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

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ncchemist's profile pic

ncchemist | eNotes Employee

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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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zala96 | Student, Undergraduate | (Level 1) Honors

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The basic structure of the cell membrane consists of layers. The cell membrane is a phospholid bi-layer. It is made up of proteins and lipids (also known as the phospholipid). The bi layer consists of primarily a thin layer of amphipathic proteins/lipids which are arranged so that the hydrophobic tails shielded from surrounding fluid (the polar fluid).

The reason that the cell membrane is considered to be 'semi-permeable' is because there are certain materials that can pass through the cell membrane (e.g. hydrocarbons and oxygen cross through the cell membrane as they can be dissolved into the lipid layer - other examples include small polar molecules. Some materials that are unable to pass include the larger polar molecules and ions such as H+, Na+ and Cl-)

Passive transport is the method of moving molecules from  a higher concentration to a lower concentration. A popular example of this is osmosis - where the water moves through the semi-permeable membrane. 

Active transport is the type of transport that requires energy (where as passive transport didn't require any). In active  transport, a protein called carrier proteins bind together with particle proteins (this particle then releases the energy). The particle then releases the transport protein to the other side of the memebrane.