Describe the structure of the plasma (cell) membrane. Why is the plasma membrane described as "semi-permeable"?

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You are only allowed to ask one question at a time so I edited the last part.  Cellular membranes are composed of a phospholipid bi-layer.  The phospholipids are composed of a polar, phosphate based "head" portion attached to a non-polar, hydrocarbon based "tail" portion.  Groups of these phospholipids come together as...

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You are only allowed to ask one question at a time so I edited the last part.  Cellular membranes are composed of a phospholipid bi-layer.  The phospholipids are composed of a polar, phosphate based "head" portion attached to a non-polar, hydrocarbon based "tail" portion.  Groups of these phospholipids come together as a double thick layer with the hydrophobic tails in the interior and the hydrophilic heads on the exterior.  The membrane is described as semi-permeable since only certain kinds of molecules can pass through the cell membrane on their own.  Other molecules require help to pass through the membrane.  As a result, there are many proteins embedded in the membrane called transport proteins that allow particular substances to pass through the membrane while preventing unwanted materials from passing.

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