Multicellular Organisms
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Multicellular organisms
In a single-celled organism, all the
functions necessary for life must
be carried out in one cell.
In multicellular organisms, cells
specialize to perform different tasks.
Cells that have adapted to a specific function are known
as specialized cells.
Specialized cells are grouped into tissues, which combine
to make organs and organ systems.
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Why specialize?
Unlike unicellular organisms, multicellular organisms need
specialized cells in order to survive.
In multicellular organisms, most cells are not in direct
contact with the environment. Therefore, specialized
cells, tissues and organs must:
 communicate between cells
 supply cells with nutrients
 control exchanges with
the environment.
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How are organisms organized?
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What makes an organism?
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What are stem cells?
All multicellular organisms
begin as stem cells. These are
unspecialized cells capable of
developing into many different
types of cell.
Stem cells found in embryos are
called embryonic stem cells
and develop into all the different
types of cell in the body.
In the earliest stages of
development, stem cells
simply divide to produce
more stem cells.
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Changing cells
When the embryo contains about 500 cells, the cells stop
being the same and they stop getting smaller with each
division. They start to differentiate into different types of cell.
At this point, stem cells no longer form two new stem cells
when they divide. Instead, one of the two daughter cells
becomes a progeny or tissue cell.
stem cell
stem cell
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tissue cell
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Becoming specialized
Tissue cells continue to
divide and differentiate,
each time becoming more
and more specialized.
stem cell
tissue cells
Some will
become nerve
cells, others will
become blood
cells, muscle
cells, bone cells,
etc.
nerve cells
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red blood
cells
cardiac muscle
cells
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How tissues work together
Epithelial tissue forms a protective
covering for different parts of the body.
Glandular tissue secretes important
substances, such as hormones.
Muscular tissue contracts to
cause movement.
These tissues are all present in the small intestine.
Epithelial tissue forms the intestinal walls. Glandular
tissue secretes mucus to protect the epithelium from
digestive enzymes. Muscular tissue contracts to move
digested food along its tract.
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Glossary
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Multiple-choice quiz
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