Nutrition in Animals — Study Notes

1. Introduction

Just like plants, animals also need food to live, grow and get energy. But unlike green plants, animals cannot make their own food. They depend on plants and other animals for food, so all animals are heterotrophs. The process of taking in food and using it to obtain energy, grow and repair the body is called animal nutrition. In this chapter we study how different animals obtain and digest their food, with special focus on the human digestive system.

Animal nutrition involves several steps. First the animal takes in food, then breaks it down into simple soluble substances that the body can absorb, and finally the absorbed nutrients are used by the cells. The way animals obtain food varies greatly; for example, hummingbirds sip nectar with their beaks, bees suck nectar through a tube-like proboscis, snakes swallow their prey whole, and some aquatic animals filter food from water. These variations show how different animals are adapted to their food habits.

2. Steps of Nutrition

Nutrition in animals takes place through a series of steps. These steps are common to most animals, though their details may vary. The main steps are ingestion, digestion, absorption, assimilation and egestion. In the unicellular organism Amoeba, all these steps occur within a single cell.

Ingestion is the taking in of food into the body. Digestion is the breakdown of complex food substances into simple, soluble substances that can be absorbed by the body. Absorption is the process by which the digested food is taken up by the body cells through the walls of the intestine. Assimilation is the utilisation of the absorbed food to build the body and release energy. Finally, egestion is the removal of undigested and unabsorbed waste food from the body. The undigested waste material is expelled through the anus as faeces. Together these five steps complete the process of nutrition in animals.

3. Digestion in Humans

Human beings take food through the mouth, and the process of nutrition proceeds through a long tube called the alimentary canal. The alimentary canal consists of the mouth, oesophagus (food pipe), stomach, small intestine and large intestine, along with associated organs like the salivary glands, liver and pancreas which secrete digestive juices.

The process begins in the mouth where food is chewed and mixed with saliva secreted by the salivary glands. Saliva contains an enzyme called salivary amylase that breaks down the starch in food into sugar. The tongue helps in chewing, swallowing and tasting food. The teeth are of different types: incisors for cutting, canines for tearing and molars for grinding food. After chewing, the food is swallowed through the oesophagus into the stomach. The stomach is a J-shaped, thick-walled muscular bag that churns the food and mixes it with digestive juices. The gastric juices in the stomach contain hydrochloric acid, which kills germs and makes the medium acidic, and an enzyme called pepsin which digests proteins.

From the stomach, the partially digested food moves into the small intestine. Here the liver secretes bile which breaks down fats into small droplets, and the pancreas secretes pancreatic juice which digests carbohydrates, fats and proteins. The inner wall of the small intestine has millions of tiny finger-like projections called villi, which greatly increase the surface area for absorption. The digested food passes through the walls of the small intestine into the blood through the villi, which is the process of absorption. The blood then carries the absorbed nutrients to all the cells of the body, where they are assimilated. The undigested and unabsorbed food moves to the large intestine, where water and some salts are absorbed. The remaining waste material is stored and finally passed out through the anus as faeces, completing egestion.

4. Digestion in Grass-Eating Animals

Grass-eating animals like cows, buffaloes and goats are called ruminants. Grass is rich in cellulose, which is a tough carbohydrate that our body cannot digest. Ruminants, however, can digest cellulose with the help of microorganisms. These animals quickly swallow the grass and store it in a part of the stomach called the rumen. In the rumen, bacteria digest the cellulose present in the grass. Later the animal brings the partially digested food (cud) back into its mouth in small lumps to chew it again; this process is called rumination, and such animals are said to be 'chewing the cud'.

After chewing the cud again, the food is swallowed once more and passes to the other chambers of the stomach and then into the intestine for further digestion and absorption. In this way, ruminants can obtain nutrition from a food source that many other animals, including humans, cannot digest.

5. Feeding and Digestion in Amoeba

Amoeba is a microscopic, unicellular organism that lives in pond water. It feeds on tiny organisms and other microscopic food particles floating in water. Amoeba has no mouth, so it takes in food by a special method. It surrounds the food particle with its finger-like projections called pseudopodia, engulfing the food completely. This is called ingestion by phagocytosis. The food is enclosed in a food vacuole where digestive juices are secreted to digest it. The digested food diffuses into the cytoplasm and is absorbed, and the undigested remains are expelled from the body. In this single-celled organism, all five steps of nutrition - ingestion, digestion, absorption, assimilation and egestion - happen within one cell.

Quick Revision Tables

Mind Map

Important Diagrams (SVG)

Diagram 1: Human Alimentary Canal

Diagram 2: Digestion in Amoeba

Common Mistakes

1. Confusing absorption with digestion; digestion breaks food down, absorption takes the simple food into the blood.
2. Thinking that bile digests fats completely; bile only breaks fats into small droplets, the actual digestion is done by enzymes.
3. Believing that starch digestion begins in the stomach; it actually begins in the mouth with salivary amylase.
4. Forgetting that hydrochloric acid in the stomach creates an acidic medium and kills germs, not the digestion of starch.
5. Writing that ruminants swallow grass twice before it reaches the stomach; actually grass first goes to the rumen, then the cud is re-chewed.
6. Mixing up the large intestine and small intestine functions; water is absorbed in the large intestine, food in the small intestine.
7. Thinking that villi are present in the stomach; villi are found in the inner wall of the small intestine.
8. Believing that the oesophagus digests food; it only carries food from the mouth to the stomach.
9. Forgetting that Amoeba engulfs food using pseudopodia, not a mouth.
10. Confusing rumen with the stomach as a whole; rumen is only the first chamber of a ruminant's stomach.

Exam Tips

1. Memorise the five steps of nutrition in order and be ready to define each one.
2. Draw a neat, labelled diagram of the human alimentary canal and practise labelling every organ.
3. Learn which digestive juice comes from which organ and what it does - this is a favourite exam question.
4. Remember the specific role of villi: they increase the surface area for absorption in the small intestine.
5. Explain the process of rumination step by step for grass-eating animals.
6. Understand why humans cannot digest cellulose but ruminants can.
7. Describe the nutrition in Amoeba in a logical sequence, using the terms pseudopodia and food vacuole.
8. Give at least two examples of each type of animal based on food habits (herbivore, carnivore, omnivore).
9. Know the location and action of salivary amylase and pepsin precisely.
10. Write answers using keywords like ingestion, assimilation and egestion to score full marks.

Conclusion

Animals are heterotrophs that depend on plants and other animals for food. Nutrition in animals follows the steps of ingestion, digestion, absorption, assimilation and egestion. The human digestive system is well organised, with each organ contributing a specific juice or action, and the small intestine with its villi is the main site of absorption. Ruminants show a wonderful adaptation for digesting cellulose with the help of microbes, while a single-celled Amoeba manages all nutritional steps in one cell. This chapter gives us a clear picture of how food is processed in the body and forms the basis for understanding health and hygiene.