Membranes

Chapter 6

Phospholipid Bilayer

•Phospholipids - glycerol (polar head = water-soluble) w/ two attached fatty acid chains (nonpolar tail = water-insoluble) .

–polar head forms hydrogen bonds w/ water, nonpolar tails pack together.

•Lipid bilayer

Phospholipid Bilayer

Fluid Mosaic Model

•Proteins in lipid bilayer.

–Transmembrane Proteins

–Network of Supporting Fibers

–Exterior Proteins and Glycolipids

Cell Membrane Proteins

•Six Major Classes

–Transporters

–Enzymes

–Receptors

–Identity Markers

–Adhesion Proteins

–Cytoskeleton Attachments

Structure of Membrane Proteins

•Anchoring Proteins - Attached to membrane surface.

•Transmembrane Proteins

–Anchors

–Channels and Carriers

–Pores

Diffusion

•Random motion causes net movement of substances from regions of high concentration to regions of lower concentration.

–Continues until equilibrium reached.

Facilitated Diffusion

•Polar molecule transported across plasma membrane through specific channel.

– Selective permeability

•Ions move across membrane in ion channels.

–Movement determined by relative concentrations and voltage across membrane.

Facilitated Diffusion

•Carriers facilitate movement across membrane

–Driven by concentration gradients

–Specific, passive, saturates.

Osmosis

•Solutions - water (=solvent) and solutes diffuse down concentration gradient.

–Most solutes cannot cross membrane.

–Water flows through membrane = osmosis.

•Water molecules very small and uncharged

Osmosis (cont.)

•Osmotic Concentration - Concentration of all solutes in a solution (relative to cell).

–Hyperosmotic - Solution > conc. of solute.

–Hypoosmotic - Solution < conc. of solute.

–Isosmotic - Osmotic conc. of both solutions are the same.

Osmosis

Osmotic Pressure

•Hydrostatic Pressure - cytoplasm pushing out against cell membrane.

•Osmotic Pressure - pressure necessary to stop osmosis.

•Maintaining Osmotic Balance

–Extrusion

–Isosmotic Solutions

–Turgor

Bulk Passage

•Endocytosis - Plasma membrane envelops food particles.

–Phagocytosis - Particulate form.

–Pinocytosis - Liquid form.

–Receptor-Mediated Endocytosis - Molecules bind to receptors on plasma membrane.

•Exocytosis - Discharge from vesicles at cell surface.

Exocytosis

Active Transport

•Movement across membrane against concentration gradient.

–Uses energy.

•Cell takes up molecules of substance already in higher concentrations inside than extracellular fluid.

•Sodium-Potassium Pump

–1/3 of non-cell division energy to drive Na⁺ & K⁺ pump

–Low concentration of Na⁺ inside cell

Coupled Transport

•Active transport that moves molecules w/ Na⁺ moving down their concentration gradients (Cotransport).

–Establish the down gradient.

•Na⁺/K⁺ pump.

–Transverse the upgradient (Na⁺ brings sugars, amino acids & other nutrients into cell)