Digestion vs Absorption
Food that animals eat goes through four main processes known as Ingestion, Digestion, Absorption, and Defecation. Upon ingestion, digestion happens first, and absorption of nutrients in digested food takes place next to generate energy. Both these processes take place in the alimentary tract of an animal. These two processes are completely different processes, however it is impossible for absorption to take place without digestion.
Digestion is generally breaking down of food; containing process or series of processes. There are two main steps of digestion known as mechanical and chemical. In digestion, simplification of large molecules into small monomers takes place, i.e. it is a process of catabolism. However, there are mainly two forms of digestive systems; primitive organisms have external digestion, while more evolved advanced animals have internal digestive systems. In advanced animals, digestion starts in mouth and continues through to stomach, and concludes at jejunum. While food passes through oesophagus, the peristaltic movements help to breakdown it into smaller particles. Inside the stomach, the chemical digestion becomes predominant with secretion of digestive enzymes and acids with optimum temperature. Protein digestion starts in the stomach and ends up in the small intestine after converting proteins to amino acids. Lipid digestion starts and ends in the small intestine, which converts lipids into glycerol and fatty acids. Mouth starts carbohydrate digestion, and it concludes at the small intestine after forming simple sugars. After all the digestive processes, the nutrients in food are ready for absorption.
Absorption is transferring the digested molecules across the gastrointestinal tract into the blood stream. Absorption starts in the stomach, continues through the small intestine, and concludes at the large intestine. There are four main mechanisms responsible for absorption viz. Active transport, Passive diffusion, Endocytosis, and Facilitative diffusion. Simple columnar epithelial tissue covers the interior wall of the intestine with folds called Plicae Circulars those increase the area of absorption. Additionally microscopic finger like processes called Villi and Microvilli are present in the folds with each having a network of capillaries. These capillaries transfer nutrients from food into the blood stream. Jejunum and ileum absorb most of the nutrients while large intestine does majority of the water absorption. Finally, after absorption at the large intestine the undigested and unabsorbed portion is ready for defecation.
What is the difference between Digestion and Absorption?
• Digestion is a mechanical and chemical break down of large food contents in to smaller particles that are easy to be absorbed. Absorption is transferring the digested molecules across the gastrointestinal tract to the blood stream.
• Digestion starts from the mouth, but the absorption starts from the stomach.
• Digestion takes place inside the intestinal tract, but the absorption happens in the line of the intestine.
• Digestion needs enzymes, but absorption does not need enzymes.
• Digestion is always an active process and needs energy, but some of the absorption mechanisms do not need energy.
• Digestion changes the chemical and the mechanical structure of the food, whereas absorption does not.
• Absorption relates with blood, while digestion does not.
• Most of the digestion takes place at duodenum, whereas absorption occurs mainly at ileum and jejunum.
• Absorption operates on both simple molecules of digestive and non-digestive matter, while digestion functions upon large and complex digestive matter only.