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Section 5 -Biomembranes.docx

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Western University
Biology 2382B
Robert Cumming

Biomembranes  Membganes define what a cell is – allow specialized cellular functions to occur in a localized manner o Everything of the cell is localized within the membrane  Basic components: o Lipids o Sterols o Proteins  Lots of proteins in membranes, some are made up of mostly membranes  Due to amphipathicity, phospholipids spontaneously form lipid bilayers in aqueous solution where properties of the fatty acids confer properties onto the bilayers Fatty Acids  Long hydrocarbon chain attached to a polar carboxyl head group o Amphipathic  Fatty carbon tail and acid group at the end  Often Cx:y where X = Number of carbon molecules and y = number of double bonds o # of carbons vary – often these carbons are linked together by single bonds  No double bonds = saturated  One double bond = unsaturated  More than one double bond = Polyunsaturated  Fatty acid chain composition and the composition of the head groups will both effect the functionality of the membrane Effects of Double bond on Structure  Tm (Melting point temperature) – increases with chain length, decreases with unsaturation  Saturated chains can tightly pack – there are numerous van der waal’s forces  Cis double bonds common, trans are rare (not found yet)  Over 100 fatty acids are known  Only a few fatty acids are common in membranes  Melting point increases with increasing chain length, but melting point decrease with increasing unsaturation  Purpose of the cis double bond – to reduce the melting temperature  Plasma membrane is made up of different fatty acids such that it is fluid enough while solid enough to serve a purpose 3 classes of membrane lipids Group called phospholipids – phospholipid means: a. The head group is attached with a phosphate b. General term for most of the lipids you’re going to find in a plasma membrane 1. Phosphoglycerides a. Most common, 2 esterfied fatty acid tails and polar head group i. Can attach different polar head groups (PC,PS,PT,PI) b. Less common, 2 fatty acid tails but only one esterfied (plasmalogen) i. Plasmalogen – one of the fatty acids is not linked with an ester group ii. Still can have different head groups, but does not have 2 ester linked fatty acids – can have 2 fatty acids but only one is ester linked c. HAS TO BE 2 FATTY ACIDS! 2. Sphingolipids a. Amino alcohol with long hydrocarbon chain and fatty acyl chain attached via amide bond b. Can be glycolipid (sugar head group) i. If the head group is a sugar then the lipid is called a glycolipid – this is important in the nervous system ii. Once you c. Not based on glycerol – starting point is not glycerol i. Starting point is an amino alcohol called sphingocine ii. Sphingocine itself has a long fatty acid like chain – starting with sphingocine, add another fatty acid chain iii. This fatty acid chain is added through an amide bond 3. Sterols a. Four ring hydrocarbon b. Cholesterol is the most important one c. 4 hydrocarbon rings – cholesterols is the main sterol in plant, animal and fungi cells d. Amphipathic – can intercalate between phospholipids, where they orient themselves with their polar hydroxyl groups facing the same direction as the polar head groups of the fatty acids i. Plasma membrane made of PC – this plasma membrane will have a certain diameter ii. This means that things diffusing across will do so at a certain speed iii. Collesterol can increase the thickness of plasma
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