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Lecture 5_.docx

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Western University
Biology 1001A
Tom Haffie

Meaning of hydrophilic, hydrophobic, fatty acid, saturated, membrane fluidity, hydrogenation, desaturase, membrane permeability, transmembrane protein, simple diffusion, facilitated diffusion, active transport, “ATP-Binding Cassette” (ABC) transporter, cystic fibrosis, Cystic Fibrosis Trans Conductance Regulator (CFTR), ∆F508, chaperone protein, “ER quality control”, proteases, proteases - Role of fatty acids in membrane structure - ● The fatty acid tails are hydrophobic and because of this, lipid bilayers form spontaneously as the fatty acids wanna stay away from aqueous environment and come together Relationship of fatty acid saturation levels on membrane fluidity - ● Saturated fatty acids are rigid and this leads to the membrane being very rigid ● Unsaturated fatty acids have a ‘kink’or a ‘bend’in them and this leads to more fluid like membranes and the fatty acids are spaced out slightly more ○ higher the unsaturation, the more fluid the membrane will be at any given temperature Relationship of temperature on membrane fluidity - ● At higher temperatures, membranes become increasingly fluid, at lower temperatures, membranes become increasingly rigid Relationship of fluidity to membrane functions such as transport - ● If a membrane is too fluid, ions and important molecules can leak through the membrane and be lost ● If the membrane is too rigid, then the cell can’t flow at all, it can’t exchange molecules and ions with the environment as needed, and the rigidness hinders membrane transport Properties of saturated vs. unsaturated fats - ● In saturated fats, all the carbons are bonded to the maximum number of hydrogens possible ● In unsaturated fats, some carbons form double bonds with other carbons and thus aren’t bonded with the maximum number of hydrogens Role of desaturases in fatty acid biosynthesis - ● All fatty acid tails are initially formed as saturated but desaturases change them to unsaturated by adding in double bonds ● Desaturases thus regulate the unsaturation of fatty acids; by regulating desaturases, you can regulate membrane fluidity ● Many different desaturases coded for and this is because each differ in location of where they place the carbon-carbon double bond on the fatty acid Relationship of bacterial desaturase expression vs. temperature - ● At lower temperatures, desaturase expression is higher as the membranes tend to be more rigid and thus require desaturases to introduce unsaturation and lead to fluidity ● As the temperatures increase, desaturases expression decreases as membranes tend to be more fluid-like at high temperatures and do not need desaturases to regulate unsaturation and the such Role of size and charge in movement of molecules across biological membranes - ● Middle of membrane is hydrophobic and thus selectively permeable to certain molecules ● Charge and size determine diffusion across the membrane ○ Non-polar molecules diffuse through ○ Small and uncharged polar molecules simply diffuse straight through ○ Large, uncharged polar molecules do not diffuse through ○ Ions do not diffuse through as they are charged Characteristics of transmembrane proteins that enable them to interact with hydrophobic core of membrane - ● Parts of the protein have a hydrophobic surface which interacts with the hydrophobic core ○ Outside of the protein is hydrophobic ○ Inside of the protein is hydrophilic ● How is this accomplished? ○ Alpha helices are the dominant structure in the proteins; this minimizes charges of the protein backbone due to the hydrogen bonding ○ You know that a protein is a transmembrane protein as the part that interacts with the membrane tend to be predominantly non-polar ■ 17-20 amino acids spans the membrane and they are predominantly non- polar amino acids Factors influencing simple & facilitated diffusion - ● Simple diffusion is driven by free energy change; there is more potential energy where there is a higher concentration of the molecule ● Facilitated diffusion occur through specific and small pores or channel through which very specific molecules leak from a high concentration to low concentration which cannot just diffuse through the membrane as it can't interact with the hydrophobic core
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