Lecture 8 : Membrane transport

Question 1

Passive Transport

Answer 1

• Simple diffusion
  
  • No membrane proteins involved
  • Driven by CONCENTRATION GRADIENTS
• Facilitated diffusion
  
  • Involves membrane proteins
  • Driven by concentration gradients

For both, no energy (ATP) is required

Question 2

Simple Diffusion

Answer 2

Up and down a concentration gradient.

Depends on -
- Concentration gradient
- Hydrophobicity / charge (membranes are highly impermeable to ions)
- Size

So hydrophobic molecules go straight through, small molecules are easier to pass through, polar molecules find it difficult, ions just dont pass through.

Question 3

Facilitated diffusion

Answer 3

Because semi-permeable membranes are impermeable to ions, ion channels are required to transport ions across the membrane.

Involves the transport of inorganic ions / small molecules across the membrane passively along concentration / electrochemical gradients. (Electrochemical gradient is a combination of concentration gradient and membrane potential. It essentially is the force driving charged molecules to pass through a semi-permeable membrane. )

Channels :

• Discriminate by size or charge

Carrier Proteins :

• Involves binding sites for solutes

Question 4

Ion channels

Answer 4

Ion Channels :

• Specific to types of ions
• Driven by electrochemical gradient
  
  • (conc grad, membrane potential)
• Fast (can transport up to 10 million ions per second)
• Can be regulated
  
  • Open and close in response to a stimulus

K+ Channels :

• Most common ion channel present in pretty much every cell
• K+ leak channels are an important subset of K+ channels which are not gated (So continuously open)

Question 5

Uniporter Carrier Proteins

Answer 5

• Highly selective (the transport molecule binds to carrier)
• Quite slow (fewer than 1000 molecules per second)
• Example, Glut 2 in gut (glucose transporter)

Question 6

Glucose transporters

Answer 6

Glucose transporters

• uniporters, so only transport glucose
• expressed by most cells
• 12 pass membrane spanning proteins
• alternate between 2 conformations (same sort of mechanism as shown above)
• Glut 1 deficiency leads to seizures or retardation
  .
  .
  Glut 1 transports glucose into erythrocytes.
• Glucose conc higher in blood than in RBC
• Transported into cell by glut1 along conc gradient
• glut1 works in both directions, so essential conc gradient is maintained
  
  • This is achieved by converting glucose into glucose-6-phosphate upon arrival in the RBC
• glucose-6-phosphate is unrecognisable to glut1 does not diffuse back through the membrane into the blood.

Question 7

Active Transport

Answer 7

Sodium Potassium pump, do a bit on this