All plants and animals
are made of cells. Most cells are much too small to see. It takes millions of
cells, for example, to make just one of your fingers. Some tiny creatures, such
as bacteria, are made of only one cell.
If you want to see what
a cell looks like, you could use a microscope. Or, you could get an egg. A
bird’s egg is actually a single, giant cell. The eggs you eat for breakfast
come from chickens.
AN EGG IS A
CELL
An egg is a very big cell.
Take a look at an egg that has been broken into a bowl. One of the first things
you see is a yellow center. You call this center the yolk. Biologists call it
the nucleus of the cell. Almost all cells have a nucleus.
The genes of the cell
are in the nucleus. The genes in the chicken egg tell what the bird will look
like. The genes tell the cell how to do its work. Different plants and animals
have different genes.
The clear, jelly-like
stuff around the nucleus is called cytoplasm. Much of the work of the cell goes
on in the cytoplasm. This work keeps plants and animals alive.
A chicken egg is a special
kind of cell. It has a hard shell to protect the egg. Most cells are much
smaller and do not have a hard shell.
WHAT ARE THE
KINDS OF ANIMAL CELLS?
There are many kinds of
animal cells. The different parts of your body, such as your muscles, skin, and
brain, are made up of different types of cells. The cells perform different
functions. Muscle cells help you move. Skin cells keep harmful things from
getting into your body. Brain cells allow you to think.
Animal cells come in all
sizes. Some are so small that 10,000 of them would be only as wide as a single
hair. The ostrich egg is a cell that is as big as a baseball.
WHAT DO
ANIMAL CELLS LOOK LIKE?
Animal cells come in many
different shapes. Skin cells are flat. Red blood cells are round like wheels.
Muscle cells are long and thin. Nerve cells have long arms and look like octopuses.
Cells have a kind of framework.
The framework is called a cytoskeleton. The cytoskeleton gives an animal cell
its shape. It holds the insides of the cell in place.
THE INSIDES
OF ANIMAL CELLS
Cells have organs just
like you have organs. The organs of a cell are called organelles. Organelles do
all the things a cell needs to stay alive and to perform its function in the
body.
Each type of organelle
has a different job. The nucleus is the organelle that holds the genes. The
genes carry a code that tells the cell what to do. Genes tell a cell when to
divide.
The cytoplasm contains
organelles that do other jobs. Organelles called mitochondria make energy for
the cell. Muscle cells have lots of mitochondria because they use a lot of
energy. Proteins, chemicals that cells need to survive, get made in organelles
called the endoplasmic reticulum and the Golgi apparatus. Lysosomes are small,
rounded organelles that act as the cell’s recycling center and garbage
disposal.
Animal cells are surrounded
by a covering called the cell membrane. The cell membrane lets some materials
pass into and out of the cell. It keeps other, harmful materials out.
WHAT DO PLANT
CELLS LOOK LIKE?
Plant cells are square
or rectangular in shape. Plant cells have a nucleus and many of the same
organelles that animal cells have. In addition, plant cells have a sturdy cell
wall that surrounds the cell membrane. Plant cells also have a big bag inside
called the central vacuole. The central vacuole stores water and takes up most
of the cell. Green plant cells contain small green spheres called chloroplasts.
THE INSIDES
OF PLANT CELLS
Chloroplasts are organelles
that absorb sunlight and use its energy to make food for the cell. The job that
chloroplasts do is called photosynthesis.
The central vacuole stores
water filled with salts, sugars, and other nutrients. The vacuole swells up
when it’s full of this water. The full vacuole gives plants sturdy stems and
plump leaves and flowers. Without enough water inside, the cells collapse and
the plant wilts. In flowers, the material in the vacuole gives the flower its
color. The central vacuole also collects plant wastes.
The strong cell wall gives
a plant cell its rigid shape. The wall lets the central vacuole swell up
without bursting the cell.
HOW DO CELLS
MULTIPLY?
All cells of living things
come from other cells. They come from cells that divide.
There are two steps in
cell division for plants and animals. First, the nucleus divides in two. Then,
the cytoplasm divides and the cell splits into two cells, each with its own
nucleus.
There are two ways a cell
can divide. The nucleus of most cells divides by a process called mitosis.
Mitosis makes ordinary cells such as skin or blood cells. The two cells
produced by mitosis are like identical twins. Each has a full set of genes.
The nucleus of a sex cell
divides by a different process called meiosis. Cells produced by meiosis have
only a half set of genes. These sex cells, such as sperm and egg cells, can
together produce whole new animals. When a sperm fertilizes (combines with) an
egg, the new cell has a full set of genes. Half of the genes come from the
father’s sperm cell and half from the mother’s egg cell. That’s how babies get
genes from both parents.
