Why is the cell membrane described as semipermeable?

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The term semipermeable means to allow some substances through and prevent others from passing. The cell membrane is semipermeable because it prevents harmful toxins from entering and damaging the cell. At the same time,  the cell membrane allows nutrients and other helpful substances through. In this way, the cell membrane is like a security guard of the cell. 

There are four major passages through the cell: diffusion, osmosis, facilitated diffusion, and active transport. 

Diffusion is the movement of small ions and other molecules through the cell membrane. These substances move down a concentration gradient (from higher to lower concentration). Osmosis is simply the diffusion of water. Energy is not needed for either diffusion nor osmosis, thus they are forms of passive transport through the cell.

Facilitated diffusion is also a form of passive transport, moving substances down a concentration gradient. However, facilitated diffusion allows for the movement of larger particles through the cell via a protein channel. 

Active transport, on the other hand, transports substances against the concentration gradient (from low to high centration) via a protein channel. This requires the use of energy in the form of ATP. 

The shape of substances determines their ability to move through the protien channels used during facilitated or active transport. 

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