Session 1&2: cell membrane and transport

Question 1

what are the different “compartments” within cells?

Answer 1

• plasma membrane
• endoplasmic reticulum
• golgi apparatus
• nuclear envelope

Question 2

What is the disadvantage of having compartments?

Answer 2

Once you set up a compartment/barrier, you have to find a way to get through/around the barrier. This can be done using energy or transport mechanisms.

Question 3

What is the basic structure of the cell membrane?

Answer 3

bilayer of phospholipids
- hydrophilic phosphate heads (facing outward)
- hydrophobic lipid tails (facing inwards)
proteins
- partially embedded/surface proteins
- transmembrane (partially/fully across the membrane

Question 4

How do transmembrane proteins work when we have a cut?

Answer 4

transmembrane proteins can stick to each other and cause blood clots

Question 5

Because of the lipids lining the cell membrane, what do biochemical properties of molecules influence?

Answer 5

• which proteins can insert
• which drugs can diffuse across
• protein movement within plane of membrane
• viscosity of bilayer

Question 6

What is the locational difference of protein regions that are hydrophilic vs hydrophobic?

Answer 6

hydrophobic regions of proteins are found within the inside (hydrophobic) envrionment of the bilayer.
hydrophilic regions of proteins are found on the outside of the bilayer.

Question 7

What are membrane protein functions?

Answer 7

• selective transport: ion channels, drug transporters
• receptors: peptide hormones, growth factors, cell-cell attachment for tissue structure, platelet clotting, transmitting signals across membrane

Question 8

Explain the meaning of “fluid mosaic” model

Answer 8

“fluid” - corresponds to the adjustable/malleable organization of the membrane because of the varying lipids.
“mosaic” - has to do with the varying transmembrane proteins and lipids that can be associated with the bilayer (creates a mosaic appearance).

Question 9

Do lipids in the bilayer move or are they stationary?

Answer 9

Lipids in the bilayer can move within the plane of the membrane

Question 10

How do lipids vary in biochemical characteristics? What is affected?

Answer 10

Some lipids are more mobile and have a fluid-like membrane phase –> dynamic.
Some lipids are less mobile and are gel-like membrane phase –> less dynamic.

Question 11

How are phase transitions controlled?

Answer 11

Cholesterol and glycolipids inhibit transitions –> less dynamic
- cholesterol helps transition from gel to fluid: vice versa

Question 12

What is charged inside the cell and draws in water?

Answer 12

• proteins
• sugars
• salt ions
  (ion transport = water transport = more ion transport)

Question 13

Membrane Integrity: The cytoplasm is full of charged stuff that attracts water. If the migration of water and ions isn’t balanced, what will happen? How can we fix it?

Answer 13

Result: the cell will burst from overflow of water.

Solution: regulate intracellular ions via transport mechanisms (channels, diffusion, and pumps)

• maintain optimal biochemical environment (bc ion concentration affects protein structure and enzyme function)
• maintain osmotic pressure

Question 14

Membrane Transport: How do ion channels determine selectivity?

Answer 14

Ion channels are highly selective
(REMEMBER: ions are hydrated with water molecules)
Selectivity dependent on:
- ion size
- channel shape
- diameter
- interior charges of the channel

Question 15

Explain how the size and charge of a hydrated ion have an impact on how the ion will move through the channel? Use Na+ and K+ as an example.

Answer 15

In a Na+ channel, since the K+ ion is significantly larger than the Na+ ion, K+ is low efficiency bc it is too large to move through a Na+ channel.

Question 16

How is selectivity of a channel generated? (What effects channel selectivity?)

Answer 16

• ion size
• channel shape, diameter, and interior changes (comes from channel protein amino acid side chains)

Question 17

What is diffusion? How does it work?

Answer 17

Diffusion is random molecular movement in a gas or liquid background that results in PASSIVE transport from areas of high to low concentration.
- Simple and facilitated diffusion

Question 18

What is the difference between simple and facilitated diffusion?

Answer 18

simple diffusion is through a membrane protein channel (H2O soluble) or intermolecular spaces of the bilayer (lipid soluble - O2)
*WITHOUT solute-protein interaction

facilitated diffusion REQUIRES interaction of the solute with a porter protein which then aids to transport the solute across the membrane (3 types)

Question 19

facilitated diffusion: what are the three types of porter proteins?

Answer 19

uniporter - transports 1 solute
symporter - transports 2 solutes at the same time in the same direction
antiporter - 2 co-transported solutes in opposite directions

Question 20

What factors effect the rate of SIMPLE diffusion thru channel or membrane?

