Membranes and Receptors 2

Question 1

What is passive diffusion?

Answer 1

The simple diffusion of molecules across the hydrophobic domain of the lipid bilayer down their concentration gradient, a non-energy requiring process.

Question 2

What is facilitated diffusion?

Answer 2

Passive diffusion of molecules utilising carrier or channel proteins.

Question 3

What is active transport?

Answer 3

Transport of molecule across a membrane which required energy due transporting the molecule against a concentration and/or electrical potential gradient.

Question 4

Define symport:

Answer 4

A co-transporter which transports different species of solute in the same direction across a membrane.

Question 5

Define antiport:

Answer 5

A co-transporter which transports different species of solute in opposite directions across a membrane.

Question 6

How is glucose transported into cells?

Answer 6

Secondary active transport (utilising a Na+ gradient set up by an Na pump on the basal membrane) of Na+ and glucose through a symport (SGLT-1) into lumenal cells. From there the glucose can diffuse through GLUT-2 (transport protein on basal membrane) to the bloodstream. This also occurs in the kidneys.

Question 7

How are amino acids transported into cells?

Answer 7

Like glucose, amino acids use secondary active transport across a symport with Na+(using the Na+ gradient) to be transported into cells.

Question 8

How are ions transported into cells?

Answer 8

Channels or carrier proteins = facilitated diffusion

Question 9

How is cytoplasmic pH regulated?

Answer 9

It is held at a set point by the antagonistic effects of Na+-H+ exchange and Cl-HCO3- exchange proteins.

Question 10

What properties of solutes affect their movement through membranes?

Answer 10

Charge, polarity, size

Question 11

What are the general features of channel proteins?

Answer 11

a

Question 12

Define a uniport transporter:

Answer 12

A transporter which transports a single solute species from one side of a membrane to another.

Question 13

Define cotransport:

Answer 13

A transporter which transports more than one species across a membrane simultaneously or sequentially. These can be symporters (both in same direction) or antiporters (opposite directions).

Question 14

What is the relationship between the rate of passive diffusion and the concentration gradient?

Answer 14

Linear - as the concentration increases the rate of passive diffusion increases.

Question 15

What molecules passively diffuse into cells?

Answer 15

Non-polar molecules, water (osmosis)

Question 16

In some cells the transport of water is facilitated by specific water channels. What is the name of this type of water channel that is found in the kidney proximal tubules?

Answer 16

Aquaporins

Question 17

Why are many hydrophilic molecules and ions unable to passively diffuse across the lipid bilayer?

Answer 17

Tranversing the lipid bilayer requires a large free-energy change and therefore occurs very rarely. Consequently their permeability coefficients are very low. The exception to this is water which is able to passively diffuse and has a higher permeability coefficient.

Question 18

List some of the roles that specific membrane transport systems have:

Answer 18

1. Maintaining intracellular pH
2. Maintaining ionic composition
3. Regulating cell volume
4. Regulating concentration of metabolic fuels and building blocks
5. Extruding waste products and toxins
6. Generation of ionic gradients necessary for the electrical excitability of nerves and muscle

Question 19

What affects the permeability of a cell’s membrane to a polar molecule?

Answer 19

The concentration of proteins that are capable of transporting the polar molecule by facilitated transport.

Question 20

What is the role of Band 3 protein in erythrocytes?

Answer 20

It is an anion exchanger in the erythrocyte membrane (amongst other cells). It exchanges Cl- for HCO3-.

Question 21

What are the three theoretical models for facilitated transport?

Answer 21

1. Protein pores (channels)
2. Carrier molecules (ping-pong)
3. Protein flip-flop (unlikely thermodynamically)

Question 22

Describe three types of gated channels

Answer 22

1. Ligand gated ion channels - open or close in response to a ligand binding to a receptor site.
2. Voltage gated ion channels - open or close in response to a change in potential difference across a membrane.
3. Gap junctions (connexin) - closed when the cellular calcium concentration goes above 10 micromolar or the cell becomes acidic.

Question 23

What is the difference between active transport and passive transport?

Answer 23

Active transport of a molecule across a membrane requires energy whereas passive transport is spontaneous. The free energy change of a molecule across a membrane is determined by the transported species’ concentration gradient and (when the species is charged) the electrical potential across the membrane.

Question 24

How is energetically unfavourable transport (against a concentration and/or electrical potential gradient) acheived?

Answer 24

By coupling to an energetically favourable process, either:

1. Directly via use of energy from ATP, electron transport or light
2. Indirectly via use of above sources of free energy

Question 25

What is secondary active transport?

Answer 25

When the transport of one substance is linked to the concentration of another (which has been set up by active transport).

Question 26

List gradients of substances that membrane transporters may utilise for secondary active transport

Answer 26

ATP, phosphenolpyruvate, Na+, H+, light and high-potential electrons.

Question 27

What is the role of the Na+/K+ pump?

Answer 27

Sets up Na+ and K+ gradients.

Question 28

What secondary active transport processes does the Na+/K+ pump enable?

Answer 28

1. Ion homoeostasis
2. Intracellular Ca2+ ion concentration
3. Intracellular pH
4. Cell volume
5. Ion gradients underpinning resting membrane potential
6. Nutrient uptake

Question 29

Give examples of hydrophobic molecules that can diffuse across the lipid bilayer

Answer 29

oxygen, carbon dioxide, nitrogen, benzene and steroid hormones

Question 30

Give examples of small uncharged polar molecules that can diffuse across the lipid bilayer

Answer 30

water, urea and glycerol

Question 31

Give examples of large, uncharged polar molecules that cannot diffuse across the lipid bilayer

Answer 31

glucose

sucrose (galactose-glucose dimer)

Question 32

Give examples of ions that cannot diffuse across the lipid bilayer

Answer 32

H+, Na+ etc… all of them!

Question 33

What does passive transport depend upon?

Answer 33

Permeability and concentration gradient

Question 34

How much of the basal metabolic rate of a cell is spent on the sodium pump?

Answer 34

Approximately 25%!

Question 35

What is the main thing that is responsible for setting up the resting membrane potential?

Answer 35

K+ diffusion through channels NOT the Na pump which generates ~-5-10mV (cf to -70mV or resting membrane potential).

Question 36

What transporter is affected in cystic fibrosis?

Answer 36

A chloride transporter named CFTR.

Question 37

How does the toxin produced by cholera cause diarrhoea?

Answer 37

It binds to G proteins and prevents them catalysing GTP -> GDP and Pi, therefore they remain active for longer. This means that higher levels of cAMP are produced and PKA is over-activated. PKA phosphorylates CFTR and leads to ATP-mediated efflux of chloride ions into intestinal lumens, drawing out water and causing diarrhoea.

Question 38

Why is maintaining the Ca2+ gradient so important?

Answer 38

High Ca2+ concentration are toxic to the cell. Small changes in intracellular Ca2+ concentration are used for cell signalling.