Membranes as Permeability Barriers

Question 1

What is a semi-permeable membrane?

Answer 1

A layer or membrane which only allows specific molecules to pass.

Question 2

What kind of molecules can pass the lipid bilayer? Give examples.

Answer 2

Hydrophobic molecules BONCO (O2, CO2, N2, Benzene)

Small uncharged polar molecules HUG (H2O, Urea, Glycerol)

Question 3

What kind of molecules cannot pass the lipid bilayer? Give examples.

Answer 3

Large uncharged polar molecules (Glucose, sucrose)

Ions (H+, Na+, K+, Ca2+ etc…)

Question 4

What is passive transport? What is it dependent on?

Answer 4

Transport that doesn’t need energy. It is also diffused across the membrane so doesn’t rely on a transporter.
It is dependent on the permeability of the membrane and the concentration gradient.

Question 5

What are transport processes important for?

Answer 5

Maintenance of:
ionic composition
intracellular pH
Regulation of cell volume
Concentration of metabolic fuels and building blocks

Question 6

How does voltage-gated ion channels work?

Answer 6

Membrane depolarisation brings conformational change which opens the channel.

Question 7

What’s faster; facilitated diffusion or simple passive diffusion?

Answer 7

Facilitated.

Question 8

What is active transport?

Answer 8

When energy is needed usually up a concentration gradient.

Question 9

Is it active or passive transport when deltaG is positive?

Answer 9

Active transport.

A negative delta G means it’s passive transport.

Question 10

Where is Na+ concentration highest? Intracellular or extracellular? By how much?

Answer 10

Higher extracellular. 145mM vs 12mM

Question 11

Where is Ca2+ concentration highest? Intracellular or extracellular? By how much?

Answer 11

Higher extracellular. 1.5mM vs 100nM

Question 12

Where is Cl- concentration highest? Intracellular or extracellular? By how much?

Answer 12

Higher extracellular. 123 mM vs 4.2mM

Question 13

Where is K+ concentration highest? Intracellular or extracellular? By how much?

Answer 13

Higher intracellular. 155mM vs 4mM.

The only of the four that is higher intracellular is K+!

Question 14

How does PMCA work?

Answer 14

PMCA stands for Plasma membrane calcium ATPase. It’s a pump which uses active transport. ATP hydrolysis is undergone to make ADP and inorganic phosphate. This causes Ca2+ to be pumped out of the cell.

Question 15

What is uniport?

Answer 15

When only one type of ion/molecule is transported in or out.

Question 16

What other types are there?

Answer 16

Symport and antiport

Question 17

What is symport?

Answer 17

When two different molecules or ions are transported in or out via same channel at same time.

Question 18

What is antiport?

Answer 18

When one molecule or ion is transported out, and another molecule or ion is transported in.

Question 19

What is the Na+K+ATPase? How does it work?

Answer 19

It’s called in lay term Sodium pump. It’s an antiport which used ATP to pump 2K+ into the cell and 3Na+ out of the cell. It uses active transport.

Question 20

Why is the Na+K+ATPase important?

Answer 20

It generates the ion gradients that are used to allow secondary active transport and action potentials. However it is only a small contribution to resting membrane potential.

Question 21

What is the Na+Ca2+exchanger dependent on (NCX)?

Answer 21

On the Na+ gradient generated by the K+Na+ATPase.

Question 22

The K+Na+ATPase is used for another sort of antiport as well. Which?

Answer 22

Na+H+ transporter. Na+ gradient generated from Na+K+ATPase is used to transport H+ out of the cell.

Question 23

The K+Na+ATPase is also used for a symport. Which?

Answer 23

The Na+ gradient generated by K+Na+ATPase is used to transport Na+ into the cell again along with glucose.

Question 24

Give examples of Co-transport systems.

Answer 24

Na+K+ATPase
NCX (Na+Ca2+exchange)
Na+H+ exchange
Na+Glucose co-transport

Question 25

Give an example of a clinical setting where this principle is important.

Answer 25

In cystic fibrosis a channel called CFTR doesn’t work which makes Cl- leave the cell to go to the lumen of for example the vas deferens and lumen of pancreas. When Cl- isn’t able to leave there will be a lower concentration of Cl- extracellularly. This means that as much water as should follow the Cl- won’t follow and the vas deferens and lumen of pancreas ends up with less water and thicker mucous for example. Digestive enzymes get stuck in the pancreas or on their ways to the gut and semen gets stuck in the vas deferens rendering one infertile.

Question 26

How do transporters work in diarrhoea?

Answer 26

Protein Kinase A is activated by an infection such as vibrio cholera. This makes the Cl- channel to be stimulated and more Cl- is transported out of the cell into the lumen of the small intestines. This makes water follow.