Week 2: Cells and Membranes Flashcards
Lipid role in cell membrane
- 50% of mass of membrane
- 20% of lipids in mem = cholesterol
- Other lipids play roles in signalling and cell recognition
Cholesterol’s role in membrane
Key determinant of membrane fluidity
* High temp: stabilise cell membrane and increase its MP
* Low temp: inserts into phospholipids and prevents them interfering with each other to avoid aggregation (cluster)
What would cell membrane be without cholesterol?
Cold: rigid, not as fluid/flexible and may break
Hot: too fluid and will not hold shape
What are integral membrane proteins?
- Embedded in membrane (transmembrane or monotopic)
- Exposed to aq environment on one or both sides of membrane
- Section in the bilayer = hydrophobic. Parts that stick out = hydrophilic
What are peripheral membrane proteins?
Located on surface of membrane (inside or out)
Associated with a membrane through interactions with other macromolecules
What are lipid linked membrane proteins?
- Attached to lipids that are embedded within bilayer
- Include long-chain acyl or prenyl groups, GPI and cholesterol
- An individual protein may have more than one lipid anchor attached
- Lipid association with proteins can be reversible/irreversible
What is the fluid mosaic model?
mixture of lipids and intrinsic proteins in the membrane.
What is the “semi-permeable” membrane?
- Serve as barrier/gatekeeper to selectively regulate transport entering and exiting cell.
- Semi-permeable: some molecules can diffuse across, others cannot and need different mechanisms
Diffusion through membrane
- Certain molecules can pass
- Movement from high conc to low. Can be bidirectional.
- Depends on size, temp, and NO ENERGY
What can pass membrane through passive diffusion?
Rapidly: small, nonpolar materials can move through bilayer quickly. E.g: oxygen and CO2
More slowly: small polar molecules but more slowly. E.g: water and ethanol
What can’t pass the membrane on its own?
- Highly charged molecules, like ions
- Large molecules (sugars, a.a…)
- They rely on specific transport proteins embedded in membrane
What is facilitated diffusion?
- Polar and charged molecules cross the membrane using proteins in membrane (carrier or channel)
- No energy required
- Each protein transports a particular class of molecule
- Direction determined by conc gradient or electrical potential (charged molecules)
What are carrier proteins?
Binds the specific solute to be transported and undergoes conformation change
(solute-binding site is sequentially accessible on one side of bilayer then the other)
What are channel proteins?
- Mediate molecules through aq diffusion pore
- Selective, will accept only 1 type of molecule
- Need to be appropriate size and charge
What are the carrier protein subtypes?
- Uniporter: one type of molecule, one direction across mem
- Symporter: 2+ molecules, one direction
- Antiporter: 2+ molecules, in opposite directions
How does glucose travel across the membrane?
Glucose transporter
* 12-alpha helical transmembrane protein
* mainly hydrophobic amino acids to interact with lipid membrane
* some polar a.a residues that form glucose-binding site in interior of protein
What are aquaporins?
- Channel transport water across membrane in response to osmotic gradients created by active solute transport
- 6 transmembrane helices that selectively allow water/other small uncharged molecules to pass along osmotic gradient.
What are ungated channel proteins?
- Always open. No gated mechanism
- Allow Na+ to slowly move into cell or K+ to slowly move out
- Diffusion along conc gradient
What are voltage gated channels?
- Electrical stimuli opens gate
- Respond to small changes in membrane potential
- ion specific
- Located on a range of different cells
What are the functions of voltage gated channels?
- VG Na+ and K+ channels generate AP that produce nerve impulse in neurones; contraction in muscles
- VG Ca2+ allow entry of Ca2+ into cytoplsm, which acts as a second messenger and initiates number of events. e.g: contraction of cardiac and smooth muscle…
What are ligand-gated channels
- ion-channel proteins open to allow ions to pass in response to the binding of a chemical messenger (e.g: neurotransmitter)
- less selective, allows 2+ ion types through
- ligand binds to a site, distinct from ion pore (orthosteric site)
- Binding causes structural modifications, which change permeability of ion channel
Examples of ligand-gated channels
- Glutamate and ACh bind to receptors, opens the channels
- Depolarisation of postsynaptic membrane
- leads to initiation of AP
What are mechanically-gated channels?
- Conversion of physical forces into biochemical signals (e.g: vibration, sound)
- Mechanosensitive channels respond to membrane tension by altering conformation between open and closed
What is primary active transport?
Uses ATP to transport solutes across membrane against their conc gradient
Explain Na+ K+ pump
- pump is open on intracellular side (space for 3 Na)
- Na binds, triggering pump to hydrolyse ATP
- One P from ATP binds to pump
- Pump undergoes conformational change, opening to extracellular space and release 3 Na
- Switches affinity to 2 K ions
- Triggers removal of P and leads to conformational change back to original form. Opening inwards, releasing K ions
What is secondary active transport?
- Uses established gradients to transport other molecules in opposite direction
- Transporter protein couples the movement of an ion (Na+ or H+) down its electrochemical gradient to the movement of another molecule/ion against a conc/elec gradient
- Energy stored in electrochemical gradient of driving ion is used to drive transport of another solute against conc/elec gradient
- Both driving and driven species must be bound to transporter for translocation to occur
What are channelopathies?
Describes various disorders that occur due to defects in ion channel function.
Includes: ion channel mutation, and disorders caused by autoimmune attacks on ion channels
What are some examples of channelopathies?
- NS diseases
- Cardio system (long/short QT syndrome)
- Respiratory (cystic fibrosis)
- Endocrine (neonatal diabetes mellitus…)
- Urinary system (autosomal-dominant polycystic kidney disease)
- Immune system (myasthenia gravis)
