How Cell Membrane Lets Things in and Out?

It is the cell membrane that keeps the insides in and the outside out. Though like your house it has doors and windows in it to let things in and out. A cell membrane is very flexible and the cell can change shape quite easily. However some cells have given up this flexibility for greater strength and protection in the form of a ‘cell wall’. A cell wall is not flexible so cells that have one have a constant shape.

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Further Reading:

What does a cell membrane let in? – http://wiki.answers.com/Q/What_does_a_cell_membrane_let_in