1. 3.8 Photosynthesis
Topic 3 The Chemistry of Life

2. 3.8.1 State that photosynthesis involves the conversion of
light energy into chemical energy.
3.8.2 State that light from the Sun is composed of a range
of wavelengths (colours).
3.8.3 State that chlorophyll is the main photosynthetic
pigment.
3.8.4 Outline the differences in absorption of red, blue and
green light by chlorophyll. (Students should appreciate
that pigments absorb certain colours of light. The
remaining colours of light are reflected.)

3. 3.8.5 State that light energy is used to produce ATP, and
to split water molecules (photolysis) to form oxygen and
hydrogen.
3.8.6 State that ATP and hydrogen (derived from the
photolysis of water) are used to fix carbon dioxide to
make organic molecules.

4. 3.8.7 Explain that the rate of photosynthesis can be
measured directly by the production of oxygen or the
uptake of carbon dioxide, or indirectly by an increase in
biomass.
(The recall of details of specific experiments to indicate
that photosynthesis has occurred or to measure the rate
of photosynthesis is not expected.)
3.8.8 Outline the effects of temperature, light intensity and
carbon dioxide concentration on the rate of
photosynthesis. (The shape of the graphs is required.)

5. Photosynthesis
Photosynthesis is the process whereby light energy is
converted into chemical energy.
Photosynthesis is used by plants and some other
organisms (e.g. photosynthetic algae).
Photosynthesis uses simple inorganic substances to
produce organic substances.
– Uses: carbon dioxide, water
– Produces: glucose
– By-product: oxygen

6. Photosynthesis and Light
Most light for photosynthesis is sunlight.
Sunlight is called white light.
White light is made up of a range of wavelengths:
– ROYGBIV
– different colours have different wavelengths
These colours make up the visible section of the
electromagnetic spectrum.

7. Photosynthesis and Light

8. Photosynthetic Pigments
Some substances called pigments can absorb light
The main pigment used to absorb light in
photosynthesis is chlorophyll.
The structure of chlorophyll allows it to absorb some
colours or wavelengths better than others.
Red and blue are absorbed better than green because
green light is reflected (making plants appear green).

9. Which wavelengths of light are most effective
in driving photosynthesis?
Absorptionoflightby
chloroplastpigments
400 500 600 700
Chlorophyll a
Chlorophyll b
Carotenoids
Absorption spectra. The three curves show the wavelengths of light best
absorbed by three types of chloroplast pigments.
Wavelength of light (nm)

10. Absorption Spectrum of Chlorophyll
The absorption spectrum
indicates the amount of
light absorbed by the leaf.
The action spectrum
indicates the efficiency of
light of different
wavelengths promoting
photosynthesis.

11. Light dependent Stage of
Photosynthesis
In photosynthesis, some of the energy from sunlight is
absorbed by chlorophyll and used to split water
molecules into H+
ions, O2 and electrons.
This is called photolysis of water.
Some of the energy from sunlight is also absorbed by
chlorophyll and used to make ATP.
These two processes are part of the Light Dependent
Stage of Photosynthesis.

12. Light Independent Stage of
Photosynthesis
Carbon dioxide is absorbed for use in photosynthesis.
The hydrogen and the ATP generated in the Light
Dependent Stage are then used to fix carbon forming
glucose.
The conversion of gaseous carbon to carbon in solid
compounds is called carbon fixation.
This is called the Light Independent Stage of
Photosynthesis.

13. Summary of Photosynthesis

14. Summary of Photosynthesis

15. Measuring the Rate of
Photosynthesis
The rate at which photosynthesis is occurring can be
measured by various methods:
– Measured Directly
• Rate of oxygen production
• Rate of carbon dioxide uptake
– Measured Indirectly
• Increase in biomass

16. Effect of Temperature
Photosynthesis is a
metabolic pathway that
requires enzymes to catalyse
each step in the process.
Thus temperature affects the
rate of the action of the
enzymes and the rate of
photosynthesis.
Photosynthesis has an
optimum temperature.

17. Effect of Carbon Dioxide
At low concentrations of CO2,
the rate of photosynthesis is
reduced.
As CO2 concentration
increases so does the rate of
photosynthesis up to a point.
At high concentrations of C02
there is no further increase in
the rate of photosynthesis.

18. Effect of Light Intensity
At low light levels, the rate of
photosynthesis is reduced.
As the level of light increases
so does the rate of
photosynthesis up to a point.
At high light levels there is no
further increase in the rate of
photosynthesis.

19. Limiting Factors
For photosynthesis to occur the following criteria need to be
fulfilled:
– presence of chlorophyll
– presence of light
– presence of carbon dioxide
– presence of water
– suitable temperature
The factor furthest away from its optimum value will limit the
amount of photosynthesis.
This is then the Limiting Factor.

20. 3.8.1 State that photosynthesis involves the conversion of
light energy into chemical energy.
3.8.2 State that light from the Sun is composed of a range
of wavelengths (colours).
3.8.3 State that chlorophyll is the main photosynthetic
pigment.
3.8.4 Outline the differences in absorption of red, blue and
green light by chlorophyll. (Students should appreciate
that pigments absorb certain colours of light. The
remaining colours of light are reflected.)

21. 3.8.5 State that light energy is used to produce ATP, and
to split water molecules (photolysis) to form oxygen and
hydrogen.
3.8.6 State that ATP and hydrogen (derived from the
photolysis of water) are used to fix carbon dioxide to
make organic molecules.

22. 3.8.7 Explain that the rate of photosynthesis can be
measured directly by the production of oxygen or the
uptake of carbon dioxide, or indirectly by an increase in
biomass.
(The recall of details of specific experiments to indicate
that photosynthesis has occurred or to measure the rate
of photosynthesis is not expected.)
3.8.8 Outline the effects of temperature, light intensity and
carbon dioxide concentration on the rate of
photosynthesis. (The shape of the graphs is required.)