3.8 Photosynthesis

Topic 3 The Chemistry of Life

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.)

3.8.5 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.

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.)

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

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.

Photosynthesis and Light

Photosynthetic Pigments

Some substances called pigments can absorb lightThe 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).

Which wavelengths of light are most effective in driving photosynthesis?

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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)

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.

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.

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.

Summary of Photosynthesis

Summary of Photosynthesis

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

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.

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.

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.

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.

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.)

3.8.5 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.

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.)