Life Processes in Animals

1. What are Life Processes?

Every living animal, from a tiny ant to a huge elephant, must perform certain basic functions to remain alive. These essential functions are called life processes. They go on continuously inside the body, often without us noticing them.

Food gives an animal the energy and raw materials to grow and repair its body. Respiration releases that energy. Transport carries food, oxygen and wastes to and from every part of the body. Excretion throws out the harmful wastes produced. Together these processes keep an animal healthy and active.

2. Nutrition in Animals

Plants make their own food by photosynthesis, so they are called autotrophs. Animals cannot make their own food — they depend on plants or other animals for food. They are therefore called heterotrophs, and their mode of nutrition is holozoic (taking in solid or liquid food and digesting it inside the body).

2.1 The Five Steps of Animal Nutrition

• Ingestion — taking food into the body through the mouth.
• Digestion — breaking down complex, insoluble food into simple, soluble forms that the body can use.
• Absorption — the digested, soluble food passes into the blood.
• Assimilation — the absorbed food is used by body cells for energy, growth and repair.
• Egestion — the undigested food is thrown out of the body as faeces.

3. The Human Digestive System

The human digestive system has two parts: the alimentary canal (a long tube about 9 metres from the mouth to the anus) and the digestive glands that pour digestive juices into this canal.

3.1 Mouth (Buccal Cavity)

Digestion begins in the mouth. The teeth cut, tear and grind food into smaller pieces. The tongue rolls the food, mixes it with saliva and helps in swallowing and tasting. The salivary glands produce saliva, which contains an enzyme that begins the digestion of starch, turning it into sugar.

3.2 Oesophagus (Food Pipe)

The chewed food, now a soft ball called a bolus, is swallowed and passes into the oesophagus. The food is pushed down towards the stomach by slow, wave-like contractions of the muscular wall called peristalsis. Because of peristalsis we can swallow even while lying down or upside down.

3.3 Stomach

The stomach is a thick-walled, J-shaped bag. Its muscular walls churn the food. The inner lining releases mucus, hydrochloric acid and digestive juices. The acid kills harmful germs in the food and creates an acidic medium for the enzymes to digest proteins. The mucus protects the stomach lining from the strong acid.

3.4 Small Intestine

The small intestine is the longest part of the alimentary canal (about 7.5 m), coiled up to fit in the body. Here digestion is completed with the help of juices from the liver and the pancreas, plus its own juice.

• Liver — the largest gland in the body; it secretes bile, which is stored in the gall bladder. Bile breaks large fat droplets into tiny droplets (this is called emulsification) so they can be digested easily.
• Pancreas — secretes pancreatic juice that digests carbohydrates, proteins and fats.

The completely digested food (glucose, amino acids, fatty acids) is now absorbed into the blood through the walls of the small intestine. The inner surface has thousands of tiny finger-like projections called villi that greatly increase the surface area for absorption.

3.5 Large Intestine

The undigested food passes into the large intestine, which is shorter but wider than the small intestine. Its main job is to absorb water and some salts from the undigested food. The remaining semi-solid waste (faeces) is stored in the rectum and removed through the anus — this final step is egestion.

4. Digestion in Other Animals

4.1 Digestion in Grass-eating Animals (Ruminants)

Animals such as cows, buffaloes, goats and deer eat grass and are called ruminants. Grass is rich in cellulose, which humans cannot digest. Ruminants quickly swallow the grass and store it in a part of the stomach called the rumen, where it is partly digested and becomes cud. Later, when the animal is resting, the cud returns to the mouth in small lumps and the animal chews it again slowly. This is called chewing the cud or rumination.

4.2 Feeding and Digestion in Amoeba

Amoeba is a tiny single-celled organism found in pond water. It has no mouth, yet it eats. It changes its shape constantly and pushes out finger-like extensions called pseudopodia (false feet).

When Amoeba senses food (such as smaller organisms), it surrounds the food with its pseudopodia and engulfs it, forming a food vacuole. Inside this vacuole, digestive juices break down the food. The digested food is absorbed and used for energy and growth, and the undigested material is thrown out.

5. Respiration in Animals

The food absorbed by the body still needs to be broken down to release energy. This release of energy from food takes place during respiration. Respiration has two parts: breathing (taking in oxygen-rich air and giving out carbon dioxide) and cellular respiration (the breakdown of food in the cells using oxygen to release energy).

5.1 Breathing in Humans

In humans, air enters through the nostrils, passes down the windpipe (trachea) and reaches the two lungs in the chest. During inhalation, the chest expands and air rushes in; during exhalation, the chest contracts and air is pushed out. In the lungs, oxygen passes into the blood and carbon dioxide passes out of the blood to be breathed out.

5.2 How Different Animals Breathe

• Lungs — humans, dogs, cows, lions, elephants and other mammals; also birds and reptiles.
• Gills — fish breathe through gills, which take dissolved oxygen from water.
• Skin — earthworms and frogs can breathe through their moist skin; frogs also use lungs.
• Tracheae — insects such as cockroaches and grasshoppers breathe through a network of air tubes called tracheae, which open at tiny pores called spiracles.

6. Transport of Materials in Animals

Food, oxygen and wastes must be carried to and from every cell of the body. In humans this is done by the circulatory system, which is made up of the heart, the blood and the blood vessels.

6.1 The Heart

The heart is a muscular organ about the size of your fist, located in the chest. It pumps blood continuously to all parts of the body. The heart beats throughout life without resting. You can feel the heartbeat as a pulse in your wrist or neck.

6.2 The Blood and Blood Vessels

Blood is a red fluid that carries oxygen, digested food and other materials to the cells, and carries carbon dioxide and wastes away from them. Blood flows through three kinds of vessels:

• Arteries — carry blood away from the heart to the body. They carry oxygen-rich blood (except the artery to the lungs) and have thick, elastic walls.
• Veins — carry blood back to the heart from the body. They carry carbon-dioxide-rich blood and have valves to stop the blood flowing backwards.
• Capillaries — extremely thin vessels that connect arteries to veins; exchange of food, oxygen and wastes between blood and cells takes place here.

7. Excretion in Animals

The life processes of the body produce harmful wastes, especially nitrogen-containing wastes from the breakdown of proteins. Removing these wastes from the body is called excretion.

The human excretory system consists of two kidneys, two tubes called ureters, the urinary bladder and the urethra. The kidneys filter the blood and remove the wastes along with extra water in the form of urine. Urine passes through the ureters into the bladder, where it is stored, and is finally passed out through the urethra.

Animals excrete in different ways. Many animals remove liquid urine, fish remove wastes through their gills and skin, and birds and insects remove wastes in a more solid, paste-like form to save water. We also remove some waste through sweat from the skin and CO₂ through the lungs.

8. Putting It Together

All the life processes work together so smoothly that we hardly notice them. Food is taken in and digested (nutrition); oxygen breaks the food down to release energy (respiration); blood carries food, oxygen and wastes around the body (transport); and harmful wastes are thrown out (excretion). If any one process stops working, the animal cannot stay healthy or alive.