Digestion and Excretion The Digestive System What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements and see if you have changed your mind. Before Statement After 3. Digestion begins in the mouth. 4. Energy from food stays in the digestive system. Key Concepts What does the digestive system do? How do the parts of the digestive system work together? How does the digestive system interact with other systems? Functions of the Digestive System What did you eat for breakfast today? Do you ever think about what happens to the food you eat? Someone might have told you to take small bites and chew your food thoroughly. The size of the chewed food particles can affect how quickly your food is digested. As soon as the food enters your mouth, it begins its journey through your digestive system. All the food you eat goes through a process with four steps ingestion, digestion, absorption, and elimination. All four steps happen in the organs and tissues of the digestive system in the following order. Food is ingested. Ingestion is the act of eating, or putting food in your mouth. Food is digested. Digestion is the mechanical and chemical breakdown of food into small particles and molecules that your body can absorb and use. Nutrients and water in the food are absorbed, or taken in, by cells. Absorption occurs when the cells of the digestive system take in small molecules of digested food. Undigested food is eliminated. Elimination is the removal of undigested food and other wastes from your body. Building Vocabulary As you read, circle all the words you do not understand. Highlight the part of the text that helps you define these words. Key Concept Check 1. Describe What does the digestive system do? Reading Essentials Digestion and Excretion 247
REVIEW VOCABULARY chemical reaction a process in which a compound is formed or broken down Reading Check 2. Define What are enzymes? Reading Check 3. Explain What happens to an enzyme after it breaks down a food molecule? 4. Identify Circle the food molecule that is in the process of being broken down. Types of Digestion Before your body can absorb nutrients from food, the food must be broken into small molecules by digestion. There are two types of digestion mechanical and chemical. In mechanical digestion, food is physically broken into smaller pieces. Mechanical digestion happens when you chew, mash, and grind food with your teeth and tongue. Smaller pieces of food are easier to swallow and have more surface area than larger pieces. This helps with chemical digestion. In chemical digestion, chemical reactions break down pieces of food into small molecules. Enzymes Chemical digestion cannot occur without substances called enzymes (EN zimez). Enzymes are proteins that help break down larger molecules into smaller molecules. Enzymes also speed up, or catalyze, the rate of chemical reactions. Without enzymes, some chemical reactions would be too slow or would not occur at all. There are many kinds of enzymes. Each one is specialized to help break down a specific molecule at a specific location. The Role of Enzymes in Digestion Nutrients in food are made of different molecules, such as carbohydrates, proteins, and fats. Many of these molecules are too large for your body to use. But, because these large molecules are made of long chains of smaller molecules joined together, they can be broken down into smaller pieces. The digestive system produces enzymes that are specialized to help break down each type of food molecule. For example, the enzyme amylase helps break down carbohydrates. The enzymes pepsin and papain help break down proteins. Fats are broken down with the help of the enzyme lipase. The figure below shows how an enzyme helps break down food molecules into smaller pieces. Notice in the figure that the food molecule breaks apart, but the enzyme does not change. Therefore, the enzyme can immediately be used to break down another food molecule. Step 1 An enzyme attaches to a food particle. Food particle Enzyme Step 2 The enzyme speeds up a chemical reaction that breaks down the food particle. Step 3 The enzyme releases the broken-down food particle. 248 Digestion and Excretion Reading Essentials
Organs of the Digestive System For your body to use the nutrients in the foods you eat, the nutrients must pass through your digestive system. There are two parts to your digestive system: the digestive tract and the other organs that help the body break down and absorb food. These organs, shown at the bottom of this page, include the tongue, salivary (SA luh ver ee) glands, liver, gallbladder, and pancreas. The digestive track extends from the mouth to the anus. It has different organs that are connected by tubelike structures. Each of these organs is specialized for a certain function. The Mouth Mechanical digestion of food begins in your mouth. Your teeth and tongue mechanically digest food as you chew. But even before chewing begins, your salivary (SA luh ver ee) glands produce saliva (suh LI vuh) at the thought of food. They produce more than 1 L of saliva every day. Saliva contains an enzyme that helps break down carbohydrates. It also contains substances that neutralize acidic foods and a slippery substance that makes food easier to swallow. The Esophagus When you swallow a bite of food, it enters your esophagus (ih SAH fuh gus). The esophagus is a muscular tube that connects the mouth to the stomach. Food moves through the esophagus and the rest of the digestive tract by waves of muscle contractions called peristalsis (per uh STAHL sus). Peristalsis is similar to squeezing the bottom of a toothpaste tube. This forces toothpaste to the top of the tube. Muscles in the esophagus contract and relax. This action pushes partially digested food down the esophagus and into the stomach. Tongue Esophagus Liver Gallbladder Large Rectum Salivary glands Stomach Pancreas Small Peristalsis Create a shutterfold book to illustrate the organs of the digestive system and their functions. Mouth Esophagus Stomach Small Large Reading Check 5. Define What is the esophagus? 6. Identify Trace with a highlighter the organ that connects the mouth to the stomach. Reading Essentials Digestion and Excretion 249
