Transcript CHLOROPLASTS, CALVIN CYCLE, PHOTOSYNTHETIC …

CHLOROPLASTS, CALVIN
CYCLE, PHOTOSYNTHETIC
ELECTRON TRANSFER AND
PHOTOPHOSPHORYLATION
(based on Chapter 19 and 20 of
Stryer )
Photosynthesis
Photosynthesis
Light driven transfer of electron across a membrane
Results in a proton gradient which drives the bonding
of ADP and Pi to form ATP
Important in the production of Oxygen and the
generation of carbon compounds that make aerobic
metabolism possible
Light
CO2 + H2O
(CH2O) + O2
Photosynthetic yield
Huge amounts of free energy are stored
annually amounting to more than 1010 tons
of carbon in the form of carbohydrate
“If a years yield were amassed in the
form of sugar cane, it would form a heap
over two miles high and with a base 43
square miles”
- G.E.Fogge
Photosynthesis takes place in
chloroplasts
STRUCTURE OF
CHLOROPLASTS
Typically 5um long. Surrounded by chloroplast
envelope with outer and inner membrane and
intermembrane space.
Inside is the stroma containing soluble enzymes,
membranous structures called thylakoids, and a space inside
the membranes called the lumen.
Thylakoid membranes contain photosynthetic electron
transfer chain and ATP synthase for photophosphorylation.
They use light energy to synthesise NADPH and ATP (“light
reactions”).
The NADPH and ATP are used by the (Benson)-Calvin cycle
to convert CO2 into sugar (“dark reactions”).
The Calvin cycle is located in the stroma
PROKARYOTIC ORIGIN OF
CHLOROPLASTS
Phototrophic bacteria similar to cyanobacteria entered
into an endosymbiotic association with the eukaryotic
ancestors of algae and higher plants.
Chloroplasts are no longer autonomous since most of
their proteins are synthesised by nuclear genes.
Chlorophyll - photoreceptor
Cyclic tetrapyrrole
Photoinduced charge separation
CHLOROPHYLLS AND THE
REACTION CENTRE
The principal photoreceptor in chloroplasts is chlorophyll a
bound to integral membrane proteins.
Chlorophyll has two roles in trapping solar energy
a) most chlorophylls absorb light energy and transfer it to a
special pair of chlorophylls in a protein complex called a
reaction centre.
b) only the few chlorophylls forming the special pair mediate
the transformation of absorbed light energy into chemical
energy
Photosynthetic bacteria and photosynthetic
reaction centres of green plants (PSI and
PSII) have a common core
C
Prosthetic
groups
Cytochrome
L
M
H
Electron chain in the photosynthetic bacterial
reaction centre
Cytochrome heme
Two photosystems generate a
proton gradient and NADPH in
oxygenic photosynthesis
PSII transfers electrons from
water to plastoquinone and
generates a proton gradient
Cytochrome bf links photosystem
II to photosystem I
PSI uses light energy to generate
reduced Ferredoxin
PHOTOSYNTHETIC
ELECTRON TRANSFER
CHAIN
Consists of three transmembrane protein complexes, two
reaction centres (Photosystem II [PSII] and Photosystem I)
and a cytochrome bf complex analogous to Complex III.
These are linked by two mobile carriers
a) plastoquinone (PQ) analogous to UQ links PSII and the
cytochrome bf complex
b) plastocyanin in the lumen is a small protein linking
cytochrome bf complex and PSI (so analogous to
cytochrome c )
The source of energy is light. This energy is absorbed and
transferred to the two reaction centres, which use the energy
to make an electron in the chlorophyll special pair more
reducing.
PS II takes electrons from H2O ( Eo 1/2 O2/H2O =+0.82V)
and donates electrons to PQ(Eo PQ/PQH2 = 0V).
Reaction centre has added 0.82V of reducing energy, using
absorbed light energy to do so.
Electrons then flow downhill in energy terms to
plastocyanin (E0=+0.38V), where PSI uses light energy to
take this electron and use it to reduce NADPH (Eo= 0.32V) via ferredoxin and ferredoxin-NADP+
oxidoreductase (FNR).
Light causes electrons to flow from H2O to NADPH .
