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Photosynthesis and plant nutrition IGCSE Biology revision notes Structure of a leaf Adaptations of a leaf Feature Function Cuticle Allows light through, prevents loss of water, protection against disease Upper & lower epidermis Allow light through (upper), protect leaf from mechanical damage Palisade cells Closely packed together, many chloroplasts for efficient light absorption as they are close to the upper surface Spongy cells Large surface area, in contact with air spaces, efficient gas exchange Air spaces Allow rapid diffusion of gases Stomata Allow carbon dioxide into the air spaces Guard cells Open the stomata when conditions are right for photosynthesis (i.e. when there is enough light) and close them at other times to prevent loss of water vapour Shape of leaf: thin and with a large surface area Thin: fast diffusion of gases; large surface area means more stomata to absorb carbon dioxide and more surface to absorb sunlight Limiting factors: Light, carbon dioxide Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature. In the graphs, the limiting factor is the one in short supply: • when you increase it, the rate of photosynthesis increases (i.e. between X and Y). • If you increase it and the rate of photosynthesis does not change, something else is now the limiting factor. Y X Without enough light, a plant cannot photosynthesise quickly, even if there is plenty of water and carbon dioxide. Increasing light intensity boosts the rate of photosynthesis. Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even with plenty of light, a plant cannot photosynthesise if there is too little carbon dioxide. Y X Limiting factors: Temperature If it gets too cold, the rate of photosynthesis will decrease, as the rate of diffusion and of enzyme activity are affected by low kinetic energy. Plants cannot photosynthesise if it gets too hot because the enzymes become denatured. Greenhouses and photosynthesis In greenhouses, limiting factors for photosynthesis are easily controlled: • • • Artificial light allows photosynthesis to continue beyond daylight hours. Bright lights also provide a higher-than-normal light intensity. Artificial heating increases the rate of photosynthesis Additional carbon dioxide released into the atmosphere also allows photosynthesis to continue at an increased rate. However, the additional cost of lighting, heat and carbon dioxide has to be weighed against the increased crop yield. Paraffin lamps are traditionally used in greenhouses, providing • • • light heat carbon dioxide Minerals required for plant growth Mineral Element supplied Why needed Signs of deficiency Nitrates Nitrogen, N Nitrogen is needed to make amino acids for proteins Poor growth, older leaves turn yellow Phosphates Phosphorus, P Phosphorus is needed to make DNA and cell membranes Poor root growth, young leaves go purple Potassium compounds Potassium, K Potassium is needed in enzymes Leaves turn yellow with involved in respiration and dead spots photosynthesis Magnesium compounds Magnesium, Mg Magnesium is needed to make chlorophyll Leaves turn yellow