Green plants manufacture their food from simple organic substances by a process called photosynthesis. Plants are therefore called autotrophs.
Photosynthesis occurs only in green plants and food produced during photosynthesis is used for Respiration, food storage as in organs and the synthesis of other substances like proteins, cellulose, enzymes, hormones, etc.
To manufacture the food plants require minerals from the soil. Those required in large amounts are called macronutrients e.g. Nitrogen, potassium, etc. While those required in small amounts are called micronutrients or trace elements e.g. Zinc, cobalt, chlorine, etc. Lack of minerals Leads to deficiency diseases whose symptoms are seen on the plant.
Nitrogen is made available to plants in the form of nitrates which are used to make proteins which animals feed on. The process of recycling nitrogen back into the soil is in the form of nitrates is called nitrogen cycle.
For optimal health and growth, living organisms require varieties of food. Deficiency of any of these food leads to impaired health and finally to deficiency diseases.
Animals are heterotrophic in feeding. In animals food is broken down by enzymes which are organic catalyst. Enzymes have an optimal condition for their action which is dependent on temperature PH.
Organisms are grouped according to their feeding habits. Some are free living, Others saprophytic while others are parasitic or symbiotic.