Lecture 2

Question 1

Micro- and Macro-transfer

Answer 1

Macro – Large scale movement of molecules

• Endocytosis
• Exocytosis

Micro – Smaller scale movement
* Cell membrane transport

Question 2

Vesicular Transport

Answer 2

Vesicular transport is involved in the movement of proteins into (endocytosis) and out of the cell (exocytosis)

Question 3

Resting Membrane Potential

Answer 3

All cells have a resting membrane potential

Determined by distribution of ions across the membrane

Mostly due to sodium and potassium

Related to equilibrium potential, as determined by the Nernst equation

Ions need membrane proteins for transport across the membrane

Question 4

Equilibrium of Ions

Artificial semi-permeable membrane

Answer 4

Only permeable to potassium

Measure charge across membrane

Question 5

The Nernst Equation

Answer 5

Em = Membrane potential
R = Gas constant
T = Temperature in Kelvin
Z = Number of charges on ion
F = Faraday’s number
[X]o = Concentration of ion outside
[X]i = Concentration of ion inside

Question 6

The Nernst equation

Answer 6

Nernst predictions equilibrium membrane potential based on concentration gradient of that ion across the membrane – the Nernst potential

Concentration gradient across a cell membrane maintained by sodium-potassium pump (Na+/K+-ATPase)

Question 7

Goldman-Hodgkin-Katz Equation

Answer 7

Membrane permeable to multiple ions

Membrane potential can still be calculated

Question 8

Resting Membrane Potential

Answer 8

Generated from the asymmetrical distribution of ions:

Particularly Na+ and K+

Differential permeability of the membrane to these ions

Question 9

If permeability of the membrane is higher for K+ than Na+:

Answer 9

• Membrane potential will be closer to equilibrium potential for K+
• About -70 mV (EK = -95 mV)

Non-excitable cells have a K+:Na+ permeability ratio of 2:1

Nerve/muscle cells closer to 25:1

Question 10

Movement Across the Membrane

* Directly through lipid bilayer

Answer 10

Passive (simple) diffusion - obeys Fick’s law of diffusion, e.g. O2 from alveoli to pulmonary capillaries

Question 11

Movement Across the Membrane

* Via integral membrane proteins:

Answer 11

Facilitated diffusion: through pores, channels and carriers (uniports)

Active transport: energy (ATP) required to transport across a membrane

Secondary active transport:

• Co-transporters (symporters): movement of a solute coupled to the movement of another solute down its concentration gradient
• Counter-transporters (antiporters): coupled movement of two or more solutes in opposite directions

Question 12

Passive (Simple) Diffusion

Answer 12

Passive (non-coupled) transport:

• Solute/gas passes down concentration gradient
• At finish, inward flux = outward flux so net flux = 0

Question 13

If a substance can pass through a membrane, it is said to be permeable

Answer 13

Driving force is the electrochemical gradient

Chemical potential energy differences

Differences in charge

Question 14

Explained by the example of passive non-coupled transport:

Answer 14

Movement across a membrane not dependent on another molecule or chemical reaction

Question 15

Transporter Proteins:

Facilitated Diffusion

Answer 15

3 types of transporters that facilitate diffusion:

• Channels (non-gated)
• Gated channels
• Uniporters

All facilitate uncoupled transport of a solute down a concentration gradient

Question 16

Non-gated Channels

Answer 16

Integral membrane proteins that allow direct access to the cell:

• Porins in bacteria, mitochondria, nuclear pore complex
• Aquaporins

Question 17

Gated Channels

Answer 17

ENaC: epithelial sodium channel

K+ channels

Ca2+ channels

Almost all ion channels

Question 18

Functional components

Answer 18

• Gate
• Sensor
• Selectivity filter

Modulated by: voltage, mechanical stimuli, ligand binding

Question 19

Facilitated Diffusion – Carrier Mediated (Uniport)

Answer 19

Include:

• GLUT1 (RBC)
• GLUT2 (intestines)
• GLUT4 (skeletal/cardiac muscle)

Question 20

Facilitated Diffusion – Carrier Mediated (Uniport) Stages

Answer 20

A.	Carrier open
B.	Solute enters and binds
C.	Outer gate closes
D.	Inner gate opens
E.	Solute released
F.	Inner gate closes

Question 21

Saturation of Carrier Mediated Transport

Answer 21

Flux much greater than predicted by passive diffusion

Carrier mediated diffusion exhibits saturation kinetics

Transporters can be inhibited by structural analogues