Movement Across the Membrane

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Movement Across the Membrane

By | November 2012
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The cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the inside of the cell by allowing certain substances into the cell, while keeping other substances out. The cell membrane also provides support to the cell and helps maintain its shape. Phospholipids are a major component of cell membranes as they form the bulk structure of the membrane providing stability and fluidity. They form a lipid bilayer in which their polar hydrophilic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the tissue fluid, while their non-polar hydrophobic (repelled by water) tail areas repel the cytosol and fluid. The lipid bilayer is selectively permeable, allowing only certain molecules to diffuse across the membrane. The cell surface membrane is not simply a phospholipid bilayer. It also contains proteins, cholesterol, glycoproteins and glycolipids. Structural proteins help to give the cell support and shape. Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters and other signalling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion. It is thought that some of the proteins are fixed within the membrane, but others are not and can move around in the fluid phospholipid bilayer. This arrangement is known as the fluid mosaic model. Channel proteins are a gateway that can be either closed or open. They don’t allow unwanted substances to pass through, apart from water molecules. Intrinsic/integral proteins are in the inside of the cell across the membrane. They do not have a gateway therefore don’t allow substances to...

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