Topic 8.2
Photosynthesis
Assessment Statements
8.2.1 Draw and label a diagram showing the structure of a
chloroplast as seen in electron micrographs.
8.2.2 State that photosynthesis consists of light-dependent and
light independent reactions.
8.2.3 Explain the light-dependent reactions.
8.2.4 Explain photophosphorylation in terms of chemiosmosis.
8.2.5 Explain the light-independent reactions.
8.2.6 Explain the relationship between the structure of the
chloroplast and its function.
8.2.7 Explain the relationship between the action spectrum and
the absorption spectrum of photosynthetic pigments in green
plants.
8.2.8 Explain the concept of limiting factors in photosynthesis, with
reference to light intensity, temperature and concentration of
carbon dioxide.
Chloroplast
Internal membranes called
thylakoids are the location
Structureof the light dependent
reaction.
Stroma surrounds the
thylakoids and inside
the double membrane.
This is the location of
the light independent
reaction that includes
the Calvin cycle.
The stroma often contains
starch grains and oil
droplets both products of
photosynthesis.
Structure
Function
Thylakoid membranes
folded into grana
LSA for absorption of
light
Small space inside
thylakoids
Concentrates H+ ions
Fluid filled stroma
Compartmentalisation of
enzymes for Calvin cycle
stroma
thylakoids
grana (sing. granum)
Stages of Photosynthesis
In the light
independent reaction
(LIR) carbon is fixed
using NADPH + ATP
Used in respiration,
stored as starch,
converted to
cellulose cell walls
and other products
6CO2 + 6H2O → C6H12O6 + 6O2
Water is split in the light
dependent reactions (LDR)
to produce H+ ions which are
used to generate ATP and
NADPH
Light dependent reaction
Thylakoid membranes
Uses light - absorbed by chlorophyll
Uses H2O and produces O2
Coupled to reduction of ADP to ATP and
coenzyme NADP+ to NADPH + H+
Involves photolysis and photophosphorylation
The Light Independent Reaction
Takes place in the stroma
Enzymes involved
Temperature sensitive
Uses CO2 - carbon fixation produces glucose
Uses products of LDR - ATP and NADPH
Light has four functions
It excites the electrons, which then allows
photophosphorylation to occur.
It excites the electrons, which then allows the
reduction of NADP to NADPH.
It triggers the opening of the stomata so that
CO2 can enter.
It initiates the photolysis of water.
Photophosphorylation
The addition of phosphate using light energy.
e.g. ADP + Pi
ATP
e.g. when light energy is used in the Z scheme to
form ATP from ADP and phosphate.
Photolysis
The splitting of water into H+, e– and oxygen
using light energy.
e- e- e- e-
Reduction
Reduction is loss of oxygen or gain of H+ or
electrons (e-).
e.g. when NADP + H+
NADPH
NADP is reduced to NADPH by the addition of
H+ and electrons.
The opposite of reduction is oxidation
Light dependent reaction
Chlorophyll molecules
are arranged clusters
called photosystems
and embedded in the
thylakoid membranes
Light Dependent Reaction
Photoactivation
When light hits the chlorophyll molecule the light
energy is transferred to electrons.
The electrons become excited, and break their bonds.
This is called photoactivation.
Light is
absorbed
and
activates
the
electron
excited or
activated state
Electron
returns to
ground state
emitting a
packet of
energy
ATP Production
This generates ATP
Excited electron leaves
And is passed along a series of
the chlorophyll molecule
electron acceptors (ETC) to PSI
Exciting an electron in Protons pass through ATP
the chlorophyll molecule Synthase by chemiosmosis
Light (680nm)
strikes PSII
H+ produced by photolysis
accumulate in the thylakoid space
Electrons are replaced
through photolysis
Chemiosmosis
The diffusion of ions across a partially permeable
membrane through ATP Synthase.
As electrons pass along the ETC energy is released.
This energy is used to pump protons across the
thylakoid membrane into the thylakoid space.
As protons build up a gradient is created.
The flow of electrons from the thylakoid to the stroma
generates ATP.
ATP Synthase
Production of NADPH
Accepted by ferredoxin and
passed down the ETC
Electrons are re-excited
and leave chlorophyll
Used to reduce
NADP with a H+ ion
Light (700nm)
strikes PSI
NADPH and ATP are
passed to the LIR
Non-cyclic Phosphorylation
Non-cyclic phosphorylation makes ATP and
NADPH (needed for the LIR)
Cyclic phosphorylation only makes ATP
You need more ATP than NADPH2 for the LIR
Route of electrons
First electron donor
Photosystems
Last electron acceptor
Products
doesn’t return to same molecule
water
PSI and PSII
NADP
NADPH2 and ATP
returns to same molecule
PSI
PSI only
PSI
ATP only
Light Independent Reactions
Takes place in the
stroma
Uses light energy
trapped in the LDR
(ATP and reduced
NADP)
Metabolic cycle
Involves carbon
fixation
The process is
controlled by enzymes
Ribulose Bisphosphate
Carboxylase (Rubisco)
The Calvin Cycle - 3 stages
1 Carbon fixation
CO2 is added to a 5 carbon compound
called ribulose bisphosphate (RuBP)
RuBP splits to form glycerate 3
phosphate (G3P)
2 Reduction
G3P is reduced to triose phosphate (TP)
3 Regeneration
⅙ TP molecules is used to make hexose bisphosphate
⅚ TP molecules are used to regenerate RuBP
CO2
unstable 6C compound
P
P
carbon
fixation
P
glycerate 3 phosphate
(G3P)
P
P
ribulose bisphosphate
(RuBP)
ADP + Pi
NADPH2
Regeneration
ATP
NADP
hexose bisphosphate
P
P
Reduction
P
triose phosphate (TP)
Chlorophyll
Found within chloroplasts
Absorb and capture light
Made up of a group of five pigments
Chlorophyll a
Chlorophyll b
Carotenoids: xanthophyll and carotene
Phaetophytin
Chlorophyll a is the most abundant
Proportions of other pigments accounts for varying
shades of green found between species of
plants
Absorptio
n
Spectrum
Action
Spectrum
Blue light is high
energy and used
in photosynthesis
PSII
absorbs at
680nm
PSI
absorbs at
700nm
Green light is
reflected and not
absorbed
Different pigments absorb different wavelengths of
light, making photosynthesis more efficient. The
greater the range of wavelengths of light that
can be absorbed, the greater the light energy
obtained.
