Main->Readings->4th Grade Readings->Table of Contents->Part 1: The Digestive System
Vocabulary |
If you're hungry, you might feel hollow. Your mouth might water, and you might grab an apple and bite into it. Then you chew the apple, and swallow it. That's all. You don't think about what happens to the apple, or what happens to you when you digest the apple. In this unit, however, we are going to spend some time and think about it.
You have learned that every living thing on Earth is made of cells. In order to grow, work, and make new cells, these cells must have energy. The earliest living things were single cells. They were right next to their food, so they could just absorb it.
In many-celled animals like you and me, this will not work. There are too many cells and they are buried deep in the body. Also, unlike single celled animals, each cell of your body cannot do all the jobs of a living thing. Our cells are specialized (SPESH-uh-lyzed) which means they do only one job.
Cells work together, organized into groups that do special jobs. Groups of cells that do a single job are called tissues (TISH-yews).
Tissues need to work together with other tissues in order to get things done. When two or more tissues work together, they make up an organ (OR-guhn). Examples of organs are a heart, a brain, and a bone.
An organ by itself cannot do a whole job. For instance, it would not do any good to have a heart if you had no arteries to carry the blood to the rest of the body, or veins to bring blood back to the heart. When organs work together to do a job, we call that an organ system.
Animals have an organ system for each main job. Jobs of the body include protecting itself from the outside, being able to move, breathe, and think, bringing oxygen and food to the cells and getting rid of wastes, growing, and reproducing.
There are eleven major organ systems. Each one is made of organs, which are made of tissues, which are made of cells, and every one of those trillions of cells needs food in order to do its job.
Why do cells need food? Just for a cell to be alive, it needs energy. Food gives energy. Also, cells must repair and replace themselves in the body, and food brings the building blocks for that. Foods and other substances we need for the body to work right are called nutrients (NU-tree-ents).
The body needs some things which are not food in order to work properly. Water is a big part of the body, and is used for keeping the body cool, mixing things in the body, and helping chemicals do their job. Vitamins are chemicals the body needs in small amounts and does not make itself. Minerals such as calcium, phosphorus, sodium, and potassium help the body work properly and build itself. Fiber, which your body does not digest at all, helps your digestive system move more quickly.
What foods do cells need every day? There are several kinds. Carbohydrates are a kind of food that gives the body energy. Carbohydrates (car-bo-HIGH-drates) are often called starches and sugars, and are found in things like bread, pasta, fruit, and cereal. Another kind of food you need is protein. Proteins (PRO-teens) are building blocks for cells, and are found in foods like meat, eggs, and beans like lentils. Fats serve as storage for energy, and also help make cells. They are found in animal fat and vegetable oils.
All these things are in the foods we eat every day. However, nutrients in our food are not in a form our cells can use. A cell in your body would not know what to do with a piece of apple, or a chicken leg. First, the food would be too big. Cells are so small they can only be seen with a microscope. Second, the food is in the wrong form. It needs to be changed so the cells can use it.
How does the food get changed and get to the cells? The cells of our body cannot do the job by themselves. One organ system, the digestive system, does that job for the whole body. That job, changing food into a form the body can use, is called digestion.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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Go on to Part 2: How Food is Digested
This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org