Download Cells All plants and animals are made of cells. Most cells are much

Cells
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.
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