Class 7

Cell Membranes: Lipid Bilayer

membranes

define life

without them, no cells

cell membrane

selective barrier that separates a cell from its surroundings, enabling the molecular composition of a cell to differ from that of its environment

~5nm or 50 atoms thick

two-ply lipid sheet into which proteins inserted

nutrients in; waste products out

in bacterial cell, plasma membrane is the only ___ ___

eukaryotic cells have plasma membrane and internal membranes that enclose individual organelles

all ___ ___ prevent molecules on one side from freely mixing with those on the other

grows as cell grows

can deform without tearing, allowing cell to move and change shape

self-healing

packed with phospholipids

functions oif cell membranes

cell communication

import and export of molecules

cell growth and motility

cell communication

receptor proteins in plasma membrane enable cell to receive signals from environment

import and export of molecules

channels and transporters in membrane enable import and export of small molecules

cell growth and motility

flexibility of membrane and its capacity for expansion allow cell to grow, change shape, and move

when cell grows, so does its membrane

self-healing ability of cell membrane

if torn, it doesn’t collapse like a balloon; it reseals

internal membranes

in eukaryotes

built on same principles as plasma membrane but differ subtly in composition, especially in resident proteins

form from many different components in a eukaryotic cell

organelles with internal menmbranes

nucleus

mitochondria

Golgi apparatus

lysosomes

endosomes

peroxisomes

endoplasmic reticulum

transport vesicles

nucleus and mitochondria

each enclosed by two membranes

general membrane structure

1. Lipid bilayer→ permeability barrier.

2. Proteins, embedded in lipid bilayer, which give membranes their distinctive functions.

plasma membrane micrograph

proteins than extend from either side of bilayer form two closely spaced dark lines

thin, white layer between them is lipid bilayer

lipid molecules

not very soluble in water

are very soluble in organic solvents such as benzene

took advantage of this when studied them

are much smaller than protein molecules, but membrane usually has ~50X more ___ ___ than protein molecules

lipid bilayer discovery

researchers used benzene to extract all lipids from plasma membranes of purified red blood cells

spread lipids out in film on surface of a through, filled with water, like oil in a slick puddle

used movable barrier to push floating lipids together until formed a continuous sheet, one molecule thick

when measured surface area of monolayer→ found it occupied twice the area of the original, intact cells

deduced it was a bilayer, i.e., that lipid molecules doubled up to form a bilayer

lipid bilayer

made up of phospholipid molecules

formed by conflicting forces of hydrophilic heads’ attraction for water and hydrophobic tails seeking aggregation with other hydrophobic molecules

is self-sealing

is a flexible two-dimensional fluid

provides basic structure of all membranes

serves as permeability barrier to hydrophilic molecules on both sides of it

phospholipids

most abundant lipids in cell membranes

have phosphate-containing hydrophilic head linked to a pair of hydrophobic, hydrocarbon tails

are amphipathic

posphatidylcholine

one of the most common phospholipids in cell membranes of animals and plants

small molecule choline attached to the phosphate head

phosphatidyl part of name refers to the phosphate-glycerol-fatty acid portion of molecule

• choline→phosphate→glycerol→hydrocarbon tails

kink in hydrocarbon chains

occurs where there is a double bond between two carbon atoms

amphipathic

__ molecules have both hydrophilic and hydrophobic parts

different membrane lipids are all ___

phospholipids in membranes

cholesterol in animal membranes

glycolipids in membranes

triacylglyceros

are the main constituents of animal fats and plant oi

are hydrophobic

have three fatty acid tails

lipid bilayer formation

resolves conflicting forces of

• hydrophilic heads’ attraction for water

• hydrophobic tails seeking aggregation with other hydrophobic molecules

self-sealing

same forces that drive amphipathic molecules to form lipid bilayer make bilayer __ __

phospholipid bilayers continuously close in on themselves to form sealed compartments

the closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable

self-sealing steps

any tear in sheet creates a free edge that is exposed to water

this is energetically unfavorable, so molecules of bilayer spontaneously rearrange to eliminate free edge

