L2 - Physiology PDF Notes

L2 - Physiology PDF Notes - Membranes are composed of...

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Membranes are composed of lipids and proteins. Lipids: Lipids of biological membranes are all amphipathic molecules. That is they have a hydrophilic end and a hydrophobic end. During synthesis of the phospholipid, the carboxy end is linked to the glycerol backbone through an ester linkage . Saturated bonding leads to straight chains. Unsaturated chains contain double bonds which lead to the presence of kinks in the chain. Cholesterol is also an amphipathic molecule. The phospholipid bilayer is a sheet-like structure composed of two layers of phospholipid molecules whose polar head groups (hydrophilic) face the surrounding water and whose fatty acyl chains (hydrophobic tails) form a continuous hydrophobic interior of about 3 nm in thickness. Molecular Species Permeability of the Lipid Bilayer Lipid-soluble molecules (e.g., steroids) High permeability Some small polar molecules (e.g., water, urea, ethanol) High permeability The majority of large polar molecules (e.g., glucose, amino acids) Low permeability Large polar macromolecules (e.g., proteins, carbohydrates) Low permeability Ions (e.g., Na + , K + , Cl , Ca 2+ , Mg 2+ , etc.) Low permeability The proper way to state these features is to say that the membrane is highly permeable to lipid-soluble molecules, or that the membrane is not permeable to ions. It may also be said that membrane permeability is high for lipid-soluble molecules, and that membrane permeability is low for ions and polar molecules. Another way of stating this is that
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L2 - Physiology PDF Notes - Membranes are composed of...

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