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What are cells?

You started life as a single cell. Each person does. First, you were just one cell. You became the person you are today as a result of cell division. Cell division produces the many special cells that form a human being.

Using a Microscope to See Cells Use a microscope to observe prepared laboratory slides of

  1. Red blood cells
  2. White blood cells
  3. Skin cells
  4. Lung cells
  5. Heart muscle cells
  6. Skeletal muscle cells
  7. Bone cells

First observe the slide of tissue showing cells on the slide without the use of the microscope. Then put the slide under the microscope at low power. Switch to higher power so you can actually see the details of the cells. Make a drawing of what you see through the microscope. Include appearance, shape, and anything else that you think is important.

Cells are the basic units of life. We are made of cells. Cells can make copies of themselves. Some cells can produce other kinds of cells that are different from themselves. Cells are small, and they come in a variety of shapes and types. Cells have different functions. Cells can move, swim, and crawl.

Figure 1.1 Examples of typical mammalian cells showing their diversity in shape. (1) Striated muscle cells, (2) red blood cells, and (3) bone cells (osteocytes).

Did You Know?

Some cells produced by cell division are programed to die. That explains how blood vessels become hollow tubes. Blood vessels develop as solid tubes. The cells in the center die and are removed, allowing for space for the blood. In the early stages of development of the hand it was paddle-shaped. The cells located in what is now the space between the fingers were programed to die.

Cells can send signals to other cells. Sometimes cells need to touch each other to send a signal, and some cells have long processes or extensions so they can send signals over long distances. Other cells release chemical signals that are carried all over the body by the blood. Once a cell receives a signal, it can interpret the signal and decide what to do. The cell can decide to divide. It can decide to become a specialized cell type. For example, it can become a red blood cell that gets rid of its nucleus. It can become a nerve cell in your eye. It can become a muscle cell of your heart. A cell can even decide to die.

Cells make the important materials that build a person. Cells make bone and cells make blood. In fact, every organ in your body, including your brain and your heart, is made of special cells. Cells make the substances necessary for life.

Each part of a cell is dynamic. The pictures you have seen of cells may make them look frozen. That's because a scientist or lab technician prepared the specimen to get it ready to be viewed in a microscope. But in life, cells are very active. Cells move and have dynamic, active behaviors.

Cells have many parts. Each part of cells has a special job. For example, cells have a boundary called a membrane. The membrane keeps some materials out, and other materials in, just like the border of a country. The membrane has channels and gates that let some materials come in and some go out. Most of the activities of a cell are controlled by its most important part, the cell nucleus. The genetic material of a human cell is contained inside the nucleus. You will learn more about these parts and other parts of the cell later in this unit.

The existence of cells was first supported only after the discovery of the microscope. Do a library search to find out who invented the first microscopes and some of the scientists, besides Robert Hooke, who made important discoveries about cells.

Scientists first learned about cells more than 300 years ago. During the 1660s, an English scientist named Robert Hooke built a simple microscope out of magnifying glasses. When he used his invention to look at a very thin slice of cork, he discovered that the cork seemed to be made up of little empty “boxes.” He called these boxes cells, which comes from a Latin word meaning “little rooms.”

Many scientific discoveries have resulted from one person exploring something simply because of curiosity. Have you ever watched an ant trail for a long time or observed the patterns on the surface of the moon? Describe in detail your observations of something you've “studied” simply because you were curious.

Robert Hooke's discovery provided scientists with the knowledge that cells existed. However, it wasn't until the late 1830s that several scientists realized how important Robert Hooke's findings were. At that time, scientists observed that all plants and animals were made of cells. About 20 years later, scientists discovered that all cells come from other cells. In other words, cells reproduce. These two ideas form the basis of one of the most important theories in biology-the cell theory. The cell theory describes what cells are and how they function. It states that cells are the units of structure and function of organisms. In addition, the theory states that all cells come from pre-existing cells.

The Organization of Living Things

Some organisms, such as an amoeba, are made of only one cell. Singlecell organisms are called unicellular organisms. Most organisms, however, are made of many cells. Organisms that contain many cells are called multicellular organisms. Humans are an example of a multicellular organism-your body contains trillions of cells!

What Do You Think?

Consider the very different functions of blood cells, nerve cells, and muscle cells. (a) Do you think all these cells look alike? Why or why not? (b) Do you think one kind of cell could perform the functions of another? Why or why not?

