Digestion, Absorption and Utilization of Food An Overview

You already have an idea of the processes of digestion and absorption. What is the purpose of the process of digestion? it is through this process that the complex substances in food are broken down into simpler substances which the body can take in and use. Some of these complex substances such as carbohydrates, proteins and fats undergo some change during digestion. However, water, minerals and vitamins present in the food can be absorbed directly without undergoing any digestive change. Digestion and absorption take place in the digestive tract of the body. Look at Figure below. It shows you the essential parts of the digestive tract.

The digestive system

Food takes the following path in the digestive tract.

Mouth ------------ O esophagus -------------- Stomach ------------ Small intestine ---------- Large intestine  ---- Rectum -------- Anus

The liver and pancreas are not a par! of the digestive tract but they secrete certain juices or secretions which do play a role in the process of digestion and absorption of food.

The whole process of digestion is accomplished in the time food remains in the digestive tract. The process of digestion starts from the ingestion or intake of food in the mouth. From here food passes to the other parts of the digestive tract. Each of these parts of the digestive tract (except the o esophagus, rectum and anus) contain some kind of digestive juice. These juices are either secreted by that particular part of the tract or poured in from the other organs or.glands of the body such as liver and pancreas. These digestive juices contain chemical substances known as enzymes. Enzymes are proteins which help chemical reactions to proceed faster. They bring about changes in food substances without themselves undergoing any change.

DIGEST!ON OF FOOD : Let us now have a look at the overall process of digestion. Digestion takes place step by-step at various sites in the digestive tract as  you will learn from the following discussion.

Mouth : As you have already noticed, the process of digestion begins in the mouth where food is chewed by the teeth and mixed with saliva. While the food is still in the mouth. it is acted upon by an enzyme. amylase. which acts only on cooked carbohydrates and partially digests them or breaks them up into smaller units.

Stomach: The chewed food mixed with saliva then passes into the stomach through the tube-like structure called the o esophagus. Here it pets mixed with the digestive juice present in the stomach called gastric juice. Besides enzymes and water, gastric juice also contains small amounts of an acid called hydroelectric acid which makes it acidic in nature. Mixing of food with the gastric juice converts the food into a thin soup-like consistency. Gastric juice contains an enzyme which acts on proteins and brings about their partial digestion. Other nutrients in food remain chemically unchanged.

Small intestine: The next stop in the digestive tract is the small intestine. The partially digested mass of food passes from the stomach into the small intestine. The small intestine not only contains intestinal juice (which is secreted from the small intestine itself) but also secretions from the liver and pancreas. The secretion

from the liver is called bile and from the pancreas is known as pancreatic juice. Bile aids in the digestion and absorption of fats (you will learn about the role of bile in fat digestion in Unit 3). Both pancreatic and intestinal juices contain enzymes which break down fats, proteins and carbohydrates into simpler substances. These simple substances ultimately reach the bloodstream.

Large intestine: The food which is not absorbed in the small intestine along with a large amount of water passes on to the large intestine. Here most of the excess water is reabsorbed and the remaining water and solid matter is eliminated from" the body as faces

.

Absorption of  Food : Where is food absorbed? You would have realized by now that absorption takes place in the small intestine. The end products of digestion or the nutrients present in the small intestine can be used by the body only when they enter the bloodstream. This process of movement of digested food or nutrients  across the intestinal wall to the bloodstream is timed absorption of food. The wall of the small intestine is made up of numerous folds or finger-like projections known as villi The presence of these villi tremendously increase the total area from which absorption can take place. Most of the nutrients arc absorbed from the  upper part of [he small intestine though some are absorbed from the gives a summary of the processes of digestion and absorption.

Summary of digestive process

As you know the end products of digestion move into the bloodstream after absorption. The blood circulating in the body and the nutrients it carries reaches every cell of the body. Blood can there fore be considered some what like a bus carrying passengers from one stop (i.e the intestine ) to other (i.e the cell). Once they reach the cell the nutrients perform their specific functions.

Transport of nutrients

Utilization of Food :  How is food utilized? In order to be utilized for specific functions the absorbed end products or'the nutrients from the food we eat further undergo chemical changes. They are either further broken down to release energy or are used to form more complex substances. These substances can then be used to manufacture more complex substances or can be stored in the body. We use certain terms to refer to these process. Let us learn these.terms.

Metabolism is a general term. It is just a small word referring to all the chemical changes that take place in the cells after the end products of digestion are absorbed You know it is of two types-breakdown of complex substances into simpler ones and manufacture of complex substances from simple ones. We have two terms referring to each of these processes. Let us learn these.

Anabolism is the term used to refer to all the chemical reactions by which simple substances are used to manufacture more complicated ones and Catholicism  includes all the chemical reactions by which complex substances are further broken down to simpler components.