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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

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stem cell tissue cell

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Becoming specialized

Tissue cells continue to divide and differentiate, each time becoming more and more specialized.

Some will become nerve cells, others will become blood cells, muscle cells, bone cells, etc.

nerve cells

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stem cell tissue cells 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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