Cell Membranes Flashcards
What the role of Membranes? (5)
- Act as barriers
- enclosed reaction compartments (organelles)- spaces in the cell where reactions can occur. e.g. mitochondria, nucleus, ER.
- Maintain concentration gradients.
- Determines what enters and leaves the compartment.
- Provides an interaction surface with the
environment.
why do membranes form?
due to insolubility of water (strong Dipole) in
hexane (C6H14- apolar-organic solvent) and vice versa.
Aqueous Solubility
Charged or polar molecules=highly soluble= they can interact with H20 molecules- hydrophillic
Apolar molecules= highly INsoluble= no interaction with the water dipole= hydrophobic
polarity of water and hexane
water is polar
hexane is apolar
Definition of Amphipathic Molecules.
Compounds that have both polar and non polar regions making them hydrophilic (dissolve/mixes with water) and Lipophilic (tendency to combine/dissolve in lipids or fats).
Components of amphiphatic moleucles.
Polar head group= Hydrophilic
Non polar tail= Lipophilic
what is phase separation.
phase separation is where the lipid and protein components of the membrane segregate into distinct regions or phases.
Phase separation between water and hexane. what is it caused by?
Occurs because they are immiscible liquids, meaning they do not mix due to differences in polarity- insolubility of water in hexane and vice versa
where do Amphiphatic molecules arrange themselves.
arrange themselves at the interphase between organic and water phase.
CHECK IMAGE!
Example of Amphiphilic molecule.
Surfactants/detergents: they contain polar and non-polar parts.
what structure is formed by amphipathic molecules in aqueous solutions?
what is the purpose of this structure?
forms a structure called Micelle where polar heads are facing outside and non-polar tale facing inside - this structure minimises exposure of (shields) lipophilic residues to aqueous surroundings.
what are lipophilic residues
are parts of molecules that are attracted to lipids (fats) and repel water.
Inverse Micelle * when is it formed?
formed in Non-polar solvent= heads inside and tails outside.
Image of Micelle structure.
How do detergents work?
1- Soap or detergent dissolves in water
2- Surfactant ions orientate themselves in grease and water.
3- Agitation begins to separate grease from surface
4- Process Continues
5- Cleaning complete
Locations of Membranes in Eukaryotes.
Single lipid bilayer cell membrane.
Internal compartments surrounded by specialised membranes (membrane bound).
e.g. mitochondria, nucleus, endoplasmic
reticulum.
Location of Membranes in Prokaryotes.
Some have a single membrane.
Outer membrane – protection.
- Fairly permeable to small molecules.
Inner membrane – permeability barrier.
Region between membranes is termed periplasm.
Definition of Hydrophillic
water loving, interactions with water is
thermodynamically favourable
Definition of Hydrophobic
water hating/fearing, apolar
molecules unable to interact with water.
Definition of Lipophilic
lipid loving, tendency to dissolve/mix in lipids or fats
Amphiphilic/Amphipathic
a molecule having both hydrophilic (polar)
and hydrophobic (apolar) parts.
Phospholipids are naturally occurring amphipathic molecules. what are they made up of ?
Have a glycerol backbone.
Two fatty acyl groups.
Phosphate group.
Head group.
IMAGE ! Including Examples
Cholesterol (6)
function?
fluidity at high temp?
fluidity at low temp?
maintains…
present/ absent where?
Intercalates with acyl chains
and reduces mobility.
- Fluidity decreases at high
temperatures.
- Increases fluidity at low
temperatures.
- Maintains fairly constant fluidity
across temperature ranges.
- Present in eukaryotic
- absent in prokaryotic and
intracellular membranes (membrane bound organelles)
Labelling and Identifying Cholesterol
What does the fluidity of membranes depend on?
- The composition of acyl chains.
- Temperature.
Spontaneous Formation of Membranes.
LIPID
SHEETS
DRIVEN
LEADS
- Lipid bilayers form spontaneously.
- Growth of bimolecular sheet is
spontaneous and rapid in water. - Driven by hydrophobic interactions.
- Leads to the creation of cell
membranes.
Lipid structures in aqueous environment
DIAGRAM IN BOOK!
Planar Bilayer (Flat)
Micelles
Vesicles/liposomes
Movement of lipids in membranes- LATERAL
- Movement within a leaflet.
- 2D lateral diffusion.
- Rapid e.g. a lipid can diffuse the length of a
bacterial cell in 1 second at 37oC.
Movement of lipids in membranes- TRANSVERSE
- Transverse diffusion (flip-flop).
- Movement between leaflets.
- Polar head has large solvation shell that must be broken down which is why it Requires lots of energy and makes it less efficient AND SLOWER (6-10HRS) than lateral diffusion.
- Catalysed by flippases
Bilayer membranes:
- Two lipid sheets (monolayers or leaflets)
- Polar heads on outside
- Hydrophobic tails inside
- Typically 4-6 nm thick
Liposomes/Vesicles
- Aqueous environments enclosed by lipid
bilayers - Very useful experimental tools
- Can be formed from planar bilayers that vary in size from 50 nm to >10 μm
Composition of Biological Membranes.
lipids /carbohydrates/proteins
Lipids:
- 25-50% by mass.
- Phospholipids,
glycosphingolipids,
cholesterol.
Carbohydrates :
- Glycolipids and
glycoproteins.
Proteins:
- 50-75% by mass
Are membrane symmetric or asymmetric.
asymmetric
——- together with —– define the ——————————————–.
Proteins together with lipids define the characteristics
of a membrane
LABEL GLYCOCALYX
what are the two types of membrane proteins?
integral
peripheral
Integral membrane proteins
are they amphillic?.
+ STRUCTURE
- YES THEY Are amphiphilic
- a-helix (recognition, receptors)
- helical bundle ( enzymes, transporters, receptors)
- B- barrel (transporter-channel proteins).
Peripheral membrane proteins + STRUCTURE
- amphiphilic a-helix
- Hydrophobic loops
- Lipid modifications/tails
- Electrostatic / ionic interactions
A hydrophobic molecule is NOT soluble in:
1. Toluene
2. Saline
3. Hexane
4. Petroleum
Hexane
Which ONE of the following molecules is NOT a
component of a prokaryotic plasma membrane?
1.Phosphatidylserine
2.Phosphatidylcholine
3. Cholesterol
4. Proteins
Cholesterol
Lipids in bilayers can readily…
1. Change places with their neighbour
2. Leave the bilayer
3. Swap leaflets
4. Exchange headgroups
Change places with their neighbour (movement within a leaflet)
Apolar molecules are poorly soluble in water
because of their…
1. size
2. aggregation (CLUSTER TOGETHER)
3. density
4. disturbance of water structure
disturbance of water structure
Which ONE of the following statements about
membrane components is INCORRECT?
1. Integral membrane proteins are amphiphilic
2. Cholesterol is amphipathic
3. Poly-sugar side chains are usually attached to the
cytosolic side of integral membrane proteins
4. Phospholipids are asymmetrically distributed between
membrane leaflets
Poly-sugar side chains are usually attached to the
cytosolic side of integral membrane proteins
saline
Saline is a solution of salt (sodium chloride, NaCl) in water.