This also leads to generation of a proton-motive force and
thus ATP synthesis (photophosphorylation
PROTON PUMPS IN
PHOTOSYNTHETIC
ELECTRON TRANSFER
There are three proton pumps
1) oxidation of water by PSII in the lumen releases protons
2) the cytochrome bf complex pumps protons from the stroma
to the lumen
3) the reduction of NADP+ in the stroma removes protons
So a proton-motive force is created with the lumen the P side,
the stroma the N side.
Protons flow back from the lumen to the stroma via an ATP
synthase similar to that found on the IMM, leading to ATP
synthesis.
Summary
Takes place in chloroplasts
 Light absorption by chlorophyll Induces
electron transfer
 Two photosystems generate Proton Gradient
and NADPH
 Proton gradient drives ATP synthesis

BENSON-CALVIN CYCLE
(The dark reaction)
ATP and NADPH formed in the “light reactions” of
photosynthesis are used to convert CO2 into hexose ( a 6
carbon sugar) and other carbon compounds. The “dark
reactions” of photosynthesis start
Calvin Cycle
Catalysed by rubisco
6C
3C
4C
5C
1 The “dark reactions” of photosynthesis start with the
reaction of CO2 and ribulose 1,5-bisphosphate (5C) to
form two molecules of 3-phosphoglycerate (3C).
This reaction is catalysed by ribulose 1,5-bisphosphate
carboxylase/oxygenase (usually called rubisco).
This carboxylase is also an oxygenase, because oxygen
can also react with the enzyme to form
phosphoglycolate (2C) and 3-phosphoglycerate. The
recycling of phosphoglycolate leads to the wasteful loss
of organic C as CO2 and the consumption of O2, in a
process known as photorespiration.
ENERGY REQUIRED TO FIX
CARBON IN THE CALVIN
CYCLE
Six rounds of the Calvin cycle are required to synthesise 1
hexose (6C) from 6 CO2.
So a balanced equation for the net reaction of the Calvin
cycle is
6 CO2 + 18 ATP + 12 NADPH + 12 H2O
C6H12O6+ 18 ADP + 18 Pi + 12 NADP+ + 6H +
1,3-phosphoglycerate (3C) is converted into fructose 6-phosphate (6C)
in four reactions which resemble similar steps in gluconeogenesis (the
synthesis of glucose from non-carbohydrate sources such as lactate,
amino acids and glycerol).
Several of these reactions in the Calvin cycle and gluconeogenesis
(synthesis of 6C) are shared with glycolysis (breakdown of 6C), except
the essentially irreversible reactions in glycolysis have to be bypassed
by new reactions. These steps in the Calvin cycle consume 2ATP and
2NADPH per 6C formed from 2(3C).
The remaining task is to regenerate ribulose 1,5-bisphosphate (the CO2
acceptor), in other words to construct a 5C sugar from 6C and 3C
sugars, a rearrangement that consumes 1ATP
STARCH AND SUCROSE ARE
THE MAJOR
CARBOHYDRATE STORES IN
PLANTS
Starch is a polymer of glucose residues.
Starch is synthesized and stored in the chloroplasts.
Sucrose is a dissacharide.
Sucrose is synthesised in the cytosol, using triose
phosphates (3C) exported from the chloroplasts.
THIOREDOXIN
CO-ORDINATES “LIGHT”
AND “DARK” REACTIONS
Carbon dioxide assimilation and other biosynthetic
reactions are switched on in the light by reduced
thioredoxin
A 12-kd protein reduced by ferredoxin which is reduced in
turn by PSI in the light.
The reduced thioredoxin reduces disulphide bridges which
control the activities of biosynthetic enzymes.
Enzymes involved in carbohydrate degradation are switched
off in the light when reduced thioredoxin reduces their
disulphide bridges.
The rate-limiting step in the Calvin cycle is that catalysed by
rubisco.
The activity of this enzyme increases in the light because the
pH of the stroma increases from 7 to 8, and the level of Mg2+
increases in the stroma, as H+ are pumped into the lumen.
Summary
Calvin cycle synthesises hexoses from
Carbon dioxide and water
 Activity of cycle depends on environmental
conditions
