Nutrition in Animals Summary Class 7 NCERT Summary Chapter 2

Introduction

Animals and humans exhibit heterotrophic nutrition because they obtain their food either directly from plants or indirectly by consuming other organisms that feed on plants.

Nutrition

Nutrition is the process by which living organisms obtain and use food to get energy and the materials needed for growth, repair and maintenance of the body. In animals, nutrition is a multi-step process that converts complex food into simple usable forms.

• Ingestion: The intake of food into the body, usually through the mouth.
• Digestion: The mechanical and chemical breakdown of complex food molecules into smaller, absorbable molecules.
• Absorption: The movement of digested products from the alimentary canal into the blood (or body fluids) through the walls of the intestine.
• Assimilation: The utilisation of absorbed nutrients by body cells to produce energy and to build complex substances such as proteins, fats and other cellular materials.
• Egestion: The removal of undigested and unabsorbed food material from the body as faeces.

Heterotrophic Nutrition

Heterotrophic organisms obtain their food from other living organisms. Based on their diet, animals are classified as:

• Herbivores: Animals that feed mainly on plants (e.g. cow, deer).
• Carnivores: Animals that feed mainly on other animals (e.g. tiger, hawk).
• Omnivores: Animals that feed on both plants and animals (e.g. bear, rat, human).

Nutrition in Humans

Human nutrition involves a well-organised alimentary canal and associated glands that carry out the steps of nutrition: ingestion, digestion, absorption, assimilation and egestion. The major parts and functions are described below.

Mouth

• The mouth contains the teeth, tongue and salivary glands.
• Teeth perform the mechanical breakdown of food by cutting, tearing and grinding. This increases the surface area for digestive enzymes to act.
• Saliva, produced by the salivary glands (located under the tongue and around the mouth), contains the enzyme salivary amylase which begins the chemical digestion of starch to simpler sugars.
• The tongue helps in mixing food with saliva, tasting and pushing the food into the throat for swallowing.

Teeth

• There are four types of teeth, each adapted to a specific function.
• Incisors are used for cutting and biting food.
• Canines are pointed and used for tearing and piercing food.
• Premolars and molars are broad and used for chewing and grinding food.
• Humans develop two sets of teeth during their life: the temporary milk (deciduous) teeth and later the permanent teeth.

Oesophagus and Peristalsis

• After swallowing, food moves from the mouth to the stomach through the oesophagus.
• The movement is effected by rhythmic muscular contractions of the oesophagus walls called peristalsis, which push the food downwards.

Stomach

• The stomach is a muscular, bag-like organ that stores food and mixes it with gastric juices.
• The inner lining of the stomach secretes gastric juice containing hydrochloric acid and digestive enzymes.
• Hydrochloric acid creates an acidic medium that aids protein digestion and kills many microbes taken in with food.
• Mucus secreted by the stomach lining protects the stomach walls from being digested by its own acid and enzymes.
• The stomach secretes digestive juices that help break down proteins into simpler substances.
• Partially digested food from the stomach (chyme) gradually moves into the small intestine for further digestion and absorption.

Digestion in the Small Intestine

• The small intestine is the longest part of the alimentary canal (about 7.5 m in an adult human) and is the major site for digestion and absorption.
• It receives secretions from two accessory glands: the liver (bile) and the pancreas (pancreatic juice), which help in the digestion of fats, carbohydrates and proteins.
• Further chemical digestion occurs here: starches are broken down to sugars, proteins to amino acids and fats to fatty acids and glycerol.
• The inner surface of the small intestine is folded and covered with tiny finger-like projections called villi.
• Villi greatly increase the surface area for absorption and contain a network of blood capillaries and a lymph vessel called a lacteal.
• Digested carbohydrates and proteins pass into the blood capillaries of villi and are transported to body cells; The digested food is absorbed by the villi and transported through the blood to different parts of the body.

Enzymes and Their Roles

• Salivary amylase (in saliva) begins starch digestion to maltose.
• Pepsin (in stomach) begins protein digestion to polypeptides.
• Pancreatic juice acts on carbohydrates, fats and proteins and changes them into simpler forms. The intestinal juice completes the digestion of all components of food.
• Bile from the liver is not an enzyme; it emulsifies fats (breaks large fat droplets into smaller droplets) to increase the surface area for lipase action.

Liver

• The liver is the largest gland in the body and produces bile, which is important for emulsification of fats.
• Bile is stored in the gall bladder and released into the small intestine when fat is present.

Pancreas

• The pancreas is a large cream-coloured gland that secretes pancreatic juice into the small intestine.
• Pancreatic juice contains enzymes such as amylase (starch → maltose), lipase (fats → fatty acids + glycerol) and proteases (proteins → peptides → amino acids) that help complete digestion of food.

Digestion in the Large Intestine

• The undigested and unabsorbed material, together with some water, moves from the small intestine into the large intestine (colon).
• The principal function of the large intestine is to absorb water and some dissolved salts from this material.
• Beneficial bacteria in the large intestine help break down some remaining substances and produce certain vitamins.
• The leftover semi-solid waste is stored in the rectum and expelled from the body through the anus as faeces; this removal is the process of egestion.

Summary

Human nutrition is a coordinated process involving mechanical and chemical breakdown of food, absorption of nutrients and their distribution to body cells for energy and growth, followed by removal of waste. The mouth, oesophagus, stomach, small intestine, large intestine and accessory glands (salivary glands, liver and pancreas) all play specific and essential roles in these steps. Understanding these parts and their functions helps explain how the body obtains and uses food to maintain life.