Lecture 3 Lipid Bilayer Osmotic Pressure Cell+Wall

# Lecture 3 Lipid Bilayer Osmotic Pressure Cell+Wall - BME...

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BME 418, Quantitative Cell Biology Alan J. Hunt Lecture #3: Bilayers continued, Osmotic pressure, bacterial cell-wall, evolution A potential problem for cells is that a lipid bilayer cannot withstand much force. To determine how this constrains a cell consider the elastic modulus : Since a membrane has a fairly uniform thickness, and in any case is not isotropic in the direction passing through the plane of the membrane, the convention is to treat it two dimensional so that A is given in units of meters rather than meters squared. Two techniques have been used to measure this in cells (draw): 1) suction pipette, 2) optical tweezers. After appropriate transformations, the elastic modulus has been variably determined to be: µ = 2.5 ± 0.4 µN/m (optical tweezers), about 2x higher for suction pipette. Strictly speaking this is not an elastic modulus since it does not have the correct units (meters rather than meters squared). Still, this is a useful number for calculating the stress a cell can bear. As might be expected for a very thin, fluid structure, a cell membrane is easily stretched.

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BME 418, Quantitative Cell Biology Alan J. Hunt Could this be a problem for bacteria? Consider the environmental variation with which bacteria must cope. For example, swimming in very dilute aqueous environments such as rivers and lakes, a bacteria will have to cope with Osmotic pressure. Remember to be very careful with the units. A frequent trouble spot is that the volume is measured in liters in chemistry, but measured in m 3 or cm 3 in physics. Since the gas constant R is given in J/(mol*K), if we want our answer in conventional physical units (Pascals, N/m 2 ) then we need to convert from moles/liter to moles/m 3 (i.e. multiply by 1000) To determine how this translates to stress on the membrane, model a bacteria as a sphere and
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Lecture 3 Lipid Bilayer Osmotic Pressure Cell+Wall - BME...

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