Membrane permeability and transport

Question 1

Molecule permeability

Answer 1

Depends on size and polarity

1. Small, non-polar molecules (ex. O2, CO2, steroids)
2. Small uncharged polar molecules (ex. H2O, ethanol, glycerol)
3. larger, uncharged polar molecules (amino acids, glucose, nucleotides)
4. ions (H+, Na+, HCO3-, Ca2+ etc.) - need transporters to pass through

-Gases diffuse rapidly across membrane
- smaller, more hydrophobic will move faster

Question 2

Types of membrane transport proteins

Answer 2

1. channels
2. passive transporters
3. active pumps

Question 3

Channels

Answer 3

• Passive transport; acts like a pore and selects what goes through based on size and charge, and its electrochemical gradient
• Can be leaky (commonly K channels) or gated (Ligand, 2nd messenger, mechanosensitive, voltage).

Question 4

Ligand-gated channel

Answer 4

Ligand (ex. acetylcholine or GABA) needs to bind for channel to open

Question 5

2nd messenger channel

Answer 5

2nd messenger (ex. Ca 2+, IP3) needs to bind for channel to open

Question 6

Mechanosensitive channel

Answer 6

(Ex. on stereocilia in hair cells, gut, or skin)- physical stress opens channels

Question 7

Voltage-gated channel

Answer 7

Voltage (ex. Na+, K+, Ca2+)- channels open based on voltage differences

Question 8

Passive transporters

Answer 8

Transporter mediated! More selective. Based on how well a molecule fits into the binding site (similar to enzymes). If it fits, then it will be transported.

Question 9

Active pumps

Answer 9

Require energy (ATP) to pump molecules against electrochemical gradient

Ex. Na/K ATPase pump – most important type, account for 30% or more of total ATP consumption in animal cells

Question 10

Sodium/Potassium Pump

Answer 10

3 Na ions enter pump from cell, ATP used and Phosphate group binds which changes confirmation so that entrance is to outside of cell, 2 K ions enter pump from outside of cell, Phosphate group leaves which changes confirmation back to original allowing K to exit into the cell

Creates a voltage difference across the plasma membrane

Question 11

Active ion pumps role in avoiding osmotic disaster

Answer 11

1. Anionic sites must be balanced out by intracellular cations
2. Osmotic pressure inside the cell is always higher than outside so water always moving inside.
3. Pump is always actively pumping Ca and Na out of cell which allows for a steady state/balance

Question 12

Secondary Active Transport

Answer 12

Symporter
Antiporter
Uniporter

Question 13

Symporter

Answer 13

*Eg. Glucose-Na symporter

Coupled transport by gradient driven pumps
Both glucose and Na are coupled and brought through the pump together

Question 14

Antiporter

Answer 14

Eg. Sodium-Hydrogen Antiporter
Coupled transport by gradient driven pumps.
Na moves through pump from high to low concentration, creating enough energy to then bring the hydrogen ion across from low to high concentration

Question 15

Uniporter

Answer 15

Uniporter
Eg. Glucose into erythrocytes
Allows glucose into the cell at a much faster rate than diffusion