Reading Check 7. Describe Why is the stomach s ability to store food beneficial? 8. Identify Where does food go after it leaves the stomach? The Stomach Partially digested food leaves the esophagus and enters the stomach. The stomach, as shown below, is a large, hollow organ. One function of the stomach is to store food. This allows you to go many hours between meals. The stomach is like a balloon. It can stretch when filled. An adult stomach can hold about 2 L of food and liquid. The stomach also helps with chemical digestion. The walls of the stomach are extremely folded. These folds enable the stomach to expand and hold large amounts of food. The cells in these folds produce chemicals that help break down proteins. The stomach contains an acidic fluid called gastric juice. Gastric juice makes the stomach acidic. Acid helps break down some of the structures that hold plant and animal cells together, such as the cells in meat, lettuce, and tomatoes. Gastric juice also contains pepsin. Pepsin is an enzyme that helps break down the proteins in foods into amino acids. Food and gastric juices mix as muscles in the stomach contract through peristalsis. As food mixes with gastric juice in the stomach, it forms a thin, watery liquid called chyme (KIME). 4 1 Food leaves the stomach and enters the small. Food moves through the esophagus to the stomach. 2 3 Stomach muscles contract through peristalsis and mix food with gastric juices. The folds in the stomach enable it to expand to hold food. 250 Digestion and Excretion Reading Essentials
The Small Intestine Chemical digestion begins in the mouth and the stomach. But most chemical digestion occurs in the small. The small is a long tube that is connected to the stomach. Chemical digestion and nutrient absorption take place in the small. The small is named for its small diameter about 2.5 cm. The small is about 7 m long. Chemical digestion of proteins, carbohydrates, nucleic acids, and fats takes place in the first part of the small, called the duodenum (doo uh DEE num). The remainder of the small absorbs nutrients from food. Like the stomach, the wall of the has many folds. The folds of the small are covered with fingerlike projections called villi (VIH li) (singular, villus). Each villus contains small blood vessels. Nutrients in the small diffuse into the blood through these blood vessels. You can see the small vessels in a villus in the figure below. Locate the pancreas and liver in the figure below. These organs produce substances that enter the small and help with chemical digestion. The pancreas produces the enzyme amylase and a substance that neutralizes stomach acid. The enzyme amylase helps break down carbohydrates. The liver produces bile. Bile makes it easier to digest fats. The gallbladder stores bile until it is needed in the small i ntestine. Liver Gallbladder Pancreas Small Folds in small covered with villi Blood vessels Lymph vessel One villus Key Concept Check 9. Name What organs work together to help with chemical digestion? 10. Identify the structures in the folds of the small. Reading Essentials Digestion and Excretion 251
Math Skills A percentage is a ratio that compares a number to 100. For example, the total length of the s is 8.5 m. That value represents 100%. If the rectum is 0.12 m long, what percentage of the s is made up of the rectum? The ratio is 0.12 m 8.5 m. Find the equivalent decimal for the ratio. 0.12 m 8.5 m = 0.014 Multiply by 100. 0.014 100 = 1.4% 11. Use Percentages The total length of the s is about 8.5 m long. If the small is 7.0 m long, what percentage of the s is made up of the small? Key Concept Check 12. Predict What might happen to other body systems if the digestive system did not function properly? The Large Intestine Any food that is not absorbed in the small moves by peristalsis into the large. The large is also called the colon. The large has a larger diameter (about 5 cm) than the small. It is much shorter than the small, however. It is only about 1.5 m long. Most of the water in ingested foods and liquids is absorbed in the small. As food travels through the large, even more water is absorbed. Materials that pass through the large are the waste products of digestion. The waste products become more solid as even more water is absorbed. Peristalsis continues to force the semisolid waste into the rectum, the last section of the large. Muscles in the rectum and anus control the release of this semisolid waste, called feces (FEE seez). Bacteria and Digestion You might think that all bacteria are harmful. However, some bacteria have an important role in the digestive system. Bacteria in the s digest food and produce important vitamins and amino acids. Bacteria in the s are necessary for proper digestion. Without these bacteria, food would not be digested well. The Digestive System and Homeostasis Recall that nutrients from food are absorbed in the small. The digestive system must be functioning properly for this absorption to occur. The nutrients that are absorbed are needed for other body systems to maintain homeostasis. For example, the blood in the circulatory system absorbs the products of digestion. The blood carries the nutrients to all other body systems, providing them with materials that contain energy. 252 Digestion and Excretion Reading Essentials
Mini Glossary chemical digestion: the breakdown of food into small molecules by chemical reactions chyme (KIME): a thin, watery liquid; formed when food mixes with gastric juices in the stomach digestion: the mechanical and chemical breakdown of food into small particles and molecules that your body can absorb and use enzyme (EN zime): a protein that helps break down larger molecules into smaller molecules, and also helps speed up, or catalyze, the rate of chemical reactions esophagus (ih SAH fuh gus): a muscular tube that connects the mouth to the stomach mechanical digestion: the physical breakdown of food into smaller pieces peristalsis (per uh STAHL sus): waves of muscle contractions that move food through the esophagus and the rest of the digestive tract villi (VIH li): fingerlike projections that cover the folds of the small 1. Review the terms and their definitions in the Mini Glossary. Write a sentence describing the difference between mechanical and chemical digestion. 2. Fill in the table below, highlighting the main structures and function of the organs of the digestive system. Organ Function Special Features Mouth Esophagus Stomach Small Large What do you think moves food from mouth to stomach Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? teeth, tongue, saliva makes chyme with gastric juice villi highly folded ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Digestion and Excretion 253