Question

What is digestion? Why do only animals require a digestive system?

Answer 1

Food consumed by animals are made up of complex macromolecules which cannot be metabolized in their natural form. They need to be broken down into simple substances so that they can be absorbed into the blood.

Complete answer:
All living beings require nutrients to survive. Plants obtain these nutrients through photosynthesis while animals obtain these nutrients from the food they consume. The food animals eat comprises macromolecules like proteins, fat, and complex carbohydrates. Animals need to convert these macromolecules into simple ones. They do so by a special organ system present in the body known as the digestive system. Digestion is the process of breakdown of the complex molecules present in food into simple ones that can be utilized by the body. It involves two processes:- Mechanical digestion and Chemical digestion.
The digestive system is one of the largest organ systems in animals. It consists of two parts, the Alimentary canal, and Digestive glands.

Additional information:
The alimentary canal has an anterior opening called a mouth through which food enters. After the intake of food by mouth, the teeth masticate the food and the tongue helps in swallowing. The saliva present in the mouth contains an enzyme called salivary amylase which helps in the breakdown of starch into maltose. The food now reaches the stomach through the esophagus where proteins are converted into proteases and peptones by the enzyme pepsin. The food then reaches the small intestine where the majority of digestion takes place. In the small intestine enzymes such as lipase, trypsin, amylase, and nucleases digest fats, proteins, carbohydrates, and nucleic acids respectively. After digestion of these macromolecules, the simpler molecules are absorbed in the small intestine and the undigested and unabsorbed substances are passed on to the large intestine. In the large intestine, absorption of minerals and water happens. The remaining substances called feces are stored in the rectum until defecation.
The digestive glands include the salivary gland, the liver, and the pancreas. Salivary glands produce saliva. The liver secretes bile. The pancreas secretes pancreatic juice which contains enzymes.

Note:
The alimentary canal and the digestive gland together make up the digestive system. Both the parts of the digestive system are equally important. The secretions from the digestive glands aid the process of digestion taking place in the alimentary canal.