Membrane Structure and Function

Question 1

Why are lipid bilayers highly impermeable to ions and most polar molecules?

Answer 1

• The ability of small molecules of cross a membrane is a function of its hydrophobicity
• Ions cannot cross membranes because of the energy cost of shedding their associated water molecules

Question 2

Membranes performing different functions contain different kinds and amounts of proteins:

Answer 2

Plasma membranes: ~50% proteins

Internal membranes of mitochondria and chloroplast: ~75% proteins

Question 3

What are some of the important cellular processes that membrane proteins are responsible for?

Answer 3

• Pumps
• Channels
• Receptors
• Enzymes

Question 4

Membrane proteins make membranes selectivity permeable. Why?

Answer 4

Membrane proteins allow transport of molecules and information across the membrane.
- Membranes vary in protein content from as little as 18% to as much as 75%

Question 5

What are integral membrane proteins?

Answer 5

Proteins that are embedded in the hydrocarbon chains of membrane lipids.
Released only when the membrane is disrupted.

Question 6

What are the factors that determine whether a small molecule will cross a membrane?

Answer 6

1) The [gradient] of the molecule across the membrane

2) The molecule’s solubility in the hydrophobic environment of the membrane

Question 7

By what process do lipophilic molecules cross the membrane?

Answer 7

Simple diffusion

Question 8

By what processes can polar molecules cross the membrane?

Answer 8

• Passive transport
• Facilitated transport
• Active transport

Question 9

Passive-transport systems called ion channels

Answer 9

Ion transport rates are 1000x faster than ion pumps

Question 10

Voltage-gated channels

Answer 10

Open in response to changes in membrane potential

Question 11

Ligand-gated channels

Answer 11

open in response to the binding of small molecules (ligands) such as neurotransmitters

Question 12

What is passive transport?

Answer 12

Movement with the [gradient]

No added energy is needed

Question 13

What is facilitated diffusion?

Answer 13

Diffusion through a protein channel

Question 14

What is active transport?

Answer 14

Movement against the [gradient]

Additional energy is needed

Question 15

What are primary transporters?

Answer 15

Hydrolyze ATP and transport molecules against the [gradient]

Question 16

What are secondary transporters/cotransporters?

Answer 16

Couple transport of molecules with the [gradient] to transport of a molecule with its [gradient]

Question 17

What is a symporter?

Answer 17

Transport molecules in the same direction

Question 18

What is an antiporter?

Answer 18

Transport molecules in opposite directions

Question 19

Na+ - K+ Pump (Na+ - Ka+ ATPase)

Answer 19

The hydrolysis of ATP by the pump provides the energy needed for the transport of 3Na+ ions out of the cell and 2K+ ions into the cell, generating the gradient

Question 20

The structure of the potassium ion channel reveals the basis of specificity:

Answer 20

• The potassium channel selectively and rapidly transports K+ across the cell membrane. Larger ions are not transported because they are too big to enter the channel
• Smaller ions are excluded because they cannot interact with the selectivity filter. Such ions are small enough that the energy of desolvation cannot be compensated for by interactions with the selectivity filter.