Download Cells and Cell Functions

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Cells and Cell Functions
Life Science
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What do cells do?
Cells are the basic unit of life. Most are
too small to see without a microscope, yet
each holds even smaller structures. These
structures carry out the activities that keep
a cell alive.
Unicellular organisms are made of only
one cell. That single cell carries out all the
organism’s life functions. Multicellular
organisms can have trillions of cells.
With so many cells, specialization occurs.
That is, cells have different jobs to do. For
example, an animal may have bone cells
and blood cells. A plant may have leaf cells
and root cells. Cells play several roles in
an organism’s life.
Cells inside a
rhinoceros and
a bird are about
the same size.
However, a
rhinoceros needs
more cells to
grow larger.
Growth Multicellular organisms need more
cells to grow in size. When a cell reaches
a certain size, it divides to form two cells.
The two new cells divide to form four cells,
and so on. Some organisms grow throughout
their lives. Others stop growing when they
become adults.
Repair Cells can be damaged or die. That
makes repair and replacement necessary.
For example, you may have cut a finger
at some time. Skin cells in the lower part
of the epidermis, or outer layer of skin,
divide to produce new skin cells. These
skin cells fill the cut to repair the skin.
Even without a cut, skin cells die and flake
off all the time. Skin cells divide to make
new cells to take their place.
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• • • • • • • • • •Reproduction
Almost all multicellular
Show What You
Know
organisms
reproduce sexually. Sexual
reproduction involves two parents. Each
parent produces sex cells. The female
parent produces egg cells; the male parent
produces sperm cells. An egg and sperm
unite in a process called fertilization.
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Comprehensive Science Assessment Grade 6 © Options Publishing
Show What You Know
Each spring, a male, white-tailed deer begins to grow new antlers made of bone.
A covering of skin called velvet protects the antlers as they grow. When growth
stops, the deer rubs the velvet off. In winter, the antlers drop off, and in spring,
new antlers begin to grow. Describe the different functions of cells during this
yearly process.
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Life Science
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Types of Cells
What kinds of shapes do cells have?
Cells within multicellular organisms
have different functions, or jobs. All the
cells together keep an organism alive.
To make their own food, plants need
water, nutrients from the soil, and energy
from sunlight. Many plant cells have shapes
and structures adapted for these jobs. For
example, photosynthesis usually occurs
in a plant’s leaf cells. Such cells have many
chloroplasts, tiny structures that capture
the energy in sunlight. The process of
photosynthesis produces sugars, or food
for the plant. Bundles of long cells shaped
like drinking straws carry food to the rest
of the plant and bring water and nutrients
up from the roots.
Animal cells have adaptive shapes and
structures, too. Skin cells are thin and flat,
forming a smooth body covering. Skeletal
muscle cells called muscle fibers are shaped
like thin cylinders. One muscle can be made
of hundreds of thousands of muscle fibers.
These contract, or shorten, to make muscles
move. Neurons, or nerve cells, carry
messages to and from the brain. The body
of a neuron receives electrical pulses with
branched parts called dendrites. It passes
the pulse on to other neurons through a
long structure called an axon. Red blood
cells are disc-shaped, making travel through
blood vessels easier. They deliver oxygen
to body cells and carry waste products away.
Animal muscle cells
Plant stem cells
Plant leaf cell
Animal blood cells
Animal nerve cell
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Show What You Know
Describe how one of the kinds of cells shown above is adapted for its function.
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Comprehensive Science Assessment Grade 6 © Options Publishing
Cells in multicellular
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organisms are specialized.
Show What You Know
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Life Science
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Animal Cells
What are the parts of an animal cell?
A cell may be too small to see without
a microscope, but it can be packed with
many thousands of organelles. Organelles
are tiny structures, each with its own job
to do. They are suspended in cytoplasm,
a gel-like substance. Then a thin layer
called a cell membrane wraps around
everything, keeping the cell’s organelles
inside, while letting other substances pass
in and out. Look at the diagram to learn
more about an animal cell and its contents.
• • • • • • • • • • • • • • • • • • • • • • • • • A• Typical
• • • • • Animal
• • • • • Cell
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The cytoplasm is everything inside
the cell membrane. It includes all of
the organelles except the nucleus.
The cell membrane surrounds the
cell and controls which substances
move in and out of the cell.
The nucleus is the control
center. It contains the cell’s
hereditary information, or DNA.
The DNA has two functions.
It determines the characteristics
of the organism to which it
belongs. And it directs
activities inside the cell.
Each Golgi complex is a stack of sacs
wrapped in membranes. Materials,
such as proteins, are packaged here.
They are then sent to other parts of
the cell, or to the cell’s surface, where
they are moved out of the cell.
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Show What You Know
The endoplasmic reticulum, or ER, is a
network of sacs, tubes, and channels. It
works like a highway system that moves
materials throughout the cell.
Think of mitochondria
as a cell’s power plants.
Sugar is broken down here,
and its energy is released
to the cell.
Proteins are made in the
ribosomes. A cell uses
proteins to grow and carry
out its activities. Ribosomes
may cluster in the cytoplasm.
They may also be attached
to the ER.
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Show What You Know
Comprehensive Science Assessment Grade 6 © Options Publishing
Match each organelle with its function.
1.
mitochondria
a. makes proteins
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ribosome
b. controls the cell’s activities
3.
endoplasmic reticulum
c. regulates what enters and exits a cell
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nucleus
d. moves materials around a cell
5.
cell membrane
e. involved in energy production
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Life Science
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Plant Cells
What structures are unique to plant cells?
Plant cells share many of the same
organelles as animal cells. They have a
nucleus, cell membrane, mitochondria,
Golgi bodies, ribosomes, ER, and cytoplasm.
But plant cells also contain three additional
structures that are not found in animal cells:
chloroplasts, a central vacuole, and a cell
wall. Look at the diagram below.
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• •Typical
• • • • •Plant
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Chloroplasts contain the
green pigment chlorophyll,
which captures the energy
in sunlight and uses it to
make food for the plant.
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Show What You Know
Some animal cells have many
small vacuoles, organelles that
store materials or help move
materials in a cell. In many
grown plants, however, vacuoles
are much larger. Although they
store water, nutrients, and
wastes, they are also important
for support. A single large, fluidfilled central vacuole can take up
80% of the room in a plant cell.
A rigid, or firm,
cell wall surrounds
the cell membrane.
This cell wall protects
the cell and gives it
support. This support
allows a tree to
grow tall.
Show What You Know
Complete the chart to identify the cell parts unique to a plant cell.
Cell Part
Function
Contains chlorophyll for making food
Makes cell rigid, or firm
Stores nutrients and wastes
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Comprehensive Science Assessment Grade 6 © Options Publishing
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