2. Photosynthesis
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.
(These should not be called “light” and “dark” reactions.)
8.2.3 Explain the light-dependent reactions.
(Include the photoactivation of photosystem II, photolysis of
water, electron transport, cyclic and non-cyclic
photophosphorylation, photoactivation of photosystem I,
and reduction of NADP+)
3. Photosynthesis
8.2.4 Explain photophosphorylation in terms of
chemiosmosis.
8.2.5 Explain the light-independent reactions.
(Include the roles of ribulose bisphosphate (RuBP)
carboxylase, reduction of glycerate 3-phosphate (GP) to
triose phosphate (TP), NADPH + H+
, ATP, regeneration of
RuBP, and subsequent synthesis of more complex
carbohydrates)
4. Photosynthesis
8.2.6 Explain the relationship between the structure of the
chloroplast and its function.
(Limit this to the large surface area of thylakoids for light
absorption, the small space inside thylakoids for
accumulation of protons, and the fluid stroma for the
enzymes of the Calvin cycle)
5. Photosynthesis
8.2.7 Explain the relationship between the action spectrum
and the absorption spectrum of photosynthetic pigments in
green plants.
A separate spectrum for each pigment (chlorophyll a,
chlorophyll b, and so on) is not required.
8.2.8 Explain the concept of limiting factors in
photosynthesis, with reference to light intensity, temperature
and concentration of carbon dioxide.
6. Chloroplast Structure
Chloroplasts have an envelope, inner and outer membranes.
Inside, thylakoid membranes form stacks called grana (granum – sing)
Thylakoids provide a large surface area for light absorption.
Thylakoids also have a small internal space.
Chloroplasts have smaller ribosomes (70S) and circular DNA.
The fluid filling the chloroplast is called the stroma. It contains the
enzymes for the Calvin Cycle.
9. Photosynthesis
Photosynthesis is the process that plants, algae and some
bacteria use to produce all of the organic compounds they
need.
Photosynthesis is a metabolic pathway, consisting of
many chemical reactions.
Photosynthesis consists of;
The light dependent reactions
produce intermediate compounds for the light independent
reactions
The light independent reactions
where glucose, amino acids and other organic compounds are
made.
10. Light Dependent Reactions
Chlorophyll absorbs light and the energy from the light
raises an electron in the chlorophyll molecule to a higher
energy level.
An excited electron
The chlorophyll is photoactivated
Chlorophyll is located in the thylakoid membrane and it is
arranged in groups of hundreds of molecules called
photosystems.
There are two types of photosystem:
Photosystem I
Photosystem II
11. Light Dependent Reactions
These occur in the thylakoid membranes
Photosystem II absorbs light energy.
This energy is used to excite electrons from chlorophyll to a higher
energy level.
The excited electron from photosystem II is passed along a chain of
electron carriers in the thylakoid membrane.
The electrons lost from photosystem II are replaced by splitting water
– photolysis. Oxygen gas is released and the protons can either
combine with NADP+
at the end of the pathway or be pumped into
the thylakoid interior.
As the electrons are passed from one carrier to the next, they give up
some of their energy. One of these is cytochrome, which acts as a
proton pump to pump protons into the thylakoid interior
12. Light Dependent Reactions
The protons then diffuse back through ATP synthase,
making ATP.
This is called non-cyclic photophosphorylation.
The coupling of electron transport to ATP synthesis is by
Chemiosmosis (like in the mitochondrion).
The electrons are then passed to photosystem I, where
light energy is absorbed to boost them to a higher level.
The electrons are passes through a short chain of carrieres
to NADP+
, in the stroma, forming NADPH + H+
.
13. Light Dependent Reactions
Alternatively the electrons can fall back through carriers
to photosystem I.
As the electrons flow along a chain of carriers they cause
pumping of protons across the thylakoid membrane.
A proton gradient is formed and this allows production of
ATP by ATP synthase.
ATP synthesised in this way is called cyclic
photophosphorylation.
16. Photophosphorylation
This is the synthesis of ATP using the energy from light.
The principle is the same as for oxidative phosphorylation
in the mitochondria.
Electrons flow through the carriers, including photosystem II
and I
As the pass through the cytochrome carrier, protons are pumped
into the interior of the thylakoid.
The thylakoid interior is small to increase the concentration of
protons.
The protons then flow back by diffusion down a concentration
gradient into the stroma through ATP synthase
ATP is generated from ADP + P
18. Cyclic and Non-cyclic
Photophosphorylation
The difference between cyclic and non-cyclic
photophosphorylation is that with non-cyclic, the electrons
continue on the end in NADP+
, whereas in cyclic they are
returned to the chlorophyll in photosystem I.
The cyclic pathway has TWO advantages:
It provides additional ATP needed to drive the light independent
reactions. The Calvin Cycle requires ATP and NADPH + H+
in
the ratio of 3:2. Non-cyclic photophosphorylation gives a ratio of
1:1.
