Energy is required by living beings for performing different activities. Energy is contained in the food.
Food can be defined as a collection of chemicals taken by an organism for the purpose of the growth, repair, replacement of body cells, energy releases and maintenance of all the life processes.
The process by which organisms obtain and utilize food for their growth, development, and maintenance is called nutrition and the chemical constituents present in the food are called nutrients.
Digestion is the breaking down of complex constituents of food by enzymes into simpler soluble forms that can be absorbed and utilized by the cells of the body.
Types of nutrition:
There are mainly two types of nutrition autotrophic nutrition and heterotrophic nutrition.
Green plants and certain bacteria can manufacture their own food (organic substances) from inorganic substances (CO2 and H2O) using energy from sunlight. This mode of nutrition is called autotrophic mode of nutrition.
In this mode of nutrition the food (organic and inorganic substances) obtained by feeding on other organisms. Heterotrophic Nutrition is further classified as holozoic nutrition, saprophytic nutrition, and parasitic nutrition.
Holozoic Nutrition: In this mode, the organisms engulf the food into the body, digest it and absorb the soluble products of digestion, e.g. humans.
Saprotrophic Nutrition: In this mode, the organisms secrete digestive enzymes on to dead organic material and absorb the products of digestion, e.g. certain bacteria and fungi.
Parasitic Nutrition: In this mode, the food is derived from other living organisms by living on or inside their body e.g. certain bacteria, roundworm, tapeworm, Cuscuta, etc.
Types of Nutrients:
Nutrients include both organic and inorganic compounds. The organic compounds include carbohydrates, proteins, fats, and vitamins. Inorganic compounds contain minerals and water.
Depending upon the quantity of nutrient in food the nutrients are classified as macronutrients and micronutrients.
Those nutrients which are needed in large amounts are called macronutrients. Carbohydrates (sugar), lipids (fats), and proteins are macronutrients. They are the main source of energy and source of carbon (sugars) and organic nitrogen (amino acids) to the organism.
Micronutrients are nutrients which are needed in a small amount. Vitamins and minerals are micronutrients. They play important roles in human development and well-being, including the regulation of metabolism, heartbeat, cellular pH, and bone density. Lack of micronutrients can lead to stunted growth in children and increased risk for various diseases in adulthood.
Steps in Animal Nutrition (Holozoic nutrition):
The breakdown of complex food constituents and their absorption is accomplished by the digestive system. The processes involved in nutrition are :
Ingestion: In this food is taken from outside into alimentary canal through the mouth. It involves taking in food, chewing or sucking it and swallowing.
Digestion: It is the breaking down of complex constituents of food by enzymes into simpler soluble forms that can be absorbed and utilized by the cells of the body.
Absorption: Absorption step involves absorbing digested food through the intestinal wall to reach the body tissues.
Assimilation: Utilization of digested food nutrients by the body tissues for energy and synthesis of new protoplasm for growth and repair.
Egestion: The process of removal of undigested and unabsorbed food from the body is called egestion or defecation.
Types of Digestion:
Generally, two types of digestion are seen in heterotrophs: (a) Intracellular (b) Extracellular
Intracellular Digestion (Intra = inside):
In this type of digestion, all the five steps of nutrition (ingestion, digestion, absorption, assimilation, and egestion) occur inside the cell itself, as in Amoeba, Paramecium and other unicellular animals.
Intracellular digestion is defined as the process in which animals that lack a digestive tract bring food items into the cell for digestion and for nutritional needs.
Intracellular digestion in Amoeba:
Food particles such as minute bacteria are enclosed (caught) by pseudopodia to form a food vacuole (Ingestion).
Enzymes from the cytoplasm are secreted into the food vacuole (phagosome) to break down complex food. (Digestion). The food vacuole changes into a digestive vesicle (phagolysosome) within a cell by the fusion of phagosome containing ingested material and a lysosome containing hydrolytic enzymes. It leads to the digestion of food.
Digested food is absorbed into the cytoplasm. (Absorption)
The absorbed food is used up wherever required in the cell. (Assimilation)
The undigested unabsorbed food is expelled, when the food vacuole comes near the cell surface and bursts open. (Egestion)
Food vacuoles are temporary structures and every time the Amoeba feeds, a new food vacuole is produced.
Extracellular Digestion (extra = outside):
In this type, the digestion occurs outside the cell. All animals (excluding sponges) carry out extracellular digestion.
They have either a cavity, a tube, or a food canal (alimentary canal) which receives the ingested food. Digestive enzymes are poured over the food during its passage through the alimentary canal, and the products of digestion are absorbed back into the cells through the walls of the intestine. The undigested, unabsorbed food is thrown out of the digestive cavity.
Fungi and other decomposers perform extracellular digestion. They suck the life out of their host by releasing chemicals which break down the food they are living on. Once broken down their cells absorb the nutrients released.
Joint Intracellular and Extracellular Digestion:
In Hydra and other Cnidarians, the food (tiny prey) is caught by the tentacles and ingested through the mouth into the single large digestive cavity, called gastrovascular cavity.
Enzymes are secreted from the cells bordering this cavity and poured on the food for extracellular digestion. Small particles of the partially digested food are engulfed into the vacuoles of the digestive cells lining gastrovascular canal for intracellular digestion.
Any undigested and unabsorbed food is finally thrown out of the mouth.