chains
Photosystem I
CALVIN
CYCLE
A summary of the
chemical processes of
photosynthesis
Stroma
Cellular
respiration
Cellulose
Starch
LIGHT REACTIONS
CALVIN CYCLE
Other
organic
compounds