02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

State that photosynthesis involves the conversion of light energy into chemical energy.

State that light from the Sun is composed of a range of wavelengths (colours)

State that chlorophyll is the main photosynthetic pigment.

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. It is not necessary to mention wavelengths or the structure responsible for the absorption).

State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

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.

Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis (The shape of the graph is required).

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.

Ref:Orange book pg. 95Green book pg. 48

Biozone: 35 “Photosynthesis”

Give a concise definition of photosynthesis.Write out the word equation.Write out the balanced chemical equation.

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.2 State that light from the Sun is composed of a range of wavelengths (colours)

Ref:Orange book pg. 96Green book pg. 47

White light is a mixture of light of different wavelengths. The energy of light is inversely proportional to its wavelength, and so light of short wavelength has more energy than light of long wavelength. Green plants convert the energy of some wavelengths of visible light into the energy, which bonds together the atoms of complex organic molecules.

? List the colours of the visible light spectrum from longest to shortest wavelength.

? Which colours do plants not absorb (they reflect then colours)?

? From which parts of the spectrum to plants absorb

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.3. State that chlorophyll is the main photosynthetic pigment.

Ref:Orange book pg. 96Green book pg. 48

White light is separated into the different colours (wavelengths) of light by passing it through a prism. Wavelength is defined as the distance from peak to peak (or trough to trough). The energy of is inversely proportional to the wavelength - longer wavelengths have less energy than do shorter ones. The order of colours is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the colour. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet.

ChlorophyllA pigment (in this sense) is any substance that absorbs light. The colour of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colours reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment.

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

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. It is not necessary to mention wavelengths or the structure responsible for the absorption).

Ref:Orange book pg. 96Green book pg. 48-49

ChlorophyllA pigment (in this sense) is any substance that absorbs light. The colour of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colours reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment.

? From which parts of the spectrum is most light absorbed?? from which parts of the spectrum is most light reflected?

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Ref:Orange book pg. 97Green book pg. 49

The sun is the main source of energy available to the living world. Light energy is converted to the potential energy of complex organic molecules by green plants in a complicated pathway of reactions called photosynthesis. Respiration allows this potential energy to be released in the biochemical reactions of respiration to provide the energy living things need to survive.

As in respiration, the pathways of photosynthesis are organized, to allow the most efficient transfer of hydrogen ions, electrons and ATP.

Photosynthesis is essentially the reverse of respiration. It is usually simplified to:

Lightcarbon dioxide + water glucose + oxygen Chlorophyll

6CO2 + 6H2O C6H12O6 + 6O2

This simplification hides numerous separate steps. To understand photosynthesis in detail we can break it up into 2 stages:

Light-Dependent ReactionsThe light-dependent reactions use light energy to split water producing oxygen and energetic hydrogen atoms. ATP is also produced. This stage takes place within the thylakoid membranes of chloroplasts, and is very much like the respiratory chain, only in reverse.

Photolysis splitting with light (in this case, splitting of light with water).

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.6 State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

Ref:Orange book pg. 97Green book pg. 49

Light-Independent ReactionsThe light-independent reactions don’t need light, but do need the products of the light-dependent stage (ATP and H from the photolysis of water), so they stop in the absence of light. This stage takes place in the stroma of the chloroplasts and involves the fixation of carbon dioxide and the synthesis of glucose.

02/06/2011 06:41IB Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

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.

Ref:Orange book pg. 98-99Green book pg. 49-50

You have already carried this practical out.

The apparatus below is an example of how the rate of photosynthesis can be measured directly by the production of oxygen. .

The pondweed releases bubbles of gas from a freshly cut stem when illuminated.

When the light source is moved closer to the pondweed, the intensity of the light increases with the inverse square law. This means that if you half the distance between the light source and the pondweed, the intensity of the light increases four-fold.

02/06/2011 06:41B Standard level Biology Dulwich College ShanghaiTopic 3: The Chemistry of Life

Photosynthesis

3.8.8. Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis (The shape of the graph is required).

Ref:Orange book pg. 98-99Green book pg. 50-51

The rate at which oxygen is given off by green plants can be used to measure the rate of photosynthesis. Aquatic plants such as Elodea are particularly suitable for this purpose. Bubbles of oxygen are given off from the cut end of the stem of Elodea when it is immersed in illuminated pond water or a dilute solution of sodium hydrogencarbonate CO2 production.

The Effect of Light Intensity on the Rate of Photosynthesis

At low intensities of light a straight line relationship between the rate of photosynthesis and light intensity is observed. However, once a critical light intensity is reached the rate of photosynthesis remains constant. The results surprised scientists who reasoned that if photosynthesis is driven by solar energy then it should go faster if more intense light is provided. They concluded that factors other than the availability of light limit the rate of photosynthesis at high light intensities.

Knowing that carbon dioxide is an essential raw material for photosynthesis and that the process is probably catalysed by enzymes, They went on to investigate the effects of carbon dioxide concentration and temperature on the rate of photosynthesis. The results from these investigations are shown in the graphs below.

Clearly photosynthesis is controlled by a combination of factors and it is apparent that the photosynthetic rate is limited by whichever factor is nearest to its minimum value – this is the law of limiting factors.

Scientists did not attempt to find out whether the availability of water affects the rate of photosynthesis. In non-wilted plants there is always sufficient water for photosynthesis. The closure of stomata of wilted leaves may however limit the rate of photosynthesis by preventing the diffusion of carbon dioxide to the mesophyll tissue. In normal circumstances temperature, light intensity and carbon dioxide concentration are the factors most likely to limit the rate of photosynthesis.

TemperaturePhotosynthesis consists of light-dependent and light-independent stages. It is the enzyme catalysed reactions of the light-independent stage which are affected by temperature. The graph below shows the effect of temperature on photosynthesis at high and low light intensities. Notice that increasing the temperature has no effect on the rate of photosynthesis at low light intensity. This is because at low light intensity, even if the temperature is at an optimum the light-dependent reactions will not be carried out at their maximum and so ATP and NADPH production will not be at their maximum. This has a knock on effect on the light-independent stage as these products are needed to synthesis glucose.

Concentration of Carbon Dioxide

On a warm sunny day the concentration of carbon dioxide in the air is probably the factor which limits photosynthesis more than any other. Enriching air with carbon dioxide has a significant effect on crop plants grown in green houses. It is now common practice for commercial growers of salad plants to raise the concentration of carbon dioxide of the air in greenhouses during daylight. The gas is either pumped in directly of is released by the burning of fuel such as paraffin, propane and natural gas which are used to heat the greenhouses. Concentrations of up to 0.2% carbon dioxide produce increases in crop yield which are economically worthwhile.

02/06/2011 06:41Photosynthesis-

Use these questions as a guide to take detailed notes on photsynthesis:

State the energy transfer that takes place during photosynthesis.

How do the reactants required for photosynthesis get into the plant?

State a simple word and symbol equation for photosynthesis.

State the colours (wavelengths) of light that make up the visible spectrum?

Name the pigment present in chloroplasts, which absorbs light energy from the sun.

Where in the plant is this pigment found?

Which wavelengths (colours) of light are absorbed mostly by chlorophyll?

What happens to the other wavelengths of light that shine on the leaf?

What is meant by ’photolysis’?

What is meant by the absorption spectrum?

State the 2 stages of photosynthesis, where each happens and what happens during each stage.