IGCSE Double Award Extended Coordinated Science Biology 4.2 - Plant Nutrition Photosynthesis You need to know the definition of photosynthesis as: the fundamental process by which plants manufacture carbohydrates from raw materials using energy from light. Photosynthesis is the reaction that happens in chlorophylls in plant cells - Chlorophylls trap light energy and convert it into chemical energy - The chemical energy is stored by the formation of carbohydrates. You need to know how to write the word equation and the chemical symbolic equation for the photosynthesis. Carbon Dioxide + Water == light and chlorophyll => Glucose + Oxygen 6 CO 2 + 6 H 2 O == light and chlorophyll => C 6 H 12 O 6 + 6 O 2 (You may notice that photosynthesis is the reverse reaction of respiration : C 6 H 12 O 6 + 6 O 2 -> 6 CO 2 + 6 H 2 O) You need to know how the plant gets the carbon dioxide and water required for photosynthesis - The carbon dioxide comes from simple diffusion of carbon dioxide in the air spaces in the leaves. - The water comes from the root then the water is transported to the photosynthesising cell (Unit B5.1)
The Leaf You need to know the structure of a leaf, and each of the parts functions and specialisations. - Waxy Cuticle - A waxy waterproof layer that covers the outer surface of the epidermis and the leaf. - Cuts down water loss by evaporation at the leaf surface - Epidermis (Upper and Lower) - Single layer of cells that contain no chloroplasts, so light can pass through to lower layers. - Acts as a protective outer layer. - Palisade Layer - The upper layer of the leaf (excluding epidermis) - Palisade cells have a lot of chloroplasts so that most of photosynthesis can occur at the closest part to the light source. - Palisade cells are arranged vertically so the more chlorophylls have more chance of absorbing light as light passes through. - Spongy Mesophyll - More round-shaped cells with lots of airspaces between them. Can still photosynthesise. - The airspaces are what allows plant to carry out gas exchange (Unit 5.1) - Vascular Bundle - The bundle of tubes that are responsible for transport around plants, includes: - Xylem: Vessel that transports water and mineral salts. - Xylem walls are tough and hard, so they are also used to support the whole leaf. - Phloem: Vessel that transports dissolved foods and nutrients (glucose etc.) - Stoma (pl. stomata) - Tiny holes in the lower epidermis that allows gases to diffuses in and out of the mesophyll layer - Also responsible, along with the mesophyll layer, for plant gas exchange - Guard Cells - Cells that control the opening and closing of the stomata. - Are light sensitive
Factors Affecting Photosynthesis You need to know that the rate of photosynthesis is affected by factors like: light intensity and carbon dioxide levels. Before investigating the effect of light intensity and carbon dioxide levels, it should be noted that: - Photosynthesis cannot occur without any of: carbon dioxide, water, light, and chlorophyll. - If any one of these four are not available, photosynthesis does not occur. - So, cells that do not have chlorophylls, like root hair cells, cannot photosynthesise. - Or if all chloroplasts were removed from a plant cell, it will not be able to photosynthesise. Effect of light intensity on rate of photosynthesis - As light energy is crucial for photosynthesis, - it is logical that as light intensity increases, the rate of photosynthesis increases. - This result can be investigated with the following experiment: - As oxygen is a product of photosynthesis, the rate of bubble production can be used as a measurement to find the rate of photosynthesis (more bubbles per time means faster rate of photosynthesis) - The light source can be moved to adjust the light intensity to the plant.
- If this experiment is done and the results were graphed, it would give a similar graph to above. Effect of carbon dioxide levels on rate of photosynthesis - As carbon dioxide is one of the reactants of photosynthesis, - The concentration of carbon dioxide directly affects the rate of photosynthesis. - If the concentration of carbon dioxide is low, there is not enough carbon dioxide and carbon dioxide becomes the limiting factor of the reaction. - And as the concentration of carbon dioxide increases, the rate of the reaction (photosynthesis) increases as there are more carbon dioxide molecules to react. - However, after a certain point, the graph levels out - This is because another factor, (light, water level) has become the limiting factor, and the increase in carbon dioxide concentration will not affect the rate of photosynthesis anymore. Effect of temperature on rate of photosynthesis - Like any reaction, you can expect the increase in temperature to increase the rate of photosynthesis. - However, since there are enzymes that catalyse photosynthesis, as the temperature reaches a critical value, the rate decreases. - Because the enzymes are denatured and no longer catalyse photosynthesis.
Plant Nutrition You need to know the importance and the deficiency effects of two main nutrients, nitrate and magnesium ions. - Nitrate ions ( NO 3 - ) - Nitrate ions are used to synthesise proteins, and proteins are essential for a plant to make enzymes and etc. - Nitrate ions provide the essential nitrogen in the protein chain. - Nitrate ions are dissolved in water and are absorbed from the soil by the roots. - Nitrate deficiency will cause the plant to suffer from slow growth and abnormal growth. - Magnesium ions ( Mg 2+ ) - Magnesium ions are used to make chlorophyll, and chlorophyll is essential for photosynthesis. - Chlorophyll molecules contain magnesium, so without magnesium, they cannot be made. - Magnesium ions are also dissolved in water and are absorbed from the soil by the roots. - Magnesium deficiency will cause the plant to turn yellow, due to the lack of green chlorophylls.
Fertilisers You need to know the use of fertilisers and the dangers of using them. - Fertilisers are used to prevent nutrient deficiency and also to boost and aid plant growth. - Most fertilisers contain multiple elements for supplement - like NPK fertilisers which contain, nitrogen, phosphorus, and potassium. - However nitrogen-containing fertilisers can be dangerous when overused - causing eutrophication. - Eutrophication is when the excess fertilisers get washed off and flow into bodies of water. - Causing a chain of events leading to the death of the body of water. Eutrophication 1. Input of nitrates and phosphates into the water (mostly excess fertilisers) 2. Because of the boosted nutrients, there is massive growth of algae at the surface (Algal blooms) 3. This massive growth of algae creates a layer of algae, which block sunlight from reaching inside. 4. Plants below the surface receive less light, cannot synthesise, therefore die. 5. Decomposer bacteria break down the dead plant matter, - Removes oxygen from the water (they respire) 6. Now the oxygen level in the water is low, the fish will start to die, - Leading to more decomposing - Leading to more lowering of water-oxygen level. - This cycle continues, ultimately leaving a dead body of water.
The syllabus says you should be able to, (SO check if you can): - Define photosynthesis - Explain the function of chlorophyll - State the word equation for the production of simple sugars and oxygen. - State the balanced equation for photosynthesis in symbols. - Investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls. - Describe the intake of carbon dioxide and water by plants. - Identify and label the cuticle, cellular and tissue structure of a dicotyledonous leaf, as seen in cross-section under the light microscope and describe the significance of the features of a leaf in terms of functions, to include: - distribution of chloroplasts Photosynthesis - stomata, palisade and mesophyll cells gas exchange - vascular bundles (xylem and phloem) transport and support. - Describe the importance of: - nitrate ions for protein synthesis - magnesium ions for chlorophyll synthesis. - Investigate and state the effect of varying light intensity on the rate of photosynthesis (e.g. in submerged aquatic plants). - Explain the effects of nitrate ion and magnesium ion deficiency on plant growth. - Describe the uses, and the dangers of overuse, of nitrogen-containing fertilisers.