Cell Membranes

Question 1

What the role of Membranes? (5)

Answer 1

• 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.

Question 2

why do membranes form?

Answer 2

due to insolubility of water (strong Dipole) in
hexane (C6H14- apolar-organic solvent) and vice versa.

Question 3

Aqueous Solubility

Answer 3

Charged or polar molecules=highly soluble= they can interact with H20 molecules- hydrophillic

Apolar molecules= highly INsoluble= no interaction with the water dipole= hydrophobic

Question 4

polarity of water and hexane

Answer 4

water is polar
hexane is apolar

Question 5

Definition of Amphipathic Molecules.

Answer 5

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).

Question 6

Components of amphiphatic moleucles.

Answer 6

Polar head group= Hydrophilic
Non polar tail= Lipophilic

Question 7

what is phase separation.

Answer 7

phase separation is where the lipid and protein components of the membrane segregate into distinct regions or phases.

Question 8

Phase separation between water and hexane. what is it caused by?

Answer 8

Occurs because they are immiscible liquids, meaning they do not mix due to differences in polarity- insolubility of water in hexane and vice versa

Question 9

where do Amphiphatic molecules arrange themselves.

Answer 9

arrange themselves at the interphase between organic and water phase.

CHECK IMAGE!

Question 10

Example of Amphiphilic molecule.

Answer 10

Surfactants/detergents: they contain polar and non-polar parts.

Question 11

what structure is formed by amphipathic molecules in aqueous solutions?
what is the purpose of this structure?

Answer 11

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.

Question 12

what are lipophilic residues

Answer 12

are parts of molecules that are attracted to lipids (fats) and repel water.

Question 13

Inverse Micelle * when is it formed?

Answer 13

formed in Non-polar solvent= heads inside and tails outside.

Question 14

Image of Micelle structure.

Question 15

How do detergents work?

Answer 15

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

Question 16

Locations of Membranes in Eukaryotes.

Answer 16

Single lipid bilayer cell membrane.

Internal compartments surrounded by specialised membranes (membrane bound).

e.g. mitochondria, nucleus, endoplasmic
reticulum.

Question 17

Location of Membranes in Prokaryotes.

Answer 17

Some have a single membrane.

Outer membrane – protection.
- Fairly permeable to small molecules.

Inner membrane – permeability barrier.

Region between membranes is termed periplasm.

Question 18

Definition of Hydrophillic

Answer 18

water loving, interactions with water is
thermodynamically favourable

Question 19

Definition of Hydrophobic

Answer 19

water hating/fearing, apolar
molecules unable to interact with water.

Question 20

Definition of Lipophilic

Answer 20

lipid loving, tendency to dissolve/mix in lipids or fats

Question 21

Amphiphilic/Amphipathic

Answer 21

a molecule having both hydrophilic (polar)
and hydrophobic (apolar) parts.

Question 22

Phospholipids are naturally occurring amphipathic molecules. what are they made up of ?

Answer 22

Have a glycerol backbone.
 Two fatty acyl groups.
 Phosphate group.
 Head group.

IMAGE ! Including Examples

Question 23

Cholesterol (6)
function?
fluidity at high temp?
fluidity at low temp?
maintains…
present/ absent where?

Answer 23

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)

Question 24

Labelling and Identifying Cholesterol

Question 25

What does the fluidity of membranes depend on?

Answer 25

• The composition of acyl chains.
• Temperature.

Question 26

Spontaneous Formation of Membranes.
LIPID
SHEETS
DRIVEN
LEADS

Answer 26

• 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.

Question 27

Lipid structures in aqueous environment

DIAGRAM IN BOOK!

Answer 27

Planar Bilayer (Flat)

Micelles

Vesicles/liposomes

Question 28

Movement of lipids in membranes- LATERAL

Answer 28

• 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.

Question 29

Movement of lipids in membranes- TRANSVERSE

Answer 29

• 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

Question 30

Bilayer membranes:

Answer 30

• Two lipid sheets (monolayers or leaflets)
• Polar heads on outside
• Hydrophobic tails inside
• Typically 4-6 nm thick

Question 31

Liposomes/Vesicles

Answer 31

• 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

Question 32

Composition of Biological Membranes.
lipids /carbohydrates/proteins

Answer 32

Lipids:
- 25-50% by mass.
- Phospholipids,
glycosphingolipids,
cholesterol.

Carbohydrates :
- Glycolipids and
glycoproteins.

Proteins:
- 50-75% by mass

Question 33

Are membrane symmetric or asymmetric.

Answer 33

asymmetric

Question 34

——- together with —– define the ——————————————–.

Answer 34

Proteins together with lipids define the characteristics
of a membrane

Question 35

LABEL GLYCOCALYX

Question 36

what are the two types of membrane proteins?

Answer 36

integral
peripheral

Question 37

Integral membrane proteins.
+ STRUCTURE

Answer 37

• Are amphiphilic
• a-helix (recognition, receptors)
• helical bundle ( enzymes, transporters, receptors)
• B- barrel (transporter-channel proteins).

Question 38

Peripheral membrane proteins + STRUCTURE

Answer 38

• amphiphilic a-helix
• Hydrophobic loops
• Lipid modifications/tails
• Electrostatic / ionic interactions

Question 39

A hydrophobic molecule is NOT soluble in:
1. Toluene
2. Saline
3. Hexane
4. Petroleum

Answer 39

Hexane

Question 40

Which ONE of the following molecules is NOT a
component of a prokaryotic plasma membrane?
1.Phosphatidylserine
2.Phosphatidylcholine
3. Cholesterol
4. Proteins

Answer 40

Cholesterol

Question 41

Lipids in bilayers can readily…
1. Change places with their neighbour
2. Leave the bilayer
3. Swap leaflets
4. Exchange headgroups

Answer 41

Change places with their neighbour (movement within a leaflet)

Question 42

Apolar molecules are poorly soluble in water
because of their…
1. size
2. aggregation (CLUSTER TOGETHER)
3. density
4. disturbance of water structure

Answer 42

disturbance of water structure

Question 43

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

Answer 43

Poly-sugar side chains are usually attached to the
cytosolic side of integral membrane proteins

Question 44

saline

Answer 44

Saline is a solution of salt (sodium chloride, NaCl) in water.