Lecture 2.1

Question 0

How does the rate of passive transport relate to concentration gradient?

Answer 0

Rate increases linearly with increasing concentration gradient

Question 1

What does the process of passive diffusion depend on?

Answer 1

Permeability of membrane to an ion and the concentration gradient of that ion

Question 2

What is the permeability coefficients for ions and hydrophilic molecules?

Answer 2

Very low at around 10 to the minus 10 cm per second

Question 3

Are membranes permeable to water? What is the permeability coefficient for water?

Answer 3

Surprisingly yes with a permeability coefficient of 5 x 10 to the minus 3 cm per second

Question 4

How does water cross a lipid bilayer?

Answer 4

Through passive diffusion against the concentration gradient of solute - known as the osmotic gradient. In some tissues such as kidney proximal tissues, the movement of water across some membranes is facilitated by aquaporins

Question 5

How do membranes act as permeability barriers?

Answer 5

Don’t allow the free passing of ions/hydrophilic molecules as they would require a lot of free energy to pass the hydrophobic region of the membrane. Instead, their movement is mediated and regulated.

Question 6

What are the transport of ions and hydrophilic molecules important for?

Answer 6

1. Controlling Intracellular pH
2. Controlling ionic concentrations
3. Control cell volume
4. Concentration of metabolic fuels and building blocks
5. Extrusion of waste products of metabolism and toxic substances
6. Generation of ion gradients needed for electrical excitability of cells

Question 7

How can we increase the permeability of a membrane to an ion?

Answer 7

By altering the presence of specific proteins in a membrane

Question 8

Give an example of where the presence of a protein increases a membrane’s permeability to an ion. Hint: erythrocyte membrane

Answer 8

The phosphopatidylserine membrane is not very permeable to chloride ions but the presence of band 3 protein in the erythrocyte membrane increases the transport of chloride ions by 10 to the 7 fold respectively. It does this by not only forming a chloride selective pore (channel) but also by exchanging chloride for hydrogen carbonate ions - essential to the function of an erythrocyte

Question 9

What models of facilitated diffusion are there?

Answer 9

1. Protein pores i.e. channels - can be voltage or ligand gated
2. Carrier molecules - ping pong transport
3. Protein flip flop or rotating protein carrier - both of which are thermodynamically unfavourable

Question 10

What is the difference between protein carrier flip flop and protein carrier rotation?

Answer 10

With flip flop, the carrier only sits on one side of the membrane and flips to the other side. With rotating carrier, the carrier sits across the whole membrane and simply rotates 180 degrees

Question 11

Why is facilitated diffusion a saturable process?

Answer 11

1. Each transporter can only deal with one or very few ions at a time
2. There are a finite number of one type of transporters in each membrane

Question 12

What happens when all the transporters in a membrane are busy?

Answer 12

As the concentration gradient increases, a maximum rate of transport will be reached. At this point, facilitated diffusion becomes a saturated process.

Question 13

Can facilitated diffusion alter the equilibrium point for a transported spices?

Answer 13

No.

Question 14

Name the three types of gated transporters.

Answer 14

1. Ligand gated
2. Voltage gated
3. Gap junctions aka connexins - close when cellular [calcium] > 10 micrometer or cell becomes acidic

Question 15

What is the difference between active and passive transport?

Answer 15

Passive transport occurs spontaneously, down the concentration gradient of the transported species (-ve delta G)
Active transport requires energy and occurs against the concentration gradient of the transported species through a carrier (+ve delta G)

Question 16

What process are simple and facilitated diffusion types of? By which means do simple and facilitated diffusion occur?

Answer 16

Passive transport.
Simple diffusion - molecule diffuses through membrane without assistance or through a channel.
Facilitated diffusion - molecule diffuses through a carrier in the membrane

Question 17

What determines whether a molecule undergoes passive or active transport across a membrane?

Answer 17

1. Concentration gradient of the molecule

2. Electrical potential across the membrane i.e. MP if the transported molecule is charged

Question 18

How is transport across an unfavourable chemical or electrical gradient overcome in active transport?

Answer 18

By coupling to a thermodynamically favourable reaction

Question 19

Where does the energy required for active transport come from?

Answer 19

Directly or indirectly from ATP, electron transport or light

Question 20

What is secondary active transport?

Answer 20

Active transport of an molecule that is linked to the concentration gradient of another molecule via a co-transporter. Indirectly uses the hydrolysis of ATP.

Question 21

What type of ionic gradient is usually employed across a membrane in secondary active transport?

Answer 21

Sodium

Question 22

Name 3 co transport systems. What type of transporters are they?

Answer 22

1. Sodium-glucose in kidneys and small intestine. Inward movement of sodium provides energy for inward movement of glucose. Symport transporter.
2. NCX - inward movement of sodium down its concentration gradient provides the energy for the outward movement of calcium up its concentration gradient. Anti port transporter.
3. Sodium-hydrogen exchanger - inward flow of sodium down concentration gradient causes outward flow of hydrogen - results in cell alkalinisation. Anti port.

Question 23

What is a uniport transporter?

Answer 23

A transporter that moves a speices from side of a membrane to the other

Question 24

What are co transporters? What are the two types that they exist as?

Answer 24

Co transporters - the transport of one molecule depends on the simultaneous or subsequent transport of another molecule.
Symport - both molecules are transported in the same direction
Anti port - each molecule is transported in different directions

Question 25

What type of molecules can cross a lipid bilayer? Give examples.

Answer 25

Hydrophobic molecules - oxygen, carbon dioxide, nitrogen, benzene
Small, uncharged molecules - water, urea, glycerol

Question 26

What molecules are a lipid bilayer not permeable to?

Answer 26

Large charged molecules - glucose and sucrose

Ions

Question 27

Is a permeability coefficient of 10 to the minus two high or low?

Answer 27

High permeability. Would expect it of a hydrophobic or small uncharged molecule.

Question 28

What is the structure of the sodium pump (Na+-K+-ATPase)?

Answer 28

It is a P-type ATPase as it phosphorylates aspartate to produce a phosphoenzyme intermediate. It’s made of two subunits (alpha and beta.
Alpha subunit - has binding sites for sodium, potassium, ATP and ouabain
Beta subunit - contains glycoproteins which direct the pump to the surface

Question 29

What is the involvement of the sodium pump in generating the resting MP?

Answer 29

Resting membrane potential isn’t generated by the sodium pump - this is down to the passive diffusion of potassium down the concentration gradient down its channels. The sodium pump creates high Intracellular potassium but this only generates about 5-10mV through the electrogenic pump.