1.3 Membrane structure

Question 1

what is the role of cholesterol in membranes

Answer 1

cholesterol in mammalian cells / ANIMAL CELLS

cholesterol is a steroid - mostly hydrophobics so centre of membrane but hydroxyl group is hydrophilic! so attracts to the phosphate heads

AMPHIPATHIC

1. reduces membrane fluidity
2. reduces permeability to some solutes

cell membranes do not exactyl correspond to one state of matter - hydrophobic tails are liquid whereaese hydrophilic phosphate heads act solid

if too fluid –> less able to control substances

if not fluid enough –> movement of substances is too restricted

• cholestrol disrupts regular packing of hydrogcarbon tails of phospholipid - no cystallization
• also restricts molecular motion and therefore the fluidity of the membrane
• also reduces the permeablility of hydrophilic particles e.g. sodium ions and hydrogen ions
• due to shape helps membrane curve into concate shape and therefore formation of vesicles during endocytosis

Question 2

draw the fluid mosaic model of membrane structure

Answer 2

include

• phospholipids as a upside down u with a circle at the top
• peripheral protein
• integral protein
• glycoprotein
• choelsterol
• pumps
• phosphate hydrophilic head
• hydrocarbon tail - hydrophobic
• chanel protein

Question 3

what is a phospholipid bi layer

Answer 3

phospholipid bilayer forms due to its amphipathic property

hydrophilic phosphate head

hydrophobic hydrocarbon tail

phospholipid has 2 hydrocarbon chains

Question 4

what is the davson and danielli model and what were the problems with it? what evidence was it based on?

Answer 4

idea

a sandwhich of proteins and a phospholipid bi layer within

good because

• despite being thin shows how membrane are an effective barrier
• railroad track appearance in electron micrographs suggests proteins are surrounding inside layers

PROBLEMS

1. freeze etched electron micrographs rapid freezing and fracturing of cells along lines of weakness. (centre of membrane) glubular structures scattered through membrane - proteins
2. structure of membrane proteins biochemistry allowed proteinst to be extracted from membrane - varied in size / globular so unlikely to form a continuous layer of periphery of membrane - also parts of protein hydrophobic so would be attracted to hydrophobic centre
3. fluorescent antibody tagging attached to antibodies that bind to membrane proteins , two colours for two cells, cells fused and the two colours mixed throughout membrane - shows proteins are free to move

needed to add

• intergal proteins
• fluidity

Question 5

what was the model of the membrane order and describe the first one

Answer 5

gorter and grendel

davson and danielli

singer and nicolson

Gorter and grendel - deduced the bilayer by extracting the phospholipids from a rbs membrane

–> many errors but they cancelled out

–> did not expalin location of proteins

Question 6

what are the functions of membrane proteins

Answer 6

first is to form a barrier through which ions and hydrophilic molecules cannot easily pass - carried out by bilayer

carried out by membrane proteins

1. hormone binding sites
2. immobilized enzyme with active site on the outside
3. cell adhesion (form tight junctions between cells)
4. cell-to-cell communication
5. channels for passive transport - allow hydrophilic particles across by facilitated diffusion
6. pumps for active transport using ATP

Question 7

what are the groups of membrane proteins

Answer 7

integral / intrinsic

hydrophobic on at least some parts so are embedded due to hyrophobic R group

many are transmembrane (across whole membrane)

e.g.

1. channel proteins - diffusion of polar moelcuels adn ions through hydrophilic channel
2. carrier proteins / pump - change shape to allow diffusion / active transport
3. glycoproteins - used for cell adhesion / receptor for chemicals

peripheral / extrinisic

hydrophilic on surface so not embedded

most of them attacehd to surface of an intergral protein and this attachement is often reversible due to hydrophilic R group on surface which interact with polar heads of intrinsic proteins

some have a single hydrocarbon chain which is inserted into the membrane and anchors the protein

more info

pump proteins in root cells have are oriented so that they pic up potassium ions and pump them into the soil

the more active a membrane the more proteins

Question 8

why are membranes useful (internal membranes)

Answer 8

• proteins can be turned on and off to allow different substances in and out so affects metabolism
• compartmentalisation
• distinct environments/ specific conditions e.g. pH
• provide a large SA for attachment of enzymes etc.