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Photosynthesis
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Introduction

Almost all the energy transferred to all the ATP molecules
in living organisms originally comes from the energy in
sunlight
Introduction

Green plants, some protoctista and some bacteria are
able to transfer sunlight energy into energy trapped in
the molecular structure of carbohydrates.
Introduction


This is the process called photosynthesis.
Once carbohydrates such as glucose have been made,
plants can convert some of them to other organic
substances such as oils, nucleic acids and proteins
Introduction

Animals cannot make organic molecules from inorganic
one and so rely entirely on plants for their supply of
organic molecules.
Photosynthesis – a summary

Photosynthesis can be summarised by the equation:
nCO2 + nH2O

light
(CH2O)n + nO2
carbohydrate
This shows that photoautotrophs synthesise carbohydrate using
carbon dioxide, water and light energy.
Q. Is photosynthesis a reduction or an oxidation?
A. CO2 is reduced; water is oxidised.
This simple summary hides the fact that photosynthesis
is a series of reactions controlled by specific enzymes.
Stages of photosynthesis
The reactions of photosynthesis
can be divided into two distinct stages.
The light dependent
stage (LDS).
The light independent
stage (LIS).
In these reactions,
ATP and a reduced
coenzyme (NADPH)
are made.
In these reactions,
the products of the
light dependent
reactions are used
to reduce carbon
dioxide to
carbohydrate.
Oxygen is a waste
product of this
stage.
An overview
oxygen (O2)
water (H2O)
light energy
light-dependent
ADP
stage
Pi
oxidised NADP
carbon dioxide (CO2)
ATP
reduced NADP
Note: the light-independent
stage is also known as
the Calvin cycle.
light-independent
stage
carbohydrates
ADP
inorganic phosphate
oxidised NADP
The chloroplast
Starch grain.
Produced from
sugars made in
photosynthesis
Small circular DNA
Granum. A stack
coding for some
of thylakoid
chloroplast proteins
membranes
outer membraneRibosomes. Smaller
than cytoplasmic
inner membrane chloroplast envelope
ribosomes.
Stroma. The site of
the Calvin cycle
Lipid droplet.
Made from the
sugars made in
photosynthesis
Lamella. A pair of
membranes
Thylakoid.
A
Thylakoid space. Space
containing chlorophyll
membranous
sac
between lamellae.
Structure to function: chloroplast
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Internal compartmentalisation.
The two stages of photosynthesis
are effectively separated, thus
allowing rate-determining factors
such as pH and enzyme
concentrations to be optimized
DNA and ribosomes mean
chloroplast can code for and
produce its own proteins such as
RuBPC
Double membrane provides
control of substances
entering/leaving the organelle
Thylakoid membranes provide a
large surface area for light
absorption
Trapping light energy



Light energy is trapped by photosynthetic pigments.
Different pigments absorb different wavelengths of light.
The photosynthetic pigments of higher plants form two groups: the
chlorophylls and the carotenoids.
Pigment
Chlorophylls:
Carotenoids:


Colour
chlorophyll a
Yellow-green
chlorophyll b
Blue-green
ß carotene
Orange
xanthophyll
Yellow
Chlorophylls absorb mainly in the red and blue-violet regions of
the light spectrum. They reflect green light which is why plants
look green.
The carotenoids absorb mainly in the blue-violet region of the
spectrum.
chlorophyll a
carotenoid
Thylakoid membranes
the possible arrangement of chlorophyll and associated molecules within the
thylakoid membranes based on studies of isolated grana
chlorophyll combined with protein
electron carriers
stalked particles containing the enzymes for catalysing
the synthesis of ATP
Trapping light energy

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
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The photosynthetic pigments fall into two categories:
primary pigments and accessory pigments
The primary pigments are two forms of chlorophyll a with
slightly different absorption peaks.
The accessory pigments include other forms of
chlorophyll a, chlorophyll b and the carotenoids. The
pigments are arranged in light-harvesting clusters called
photosystems.
In a photosystem, several hundred accessory pigment
molecules surround a primary pigment molecule and the
energy of the light absorbed by the different pigments is
passed to the primary pigment.
The primary pigments are said to act as reaction
centres.
Absorption spectra:

An absorption spectrum is a graph of
the absorbance of different
wavelengths of light by a pigment.
Action spectra:

