Membrane Structure & Function

Ch. 7

Membrane & Function

Lipid Bilayer Minimizes number of hydrophobic groups exposed to

water Fatty acid tails don’t have to spend energy fighting

water molecules

Fluid Mosaic Model

Mosaic – because cell membranes have a mixed composition (phospholipids, glycoplipids, sterols, proteins)

Fluid – motions and interactions of the parts of the membrane

Hydrophobic interactions in membrane are weaker than covalent bonds

Most phospholipids and some proteins are free to drift sideways

Membrane Proteins

Transport proteins – allow water-soluble substances to move through Bind molecules or ions on one side/release on the other

Receptor proteins – Bind extracellular substances like hormones that change cell activity

Recognition Proteins – “Molecular Fingerprint” Ex: “Self” sign for immune system

Adhesion Proteins – Cells of same type stick together (tissue)

Crossing Cell Membranes

Selective Permeability

Allows some substances to cross, but not others.

Water, oxygen, carbon dioxide, small nonpolar molecules can cross cell membrane

Glucose, ions, other large polar molecules do not move freely across bilayer

Concentration Gradients

Concentration – number of molecules or ions of a substance in a region

Gradient – number in one region is different than in another

Concentration Gradient – difference in number of molecules or ions in two adjoining regions

Things will naturally travel from

high concentration low concentration

Diffusion – movement of like molecules or ions down a concentration gradient (high low)

Factors that influence rate

Steepness of gradient ↑ Diffusion ↑

Temperature ↑ Diffusion ↑

Molecule size ↓ Diffusion ↑

Electric gradient (opposite charges attract)

Osmosis

Osmosis – Diffusion of water due to a water concentration gradient between two regions through a selectively permeable membrane

The side with more solute has a lower water concentration

Hypertonic Solution – more solutes Hypotonic Solution – less solutes Water goes: hypotonic hypertonic Isotonic Solution – solutes the same, no net movement

Protein Mediated Transport

Passive Transport

Transport proteins allow solutes to move both ways across cell membrane

Concentration ↑ binding ↑ transport ↑

Travels from higher concentration to lower concentration until even

Active Transport

Moves things AGAINST concentration gradient

Requires ENERGY in order to “force” it against the concentration gradient (usually ATP)

Donates phosphate group to a transport protein

Binding site on other side improvesSolute bindsProtein changes shape, solute moves to

other side, binding site becomes less attractive

Solute and phosphate releasedProtein goes back to original shape

Exocytosis & Endocytosis

Exocytosis – cytoplasmic vesicle moves to cell surface & fuses with plasma membrane. Vesicle loses shape, but contents released to surroundings

Endocytosis – cell takes in substances next to its surface Small indentation, balloons inward, pinches off

Three types:

Receptor MediatedMembrane receptors chemically recognize &

bind substances Bulk Phase

Vesicle forms around small volume of extra-cellular fluid regardless of what’s in it

PhagocytosisActive form of Endocytosis where cell takes

in microorganisms and other debris

** Membrane cycling – maintain surface area of membrane