Download Cells—The Building Blocks of Life

SCIENCE SKILLS
“All living things are alike at the most basic structural level—they are all
made up of cells.”
Cells—The Building Blocks of Life
What Are Cells?
As you learned in the program, all living things are made of cells, the basic unit of life.
Some living things, like animals and plants, are made of millions of cells. Such
organisms have many different types of cells. In humans, for example, there are blood
cells, skin cells, and bone cells. Each looks different and performs a different job. In a
plant there are different types of cells in the leaves, roots, stems, and flowers. Other
living things are made of only one cell. For example, bacteria are one-celled organisms.
Most cells are tiny. You cannot see them with your eyes alone. Instead, you need a
microscope to see them. However, some cells are quite large. In fact, you probably have
a few of these large cells in your refrigerator. Eggs are the reproductive cells of
chickens. Each egg is a single, large cell. The largest cell is the ostrich egg. It is about
20 inches across!
Although there are many types of cells, all cells have several things in common:
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Cells use energy.
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Cells move and/or have parts that move.
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Cells can grow, divide, and die.
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Cells have an outer covering, a jellylike inside, and genetic material, which
stores information.
The Structure of Cells
All cells have an outer covering called the cell membrane. The cell membrane protects
the cell from the environment around it. It allows certain things, such as water and food,
to enter the cell. It allows other things, such as wastes, to leave the cell. Plant cells also
have a cell wall outside the cell membrane. The cell wall is tough. It helps give the plant
shape and support it. Animal cells do not have cell walls.
Inside most cells is a large structure called the nucleus. The nucleus is the control
center of the cell. It contains the cell’s genetic material, which controls the cell’s
activities. In some cells, like bacteria, there is no nucleus. Instead, the genetic material
occurs as a single strand that forms a ring inside the cell. For more information about
the genetic material called DNA, see Science Resources, page 122.
The rest of a cell is mostly cytoplasm. This is a thick, watery material like jelly.
It surrounds the nucleus and contains the cell’s other structures.
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PROGRAM 16
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LIFE SCIENCE
Look at the diagram of a typical plant cell. You can see the cell membrane and cell wall,
which protects the cell from its surroundings. The cytoplasm contains several other structures:
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Nucleus
Cytoplasm
Vacuole
Mitochondrion
Cell wall
SCIENCE SKILLS
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Mitochondria (plural of
mitochondrion) break down
food to make energy.
They are often called the
cell’s “powerhouses.”
The chloroplasts help plants
make food. Animal cells
do not have chloroplasts.
A vacuole is a storage area.
Most plant cells have one
large vacuole full of water.
Some animal cells do not
have vacuoles.
Cell membrane
Chloroplast
A TYPICAL PLANT CELL
LIFE SCIENCE ■ PRACTICE 1
A. Based on the information on page 20 and the picture above, write True or
False next to each statement.
________ 1. The cell is the basic unit of all living things.
________ 2. All the cells in the human body are alike.
________ 3. Most cells are so tiny they can be seen only with a microscope.
________ 4. The nucleus is the control center of a cell.
________ 5. The mitochondria are called powerhouses because they produce
energy from food.
________ 6. The chloroplasts surround the plant cell’s cytoplasm.
________ 7. A bacterial cell has no nucleus; the genetic material occurs as
a simple ring inside the cell.
________ 8. All cells are capable of growing and dividing.
B. Describe two ways that plant cells differ from animal cells.
9.
10.
Answers and explanations start on page 110.
PROGRAM 16
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LIFE SCIENCE
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