Week 2: Cells and Membranes

Question 1

Lipid role in cell membrane

Answer 1

• 50% of mass of membrane
• 20% of lipids in mem = cholesterol
• Other lipids play roles in signalling and cell recognition

Question 2

Cholesterol’s role in membrane

Answer 2

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)

Question 3

What would cell membrane be without cholesterol?

Answer 3

Cold: rigid, not as fluid/flexible and may break
Hot: too fluid and will not hold shape

Question 4

What are integral membrane proteins?

Answer 4

• 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

Question 5

What are peripheral membrane proteins?

Answer 5

Located on surface of membrane (inside or out)
Associated with a membrane through interactions with other macromolecules

Question 6

What are lipid linked membrane proteins?

Answer 6

• 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

Question 7

What is the fluid mosaic model?

Answer 7

mixture of lipids and intrinsic proteins in the membrane.

Question 8

What is the “semi-permeable” membrane?

Answer 8

• 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

Question 9

Diffusion through membrane

Answer 9

• Certain molecules can pass
• Movement from high conc to low. Can be bidirectional.
• Depends on size, temp, and NO ENERGY

Question 10

What can pass membrane through passive diffusion?

Answer 10

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

Question 11

What can’t pass the membrane on its own?

Answer 11

• Highly charged molecules, like ions
• Large molecules (sugars, a.a…)
• They rely on specific transport proteins embedded in membrane

Question 12

What is facilitated diffusion?

Answer 12

• 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)

Question 13

What are carrier proteins?

Answer 13

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)

Question 14

What are channel proteins?

Answer 14

• Mediate molecules through aq diffusion pore
• Selective, will accept only 1 type of molecule
• Need to be appropriate size and charge

Question 15

What are the carrier protein subtypes?

Answer 15

1. Uniporter: one type of molecule, one direction across mem
2. Symporter: 2+ molecules, one direction
3. Antiporter: 2+ molecules, in opposite directions

Question 16

How does glucose travel across the membrane?

Answer 16

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

Question 17

What are aquaporins?

Answer 17

• 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.

Question 18

What are ungated channel proteins?

Answer 18

• Always open. No gated mechanism
• Allow Na+ to slowly move into cell or K+ to slowly move out
• Diffusion along conc gradient

Question 19

What are voltage gated channels?

Answer 19

• Electrical stimuli opens gate
• Respond to small changes in membrane potential
• ion specific
• Located on a range of different cells

Question 20

What are the functions of voltage gated channels?

Answer 20

• 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…

Question 21

What are ligand-gated channels

Answer 21

• 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

Question 22

Examples of ligand-gated channels

Answer 22

• Glutamate and ACh bind to receptors, opens the channels
• Depolarisation of postsynaptic membrane
• leads to initiation of AP

Question 23

What are mechanically-gated channels?

Answer 23

• Conversion of physical forces into biochemical signals (e.g: vibration, sound)
• Mechanosensitive channels respond to membrane tension by altering conformation between open and closed

Question 24

What is primary active transport?

Answer 24

Uses ATP to transport solutes across membrane against their conc gradient

Question 25

Explain Na+ K+ pump

Answer 25

1. pump is open on intracellular side (space for 3 Na)
2. Na binds, triggering pump to hydrolyse ATP
3. One P from ATP binds to pump
4. Pump undergoes conformational change, opening to extracellular space and release 3 Na
5. Switches affinity to 2 K ions
6. Triggers removal of P and leads to conformational change back to original form. Opening inwards, releasing K ions

Question 26

What is secondary active transport?

Answer 26

• 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

Question 27

What are channelopathies?

Answer 27

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

Question 28

What are some examples of channelopathies?

Answer 28

• 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)