An action spectrum is a graph of the
rate of photosynthesis at different
wavelengths of light.
Absorption spectra:
chlorophyll a
chlorophyll b
absorbance
carotenoids
400
450
500
550
600
Wavelength of light (nm)
650
700
Rate of photosynthesis
Action spectrum:
400
450
500
550
600
Wavelength of light (nm)
650
700
Photosystems


Photosystem I
This is arranged
around a molecule
of chlorophyll a with
a peak absorption
at 700nm
The reaction centre
of photosystem I is
therefore known as
P700


Photosystem II
This is arranged
around a molecule
of chlorophyll a with
a peak absorption
at 680nm
The reaction centre
of photosystem II is
therefore known as
P680
chlorophyll a
Found in PS1
absorbance
Found in PS2
400
450
500
550
600
Wavelength of light (nm)
650
700
A photosystem:
light
thylakoid
membrane
photosystem
accessory pigments
primary pigment reaction centre
P700 or P680
An overview
oxygen (O2)
water (H2O)
light energy
light-dependent
ADP
stage
Pi
oxidised NADP
carbon dioxide (CO2)
ATP
reduced NADP
Note: the light-independent
stage is also known as
the Calvin cycle.
light-independent
stage
carbohydrates
ADP
inorganic phosphate
oxidised NADP
The light-dependent reaction

Occurs in the thylakoids

Results in photophosphorylation

This can be either cyclic
photophosphorylation (CPP) or
non-cyclic photophosphorylation
(NCP)

CPP produces
– ATP
– Hydrogen ions

NCP produces
– Oxygen
– Reduced NADP
– ATP

Once the light energy is passed to the reaction centre,
electrons are energised to a level where they are emitted
from the chlorophyll.

These are used in the light dependent stage to:
– Produce reduced NADP [NADPH]
– Transfer light energy to ATP by the process of
photophosphorylation

Depending on the route taken by the released electrons
this can be either
– non-cyclic photophosphorylation or
– cyclic photophosphorylation.

Non cyclic photophosphorylation produces oxygen,
NADPH and ATP

While cyclic photophosphorylation produces just ATP and
hydrogen ions
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At the same time water molecules are split to produce
electrons
These replace the electrons ejected from the chlorophyll
and hydrogen ions.
Oxygen is given off as a waste product
Cyclic photophosphorylation
Electrons are cycled (PSI  carriers  PSI  carriers etc.)
electron carrier
ADP + Pi
electron carrier
electron carrier
ATP
2e-
electron carrier
PSI
light
electron carrier
ADP + Pi
electron carrier
2e-
electron carrier
ATP
2e-
electron carrier
PSI
to LIS
light
NADP  NADPH
2ePSII
light
H2O  21 O2 + 2e- + 2H+
waste product
NON-CYCLIC
PHOTOPHOSPHORYLATION
The production of ATP in non-cyclic photophosphorylation
When light energy is passed
to the reaction centre in
PSII, electrons are
energised to a level where
they are emitted from the
chlorophyll
The production of ATP in non-cyclic photophosphorylation
As the result of the flow of
electrons from PSI to PSII and
the breakdown of water there
is a build up of hydrogen ions.
The production of ATP in non-cyclic photophosphorylation
These accumulate within the
thylakoid space and create a
concentration gradient.
The production of ATP in non-cyclic photophosphorylation
The consequent passage of
H+ across the thylakoid
membranes provides the
energy for the production of
ATP in the presence of
ATPase. (chemiosmosis).
The production of NADPH in non-cyclic photophosphorylation
When light is absorbed by the
chlorophyll in photosystem I
(PSI), an electron is ejected
and taken up by an electron
acceptor (ferredoxin).
The production of NADPH in non-cyclic photophosphorylation
This in turn passes the
electron to a molecule of
NADP, which is thus reduced
to NADPH.
Non-cyclic photophosphorylation
This process would
eventually stop if the
released electrons were
not replaced in PSI.
This happens as a result of
light energy displacing
electrons from PSII.
Non-cyclic photophosphorylation
Electrons from PSII are
passed along a series of
electron carriers
(cytochromes) and
eventually replace the lost
electrons in PSI
If there is sufficient NADPH then the
plant will automatically switch to
CYCLIC PHOTOPHOSPHORYLATION
In this, the electrons follow
a different route; PSI is
both the donator and
acceptor of the electrons;
i.e. they follow a cyclical
route.
As light enters PSI electrons
are lost from the chlorophyll
and pass along a chain of
electron carrier molecules
before re-entering PSI.
The accumulation of H+ still
occurs in the thylakoid space,
with the consequent synthesis
of ATP, but no NADPH is
formed
The Hill Reaction
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The photolysis of water was first shown by Robert Hill in 1939
Working on isolated chloroplast he showed that they had
‘reducing power’
In the presence of an oxidising agent, oxygen was liberated
from water
He used a substance that changed colour on reduction
This can be demonstrated with a variety of substances but is
usually shown using the blue dye DCPIP
(dichlorophenolindophenol)
This is substituting for the plant’s NADP.
DCPIP becomes colourless when reduced
oxidised DCPIP
reduced DCPIP
H2O
1
2
O2
The Hill Reaction
Chloroplasts in the light
Chloroplasts in the dark for
5 minutes and then in the light
Colorimeter readings
blue
colourless
time
An overview
oxygen (O2)
water (H2O)
light energy
light-dependent
ADP
stage
Pi
oxidised NADP
carbon dioxide (CO2)
ATP
reduced NADP
Note: the light-independent
stage is also known as
the Calvin cycle.
light-independent
stage
carbohydrates
ADP
inorganic phosphate
oxidised NADP
The light-independent reaction

