Lecture 5.1: Cell Membranes Flashcards
Amphipathic Molecules
Contain both a hydrophilic and a hydrophobic moiety
Phospholipids are the primary class
Phospholipids
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
Phospholipid Head Groups (4)
• Choline
• Serine
• Ethanolamine
• Inositol
Glycolipids
Sugar containing lipids
Replace phosphocholine moiety with a sugar = glycolipid
Glycolipids: Cerebrosides
Sugar monomer head group
Glycolipids: Cerebrosides
Sugar monomer head group
Glycolipids: Gangliosides
Oligosaccharide head group
Cholesterol: Structure + Function
Polar head group
Rigid planar steroid ring structure
Non-polar hydrocarbon tail
Comprises ca 45 % membrane lipid
Modulates membrane fluidity under physiological conditions
Influence of cis double bonds in bilayer structure
Unsaturated hydrocarbon chains with cis double bonds reduce phospholipid packing
Whilst saturated hydrocarbon chains are ‘straight’
Phospholipid Motions
Flexion
Rotation
Lateral diffusion
Flip flop (rare)
Types of Membrane Proteins (2)
Peripheral
Integral
Peripheral Membrane Proteins
• Bound to surface
• Electrostatic and hydrogen bond interactions
• Removed by changes in pH or in ionic strength
Integral Membrane Proteins
• 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
Membrane Transport Roles (6)
• 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
Membranes as Permeability Barriers
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
Type of Channel: A pore between cells that allows for electrical and chemical continuity
Gap Junction
Type of Channel: A channel that transports one molecule across a membrane
Uniporter
Type of Channel: A channel that opens when a chemical binds to a receptor (ligand or agonist binding site)
Ligand Gated
Type of Channel: A channel that transports two different molecules in opposite directions
Antiporter
Type of Channel: A channel that opens when the potential difference across a membrane changes
Voltage Gated
Type of Channel: A channel that transports two different molecules across a membrane in the same direction
Symporter
Simple Diffusion
Movement of a molecule across a membrane down a concentration gradient
Facilitated Diffusion
Movement of a molecule across a membrane down a concentration gradient through a channel
Active Transport
Active transport allows the transport of ions or molecules against an
unfavourable concentration and/or electrical gradient
Energy directly or indirectly from ATP hydrolysis
Co-Transport
More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle
Membrane Proteins: Functions
• 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
Haemolytic Anaemias: Hereditary Spherocytosis
– Spectrin depleted by 40-50%
– Erythrocytes round up
– Less resistant to lysis
– Cleared by spleen
Haemolytic Anaemias: Hereditary Elliptocytosis
– Hereditary Elliptocytosis
– Defect in spectrin molecule
– Unable to form heterotetramers
– Fragile ellipsoid cells
– Similarly 4.1 deficiency (see Yawata et al.)
Sodium Pump: what is involved? what blocks it?
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
Secondary Active Transport Systems
• Not ATPases themselves
• Energy indirectly from ATP
• Utilise gradients established by pumps such as Na+-K+-
ATPase
• Co-transport systems
