M&R Session 2 (Lecture 2.1)

Question 1

Name examples of hydrophobic molecules. Can they freely diffuse across a bilayer?

Answer 1

O2, CO2, N2, Benzene. Yes

Question 2

Name examples of small uncharged polar molecules. Can they freely diffuse across a bilayer?

Answer 2

H20, Urea, Glycerol. Yes

Question 3

Name examples of large uncharged polar molecules. Can they freely diffuse across a bilayer?

Answer 3

Glucose, Sucrose. No

Question 4

Name examples of ions. Can they freely diffuse across a bilayer?

Answer 4

H+, K+, Na+, Mg2+, Ca2+, Cl-, HCO3-. No

Question 5

What is passive transport dependent on?

Answer 5

Depends on permeability and concentration gradient. (Formula J=P(C1-C2) therefore linear relationship between rate of transport and concentration gradient).

Question 6

What roles do transport processes have?

Answer 6

1) Maintenance of ionic composition
2) The maintenance of ionic composition
3) Regulation of cell volume
4) The concentration of metabolic fuels and building blocks
5) The extrusion of waste products of metabolism and toxic substances
6) The generation of ionic gradients necessary for the electrical excitability of nerve and muscle

Question 7

Why is it unlikely that a charged and/or hydrophilic molecule would cross a membrane?

Answer 7

Thermodynamically unfavourable

Question 8

Why do permeability coefficients vary? The erythrocyte membrane is very permeable to choride ions. Why?

Answer 8

Depends on whether there are any specific protein channels or pores that increase the permeability of membranes to substances.

Band 3 is present (AE) which exchanges HCO3- for Cl-.

Question 9

What are the models for facilitated transport?

Answer 9

1) Protein channels
2) Carrier molecules (ping-pong with conformational change)
3) Protein flip-flop (unfavourable = wrong!!!!)

Question 10

Name the gated pores with an example.

Answer 10

1) Ligand-gated ion channels e.g. nAchR for Na+ (opens upon Ach binding), ATP sensitive K+ channel (closes upon ATP binding).
2) Voltage-gated ion channels e.g. Na+ channel (opens when cell membrane depolarises allowing Na+ into the cell)

Question 11

In what ways are facilitated transport similar to enzymes?

Answer 11

1) Saturable process as each carrier can interact with a finite number of molecules. There are also a limited number of carriers in the membrane so when the concentration gradient increases, a maximum (Vmax) rate is achieved.
2) There is high and low affinity carriers.

Question 12

What determines whether a molecule is transported by passive or active transport?

Answer 12

Free energy change of the transported species. Depends on concentration gradient of the species and membrane potential (more negative=passive)

Question 13

How is ATP used to drive active transport?

Answer 13

Hydrolysis of ATP gaining energy directly or indirectly from it.

Question 14

State the intracellular and extracellular free ion concentrations of Na+, K+, Cl- and Ca2+.

Answer 14

Intracellular:

Na+ [12mM]
K+ [155mM]
Cl- [4.2mM]
Ca2+ [10-7mM]

Extracellular:

Na+ [145mM]
K+ [4mM]
Cl- [123mM]
Ca2+ [1.5mM]

Question 15

What is primary active transport? Give an example.

Answer 15

The energy source (ATP) is hydrolysed and used directly to transport a species against its concentration gradient e.g. Ca2+ ATPase (efflux of Ca2+) e.g. PMCA

Question 16

What is PMCA?

Answer 16

Plasma membrane calcium ATPase

Question 17

What is co-transport?

Answer 17

More than one type of ion or molecule may be transported on a membrane transporter per reaction cycle.

Question 18

What are the different ways in which transportation can take place?

Answer 18

1) Uniport
2) Symport
3) Antiport

Question 19

What is secondary active transport?

Answer 19

Primary energy source is used indirectly to drive the transport of a species down its concentration gradient. e.g. a sodium gradient is used to exchange with other ions

Question 20

What is the Na+ pump? What is it also known as?

Answer 20

Na+-K+-ATPase (exchanges 3 Na+ (efflux) for 2 K+ (influx)) using ATP directly. Also known as a P type ATPase (ATP phosphorylates aspartate on protein)

Question 21

What makes up the Na+ pump?

Answer 21

1) Alpha subunit - K+, Na+, ATP, ouabain binding sites

2) Beta subunit - glycoprotein directs pump to surface.

Question 22

What is the main method of generating membrane potential?

Answer 22

K+ efflux through channels.

Question 23

What does the NCE do?

Answer 23

Exchange 3Na+ (influx) for one Ca2+ (efflux) not using ATP directly.

Question 24

What does the NHE do?

Answer 24

Exchange Na+ (influx) and H+ (efflux) to regulate pH (alkanises a cell). Uses Na gradient generated by Na + pump

Question 25

What does the NGCT do? Where is it found?

Answer 25

Co-transports Na+ and glucose (against concentration gradient) into a cell using the Na+ gradient (symport). Found in kidney and small intestine.