Primary pigments excite electrons. e.g.chlorophyll
a (NB there are different forms of chlorophyll a,
e.g. 720 nm and 680 nm).
Accessory pigments channel electrons to primary
pigment for photoexcitation. e.g.: chlorophyll b,
carotene, xanthophyll.
Review
Limiting Factors
The rate of photosynthesis is affected by light intensity, carbon
dioxide concentration and temperature.
Under a given set of conditions only one factor will affect the
rate of photosynthesis this factor is at its minimum and is
called the limiting factor
The overall rate of photosynthesis is determined by the step
that is proceeding most slowly (rate-limiting step).
Each factor e.g. light, temperature etc. can become the
limiting factor in any on the rate-limiting steps.
(a) Draw a labelled diagram of the structure of a
chloroplast as seen with an electron microscope (4)
Award [1] for each of the following clearly drawn and correctly
labelled.
double/inner and outer membrane/envelope—shown as two
concentric continuous lines close together;
granum/grana —shown as a stack of several disc-shaped
subunits;
(intergranal) lamella — shown continuous with thylakoid
membrane;
thylakoid — one of the flattened sacs;
stroma;
(70S) ribosomes/(circular) DNA / lipid globules / starch
granules /thylakoid space;
Effect on the concentration of TP, GP and RuBP
Factor
light
intensity
carbon
dioxide
concentration
temperature
Effect on
TP
Effect on
GP
Effect on
RuBP
Factor
Effect on TP
Effect on GP
Effect on RuBP
Light
intensity
Decreasing light intensity means
less ATP and reduced NADP, so
less TP is made since ATP and
reduced NADP are needed to
make TP from GP.
Decreasing light intensity means
more GP because RuBP can be
converted to GP but without ATP
and reduced NADP GP will not
be used up to make TP.
Decreasing light intensity means
less ATP and reduced NADP, so
less RuBP because RuBP is still
being used up to make GP but
RuBP is not being regenerated as
GP cannot be made into TP, which
is needed to make RuBP.
Carbon dioxide
concentration
As carbon dioxide increases TP
increases. Because more CO2 is
fixed, so more GP is made, so
more TP.
As carbon dioxide increases GP
increases. Because more CO2 is
fixed, so more GP is made.
As carbon dioxide increases RuBP
decreases. Because more CO2 is
fixed, so more GP is made and
more RuBP is used up.
Temperature
As temperature increases TP
increases. But at high
temperatures TP will decrease
because the enzyme RuBisCO
denatures and less carbon
dioxide fixed, so less GP will be
made and so less TP is made.
As temperature increases GP
Increases. But at high
temperatures will decrease
because the enzyme RuBisCO
denatures and less carbon
dioxide fixed, so less GP will be
made and so less TP is made.
As temperature increases RuBP
decreases because as the rate of
enzyme action increases more
RuBP is used up. When the
RuBisCO denatures at high
temperature less RuBP will be used
up as CO2 is not fixed.
Outline the light-dependent reactions of photosynthesis
(6)
(chlorophyll/antenna) in photosystem II absorbs light;
absorbing light/photoactivation produces an excited/high
energy/
free electron;
electron passed along a series of carriers;
reduction of NADP+ / generates NADPH + H+;
absorption of light in photosystem II provides electron for
photosystem I;
photolysis of water produces H+ / O2;
called non-cyclic photophosphorylation;
in cyclic photophosphorylation electron returns to chlorophyll;
generates ATP by H+ pumped across thylakoid membrane /
by chemiosmosis / through ATP synthetase/synthase;
Explain the effect of light intensity and temperature on the
rate of photosynthesis.(8)
both light and temperature can be limiting factors;
other factors can be limiting;
graph showing increase and plateau with increasing light / description of this;
graph showing increase and decrease with increasing temperature /description of
this;
light:
affects the light-dependent stage;
at low intensities insufficient ATP;
and insufficient NADPHH + H+ produced;
this stops the Calvin cycle operating (at maximum rate);
temperature:
affects light-independent stage / Calvin cycle;
temperature affects enzyme activity;
less active at low temperatures / maximum rate at high temperatures;
but will then be denatured (as temperature rises further);
Award [5 max] if only one condition is discussed.