Science at key stage 3    (Year 9)

• Show pupils a range of photographs or video clips, including, if possible, time-lapse sequences, illustrating that new biomass is formed as plants grow and that they can grow to an enormous size. Ask pupils about the conditions needed for healthy plant growth and the role of the root and leaf in this. Ask them to suggest where the new material in plants comes from. Challenge ideas about the material coming from the soil, eg show photographs or specimens of plants growing without soil; ask why the soil doesn't disappear from pots in which plants are growing; look at large fruits and vegetables such as marrow, pumpkin, cucumber.
• Establish that plants use carbon dioxide from the air and water taken in through the roots to make glucose, which is usually converted to starch for storage. Remind pupils of the formulae for carbon dioxide and water and show them how the equation for photosynthesis builds up. Introduce and explain the terms 'biomass' and 'photosynthesis'.

• Remind pupils that plants need light as well as carbon dioxide and water in order to grow well. Establish with pupils that the chlorophyll in plants enables them to absorb light, which is necessary for photosynthesis. Provide them with secondary sources showing how the carbon dioxide and/or oxygen concentration around leaves varies over a 24-hour period. Ask them to find patterns in data and to relate these to night and day. Establish that the evidence supports photosynthesis taking place in the presence of daylight.

• Explain to pupils that the glucose formed is usually stored as starch and ask them about a test for starch. Demonstrate how this can be carried out on a leaf.
• Provide pupils with a variety of leaves, eg variegated leaves, leaves kept in the dark for a few days, leaves covered with cut-out templates, copper beech leaves, so they can investigate starch distribution.
• Discuss the results with pupils and ask them to explain their observations. Compile a summary of all the observations and establish that starch is stored in the green parts of leaves and those parts that have been exposed to light. Establish with pupils that the chlorophyll in plants enables them to absorb light, which is necessary for photosynthesis.

• Extend this work by providing pupils with actively photosynthesising pondweed (Elodea) and with opportunities to discuss in groups how they could detect oxygen production. Help pupils to refine their suggestions and investigate how the release of oxygen from pondweed varies with light intensity, and to present their results as line graphs where appropriate.
• Ask pupils to suggest what happens to the oxygen produced by water plants, if necessary reminding them about organisms that respire aerobically in a pond, including pondweed and aquatic plants. Help pupils to draw up a respiration and photosynthesis cycle for a pond.

• Review pupils' knowledge of cell structure with quick questions. Provide them with microscope slides or secondary sources, eg 35mm slides, photomicrographs, so that they can see the internal structure of the leaf. Ask them to compare the leaf cells close to the upper surface with other cells and to suggest how they are adapted for photosynthesis, eg comparing the darker upper surface of leaves with the lower surface.
• Discuss their ideas with them and establish the idea that the leaf is designed to absorb light efficiently in its habitat.

• Help pupils to make a summary of the key points about photosynthesis. Provide them with information on earlier ideas about the growth of plants, eg Van Helmont's conclusions about a willow tree's growth before carbon dioxide was known; the work of Joseph Priestley, and, using the key points, discuss with them how knowledge about gases in the air and cells has helped to change ideas.

• Remind pupils, eg by showing them the word and/or symbol equations, that the first product of photosynthesis is glucose and that some of this is used in plant respiration. Ask pupils about foods from plants that contain sugars and starches and, if appropriate, demonstrate, eg using models, photographs, how the structures of sugars and starches are related.
• Present pupils with a range of familiar plant products, eg cotton fibre, rice, potatoes, wooden spills, perfume, corn oil, textured vegetable protein (TVP), paper. Ask what substances could be found in each, eg cellulose, fats, proteins, starches, sugars. Explain that these form a plant's biomass together with the glucose used in plant respiration.
• Ask pupils to use secondary sources to investigate an economically important plant product or products, and present information on products from biomass as a poster, helping them to identify what is needed.

• Ask pupils to suggest reasons why plants need water taken in by roots. Provide pupils with plants with growing root systems, eg garlic cloves suspended over water, water lettuce, cress seedlings, or a video clip of growing roots. Discuss with pupils how the roots are adapted for taking in water.
• Challenge pupils to explain why plants in flooded or waterlogged soil die despite easy access to water and minerals.
• Ask pupils to examine roots with root hairs, eg cress, or secondary sources, eg prepared slides, animations, and to make their own annotated drawings showing how roots are adapted for their functions.
• Show pupils four pieces of evidence of movement of materials in plants, eg root hairs on germinating mung beans, celery sticks which have been soaked in a dye to show movement up xylem vessels, a transverse section through a stem showing vascular bundles, and a leaf. Ask them to use the evidence to explain the movement of substances from the soil and through the plant, and to present their ideas diagrammatically.
• Review earlier work on the production of biomass. Ask pupils why farmers put fertilisers on fields. Provide boxes of plant fertiliser for pupils to explore the ingredients listed, and ask them to use secondary sources to find out why plants need certain minerals, eg nitrate and potassium, for healthy growth.

• Ask pupils about the importance of maintaining the proportion of oxygen in the air, and provide them with secondary sources of information to show the extent of variation in this over long periods of time. Remind pupils of the reactions involved in respiration and photosynthesis. Ask why green plants are important to the environment and discuss the range of plants involved. Provide pupils with a case study about forest conservation and ask them to identify the advantages and disadvantages of cutting down forest from the perspectives of a range of people. Organise a debate or role play for discussion of the issues.

• Provide pupils with a large diagram/drawing of a tree and ask them to construct a flow chart around it showing:
• that plants can use carbon dioxide and water to produce biomass
• that light is needed for photosynthesis
• the role of the leaf in photosynthesis and how it is adapted for its function
• the role of the root in photosynthesis and how it is adapted for its function