Cell biology exam 3

CHP. 11 - Membrane Structure

CHP. 11 - Membrane Structure

micelles do not have

cytosol, we need the lipid bilayer in liposome to have cytosol

cell membranes have the same basic architecture:

phospholipid bilayer (about 3nm wide) with embedded proteins

components of the lipid bilayer

hydrophilic head - interacts with the water-based cytosol and outside environment

hydrophobic core - prevents unassisted movement of water-soluble substances from one side to the other

all membrane lipids are

amphipathic meaning they have a water loving and water hating region

cell membranes act as

selective barriers

in some bacteria, the plasma membrane is the

only membrane

in addition to the plasma membrane, eukaryotic cells also have

internal membranes that enclose individual organelles

all cell membranes prevent

molecules on one side from freely mixing with those on the other

what are the three classes of membrane lipids

phosphoglycerides, sphingolipids, and sterols

composition of phosphoglycerides

glycerol backbone, tails, and head

phosphoglyceride tails

two hydrophobic fatty acyl chains

• commonly 16 or 18 carbon

• saturated or unsaturated

phosphoglyceride head

4 major head groups: phosphatidylcholine (PC), phosphatidyethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI)

plasmalogens composition

one fatty acyl chain attached to glycerol by an ester linkage and one attached by an ether linkage

plasmalogens constitute

10 mol% of the total mass of phospholipids in humans, mainly as membrane structure components

plasmalogens structures

sn-1 vinyl ether

sn-2 polyunsaturated fatty acids

sphingolipids are derivatives of

sphingosine (an amino acid alcohol with a long hydrocarbon chain)

sphingolipids composition

fatty acyl chains connected by an amide bond

sphingomyelines

contain a phosphocholine head

others contain a sugar or oligosaccharide head - glycolipids

glucosylcerebroside

has a glucose head group

sterols are ___ of _____

membrane components of animals (cholesterol), fungi (ergosterol), and plants (stigmasterol)

they are precursors for different hormones

sterol structure

head group - single polar hydroxyl

tail - conjugated four-ring

hydrocarbon and short hydrocarbon chain

purpose of carbohydrates in the membrane

they stick out and serve as receptors

how are cancer cells identified

DNA killed the study of lipids, however cancer cells have unique lipid structures on them so we use those now

amoeba and bacterial DNA would be hard to distinguish, how could we tell them apart?

their membranes could have different lipids that could be separated for profiling

• not all cells have the same types of lipids/fatty acids

purpose of phosphoglyceride heads

determine the direction it points on the membrane

plasmalogens are a hybrid version of

archea since they both have ether linkages

how do we obtain plasmalogens

our bodies make them on their own, but people can take supplements to give them bigger pieces of these structures to synthesize them faster

sphingolipid use

retain moisture in the skin

shingomyelin use

cover nerve cells and is useful in the regeneration of cells

simplest sphingolipid has

just an H on the other chain leg

cholesterol role

it stiffens the membrane, decreases membrane fluidity

cholesterol in eukaryotes

sits in between phospholipids in eukaryotes

what determines membrane fluidity in prokaryotes

degree of fatty acid saturation

biological membranes

vary in lipid composition, impermeable to water-soluble molecules and ions

have a viscous consistency with fluid like properties

membrane lipids will spontaneously form

liposomes

phospholipid bilayers spontaneously

close in on themselves to form sealed compartments

why do phospholipid bilayers spontaneously close in on themselves

in a planar phospholipid bilayer, the hydrophobic tails are exposed to water along the edges which is energetically unfavorable

formation of the sealed compartment shields the hydrophobic tails from water which is energetically favorable

what are the faces of cellular membranes

plasma membrane, vesicle and some organelles, nucleus, mitochondrion, and chloroplast

plasma membrane

single bilayer

cytosolic and exoplasmic leaflets

vesicle and some organelles

single bilayer

internal aqueous space is equivalent to the outside of the cell

nucleus, mitochondrion, and chloroplast

enclosed by two membranes separated by a small intermembrane space

phosphatidyl choline

most common phospholipid in cell membranes

has 3 parts

3 parts of phosphatidylcholine

1. hydrophilic head: has choline linked to phosphate

2. two hydrocarbon chains; forms hydrophobic tails

3. glycerol; links head to the tails

compare different membrane lipids

they are all amphipathic, but differ in their hydrophilic head

galactocerebroside head

sugar galactose plus an -OH group

formation and study of phospholipid bilayers

1. treatment with organic solvent mix of chloroform and methanol to selectively soluble the phospholipids and cholesterol

2. mechanical dispersal of extract in water; lipids spontaneously form liposomes

3. planar bilayer formation over a small hole in a partition separating two aqueous phases; used to study permeability of solutes

the fluidity of a lipid bilayer depends on

its composition

phospholipid synthesis in ER membrane

Step 1: Two fatty acids synthesized on fatty acyl CoA. Hydrocarbon tails anchor the
molecule to the membrane.
• Step 2: Phosphatase – converts phosphatidic acid into diacylglycerol.
• Step 3: Phosphotransferase transfers a polar head group – Choline in this example
• Step 4: Flippase – uses ATP energy to catalyze movement of phospholipids from the
cytosolic leaflet to the exoplasmic leaflet

what does scrambalase do

catalyzes the transfer of random phospholipids from one monolayer to another

flippases mainatin the

asymmetric distribution of phospholipids

certain phospholipids are confined to one side of the membrane

phospholipid translocases

floppase: takes outside and puts in

flippase: takes inside and puts outside

scrambalase: random (activity of this enzyme are increased with calcium ions are increased in the cell)

*floppase and flippase are directed

why does the cell need scrambalase

for when it is undergoing apoptosis