Explain with example: Essential elements.

Hint: Essential elements are macro/micronutrients which are required by living organisms for growth, metabolism and development. They include carbon, hydrogen, oxygen, nitrogen and sulphur.

Complete answer:
Elements which are required by all living organisms to maintain their normal growth, metabolism and development are known as essential elements. Living organisms contain relatively large amounts of carbon, oxygen, hydrogen, nitrogen and sulphur. They must be taken from an external source as they cannot be produced in the body. These mostly enter the food chain through plants. They have a clear physiological role. For example, magnesium (a major component in chlorophyll) is required for photosynthesis, potassium is essential for opening and closing of stomata, etc.

Essential elements can be classified in two categories:
• Macronutrients- They include carbon, hydrogen, oxygen, sulphur, phosphorus, nitrogen, calcium, magnesium and silicon. In plant tissue, they are present in concentration of 1-10 mg per gram of dry matter.
• Micronutrients- They include trace elements like iron, copper, molybdenum, zinc, boron, cobalt, chlorine and manganese. They are required in very low concentration, as low as 0.1mg per gram of dry matter.

Additional information:
Essential elements are usually taken up from the surrounding environment. Carbon is mostly derived from the atmosphere as carbon dioxide. Hydrogen is mostly obtained from water. Oxygen is either taken from air or from water in the form of inorganic ions. However, plants are unable to use atmospheric nitrogen. Hence, nitrogen cycle along with nitrogen fixing bacteria help plants take up nitrogen.

Note: Essential elements are always specific and cannot be replaced by another element. These elements are directly involved in plant metabolism. Almost 20 elements have been found to be essential for growth and development of plants.