Multicellular organisms are made up of many cells, and of many types of cells. The different types of cells work together to perform the many different functions that keep you alive. For example, one type of cell is a blood cell. Red blood cells carry oxygen to different parts of your body and carry carbon dioxide away from different parts of your body. Other blood cells attack viruses and bacteria that invade your body. There are also fragments of blood cells that are in your blood to help repair wounds. Another type of cell is a nerve cell. Nerve cells control your body's reactions to events that happen inside and outside your body. Nerve cells send signals to your muscle cells to make them contract. Other nerve cells enable you to see and hear.

Sometimes groups of similar cells work together to perform a specific function. A group of similar cells working together to carry out a specific function is called a tissue. For example, the skeletal muscle in your arm is a tissue made up of many muscle cells. Each individual muscle cell can contract and relax to make your arm move. But a single cell alone cannot move your arm because your arm is too heavy. So all the muscle cells work together. Some are contracting and others are relaxing at the same time. By working together as a tissue, the cells make your arm move in a certain way.

Relative Size of a Cell How big is a cell of your body as compared to a molecule in your body such as DNA? How big is a cell of your body as compared to an organ such as your lung? To learn about relative sizes of selected parts of your body, place the following structures in decreasing order of size. (Begin with the largest structure.) Even though you have not learned about each of the following, do your best to put them in the correct order.

A. Arm

B. Blood cell

C. Cell nucleus

D. Chromosome

E. DNA

F. Human body

G. Lung

H. Nose

I. Skeleton

All cells can be grouped into four general tissue types. (1) Muscle tissue is made up of cells that can contract. (2) Epithelial tissue is made up of cells in sheets. These sheets of cells line your breathing and digestive systems, cover the organs of your body, and form your skin. (3) Connective tissue is made up of cells that support and hold things together such as cartilage, tendons, and bones. Blood cells are also connective tissue. (4) Nervous tissue is made up of cells that can process information and send electrical signals throughout your body.

Figure 1.2 Cells make up tissues. Tissues make up organs. Organs make up systems.

Figure 1.3 All cells in the human body can be grouped into four general tissue types.

Figure 1.4 The human digestive system.

Several different kinds of tissues also can work together to perform specialized functions. A group of tissues that work together is called an organ. Your heart is an organ that contains muscle, nerve, and connective tissues. These tissues work together to pump blood throughout your body. Your stomach is an organ that helps you digest the food that you eat.

Write down your three favorite activities or interests. Here's an example:

  1. Playing sports
  2. Talking on the phone with friends
  3. Listening to music.

Write a list of some of the cells that you think are involved in those activities. Make guesses and don't worry about the terminology. Later you will know more about the names and functions of cells.

Organs that work together form a system. Your body has ten different organ systems, each of which does a different job. You can digest food, for example, because of your digestive system. The digestive system is made of several different organs, including your stomach, small intestine, large intestine, and liver. Your heart and all of your blood vessels form your cardiovascular (cardio = heart and vascular = vessels) system, which pumps and circulates blood throughout your body.

Together, all the different cells, tissues, organs, and systems work together to provide a suitable “internal environment” for all of your cells. Each cell has needs, and together all of the organ systems maintain an environment inside your body that takes care of the needs of each and every cell. Knowing what cells are and how they work helps us learn more about the human body and how it functions.

Imagine a One-celled Human Imagine the impossible. Could a human being be made up of just one giant cell?

What would the problems be?

What would hold this one huge cell together?

What would the cell look like?

How could the cell have any shape?

Will the cell be able to live on land, or will it have to live in water?

How could the cell have arms and legs?

How could the cell move?

How could the cell make more cells?

What problems would this one-celled human face? Be creative. Use your imagination as well as your knowledge of human biology.

The Size of Cells

Why is a person not just one huge cell? Why are we made up of so many tiny cells? Why are cells so small? Why does it take so many billions of cells to build a person? Let's consider an alternative design. Instead of a person being made from millions of tiny cells, could a person be made up of one giant cell?

One real problem for a one-celled human would be maintaining an adequate supply of oxygen and nutrients and maintaining an adequate rate of elimination of wastes, such as carbon dioxide. The exchanges of nutrients, oxygen, and wastes, such as carbon dioxide, would have to be across the cell's outer membrane. The bigger the cell, me more it must exchange with the environment around it. Those exchanges would all have to go across its one big cell membrane. This creates a situation called the surface/volume problem. The surface/volume problem can best be explained in terms of mathematics, as you will see in Activity 1-1.

Activity 1-1: Why Are Cells Small?