Answer 20

• concentration of the solute
• kinetic motion, heat (not a big deal bc of the body’s stable body temp.)
• number/sizes of pores in the membrane
• the solute’s lipid solubility (O2 is high solubility across membrane; H2O is low solubility across membrane and high diffusion thru channels)

Question 21

What is the main differences (2) between facilitated diffusion and simple diffusion?

Answer 21

1) facilitated diffusion requires interaction with the carrier protein
2) The rate of simple diffusion increases as the concentration increases linearly. Whereas, facilitated diffusion plateaus after the carrier proteins reach max saturation.
Max capacity of carrier proteins = Vmax

Question 22

The rate of facilitated diffusion is dependent on…

Answer 22

• the rate of protein shape change (after binding molecule to carrier)
• the number of carrier proteins

Question 23

What makes pumps different from diffusion?

Answer 23

Pumps are a non-diffusion transport mechanism that uses ENERGY to move ions or other substances in combination with a CARRIER PROTEIN AGAINST the concentration gradient.
- 2 types

Question 24

Explain the two types of active transport

Answer 24

primary - energy from ATP breakdown (Na+/K+ pump)

secondary - energy derived from stored ionic differences across membrane caused by primary transport or other ion gradient (glucose/Na+ symporter)

Question 25

Explain the digitalis therapy.

Answer 25

• Digitalis binds to the Na+/K+ pump and inhibits it.
• In the muscle cell, there is the Na+/K+ pump as well as the Na+/Ca2+ pump. The Na/Ca pump will only pump Na IN the muscle cell if there is a LOW relative concentration IN the cell (from the Na/K pump).
• When the K+ isn’t able to bind (bc of digitalis), the Na/K pump doesn’t function properly and therefore the Na/Ca exchanger also doesn’t function.
• There is going to be an INCREASED concentration of Na+ IN the cell that will compete with Ca2+ and get pushed OUT of the cell while Ca2+ is exchanged IN.
• This results in an excess amount of Ca2+ INSIDE the muscle cell which causes a STRONGER contraction of the muscle.

Question 26

Explain the Na+/K+ pump as a primary active transporter

Answer 26

• At the inner cell surface, there are 3 sites for Na+ binding and domain with ATPase function.
  (Na+ binding causes conformational change that allows release of 3 Na+ entry for 2 K+)
• At the outer cell surface, there are 2 sites for K+ binding.
  More pumps are found in areas that need repolarization (heart cell/neuron) and less in areas like RBCs.

Question 27

Explain the steps of the Na+/K+ pump

Answer 27

• Three Na+ bind to the inner sites of the pump.
• ATP binds to the pump and causes a conformational change.
• Na+ is released out of the cell and 2 K+ bind to the sites in the pump.
• Dephosphorylation causes the pump to go back to previous conformation.
• K+ is able to be released into the cell.

Question 28

What are the consequences of digitalis therapy?

Answer 28

• Since digitalis binds to the extracellular domain of the pump and blocks K+ binding, this prevents Na+ from binding the next round and makes the pump unavailable.
• There is going to be increased intracellular levels of Na+.
• There is going to be more Na+ available to interact with other Na+ binding proteins (Na+/Ca++ exchanger –> more Ca++ will be exchanged into the cell –> stronger muscle contraction)

Question 29

Explain the Na+<=>Ca2+ exchanger and how digitalis affects it.

Answer 29

A transmembrane protein that moves Na+ and Ca++ in EITHER but OPPOSITE directions (determined by ion concentration) at 3Na+ to 1Ca2+ ratio.
- No ATP required bc it is facilitated diffusion w/ bi-directional anti-porter.

Increased intracellular Na+ (bc of digitalis) competes with Ca2+
- 3Na+ gets pumped out and 1Ca2+ gets pumped in
- result: intracellular Ca2+ increases

Question 30

Other than depolarization, how else is Na+ entering the cell?

Answer 30

Na+/glucose symporter
- Glucose uptake from intestinal lumen to cell via Na+/glucose symporter through facilitated diffusion
- cause: lumen Na+&raquo_space; intracellular Na+
- result: high Na+ and glucose concentration in the cell.
Glucose gets transported by uniporter down its gradient and into the blood.
To keep intracellular Na+ low, Na+ is pumped out of the cell by Na+/K+ pump.

Question 31

What does the glucose uniporter do?

Answer 31

The glucose uniporter transports glucose from inside the cell to the blood through facilitated diffusion.