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    Membrane Transport

    Membrane transport is essential to the functioning of a cell because it allows for substances, such as ions and chemicals, to pass membrane barriers. Remember that membranes are selectively permeable, meaning that some substances are able to pass freely while others cannot.

    Biological membranes are both hydrophobic and hydrophilic in nature. Thus they are amphiphilic molecules. Cell membranes are organized in bilayers, with their hydrophobic tails facing inwards and their hydrophilic heads facing outwards. This specific arrangement means that small, nonpolar molecules can freely cross the membrane. Conversely, large polar molecules such as sugars and charged ions cannot freely pass.

    There are two basic types of membrane transport: 

    1. Passive Diffusion: This is a type of transport which follows a concentration gradient. The flow of particles is from a high concentration area to a low concentration area and requires no energy input. In addition, this process depends on the characteristics of the substance requiring transport such as its size, charge and hydrophobicity. For example, nonpolar molecules and small polar molecules can move freely with a concentration gradient. Furthermore, passive diffusion includes osmosis, which describes the movement of water across a membrane and facilitated diffusion, which uses channel proteins or carrier proteins to transport particles.
    2. Active Transport: This is a type of transport which requires energy because molecules are pumped against a concentration gradient. There is both primary active transport, which directly uses ATP from a pump, and secondary active transport which does not directly utilize ATP, but rather an electrochemical gradient. Carrier proteins, such as antiporters and symporters are used in secondary active transport. Endocytosis and exocytosis are two types of transport requiring energy. Endocytosis brings particles into a cell, while exocytosis takes particles out of a cell.  
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