Introduction

What happens when a cell increases in size? How does an increase in size affect the efficiency of the functions of the cell, especially the functions of the cell membrane? You should be able to answer these questions after completing this activity.

The purpose of this activity is to help you learn about what happens to a cell when it increases in size. You explore cells having the shape of cubes (cuboidal, like dice). Cuboidal cells are found in your kidney, thyroid gland, and salivary glands.

When you finish, you should be able to propose answers to questions such as, Why are cells small (microscopic) in size? Why do cells divide?

Materials

  • Plain paper for constructing a model of a cube
  • Scissors
  • Metric ruler
  • Clear tape
  • Activity Report

Procedure

Step 1 Use the materials provided to explore the question below:

What are the surface area and volume for a cube-shaped cell?

If you dismantle a cube 1 \ cm on each side and lay it flat, it would look like the drawing in Figure 1.5 below. Use this information to complete Part A on your Activity Report.

Step 2 Build a model to show what happens to a cube-shaped cell when it doubles in size. If you dismantle a cube 2 \ cm on each side and lay it flat, it would look like the drawing in Figure 1.6. Use this illustration to help you complete Part B on your Activity Report.

Step 3 Dismantle your 2-cm cube. What happens to the relationship between surface area and volume when a cube-shaped cell doubles in size? Complete Part C on your Activity Report.

Figure 1.5

Figure 1.6

When animals use the energy they get from food, they lose a lot of that energy as heat radiating from the surface of their bodies. Explain why small animals, such as a mouse or a hummingbird, have to spend most of their time eating.

Because of the surface area/volume problem, single-cell organisms must be very small. Multicellular organisms can be bigger because they are made up of large numbers (sometimes billions and trillions) of many different kinds of cells. Many millions of cells can make up breathing systems and digestive systems. These systems depend on different types of specialized cells that work together.

Besides being small, the cells in the human body vary in shape. Some are cuboidal, such as some epithelial cells that line body cavities. Other cells are more rounded (spherical), such as red blood cells. Some cells are shaped like a rectangle (columnar), such as those found in the lining of the digestive tract. Some highly specialized cells, such as nerve cells and white blood cells, have irregular shapes. The shape of a cell is related to its function in your body. Think about this relationship as you learn about different types of cells and what they do.

Figure 1.7 Examples of cells with different shapes that make up different tissues in your body.

How Changes in Surface Area and Volume Affect Cells with Different Shapes Is the pattern of surface area and volume changes with increasing size the same for all cells regardless of shape? Use a spherical-shaped cell as a model and the formulas for surface area and volume of a sphere to explain your answer.

\text{Surface area of a sphere} = 4 \pi r^2

\text{Volume of a sphere} = \frac{4}{3} \pi r^3

To simplify the surface/volume ratios, use \pi =3.

Look at Figure 1.8.

  1. What happens to the surface area to volume ratio as cells increase in size?
  2. What effects does this change have on cell functions?
Radius (cm) Surface Area (A = 4 \pi r^2) Volume \left ( V = \frac{4}{3} \pi r^3 \right ) Surface Volume
1 \ cm 12 \ cm^2 4 \ cm^3 \frac{12}{4} or 3:1
2 \ cm ? ? ?
3 \ cm ? ? ?
4 \ cm ? ? ?

Figure 1.8 Surface Area and Volume for Spheres.

Why Are Cells Important?

Your overall health depends partly on keeping your cells healthy. Diseases, such as cancer and heart disease, stem from changes in the functions of cells. Cells live and do their work in a dynamic relationship with the entire body. There is a delicate balance of interactions between cells that allow tissues, organs, and systems to function well. The life of a cell depends on events that take place within it and on the environment surrounding it.

Cells and tissues live in a fluid environment. The body's fluids bathe all of the cells of the body. Cells get oxygen and nutrients from these body fluids. Cells discharge wastes, such as carbon dioxide, into these fluids. So the activities of cells tend to change their environment. But their health requires the environment to have enough oxygen and nutrients and not too much waste. How does this problem get solved? Different types of cells, different tissues, and different organs all contribute in different ways to keep the cell's fluid environment constant. Maintaining balance of your internal environment under changing conditions is called homeostasis.

Review Questions

  1. Describe four characteristics or functions of cells.
  2. Describe the cell theory in your own words.
  3. What is the relationship between a cell, tissue, organ, and system? Include examples.
  4. Why are cells so small?
  5. Why is homeostasis important to cells?

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