Cell Membrane Transport

Question 1

What are the types of passive transport?

Answer 1

Simple diffusion
Facilitated diffusion

Question 2

What are the types of active transport?

Answer 2

Primary (direct)
Secondary (indirect)

Question 3

What is an electrochemical gradient?

Answer 3

Drives passive transport
Depends on concentration gradient of solute
For charged molecules also depends on difference in voltage between ICF and ECF

Question 4

What is simple diffusion?

Answer 4

Movement of uncharged hydrophobic solutes (e.g. CO2) through the phospholipid bilayer down their electrochemical gradient

Question 5

What is flux (Jx)?

Answer 5

How fast the solute X moves

Question 6

What does flux depend on?

Answer 6

Permeability coefficient of X (Px)
Difference in [X] between ECF and ICF

Question 7

What are transmembrane proteins?

Answer 7

Integral membrane proteins
Composed of membrane-spanning alpha-helical domains
Can be single or multi pass

Question 8

What are the types of transmembrane proteins?

Answer 8

Pore
Channel
Carrier
Pump
All have multiple transmembrane segments surrounding a hydrophilic solute permeation pathway through membrane

Question 9

What is a pore?

Answer 9

Protein that helps to form a pathway a solute can move through

Question 10

What is a channel?

Answer 10

Gated pore

Question 11

What is a carrier?

Answer 11

Protein that a solute binds to in order to move through

Question 12

What is a pump?

Answer 12

Protein that requires ATP in order to change its configuration and move the solute through

Question 13

What are amphipathic helices?

Answer 13

Proteins that have alternating hydrophobic and hydrophilic AA
Hydrophobic surfaces face lipid membrane
Hydrophilic surfaces create central pore

Question 14

What is facilitated diffusion?

Answer 14

Movement across a membrane where the driving force is the electrochemical gradient
Allowed by pores (always open) and channels

Question 15

What do all channels have?

Answer 15

A moveable gate
A sensor - voltage, ligand, mechanical
A selectivity feature
An open channel pore once sensor activated

Question 16

What are the different types of channels?

Answer 16

Voltage-gated - senses changes in voltage
Ligand-gated - intracellular or extracellular ligands bind -> involved in cell to cell communication
Mechanical-gated - opens when the membrane is stretched

Question 17

What is carrier mediated facilitated diffusion?

Answer 17

• Driving force is electrochemical gradient
• Never has a continuous transmembrane path
• Much slower than channel or pore - carriers have two gates that open and close at different times
• Can become saturated - Jx is limited by no of carriers in membrane and speed (stays occupied for longer)

Question 18

What is active transport?

Answer 18

• Moves a solute against its electrochemical gradient
• Used by cells when they require a higher concentration of solute than is in ECF
• Requires metabolic energy through ATP
• Mediated by carriers - pumps, cotransporters, exchangers

Question 19

What is primary active transport?

Answer 19

• The driving force is a chemical reaction e.g. ATP hydrolysis - releases energy used to move solutes against elec chem grad
• e.g. Na+/K+ ATPase
• Pumps can become saturated

Question 20

What is secondary active transport?

Answer 20

• Uses cotransporters and exchangers
• Driving force is using the energy of a solute moving along its electrochemical gradient to move another against its gradient

Question 21

What are cotransporters?

Answer 21

• Move both solutes in the same direction
• Requires a driving solute whose elec chem grad provides the energy to drive movement of something else out of cell
• Often the inward Na+ gradient