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The Digestive System Study Guide

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Updated on Sep 22, 2011

Introduction

All organisms need nutritious food to live. Our digestive system takes in foodstuffs by first grinding them into small pieces by chewing and stomach action. Then the small bits participate in chemical reactions that yield the specific substances our bodies need. The chemical reactions of digestion are dependent upon molecules we call enzymes. Once the digestive process yields the specific substances our bodies need, they must be absorbed, and our digestive tract has been developed to do that. Absorption also depends on lots of surface area.

Cellular Metabolism

To sustain themselves, living organisms must consume organic matter (or food). The biochemical reactions of metabolism use food as energy and these reactions occur at the cellular level. The two main types of reactions are anabolism and catabolism.

You may have heard of the anabolic steroids that athletes take to improve their performance. These substances work because they stimulate the process of anabolism in which simple molecules are built into more complex ones. For athletes, it helps build more muscle tissue. Anabolism is thus a constructive process, and anabolic processes require energy. Conversely, catabolism is a destructive process in which complex molecules are broken down into simpler ones, releasing energy as chemical bonds are broken.

Both processes require a substance that speeds up the reaction. Such a substance is called an enzyme, which can also be described as a biological catalyst. Energy for living beings is contained in the chemical bonds of food molecules. To begin with, plant photosynthesis captures sunlight energy and locks it into the chemical bonds of organic matter. Once an animal consumes this matter, it is usually broken down (by catabolism) into simple molecules, and energy is released from the chemical bonds in the organic matter. An organism then uses those simple molecules and some of the energy to rebuild complex molecules (by anabolism) into the ones it needs.

Digestive System

Before the cells can use food for energy, the large molecules we eat must be broken down into smaller ones that can be absorbed by the body and distributed to the cells in a usable form. It is the digestive system's job to break down foodstuffs using chemical and mechanical processes. The circulatory system then aids in the distribution of small organic molecules to the body's cells. The digestive system is essentially a long, hollow tube around which the rest of the body is built. This tube in vertebrates is open at one end (the mouth) for the ingestion of food and open at the other end (the anus) for the elimination of wastes. Along the length of the tube are accessory organs that have important functions in digestion. All of these chemical reactions, whether catabolic or anabolic, are catalyzed by enzymes.

The first step in the process of digestion is for the food to be broken down into a fine mash or pulp. In this form, the food has a much greater surface area, and the fine particles are more exposed to digestive enzymes and other fluids. This starts in the mouth where chewing breaks food down and saliva moistens it to form a pulpy mass. Enzymes in the saliva immediately begin working to break down starches and complex carbohydrates into simple sugars.

After chewing, food is swallowed as a lump called a bolus, which passes through the esophagus (a tube that connects the mouth to the stomach). The smooth muscle that lines the esophagus contracts and relaxes in such a way to move the food along. We call these rhythmic contractions peristalsis.

In the stomach, food breakdown continues. Gastric pits in the wall of the stomach secrete strong acid and enzymes. These chemicals are especially good at breaking down proteins into smaller chains of amino acids. The walls of the stomach begin contracting to mix the food and digestive fluids. After a couple hours, the food mass has been turned into a pasty mixture called chyme. At this point, the chyme is ready to pass from the stomach into the small intestine. However, only some starches have been broken down into simpler sugars, and the proteins have been broken down into smaller chains of amino acids.

In the small intestine, a variety of enzymes are added to the chyme mixture, and the still undigested food is broken down into absorbable molecules. Many accessory organs such as the liver, pancreas, and gall bladder contribute enzymes and buffering fluids to the mix inside the small intestine to aid in the chemical breakdown of food. The small intestine is quite long, up to 21 or more feet in adult humans and even larger in other animals. Also, peristaltic muscular action is still responsible for moving the food along.

In addition to its length, the interior wall of the small intestine is folded in such a way as to greatly increase the surface area. Small projections or foldings of the inner wall called villi (villus is the singular form) look like little fingers extending into the interior of the small intestine. The surface of each villus is further folded into microvilli. All this surface area makes the absorption of food molecules very efficient. Each villus has its own tiny blood vessels, and food molecules move through the villi cells into the blood stream for distribution to the rest of the body.

At the end of the small intestine, most nutrients that the vertebrate body needs have been absorbed. What remains is mostly water, cellulose (from the plant tissue eaten), and other indigestible matter. This mass passes into the large intestine or colon. The main function of the colon is to absorb water, reducing the undigested matter into solid waste called feces, which is eliminated from the body in a bowel movement.

In Short

To sustain themselves, living organisms must consume organic matter (food). The biochemical reactions of metabolism use this food as energy and occur at the cellular level. Two main types of reactions exist: anabolism and catabolism.

Before the cells can use food for energy, the large molecules we eat must be broken down into smaller molecules that can be absorbed by the body and then distributed to the cells in a usable form. It is the job of the digestive system to break down foodstuffs using chemical and mechanical processes.

Practice problems of this concept can be found at: The Digestive System Practice Questions

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