Cell Membrane Function Part I

How does the membrane control what enters or leaves the cell? Passive transport (without energy input) -Diffusion -Facilitated Diffusion -Osmosis

Movement of solute (dissolved stuff) from area of high concentration to area of low concentration Diffusion Remember: passive = No energy input Continues to equilibrium (even distribution)

Facilitated Diffusion Diffusion with the help of transport proteins and protein channels

Facilitated diffusion Still moves from high concentration of solute (dissolved stuff) to low concentration till it reaches equilibrium Still does not use energy

Diffusion vs. facilitated diffusion Diffusion -small molecules (like O 2, CO 2 ) -nonpolar molecules (can dissolve in lipids) Facilitated diffusion -larger molecules (like glucose) -polar molecules (have a + end and a – end) -charged ions

Diffusion of water across a selectively permeable membrane Osmosis

Tonicity Isotonic: Both solutions are equal in concentration Hypertonic solution: Higher concentration of solute Hypotonic solution: Lower concentration of solute

Tonicity example: red blood cell The red blood cell has an internal salt concentration that matches the 9 grams of salt in 1 liter of solution Salt Sucks! So water always moves into saltier solution

Always moves from high to low concentration Requires no energy In the cell it occurs as: –diffusion of solute through lipid bilayer -- facilitated transport: solute helped through membrane by proteins –Osmosis: diffusion of water molecules Slide 3.7 Summary of passive transport

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