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Reianna Shakil L6EFBiology10/11/12

Your body contains about 1013 cells. They have all developed from the single cell with
which you began your life ­ the zygote that was formed by the fusion of an egg cell and
a sperm cell. The zygotes divided to form a tiny ball of…

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Reianna Shakil L6EFBiology10/11/12

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Reianna Shakil L6EFBiology10/11/12

Some specialised plant cells

Plants do not have stem cells ­ most of their cells retain the ability to differentiate into
other kinds of cells throughout their lives.

However, there are several parts of a plant where cells are able to divide, and places
where this occurs…

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Reianna Shakil L6EFBiology10/11/12

Tissues, organs and organ systems

The millions of cells inside a multicellular organism (i.e. you) are not scattered about
randomly. Cells that carry out the same function are usually grouped together forming a

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Reianna Shakil L6EFBiology10/11/12

tissue. Tissues may be further grouped into organs, and organs into systems. We can define
these terms as follows:

A tissue is a collection of cells together with any intercellular (`between cells') secretion
produced by them, that is specialised to perform one or more particular functions. The…

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Reianna Shakil L6EFBiology10/11/12

The cells rest on a basement membrane, which, despite its name, is not a cell
membrane at all. It is a network of collagen and glycoproteins that is secreted by the
underlying cells, and that holds the epithelial cells in position.

Squamous epithelium covers many surfaces in…

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Reianna Shakil L6EFBiology10/11/12

Pulling the activities of all the different parts of the body together, so that they work
with each other and do appropriate things at appropriate times, is essential if a
multicellular organism is to survive.
As we have seen, this communication involves cell signalling.
Much of it…

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Reianna Shakil L6EFBiology10/11/12


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