GOOD FRIENDS and GERMS

1.1 Understand the Challenge

27GF 27

cell: the structural and functional unit of all living organisms. (What a living thing is made of and what makes it work.) It is sometimes called the “building block” of life.

magnifi es: makes something look larger, but does not actually enlarge the physical size of the object.

cell theory: a theory about the relationship between cells and living things.

2.3 Explore

Observing Cells

What structure do all living things, whether they are germs or humans, have in common? The cell is the basic unit common to all living things. Every living thing is made of cells. Most germs are made of a single cell. You are made of about 100 trillion cells. To understand how germs spread disease and how they affect the cells in your body, you need to know the structure and the functions of a cell.

Cells are the “building blocks” of all plants and animals. They are very small. Most cells are too small to see with the human eye. To actually observe them, you have to make them appear larger. To observe cells, you will look at them through a microscope. A microscope magnifi es objects. It makes things appear much larger than they are.

Before you begin your observations, read about what a cell is and how it functions. Also, read about the parts of a microscope and their function.

Human blood cells under a microscope.

2.3 Explore

The Cell TheoryAfter scientists were able to study cells for some time, they put together a theory, called the cell theory, about cells and living things. The cell theory explains the relationship between cells and all living things, large and small. The theory states that

• all living things are composed of cells.

• cells are the basic units of structure and function in living things.

• all cells are produced from other cells.

The cell theory helps scientists learn more about living things by studying cells. Because all cells come from other cells, scientists can study cells to learn about reproduction and growth.

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Project-Based Inquiry Science28

Learning Set 2 • What Kinds of Things Make You Sick?

GF 28

Cells – The Basic Units of LifeScientists have spent many years observing cells. They have found that there are many different sizes and shapes of cells. Some different types of cells are shown in the photographs. Although cells come in different sizes and shapes, they have some structures in common.

Each different structure in a cell has a specifi c function (role). Each structure acts as if it were a special organ. That is why cell structures are known as organelles.

The diagram shows some of the organelles that are found in animal cells. All these structures are also found in plant cells.

The nucleus is the control center of the cell. It holds the information that directs what the cell does. The nucleus contains chromosomes. Chromosomes store the genetic material. Genetic material determines the specifi c traits of an organism. The cell membrane surrounds the cell. It controls the movement of materials into and out of the cell.

The cytoplasm is the watery fl uid inside the cell. It contains all the cell’s organelles. Mitochondria (singular, mitochondrion) provide the cell with energy. In the mitochondria, food and oxygen are converted to energy, carbon dioxide, and water by the process of cell respiration. The vacuole is a storage area. It is used to store water and nutrients.

Single-celled organismAmoeba proteus Animal cell Plant cells

organelle: a specialized structure in a cell.

nucleus: the control center of the cell.

chromosomes: contain the genetic material of the cell.

genetic material: contains the information that determines the traits of an organism; hereditary material.

cell membrane: surrounds the cell; controls the movement of materials into and out of the cell.

cellmembrane

nucleus

chromosomes

cytoplasm

mitochondria

vacuole

Animal Cell

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GOOD FRIENDS and GERMS

1.1 Understand the Challenge

29GF 29

cytoplasm: the watery fl uid that contains the organelles of the cell.

mitochondrion (plural, mitochondria): provides the cell with energy.

cell respiration: the process by which food and oxygen are converted to energy, carbon dioxide, and water. vacuole: a storage area for food and water.

chloroplast: organelle that contains the green pigment chlorophyll used in photosynthesis.

chlorophyll: a green pigment.

photosynthesis: the process by which plants make sugar and oxygen using light, water, and carbon dioxide.

Plant CellsPlant cells have all the same organelles found in animal cells. They also contain a few structures not found in animal cells.

In plants, the organelles in which food is made are called chloroplasts. The chloroplasts contain a green pigment called chlorophyll. Chlorophyll is used in photosynthesis. You may recall from previous science classes that photosynthesis is the process by which plants use light energy, water, and carbon dioxide to make sugar and oxygen. Plant cells also have a cell wall. The cell wall supports and protects the plant cell.

Stop and Think1. Name four structures found in both animal and plant cells.

What is the function of each?

2. What structures are found in plant cells that are not found in animal cells?

3. What can a plant cell do that an animal cell cannot? What structure carries out this function?

chloroplastvacuole

cell membrane

mitochondrion

nucleus

cell wall

2.3 Explore

Plant Cell

cell wall: protects and supports the plant cell.

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