• if small tear, spontaneous rearrangement will exclude water and lead to repair of bilayer

• if large tear, sheet may fold in on itself and break up into separate closed vesicles

only way amphipathic sheet can avoid having free edges is to bend and seal, forming a boundary around a closed space, thus forming vesicles, organelles, cells→ energetically favorable

fluidity

within membrane, molecules move about and change places with one another

depends on lipid bilayer’s composition

__ of lipid bilayer refers to the ease with which its lipid molecules move within the plane of the bilayer→ is important for membrane function and has to be maintained within limits

at a given temperature, ___ of a membrane depends on its phospholipid composition, in particular the nature of hydrocarbon tails→ the closer and more regular the packing of tails, the more viscous and less fluid the bilayer

flexibility

lipid bilayer is flexible

synthetic lipid bilayers

can study membranes with ___ ___ ___

are spontaneously produced by aggregation of amphipathic lipid molecules in water

in ___ ___ ___, phospholipids rarely tumble from one half of lipid bilayer or monolater to other→ flip flop

as a result of random thermal motions, lipid molecules continuously exchange places with their neighbors withing the same monolayer

studies of whole cells and isolated cell membranes show same kinds of movements that see in ___ ___ ___

liposomes

pure phospholipids form closed, spherical vesicles, called ____

25nm to 1mm in diameter

flip flop

movement of phospholipids from one half of lipid bilayer or monolayer to other

occurs less than once per month for any individual molecule under conditions similar to those in cell

lateral diffusion

as a result of random thermal motions, lipid molecules continuously exchange places with their neighbors within the same monolayer

• this leads to rapid ___ ___ of lipid molecules within the same plane of each monolayer

in synthetic bilayer, lipid may do ___ ___ ~2 microns per second

lipid molecules flex hydrocarbon tails and rotate rapidly around long axis→ can reach up to 500 rev/s

because of these motions, bilayer behaves as a two-dimensional fluid

packing of hydrocarbon tails

affected by two major properties

• length of hydrocarbon tails

• number of double bonds in hydrocarbon tails

shorter chain

reduces tendency of hydrocarbon tails to interact and increase fluidity of membrane

vary in length between 14 and 24 carbon atoms, with 18-20 being the most common

hydrocarbon tail

for most phospholipids,

• one of the ___ ___s contains only single bonds between its adjacent carbon atoms→ saturated

• whereas the other ___ ___ includes one or more double bonds→ unsaturated with respect to hydrogen

double bond

each __ __ creates kink in tail, making it difficult for tails to pack with one another, thereby increasing fluidity if the bilayer

bilayers with higher proportion of unsaturated H-C tails (more ___ ___s) are more fluid than those with lower proportions

membrane fluidity

key to cell survival, growth, and reproduction

1. enables many membrane proteins to diffuse rapidly in plane of bilayer and to interact with each other

2. permits membrane lipids to diffuse from sites where they are inserted into bilayer after their synthesis to other regions of cell

3. ensures membrane molecules distributed between daughter cells when cell divides

4. allows membranes to fuse with one another and mix their molecules

bacterial and yeast cells

have to adapt to changing temperatures and they adjust the lengths and degree of saturation of H-C tails in bilayers to maintain a membrane of relatively constant fluidity

at higher temps

cells make longer hydrocarbon tais with fewer double bonds

longer tails→ more van der Waals interactions, which make membrane more rigid under circumstances that normally give more fluidity

fats produced by plants

are generally unsaturated and loquid at room temperature

amimal fats

generally saturated and solid at room temperature

to produce margarine

vegetable oils are hydrogenated→ addition of hydrogen removes the double bonds, making the oils more solid and butter-like at room temperature

cholesterol

in animals cells, at physiological temperatures, membrane fluidity is also regulated by sterol ___

present in especially large amounts in plasma membrane, ~20% of lipids in plasma membrane by weight

