And Molecular Movement Across Them Biological Membranes And Molecular Movement Across Them

Structure and Composition of Membranes

Membrane Lipids

Amphipathic hydrophobic tails

Decrease Fluidity by: Increase Cholesterol Increase Length of FA Tails Increase FA saturation

Decrease Fluidity by: Increase Cholesterol Increase Length of FA Tails Lipid Raft Fluid membrane area Lipid raft (ridged membrane area) Transmembrane glycoprotein Transmembrane protein Carbohydrate Lipid-linked (GPI) protein Cholesterol Glycolipid Decrease Fluidity by: Increase Cholesterol Increase Length of FA Tails Increase FA saturation

Phosphatidylserine (PS) Phosphatidylethanolamine (PE) GOLGI LUMEN Phosphatidylserine (PS) Phosphatidylethanolamine (PE) Phosphatidylinositol (PI) CYTOSOL

Unless flippase(s) is/are at work, the original orientation of the bilayer established in the ER is maintained

Membrane Asymmetry Phosphatidylcholine (PC) Sphingomyelin (SM) Carbohydrate (glycolipid) Phosphatidylserine (PS) Phosphatidylethanolamine (PE) Phosphatidylinositol (PI) Membrane Asymmetry

Membrane Proteins

Integral Peripheral NaCl, , H+ , OH-

= Transmembrane protein Lipid-linked protein Monolayer associated protein = Integral Membrane Proteins

Cell Cortex Cytoplasm Extracellular Space

Fluorescence Recovery After Photobleaching apical basolateral

Membrane Carbohydrates

Molecular Movement Across Membranes

CARRIER Both are SPECIFIC Both are SATURABLE Carriers move solutes more slowly than channels

facilitated diffusion Against With facilitated diffusion No Energy Required

With Electrical Gradient Against Electrical Gradient

Active Transport

Outside Inside

Na+-K+ ATPase

Co-Transport

Na+-Glucose Symporter Against With chemical

Transport Through Channels Passive Transport

Gated Ion Channels

Movement of ions across a cell membrane sets up a “voltage” across that membrane (Vm). The Vm is the membrane potential The Vm is measured in millivolts (mV) The Vm of a typical resting cell is ~70mV The Vm can change if ion channels open or close “Excitable cells” (e.g., muscle cells, neurons) are ‘excited’ when their Vm changes rapidly. The Vm of a resting cell can be calculated if you know ion concentrations and you make some assumptions.

The Vm of a typical resting cell is ~70mV

V = 62z log10 (Co /Ci)

“Electrical” Signal Transmission Synaptic Cleft/Synapse

-70

Na+ in DEpolarize REpolarize K+ out -70 HYPERpolarize

Na+ in