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 improves Solute binds Protein changes shape, solute moves to other side, binding site becomes less attractive Solute and phosphate released Protein 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 Mediated Membrane receptors chemically recognize & bind substances Bulk Phase Vesicle forms around small volume of extra- cellular fluid regardless of what’s in it Phagocytosis Active form of Endocytosis where cell takes in microorganisms and other debris ** Membrane cycling – maintain surface area of membrane