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Transport Across Plasma Membranes.

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Ion channels are formed by proteins which have a central pore. Ion channel proteins are selective in that they will only accept one type on ion. This is because the proteins have a specific three-dimensional shape which are lined with particular arrangement of electric charges inside the channel. The diffusion of charged particles such as Ca2+, Na+,K+,Cl- are helped by these ion channels. Some of the ion channel proteins are gated and are used in order to regulate the flow of ions from one cell to another. Osmosis Osmosis is a special type of diffusion which relates to the diffusion of water molecules. Osmosis is the net movement of water molecules from a solution of less negative water potential (weak solution with more water molecules) to a solution which has more negative water potential (stronger solution with less water molecules). Water molecules diffuse down a water potential gradient. A plasma membrane is a partially permeable membrane (allows only small molecules and ions through) as it lets molecules and ions pass through itself, but it does not allow larger molecules to pass. Water molecules are small enough to pass between phospholipid molecules. However, sugar molecules are to large to diffuse through the pores in the partially permeable membrane. ...read more.


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