Occurs in the stroma
of the chloroplast

The product is sugar
which can be
converted into fats,
amino acids etc
The light-independent reaction

Also known as the Calvin cycle. This is the stage that fixes
carbon dioxide from the atmosphere

Carbon dioxide combines with a 5-carbon sugar called
ribulose bisphosphate (RuBP). The reaction is catalysed by
the enzyme RuBPC. [ribulose bisphosphate carboxylase]

The 6-carbon compound formed is unstable and immediately
splits to give two molecules of a 3-carbon compound called
glycerate-3-phosphate (GP).

GP is reduced to 3-carbon triose phosphate (TP) in the
presence of ATP and reduced NADP [from the light-dependent
stage]

The triose phosphate is then either used to regenerate RuBP
or to make carbohydrates and other metabolites for the plant
CO2
(1C)
The Calvin cycle
unstable intermediate
(6C)
RuBP
(5C)
reduced
NADP
ADP
ATP
2 x glycerate
3-phosphate
(3C)
NADP
2 x triose
phosphate (3C)
Glucose (6C), amino
acids and lipids
ATP
ADP + Pi
The Calvin cycle
Fate of triose phosphate

Some is used to regenerate RuBP

Some molecules condense to form hexose
phosphates, sucrose, starch and cellulose

Some are converted to acetyl coenzyme A
to make amino acids and lipids
The light-independent reaction
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This cycle of events was worked out by Calvin, Benson and
Bassham between 1946 and 1953
The cycle is usually called the Calvin cycle
The enzyme ribulose bisphosphate carboxylase (ribisco or
RuBPC for short), which catalyses the combination of
carbon dioxide and RuBPC is the most common enzyme in
the world!
Photosynthesis – a summary
Light-dependent stage (thylakoid membranes)
Light-independent stage (stroma)
Light energy
12H2O
6CO2
ADP + Pi
4
1
5
chlorophyll
24 electrons
+
12H2O
24H+
ATP
+
24H+
6CO2
2
3
6
6O2
6H2O
+
C6H12O6
Structure to function:the leaf
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sps
Thin, therefore rapid light
penetration
Waxy cuticle reduces water
loss
Upper epidermis transparent
to light
Palisade mesophyll arranged at
90o to surface thus minimising
amount of light absorbed by
cell walls
Chloroplasts in
mesophyll can be
moved to maximise absorption
Spongy mesophyll has many air
spaces for rapid gas diffusion
Light and shade
A variety of environmental factors can affect the size and
thickness of leaves. In many species, leaves grown under high light
intensity (sun leaves) are smaller and thicker than those grown
under low light intensity (shade leaves).
Increased thickness of sun leaves is due to greater development of
palisade parenchyma.
Shade leaf
Acer : maple
Sun leaf