07_-_cell_membrane

07_-_cell_membrane - BMEN E4001x: Quantitative Physiology I...

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BMEN E4001x: Quantitative Physiology I / Molecular and Cellular Systems Notes: 07 - Lipid bilayers as barriers & channels B&B, Chapter 3 K&S (Coursepack) sections 2.4.1 - 2.5 Membranes as a barrier An important feature is that a lipid bilayer is pretty much impermeable to ions and only slightly more permeable to water. The structures we will be detailing here allow stuff to go through a bilayer, and control of this is really the heart of how a bilayer functions. Here’s a snapshot of the compositions: Ion concentration (mM) interstitial space cell (“typical”) Na + 145 15 K + 4.5 120 Ca 2+ 1.2 1 x 10 -7 Mg 2+ .55 1 Cl - 116 20 HCO 3 - 25 15 glucose 5.9 low Note that each fluid, in bulk, must have equal number of positive and negative charges; this is the principle of electroneutrality. Inside the cell, this table illustrates a preponderance of Na + and K + ions. Balancing charges come from proteins and other biomolecules, such as DNA. Why phospholipids? (This section is not required material, but will be covered in class. Required material picks up again at transport across membranes) What are amphiphiles? Phospholipids are amphiphiles. In the simplest form, an amphiphile contains both a hydrophilic head group and a hydrophobic tail. As a model, consider sodium dodecyl sulfate (SDS), a strong detergent. Similar to other detergents, and as key to its behavior, this molecule forms aggregates, or more formally micelles, under appropriate conditions, with the hydrophobic tails together and the hydrophilic heads facing out.
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Self assembly: This is driven by the hydrophilic / hydrophobic effect, a manifestation of entropy and the special properties of water. Key concepts underlying the hydrophobic effect: Water forms a hydrogen bonded network The high density of hydrogen bonding opportunities means that there is little entropic cost for forming hydrogen bonds, but a high energetic gain by forming these bonds. In short, surfaces of water against a substance/molecule that disrupts this, such as a hydrophobic molecule or air, incurs a large cost. This is the basis of high surface tension of water. The hydrophobic effect is thus reliant on the presence of water. See P. Nelson or Dill & Bromberg (BME Thermo people) for more complete discussion. Phospholipids Nature uses a wide range of amphiphiles to carry out cellular functions. In animal cells, the main structural molecules are phospholipids. Here, we’re looking to explain why this is so.
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This note was uploaded on 12/21/2010 for the course BMEN 4001 taught by Professor Kam during the Fall '10 term at Columbia.

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07_-_cell_membrane - BMEN E4001x: Quantitative Physiology I...

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