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