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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.
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.
In multicellular organisms, cells specialize to perform different tasks.
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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.
In the earliest stages of development, stem cells simply divide to produce more stem cells.
Stem cells found in embryos are called embryonic stem cells and develop into all the different types of cell in the body.
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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
tissue cellstem 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 red bloodcells
stem cell
tissue cells
cardiac musclecells
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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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