| NUTRITION Introduction  Organisms obtain food in various ways so as to survive and carry out physiological processes like respiration, growth, osmo-regulation among others. Organisms having chlorophyll like plants and protests like Euglena make food by combining inorganic substances namely water and carbon dioxide to make carbohydrates in a process calledphotosynthesis. They use energy from the sun. Some organisms use energy from the oxidation of certain substances to make food in a process called chemosynthesis. Organisms that make their own food are referred to as autotrophs. Some organisms obtain complex food substances made by other organisms and break them down to simple substances while others make their own food from simple inorganic substances. Some organisms obtain food by breaking down dead organic substances. These are called saprophytes and the type of feeding is called saprophytism e.g fungi feeding on a dead decaying wood. Others obtain food by associating with other organisms in particular kinds of relationships. Some organisms obtain food by associating with others not of their own kind in a relationship called parasitism. In this relationship, the organism that benefits is called a parasite while the other does not benefit and is harmed; it is called a host e.g a tick on a cow. In other relationships, both organisms benefit from the relationship. This is called mutualism e.g the gut bacteria. In some relationships, one may benefit or not. This is called commensalism and the organisms involved are called commensals. All relationships where two organisms live together are called symbiosis. Organisms which obtain food by breaking down complex food substances to simple absorbable molecules are called heterotrophs. NUTRITION IN GREEN PLANTS Green plants differ from other organisms because they take in simple materials such as carbon dioxide, water and mineral salts (i.e. nitrates, phosphates and sulphates) to build more complex substances including carbohydrates, oils and proteins. The building up of carbohydrates takes place in the presence of the sunlight in a process called photosynthesis. Therefore, the raw materials for photosynthesis are water and carbon dioxide. During photosynthesis plants give off oxygen according to the equation below.Light energy 6H20 +6C02  C6 H1206 + 602Chlorophyll Some carbohydrates are later changed into lipids and proteins. This unit therefore deals with: - • The process of photosynthesis. • Rate of photosynthesis. • Factors affecting photosynthesis. • The structure of a leaf and its adaptations to photosynthesis. • Experiments on photosynthesis. • The importance of major plant nutrient elements. Factors affecting the rate of photosynthesisWhen plenty of carbon dioxide, sunlight and water are provided to a plant, photosynthesis will be at its maximum rate; sometimes the rate of photosynthesis is not as high as expected due to inadequacy of factors that include light intensity, water, temperature and carbon dioxide concentration.Providing plenty of each of these factors to a plant increases the rate of photosynthesis. However, if one of these factors is not adequate, the rate of photosynthesis may become low. A factor that is inadequate is called a limiting factor. • Light intensity: in case of dim light, the rate of photosynthesis is low. As light intensity increases, the rate of photosynthesis increases. • Carbon dioxide concentration: the more carbon dioxide a plant is given, the faster is the rate of photosynthesis, until a maximum is reached. • Temperature: photosynthesis is an enzyme-controlled process. Increase in temperature increases the rate of photosynthesis while a decrease lowers it. • Size of stomatal aperture: This regulates the amount of carbon dioxide entering a plant which it uses for photosynthesis. If the stomata are closed then photosynthesis cannot take place and when they are open, carbon dioxide enters and the rate of photosynthesis increases. Mineral salts are necessary for normal plant growthSeveral elements are required for normal plant growth and development. Some of these elements are Carbon, Hydrogen and Oxygen. Other elements are Nitrogen, Sulphur, Phosphorus, Potassium, Calcium and Magnesium. The last category of important elements includes Iron III, Manganese, Zinc, Boron and Molybdenum. These elements in the last category are called trace elements because they are needed in small amounts for normal growth. Apart from the first category, the plant obtains the nutrients it needs from the soil. The effects of these chemical elements can be discovered by growing plants in water solutions containing balanced amounts of salts necessary for healthy plant growth. This solution is called a culture solution. If any of the elements is left out; its effect can be observed. Table II: Shows the importance of certain elements are necessary for normal plant growth.
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Chapisha Maoni