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Lecture 7

Lecture 7: "Cell Membranes"

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Biology 1002B
Tom Haffie

Biology Lecture No. 7: Cell Membranes th Monday January 30 , 2012 Membrane Biology: -Although it is still unclear as to how cells evolved, it is well known that the composition of the cell membrane can be up to 50% protein and that its foundational structure is lipid-based. -The cell membrane follows the fluid mosaic model in that its structure never remains static, but constantly induces motion. Lipid Biology: -The cell membrane is composed of a phospholipid bi-layer whereby hydrophilic heads interact with the polar, aqueous environment and hydrophobic tails interact with themselves (by the hydrophobic effect). -The synthesis of this lipid bi-layer is a spontaneous reaction. The hydrophobic tails are of a lower energy state. -The cell membrane can be classified as amphipathic due to polar/non-polar capabilities. Detergents are similar in this respect as they can solvate both aqueous and lipid residues. Fatty Acid Structure: -Part of the functional lipid portion of the membrane, this encompasses the hydrophobic/hydrocarbon tails. In such molecules, each carbon is saturated with hydrogen. -It is the introduction of the double bond between carbons that discourages carbon-hydrogen bonding and renders the molecule unsaturated. Every double-bond denotes a bend in the molecular structure. -Two examples of unsaturated fatty acids are trans-unsaturated and cis-unsaturated. The “trans” version involves hydrogen atoms located across from each other, while in the “cis” variation, they are proximal. -There is a greater bend in the double bond for cis-unsaturated molecules and this type is common in nature. Trans-unsaturated molecules are more related to saturated fats and quite rare in nature (more prevalent for industrial purposes). Importance Of The Fatty Acid Structure: -The structure of fatty acids is extremely diverse in the amount of carbons in a chain and in the number and position of double bonds. Saturated hydrophobic tails are closer together, while unsaturated tails are much further apart. -The cell membrane consists of many unsaturated molecules as they promote great fluidity in the lipid bi-layer. Membrane fluidity affects function and maintaining correct fluidity is critical to life in many systems. Physiological Importance Of Unsaturation: -Fatty acids are synthesized by biological reactions occurring in the cell. When biosynthesized, fatty acids are always produced as fully saturated molecules. -Unsaturated fatty acids arise from desaturase enzymes which regulate the abundance of unsaturated molecules. Different kinds of desaturases introduce double bonds and control the diversity of fatty acids. -Ectotherms like bacteria and plants are extremely susceptible to the temperature of their environment. Such organisms rely on these enzymes to keep their cells alive, with desaturase transcript being very high in lower temperatures where membrane fluidity is often less. -Humans however, have the ability to regulate the temperature of their environment in a way that maintains optimal fluidity of the cell membrane. It is important to maintain fluidity within a certain range (not too rigid, not too fluid). -Enzymes have a high temperature profile (are most active) in high temperatures (where they face risk of denaturation) and in low temperatures (where not enough enzyme is being produced to counteract membrane solidity). Membrane Permeability: -If the cell membrane was too fluid, it would not be able to retain essential molecules. However, if the cell membrane was too viscous, transport of molecules into the cell would be increasingly difficult. -Hydrophobic molecules (O , CO , and N ) and small, uncharged, polar molecules (H O and glycerol) can 2 2 2 2 diffuse (easily navigate) through the cell membrane. -Large, uncharged, polar molecules (glucose, sucrose) and ions (Cl, K and Na ) cannot easily pass through the cell membrane. Membrane Proteins: -Membrane proteins are defined as proteins that interact with the cell membrane and produce a channel or pore. It is through these protein channels that charged molecules may pass the cell membrane. -The channel or protein surface is hydrophilic except for the surface that interacts with the fatty acid tails, which is hydrophobic. These are known as integral proteins or trans-membrane proteins. Trans-membrane Protein Prediction: -Such proteins are capable of interacting with the hydrophobic core of the membrane as they have strong alpha-helical structures (which minimize cha
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