OBJETIVOS General:

All organisms need nutritious food to live. Our digestive system takes in foodstuffs by first grinding them into small pieces by chew- ing 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 sub- stances our bodies need, they must be absorbed, and our diges- tive tract has been developed to do that. Absorption also depends on lots of surface area.

of food molecules. To begin with, plant photosynthe- sis captures sunlight energy and locks it into the chem- ical 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 mat- ter. 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 mol- ecules 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 mechan- ical 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 imme- diately 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 contract- ing to mix the food and digestive fluids. After a couple hours, the food mass has been turned into a pasty mix- ture called chyme. At this point, the chyme is ready to pass from the stomach into the small intestine. How- ever, 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 chemi- cal 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 vil- lus 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 –N U T R I T I O N A N D E N E R G Y U S E : T H E D I G E S T I V E S Y S T E M–

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.



P r a c t i c e

1. The function of the digestive system is to break

down food molecules and distribute them to the body so that

a. the liver can break down starches. b. the cells can break down proteins. c. the lungs can engage in respiration. d. the cells can engage in cellular respiration. 2. Chyme enters the small intestine

a. devoid of all nutritional value.

b. with all food molecules completely broken

down.

c. with some food molecules partially broken

down.

d. where it is not exposed to enzymes.

3. Once food reaches the cells, it is subjected to cel-

lular respiration and what other two metabolic processes?

a. circulation and breathing b. anabolism and catabolism c. integration and summation d. ingestion and inhalation

4. Anabolism is a metabolic process that a. breaks large molecules into smaller ones. b. combines small molecules into larger ones. c. distributes oxygen evenly throughout the

body.

d. consumes protein molecules.

5. Which of the following is NOT an accessory

organ of the digestive system?

a. liver b. pancreas c. gall bladder d. urinary bladder

6. The chief function of the colon is to a. absorb water from undigested waste. b. produce sugars.

c. absorb protein from undigested waste. d. produce carbohydrates.



A n s w e r s

1. d. Food molecules must be broken down into

their constituent molecules and be small enough to be absorbed, transported, and then reabsorbed by the cells. The cells will then use this energy in the chemical bonds of the broken-down food molecules for cellular respiration.

2. c. Chyme is the slurry that exits in the stomach

and enters the small intestine. At this point, some of the foodstuffs in chyme have been broken down (some starch and a lot of the proteins). More digestion takes place as the food moves through the small intestine where many enzymes are present.

3. b. The two metabolic processes that molecules

are subjected to at the cellular level are anabolism and catabolism. Cellular respira- tion is a type of catabolism because a larger molecule (glucose) is broken down to very small molecules (carbon dioxide and water). Anabolism occurs when the cell produces proteins from the free amino acids resulting from the breakdown of proteins in the diges- tive system.

4. b. Anabolism combines smaller molecules into

larger ones.

5. d. The urinary bladder is part of the excretory

system.

6. a. The absorption of water from undigested

food occurs in the colon.



I n S h o r t

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.



Ve r t e b r a t e s a n d I n v e r t e b r a t e s

Earlier, when classifying animals, it was mentioned that we separate multicelled animals into two overall, large groups. Those two groups are the vertebrates (those animals with backbones) and the invertebrates (those without backbones). Many invertebrates have no internal skeleton, but they have what is called an exoskele- ton, which is a hard outer coating. Insects, crabs, and lobsters all have exoskeletons. Muscles can attach to the exoskeleton, which acts as protective body armor. The disadvantage to an exoskeleton is that it limits the organism’s growth. Many invertebrates have worked around this problem by molting (or shedding) their exoskeletons at various stages in their life cycles so that they can grow their bodies and then grow a new exoskeleton.

In vertebrate animals, the skeleton is internal (an endoskeleton) and has evolved structures called ver- tebrae, which are hard, bony projections surrounding the spinal cord. This vertebral column is commonly

L E S S O N

Structure and