has short, rigid, steroid ring structure that lets it fill spaces between neighboring phospholipid molecules left by kinds in unsaturated H-C tails→ ___ stiffens the bilayer by doing this, making it less flexible and less fluid

distributed almost equally in both monolayers

cholesterol molecule

polar head group

rigid planar steroid ring structure

nonpolar hydrocarbon tail

endoplasmic reticulum

membrane assembly begins in the ___ ___

in eukaryotes, new phospholipids are made by enzymes bound to cytosolic surface of ___ ___

the enzymes use free fatty acids as substrates and deposit newly made phospholipids only in cytosolic half of bilayer

scramblase

type of transporter protein that removes randomly selected phospholipids from one half of bilayer and inserts them in another

catalyzes transfer of phospholipids, since flip-flop movement is not spontaneous

newly made phospholipids

redistributed equally between each monolayer

new membrane

some of __ __ will stay in ER

rest of __ __will supply fresh membrane to other compartments

• Golgi

• plasma membrane

• mitochondria

• nucleus

• peroxisome

• lysosome

• endosome

• transport vesicle

asymmetry

certain phospholipids are confined to one side of membrane

most cell membranes have ___→ two halves of bilayer often have very different sets of phospholipids

membranes emerge from ER with evenly assorted set of phospholipids

____ of cell membranes arises in Golgi apparatus

flippases

___ is preserved as membranes bud from one organelle and fuse with another or with plasma membrane

flippases

phospholipid-handling transporter

remove specific phospholipids from side of bilayer facing exterior space and flip them into monolayer facing cytosol, unlike scramblases which move random phospholipids from one half of bilayer to another

help establish and maintain asymmetric distribution of phospholipids characteristic of animal cell membranes

when membranes leave ER and are incorporated into Golgi, transporters called ___,

• selectively remove phosphatidylserine and phosphatidylethanolamine from Golgi-lumen-facing monolayer and flip them to cytosolic side

• transfer leaves phosphatidylcholine and sphingomyelin concentrated in Golgi-lumen-facing monolayer

• resulting curvature of membrane may help drive subsequent vesicle budding

conservation of orientation

all membranes have inside and outside faces

cytosolic monolayer→ always faces cytosol

noncytosolic monolayer

• either exposed to cell exterior if plasma membrane

• or to lumen of organelle

___ ___ ___ applies to phospholipids and to membrane proteins inserted into membrane

important since protein’s orientation in membrane is key to its function

vesicle budding and fusing

membranes transported by a process of ___ ___ and ____

orientation of both membrane lipids and proteins preserved during this process

vesicle budding from Golgi and fusing with plasma membrane

original cytosolic surface of lipid bilayer remains facing cytosol

noncytosolic surface continues to face away from cytosol, toward lumen of Golgi and transport vesicle — or toward extracellular fluid

similarly, glycoprotein remains in same orientation, with attached sugar facing noncytosolic side

glycolipids

among lipids, those with most lopsided distribution in cell membranes are ____

located mainly in plasma membrane

located only in noncytosolic monolayer

sugar groups of ___ face cell exterior

form part of continuous coat of carbohydrate that surrounds and protects animal cells

once ___ are created by enzymes, they are trapped in noncytosolic monolayer since there aren’t any flippases that transfer ___ to cytosolic side

when ___ reach plasma membranes, their sugars are exposed on exterior of cell

enzymes that add sugar

__ __ __ __ groups to lipids to make glycolipid molecules are confined to Golgi apparatus

these are oriented such that, sugars are added to lipid molecules only in noncytosolic monolayer

phosphatidylcholine and sphingomyelin

concentrated in noncytosolic monolayer

phosphatidylserine and phsophatidylethanolamine

found mainly in cytosolic side

phosphatidylinositols

minor constituent of plasma membrane in cytosolic monolayer, where they participate in cell signaling

membrane proteins

carry out most functions of membrane

in animals, ___ ___ make up ~50% of mass of most plasma membranes

• remainder is

  • lipid

  • carbohydrate found on some lipids (glycolipids)

  • carbohydrate found on many proteins (glycoproteins)