Membrane Physiology Wk1

Question 1

What does the cell membrane do? (4)

Answer 1

• Defines boundaries of the cell
• Encloses cell’s organelles
• Enables cell to create an internal environment that promotes the normal functions of the cell
• Creates an internal environment (intracellular compartment allowing for cell to thrive) that is different from that of the outside of the cell

Question 2

What does the cell membrane consist of? (5)

Answer 2

• Cell membranes are composed of closely opposed two layers of phospholipids
• Each unit has a hydrophilic head and a hydrophobic tail
• Hydrophobic tails align deep within surface of membrane
• Hydrophilic heads form outer surfaces of the membrane
• This means the cell membrane is impermeable to any hydrophilic particles

Question 3

What do the structural features of the cell membrane produce? (3)

Answer 3

• Intracellular compartment - compartment within confines of the cell membrane
• Extracellular compartment - compartment outside the confines of the cell membrane
• The cell membrane is able to influence the composition of the intracellular compartment, but not the extracellular compartment because it is much larger

Question 4

What quality does both extra and intracellular compartments have? (1)

Answer 4

Largely aqueous environment

Question 5

How does the cell membrane establish a resting membrane potential? (5)

Answer 5

• Selectively concentrated various combinations of species of charge-carrying particles within the intracellular compartment: organelles of the cell, proteins, defined relative concentrations of combinations of specific species of anions and cations
• This results in concentration difference of species of charge-carrying ions between compartments
• Since particles carry electrical charge, the concentration differences between compartments then give rise to an electrical potential between the compartments. (Each cell’s membrane creates a different and individual environment).
• The overall effect of difference in charge carrying ions is the establishment of electrical potential between compartments - this is known as standing electro-chemical gradient between compartments
• Electro-chemical gradient = ‘resting membrane potential’

Question 6

What is the resting membrane potential of nerve cells?

Answer 6

• At rest, the inside is negatively charged with respect to the outside of the cell
• The resting membrane potential of nerve cells is around -65mV to -70mV with respect to outside of cell
• RMP will vary between cell types, but outside is always negatively charged

Question 7

What is a defining characteristic of electrically excitable tissues?(1)

Answer 7

They are able to momentarily discharge the standing electrical potential between the compartments

Question 8

What is the process of death characterised by? (1)

Answer 8

Permanent discharge of resting membrane potentials of electrically excitable tissues

Question 9

How do cells create an intracellular compartment?(7)

Answer 9

• By having cell membranes that are selectively permeable to movements of certain particles and not others
• Some particles e.g. ions are able to cross the cell membrane
• Some particles e.g. proteins are not able to cross the cell membrane
• Factors that determine this: size of particles, electrical charge on the particles, whether particles are recognised by specialised transport systems that cross the cell membrane or not
• Transport ATPases (ion pumps) are able to identify what needs to move in or out of cells
• Presence of ion channels - selective ion channels (facilitate movement of ions) and non-selective ion channels (not the exact same as leakage channels)
• Selectively permeable membrane

Question 10

What are ion channels? (8)

Answer 10

• Protein assemblies that are lodged within the substance of the cell membrane
• These span the full thickness of the cell membrane: also known as trans-membrane spanning proteins
• They have water-filled central pore that passes from inside to outside of cell (or vice versa)
• They facilitate passive movement of ions across the cell membrane in both directions
• Some ion channels are selective as to which ions they conduct through their pores
• Other ion channels are less selective in terms of which ions they will conduct
• Port is normally filled with water to allow movement of water - impacts movements of ions
• Phospholipid bilayer formed

Question 11

Features of a typical ion channel

Answer 11

• Protein assemblies
• Central pore (cavity)
• Selectivity cavity
• Hydration cavity

Question 12

What differentiates between ion channels?

Answer 12

• Which species of ion they conduct
• How their conductance of ions is governed
• Gating mechanisms of the ion channel

Question 13

What are the most commonly studied ion channels?

Answer 13

• Voltage gated sodium channels
• Voltage gated potassium channels
• Ligand gated ion channels (most common ligands are neurotransmitters e.g. ACh)
• Mechanically-gated ion channels
• Non-gated ion channels
• Leakage channels

Question 14

What are ion pumps?

Answer 14

• Found lodged within the cell membrane
• Main function is to maintain the resting membrane potential
• They do not set up the resting membrane potential
• They require energy in the form of ATP to maintain the RMP
• Some are known as ion-exchange pumps
• Some poisons specifically target these ion pumps
  1. ATP ==> ADP + Pi
  2. Pi causes Na+/K+ channel to open
  3. 3 Na+ leaves and 2 K+ enter the cell