Chapter 11: Membrane Structure

For Exam 3

in eukaryotes

most membranes are internal through the organelles

everything has a lipid bilayer (which is same everywhere)

all biological membranes have a common general structure (important to separate inside from out)

lipid bilayer

provides structure of cells

membrane proteins

provides function of cells

cell membranes

act as selective barriers

lipid bilayers don’t completely cut off from outside; selective and need to take things in and take things out

in eukaryotic cell → more busier than bacterial (which has no internal membranes due to lack of organelles)

plasma membrane

involved in many vital functions:

receiving information, import (endocytosis) and export (exocytosis) of small molecules, capacity for movement and expansion (ability to change shape and physically crawl around)

a cell membrane consists of a lipid bilayer in which proteins are embedded

membranes are about 50% lipid (mass) and 50% protein

lipid bilayer is about 5 nanometers thick

lipid is arranged in a bilayer that also includes proteins either transmembrane or peripheral

major lipids in cell membranes

phosphoglycerides, sphingolipids, sterols

polar (hydrophilic) head

phosphate which is attached to choline

glycerol

in the middle of phosphate and tail components

nonpolar (hydrophobic) tails

2 tails, 14-24 carbons in tail

one tail → saturated (with hydrogens), no double bond

other tail → unsaturated and creates a kink (affects structure of membrane), due to cis double bond

phosphoglycerides

phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine

(2 fatty acid tails, glycerol, phosphate, (ethanolamine, serine, or choline))

sphingomyelin

choline, phosphate group, amino group which attaches to fatty acid group

longer fatty acid tail

sphingosine

long acyl chain with NH2 and 2 OH groups at one end

hydrophilic molecules in water

with form H bonds with water or have electrostatic interactions due to partial positive and negative charges from the molecules

creates a nice mixing

hydrophobic molecules in water

electrons in nonpolar molecules are shared evenly, leaving no partial charges

water will H bond to other water molecules and exclude nonpolar one

will form ordered/cage-like structure

increases free energy (which is something we do not want in cells)

why do phospholipids spontaneously form bilayers?

to protect its hydrophobic tails from water; will close into a spherical-like structure and form bilayer

polar interacts with water and decreases free energy; nonpolar is shielded from water

how do membrane phospholipids move within the lipid bilayer?

lateral diffusion, flexion, flip-flop, rotation

lateral diffusion

occurs most often; lipids move from side-to-side (laterally)

flexion