3.8 Photosynthesis (Core)

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.

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What is photosynthesis?

The conversion of light energy into chemical energy in plants.

The Visible Spectrum

Light from the sun is made of a range of colours.

This is a result of the different wavelengths of light.

Blue light has a shorter wavelength and more energy.

Red light has a longer wavelength and less energy.

ChlorophyllChlorophyll traps sunlight.

It is the main photosynthetic pigment in plants

Why do leaves look green?

visible light

Red and blue light is absorbed by the leaf.

Green light is reflected and reaches our eyes

visible light

Green light is reflected and reaches our eyes

The amount of red and blue,light absorbed can be measured using a spectrophotometer

Leaf extract (chlorophyll)

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Why are plants not always green?

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Absorption Spectrum

If we pass light through a leaf extract and measure how much of each wavelength of light is absorbed we produce an absorption spectrum.

The main colours of light absorbed are red and blue. The main colour reflected is green.

Action Spectrum

If we pass different wavelengths of light through a leaf extract and measure the rate of photosynthesis we get an action spectrum.

Comparing the Spectra

Both show peaks in the red and blue areas of the visible spectrum.

However, there is activity in wavelengths where there is no absorption.

This is evidence for the presence of other pigments, e.g. carotenoids.

chlorophyll achlorophyll b

carotenoids

Try this:

Plant pigment chromatography

Assessment Statements:

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.

What is the light used for?

To produce ATP and to split water molecules to form oxygen (released as a waste product) and hydrogen. The ATP and hydrogen are used to fix carbon dioxide into glucose.

Assessment Statement:

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.

Measuring the rate of photosynthesis

What can we measure? How?

How quickly raw materials are used:

e.g. water and carbon dioxide

How quickly products are formed:

e.g. oxygen production and biomass

Controlling Variables

Controlling temperature is always important when enzymes are involved. The water bath keeps the temperature constant.

Water plants like Elodea are ideal as we can see the oxygen forming.

Try this - Virtual Photosynthesis Lab

Photosynthesis Design Lab

Design an experiment to investigate photosynthesis using leaf discs.

Assessment Statement:

3.8.8Outline the effects of temperature, light

intensity and carbon dioxide concentration on the rate of photosynthesis.

Effect of changing light intensity

Increasing the light intensity has no effect on the rate. Other factors are limiting (carbon dioxide or temperature).

As the light intensity increases the rate of photosynthesis also increases.

At low light intensity the plant is respiring.

Effect of changing carbon dioxide

Increasing the carbon dioxide has no effect on the rate. Leaves are saturated with carbon dioxide. Other factors are limiting.

As the carbon dioxide increases the rate of photosynthesis also increases. The limiting factor is the carbon dioxide concentration.

Effect of changing temperature

Plant enzymes have an optimum of about 25oC and are denatured at 45oC

As the temperature and kinetic energy increase the rate of photosynthesis also increases. Enzymes needed for photosynthesis work better in warmer temperatures.

Maximum rate at optimum temperature