Introduction to the Topic

Welcome back, young learners! In our previous journey through the plant kingdom, we discovered how green plants prepare their own food using sunlight through photosynthesis. But what about animals? Unlike plants, animals cannot make their own food. They depend directly or indirectly on plants for their nutrition. In NCERT Class VII Science Chapter 2 - Nutrition in Animals, we explore how animals take in food, process it inside their bodies, and use it for energy, growth, and repair.

Nutrition is a complex process that involves several stages: ingestion, digestion, absorption, assimilation, and egestion. Don't worry if these terms sound heavy right now. By the end of this comprehensive guide, you will understand how every morsel of food you eat travels through your body and fuels your daily activities!

Key Concepts Explained

1. Different Ways of Taking Food

The mode of taking food into the body varies in different organisms. For instance, bees and hummingbirds suck the nectar of plants, infants of humans and many other animals feed on mother's milk, snakes like the python swallow the animals they prey upon, and aquatic filter-feeders strain tiny food particles floating nearby and feed on them. Fascinating, isn't it? Even microscopic organisms like amoeba have unique ways of capturing and engulfing tiny particles of food using finger-like projections called pseudopodia.

2. Digestion in Humans

We take in food through the mouth, digest and utilize it. The unused parts of the food are defecated. Have you ever wondered what happens to the food inside our body? The food passes through a continuous canal, which begins at the buccal cavity and ends at the anus. This canal can be divided into various compartments:

  • Buccal Cavity (Mouth): This is where digestion begins. Our teeth cut, chew, and grind food. The salivary glands secrete saliva, which contains digestive juices that break down starches into sugars. This is why chewed rice often tastes sweet!
  • Foodpipe (Oesophagus): The swallowed food passes into the foodpipe. The movement of the walls of the foodpipe pushes the food downward into the stomach.
  • Stomach: The stomach is a thick-walled bag. Its inner lining secretes mucous, hydrochloric acid, and digestive juices. The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic so that the digestive juices can act.
  • Small Intestine: The small intestine is highly coiled and is about 7.5 meters long. It receives secretions from the liver (bile juice, which helps in digesting fats) and the pancreas (pancreatic juice acting on carbohydrates, fats, and proteins). Complete digestion of all components of food takes place here.
  • Absorption in the Small Intestine: The inner walls of the small intestine have thousands of finger-like outgrowths called villi. Villi increase the surface area for absorption of the digested food, passing it into the blood vessels.
  • Large Intestine: The large intestine is wider and shorter than the small intestine (about 1.5 meters in length). Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and is removed through the anus as faeces.

3. Digestion in Grass-Eating Animals

Have you ever observed cows, buffaloes and other grass-eating herbivores chewing continuously even when they are not eating? These animals quickly swallow grass and store it in a separate part of the stomach called the rumen. Here the food gets partially digested and is called cud. But later the cud returns to the mouth in small lumps and the animal chews it. This process is called rumination, and these animals are called ruminants. Grass is rich in cellulose, a type of carbohydrate that ruminants can digest with the help of special bacteria present in their digestive system, which humans cannot.

Summary & Key Takeaways

Let us review the most important points from this chapter to keep them fresh in your memory:

  • Animal nutrition includes nutrient requirement, mode of intake of food and its utilization in the body.
  • The human digestive system consists of the alimentary canal and secretory glands. It includes the buccal cavity, oesophagus, stomach, small intestine, large intestine ending at the anus, and associated glands like salivary glands, liver, and pancreas.
  • The principal digestive juices are secreted by the stomach and small intestine, along with secretions from the liver and pancreas.
  • The digested food is absorbed in the blood vessels from the small intestine.
  • The absorbed substances are transported to different parts of the body (assimilation).
  • Water and some salts are absorbed from the undigested food in the large intestine.
  • Amoeba ingests food with the help of false feet or pseudopodia, and digests its food inside food vacuoles.
  • Ruminants like cows swallow grass rapidly, store it in the rumen, and later bring it back to the mouth to chew it slowly through rumination.