It provides ATP that can be used for other processes such as
stomatal opening by guard cells.
19. Light Independent Reactions
The light independent reactions are driven by the ATP
and NADPH + H+
made in the light dependent reactions.
Both sets of reactions continue at the same time.
The light independent reactions occur in the stroma.
It is a cyclic system called the Calvin Cycle.
21. Light Independent Reactions
Carbon Fixation:
CO2 is an essential substrate in the Light In-dependent reactions.
It enters the chloroplast by diffusion.
At the start of the Calvin cycle, CO2 combines with ribulose
bisphosphate (RuBP), a five carbon compound.
This is a carboxylase reaction and the enzyme is called rubisco.
The product of this reactions is a 6 carbon compound which
immediately splits to form TWO molecules of glycerate 3-
phosphate.
22. Light Independent Reactions
Carbohydrate Synthesis:
Glycerate 3-phosphate is converted into a carbohydrate by a
reduction reaction.
Hydrogen is supplied by NADPH.
Energy is also required in the form of ATP.
Both NADPH and ATP were synthesised in the Light
Dependent Reactions.
Gylcerate 3-phosphate is reduced to a 3 carbon sugar, triose
phosphate.
2 triose phosphate molecules are joined together to form
glucose phosphate.
Condensation reactions join the glucose phosphate molecules
together to form Starch.
23. Light Independent Reactions
Not all of the triose phosphate sugar molecules are used
to produce glucose.
Some are used to resynthesise ribulose bisphosphate
(RuBP).
In fact 5/6 of the triose phosphate molecules are used to
resynthesise RuBP.
Only 1/6 of the triose phosphate molecules produce is use
to make glucose phosphate.
Thus, 2 turns of the calvin cycle produce 1 molecule of
glucose phosphate.
26. Action Spectrum of Photosynthesis
A spectrum is a range of wavelengths of electromagnetic radiation
(the radiation emitted from the sun).
The spectrum of visible light is the range of wavelengths from 400nm
to 700 nm.
Each wavelength is colour of light:
400-525 violet-blue
525-625 green-yellow
625-700 orange-red
The efficiency of photosynthesis is not the same in all wavelengths.
The Action Spectrum shows the efficiency of the different
wavelengths of light that are used in photosynthesis.
28. The Absorption Spectrum of Photosynthesis
The Absorption Spectrum of Photosynthesis shows how
much of a particular wavelength of light that are absorbed
by the photosynthetic pigments.
The two most common pigments are chlorophyll a and
chlorophyll b.
The action spectrum and absorption show strong
similarities:
Greatest absorption in violet-blue range.
Also a high level of absorption in the red range.
The lowest absorption in the yellow-green range.
There is a close correlation between the action spectrum
and the absorption spectrum.
29. Limiting Factors
There are 3 main factors that can affect the rate of
photosynthesis:
Light intensity
Temperature
Concentration of Carbon dioxide.
Changes to one of these factors can affect the rate of
photosynthesis.
The factor that is nearest to its minimum at any one time
is called the Limiting Factor and this is the one that
affects the rate of photosynthesis.
30. Rate limiting steps
Different factors limit different steps in photosynthesis.
Identify the limiting factor for:
1. Photolysis
2. Carbon fixation
3. Photophosphorylation
4. Reduction of NADP
5. Regeneration of RuBP
31. Why the light independent stage
is not really light independent
The light independent reactions rely on products of the
light dependent reactions.
Without light there is no way to regenerate ATP and
NADPH
Without ATP and NADPH GP cannot be converted to TP
Without ATP RuBP cannot be regenerated from TP
32. IBO guide:
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.
(These should not be called “light” and “dark” reactions.)
8.2.3 Explain the light-dependent reactions.
(Include the photoactivation of photosystem II, photolysis of
water, electron transport, cyclic and non-cyclic
photophosphorylation, photoactivation of photosystem I,
and reduction of NADP+)
33. IBO guide:
8.2.4 Explain photophosphorylation in terms of
chemiosmosis.
8.2.5 Explain the light-independent reactions.
(Include the roles of ribulose bisphosphate (RuBP)
carboxylase, reduction of glycerate 3-phosphate (GP) to
triose phosphate (TP), NADPH + H+
, ATP, regeneration of
RuBP, and subsequent synthesis of more complex
carbohydrates)
34. IBO guide:
8.2.6 Explain the relationship between the structure of the
chloroplast and its function.
(Limit this to the large surface area of thylakoids for light
absorption, the small space inside thylakoids for
accumulation of protons, and the fluid stroma for the
enzymes of the Calvin cycle)
35. IBO guide:
8.2.7 Explain the relationship between the action spectrum
and the absorption spectrum of photosynthetic pigments in
green plants.
A separate spectrum for each pigment (chlorophyll a,
chlorophyll b, and so on) is not required.
8.2.8 Explain the concept of limiting factors in
photosynthesis, with reference to light intensity, temperature
and concentration of carbon dioxide.
