Nutrition and Digestion

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Nutrition is the study of nutrients and their uses in our body. Nutrients are chemical substances needed by the body for growth, maintenance, development, repair and reproduction. Nutrients are usually obtained from the food. 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

Image showing types of nutrition.

Image Showing Types of Nutrition.

Image showing types of nutrition.

Autotrophic nutrition: all green plants and certain protists have evolved a mechanism to directly use the energy of sunlight for preparing organic food in their own body from simple inorganic materials.

Heterotrophic nutrition: they obtained food by feeding on other organisms;

Holozoic: Organisms engulf the food into the body, digest it and absorb the soluble products of digestion, e.g. humans.

Saprophytic: organisms absorb fluid food through the body surface, e.g. certain bacteria and fungi.

Parasitic: food is derived from other living organisms by living on or inside their body, e.g. certain bacteria, roundworm, tapeworm, Cuscuta.

Mixotrophic: Euglena carries autotrophic and saprotrophic nutrition at the same time.

There are Five Major Steps in Animal Nutrition (Holozoic nutrition)

The food we take contains nutrients like protein, carbohydrates and fats. These substances cannot be utilized by our body. These have to be broken down into simpler and smaller molecules before they can enter into the cells. Proteins are broken down into amino acids, carbohydrates into glucose, fats into fatty acids and glycerol. Amino acids, glucose, fatty acids and glycerol are simpler substances, and can be directly utilized by our body. This breakdown of complex food constituents and their absorption is accomplished by the digestive system. The processes involved in nutrition are;

Ingestion: taking food into the body,

Digestion: food containing large, insoluble molecules is broken down into small, water soluble molecule,

Absorption: digested food passes through the intestinal wall into blood stream,

Assimilation: absorbed food is taken in by the body cells and used for energy, growth and repair,

Egestion: undigested food is removed from the body.

Two types of Digestion (Intracellular and extracellular)

The digestion is classified into two types on the basis of site of digestion of food; (a) intracellular (b) extracellular.

Intracellular Digestion (Intra = inside)

Amoeba has no mouth for ingestion of food. It ingests the food by using its pseudopodia. The food is engulfed with to form a food vacuole (ingestion). Enzymes from cytoplasm are secreted into the food vacuole to break down complex food (digestion). Digested food is absorbed directly into the cytoplasm by diffusion (absorption). Food is used to obtain energy through respiration and the remaining part of the food is used for growth (assimilation). Undigested food collects inside the cell and the cell membrane ruptures and through this undigested food is thrown out of the body. Some multicellular animals like Hydra also digest partially digested food by intracellular digestion.

Image showing Intracellular digestion in Amoeba.

Image Showing Intracellular Digestion in Amoeba.

Image showing Intracellular digestion in Amoeba.

Extracellular digestion (extra = outside)

When digestion of food occurs outside the cells within a cavity, it is called extracellular digestion. The digestive enzymes, secreted by the gland cells of endoderm, are poured into the cavity where digestion of food takes place. This type of digestion occurs in higher invertebrates and all vertebrates.

Image showing Digestive organs of a cockroach for extracellular digestion.

Image Showing Extracellular Digestion.

Image showing Digestive organs of a cockroach for extracellular digestion.

Joint Intracellular and Extracellular digestion

In Hydra and Cnidarians possess only one digestive opening, which serves as both the mouth and the anus. This opening is surrounded by tentacles and leads to an internal digestive cavity called the 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.

Image showing Hydra catching its prey.

Image Showing Hydra Catching Its Prey.

Image showing Hydra catching its prey.