Lecture 5.1: Cell Membranes

Question 1

Amphipathic Molecules

Answer 1

Contain both a hydrophilic and a hydrophobic moiety

Phospholipids are the primary class

Question 2

Phospholipids

Answer 2

Range of polar head groups (choline, amines, amino acids, sugars)

Fatty acid chains (Length between C14 and C24, C16 and C18 most prevalent, cis double bond introduces a kink)

Hydrophilic phosphate head, hydrophobic lipid tails

Question 3

Phospholipid Head Groups (4)

Answer 3

• Choline
• Serine
• Ethanolamine
• Inositol

Question 4

Glycolipids

Answer 4

Sugar containing lipids
Replace phosphocholine moiety with a sugar = glycolipid

Question 5

Glycolipids: Cerebrosides

Answer 5

Sugar monomer head group

Question 6

Glycolipids: Cerebrosides

Answer 6

Sugar monomer head group

Question 7

Glycolipids: Gangliosides

Answer 7

Oligosaccharide head group

Question 8

Cholesterol: Structure + Function

Answer 8

Polar head group
Rigid planar steroid ring structure
Non-polar hydrocarbon tail
Comprises ca 45 % membrane lipid
Modulates membrane fluidity under physiological conditions

Question 9

Influence of cis double bonds in bilayer structure

Answer 9

Unsaturated hydrocarbon chains with cis double bonds reduce phospholipid packing

Whilst saturated hydrocarbon chains are ‘straight’

Question 10

Phospholipid Motions

Answer 10

Flexion
Rotation
Lateral diffusion
Flip flop (rare)

Question 11

Types of Membrane Proteins (2)

Answer 11

Peripheral
Integral

Question 12

Peripheral Membrane Proteins

Answer 12

• Bound to surface
• Electrostatic and hydrogen bond interactions
• Removed by changes in pH or in ionic strength

Question 13

Integral Membrane Proteins

Answer 13

• Interact extensively with hydrophobic domains of the lipid bilayer
• Cannot be removed by manipulation of pH and ionic strength
• Are removed by agents that compete for non-polar interactions e.g. detergents and organic solvents

Question 14

Membrane Transport Roles (6)

Answer 14

• Maintenance of ionic composition
• Maintenance of intracellular pH
• Regulation of cell volume
• Concentration of metabolic fuels and building blocks
• The extrusion of waste products of metabolism and toxic substances
• The generation of ion gradients necessary for the electrical excitability of nerve and muscle

Question 15

Membranes as Permeability Barriers

Answer 15

Movement across lipid bilayers is restricted to hydrophobic molecules & small uncharged polar molecules

Large uncharged polar molecules and ions require proteins to facilitate their transport across membranes

Question 16

Type of Channel: A pore between cells that allows for electrical and chemical continuity

Answer 16

Gap Junction

Question 17

Type of Channel: A channel that transports one molecule across a membrane

Answer 17

Uniporter

Question 18

Type of Channel: A channel that opens when a chemical binds to a receptor (ligand or agonist binding site)

Answer 18

Ligand Gated

Question 19

Type of Channel: A channel that transports two different molecules in opposite directions

Answer 19

Antiporter

Question 20

Type of Channel: A channel that opens when the potential difference across a membrane changes

Answer 20

Voltage Gated

Question 21

Type of Channel: A channel that transports two different molecules across a membrane in the same direction

Answer 21

Symporter

Question 22

Simple Diffusion

Answer 22

Movement of a molecule across a membrane down a concentration gradient

Question 23

Facilitated Diffusion

Answer 23

Movement of a molecule across a membrane down a concentration gradient through a channel

Question 24

Active Transport

Answer 24

Active transport allows the transport of ions or molecules against an
unfavourable concentration and/or electrical gradient

Energy directly or indirectly from ATP hydrolysis

Question 25

Co-Transport

Answer 25

More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle

Question 26

Membrane Proteins: Functions

Answer 26

• Transporters – control movement across membrane
• Anchors – act as attachment points to membrane
• Recognition – markers for cell-cell interactions
• Glue – junctions connecting cells
• Enzymes – localising metabolic pathways
• Transduction – receptors carry signals into the cell

Question 27

Haemolytic Anaemias: Hereditary Spherocytosis

Answer 27

– Spectrin depleted by 40-50%
– Erythrocytes round up
– Less resistant to lysis
– Cleared by spleen

Question 28

Haemolytic Anaemias: Hereditary Elliptocytosis

Answer 28

– Hereditary Elliptocytosis
– Defect in spectrin molecule
– Unable to form heterotetramers
– Fragile ellipsoid cells
– Similarly 4.1 deficiency (see Yawata et al.)

Question 29

Sodium Pump: what is involved? what blocks it?

Answer 29

Plasma membrane associated pump
Na+, K+, ATPase
Primary transporter (active transport)
25% of Basal Metabolic Rate used for pump
Called a P-type ATPase (ATP phosphorylates Aspartate, producing a phosphoenzyme intermediate)
Blocked by ouabain

Question 30

Secondary Active Transport Systems

Answer 30

• Not ATPases themselves
• Energy indirectly from ATP
• Utilise gradients established by pumps such as Na+-K+-
ATPase
• Co-transport systems