Chapter 6 – Part (i) Lipids, and MembranesI. Lipidsa. What are lipids?i. A lipid is a molecule that is hydrophobic (will not dissolve in water)1. Most lipids are hydrophobic because they contain many C-H bonds, whichare nonpolarii. There are many classes of lipids with highly different chemical structuresb. A look at three types of lipids found in cellsi. A fat (triacylglycerol [TAG], triglyceride) is a glycerol with three fatty acids1. Each fatty acid is linked with an ester linkage to one of the carbons of the glycerol2. Fats are not polymers because the fatty acids are notlinked in a chainii.Steroids have a nonpolar four-ring structure with a polar side group. Example: cholesteroliii. Phospholipids have a polar “head” and a nonpolar “tail”1. The head is a three-carbon glycerol with a phosphate group (PO4-)2. The tail is two fatty acidsSimilarities and differences between Fats and Phospholipids?[Melting Temp of fatty acid depends on the Chain length and the degree of unsaturation. Longer chain length higher melting Temp, More double bonds Lower melting temp. Why?]c. The structures of membrane lipidsi. Lipids that can form membranes are amphipathic, with apolar hydrophilic end and a nonpolar hydrophobic endII. Phospholipid Bilayers main membrane componenta. Phospholipids in wateri. In water phospholipids can spontaneously (withoutenergy input) form two structuresii. The polar heads interact with water; the nonpolar tails are driven togetheriii. Phospholipids in water form either micelles or bilayers1. Micelles are tiny droplets of lipids with the hydrophilic heads on the outside and the nonpolar tails on the inside2. Lipid bilayers are two sheets back to back, with the nonpolar tails togetherb. Artificial membranes as an experimental systemi. Researchers use liposomes (small balls ofbilayers) and planar bilayers (flat sheets) to studythe behavior of lipid bilayersc. Selective permeability of lipid bilayersi. Small nonpolar molecules cross membranesquickly. Example: O21ii. Large polar molecules or charged molecules (including all ions) cross membranes very slowly, if at all. Examples: K+, Cl-, glucose, and sucroseiii. Some small polar molecules can cross membranes if they are uncharged. Example: H2Od. How does lipid structure affect membrane properties?i. Double bonds form “kinks”1. C=C double bonds in a fatty acid form a kink in a carbon chain2. Unsaturated lipids have one or more double bonds and have kinks3. Saturated lipids have no double bonds and are straightii. Bond saturation and length affect fluidity and permeability1. Saturated fats pack tightly together and are solid at room temperature, whereas unsaturated fats (oils) are liquid at room temperature2. Saturated fatty acids also reduce the permeability of the membraneVegetable oil is a liquid at room temperature while animal fats are solid. WHY??iii. Cholesterol dramatically reduces membrane permeabilitye. How does temperature affect the fluidity and permeability of membranes?i. At lower temperatures, hydrophobic tails pack more closely; fluidity decreases, permeability decreases, and membranes will solidify if the temperature is low enoughf. Molecules can move freely in the membranei. The fluid nature of the membrane allows phospholipids to move laterally within the bilayerii. Phospholipids move side to side readily but rarely “flip”III. Membrane Proteinsa. What are membrane proteins (glycoproteins) like?i. Membrane proteins are amphipathic, with both nonpolar and polar amino acids1. Polar amino acids “stick out” into the aqueous solution on either side2. Nonpolar amino acids are stabilized in the interior of the bilayer and keep the protein in the membrane3. Non covalent interactionsii. Secondary and tertiary protein structurescan form a channel or poreThese proteins strongly affect membrane permeabilityBioMembranes also contain glycoproteins and
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