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Chapter

Physics of the Cell Membrane

Chapter

Physics of the Cell Membrane

DOI link for Physics of the Cell Membrane

Physics of the Cell Membrane book

Physics of the Cell Membrane

DOI link for Physics of the Cell Membrane

Physics of the Cell Membrane book

Edited ByRobert Splinter
BookHandbook of Physics in Medicine and Biology

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Edition 1st Edition
First Published 2010
Imprint CRC Press
Pages 8
eBook ISBN 9780429193064

ABSTRACT

Within a drop of uid t hat i s not su rrounded b y a c ellular membrane, t he relationship between su rface tension a nd pressure is described by the law of Laplace. Given a uniform surface

tension (σ), internal pressure (P), and radius (R) of the drop, the law states that

P = 2σ___ R

. (1.1)

When mo deling t his rel ationship i n a c ell, one m ight t hink that the density of the membrane reacts to pressure di erences between the external and internal environments. However, density, w hich de scribes t he c ompressibility o f l ipids w ithin t he bilayer, rem ains c onstant u nder ph ysiologically rele vant p ressures (100 atm).1 Surface area displays somewhat weaker resistance a nd do es u ndergo s ome c hange, b ut o nly 2 –4% b efore rupturing. e tensile force (Ft) on the membrane is expressed in Equation 1.2 for this situation:

Ft = KA ΔA___ A0

, (1.2)

where ΔA is the increase in bilayer surface area from the original area A0, KA is the area expansion constant (between 102 and 103 mN/m), a nd Ft i s ten sion ( between 3 a nd 3 0 mN/m). A nd while surface area expands, membrane t hickness changes proportionally so that

ΔA___ A0

= Δh___ h0

, (1.3)

where h0 represents original membrane thickness. But the membrane response to s hear stress is what clearly describes it as an elastic s olid. U sing t wo si lica b eads a nd opt ical t raps to e xert shear st ress across a n R BC2 (Figure 1.1), elasticity c an be seen

as this membrane elongates in the direction of applied force F. It can be shown that the diameter of the RBC obeys Equation 1.4:

D = D0 − F____2πμ , (1.4)

where D is the diameter of the RBC, D0 is the original diameter, and μ is the shear stress applied by the optical trap.

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