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
