Chapter 11.1 Principles of Membrane Transport

Question 1

What restricts the passage of most polar molecules in cell membranes?

Answer 1

The hydrophobic interior of the lipid bilayer.

Question 2

How do cells transfer water-soluble molecules and ions across their membranes?

Answer 2

Cells use specialized membrane transport proteins.

Question 3

What percentage of membrane proteins in cells are involved in transmembrane transport?

Answer 3

15-30%

Question 4

What are the two main classes of membrane proteins that mediate transmembrane transport?

Answer 4

Transporters and channels

Question 5

• Move specific small molecules across membranes by undergoing sequential conformational changes.
• They bind specific solutes and undergo conformational changes to alternately expose solute-binding sites on one side of the membrane and then on the other, transferring the solute across the membrane.

Answer 5

Transporters

Question 6

• Form narrow pores allowing passive transmembrane movement, primarily of water and small inorganic ions.
• They form continuous pores across the lipid bilayer, allowing specific solutes to pass through without the need for conformational changes.

Answer 6

Channels

Question 7

Transport molecules against their concentration gradients, using energy, often creating large differences in cytosol composition compared to extracellular fluid.

Answer 7

Active transport

Question 8

How do cell membranes store potential energy?

Answer 8

Through electrochemical gradients generated by differences in ion concentrations across the lipid bilayer.

Question 9

Which types of cells have highly sophisticated ion channel functions?

Answer 9

Neurons (nerve cells)

Question 10

• What is the cytoplasmic concentration of Na+ inside a typical mammalian cell?
• What is the extracellular concentration of Na+ outside a typical mammalian cell?

Answer 10

• Na+ (inside) 5-15 mM
• Na+ (outside) 145 mM

Question 11

• What is the cytoplasmic concentration of K+ inside a mammalian cell?
• What is the extracellular concentration of K+ outside a mammalian cell?

Answer 11

• K+ (inside) 140 mM
• K+ (outside) 5 mM

Question 12

• What is the cytoplasmic concentration of Cl- inside a typical mammalian cell?
• What is the extracellular concentration of Cl- outside a mammalian cell?

Answer 12

• Cl- (inside) 5-15 mM
• Cl- (outside) 110 mM

Question 13

Besides Cl-, what other negatively charged components are found in cells? (4)

Answer 13

• HCO3^- (Bicarbonate)
• PO4^3- (Phosphate ion)
• nucleic acids
  *metabolites carrying phosphate and carboxyl groups.

Question 14

• Which type of molecules diffuse rapidly across a lipid bilayer?
• Do small uncharged polar molecules diffuse across lipid bilayers easily?

Answer 14

• Small nonpolar molecules, such as O2 and CO2
• They do, but much more slowly than nonpolar molecules.

Question 15

Can charged molecules (ions) diffuse across a protein-free lipid bilayer? Why?

Answer 15

• No, lipid bilayers are essentially impermeable to charged molecules (ions), regardless of their size.
• The charge and high degree of hydration of ions prevent them from entering the hydrophobic hydrocarbon phase of the bilayer.
• the lipid bilayer of a cell membrane is made up of hydrophobic (water-repelling) fatty acids in the middle, which are like oily substances. Since ions are charged and love water, they don’t easily move through the middle of the membrane, which is non-polar and doesn’t like water.
• Ions would need to shed their hydration shells to move through the bilayer, but that requires a lot of energy, which doesn’t happen easily.
• That’s why ions need special channels or transporters in the membrane to help them cross

Question 16

Which type of transport protein typically allows for faster transport rates? Why?

Answer 16

Channels, because they do not require stepwise conformational changes.

Question 17

Specialized channel proteins that greatly increase the permeability of membranes to water.

Answer 17

Aquaporins

Question 18

What is an example of a disease caused by mutations in a membrane transport protein?

Answer 18

Cystinuria, which results from mutations affecting the transport of cystine and certain amino acids, leading to kidney stone formation

Question 19

All channels and some transporters allow solutes to cross the membrane only passively (“downhill”), a process called

Answer 19

passive transport

Question 20

What drives passive transport of uncharged molecules across the membrane?

Answer 20

concentration gradient of the solute across the membrane

Question 21

It is the electrical potential difference across the membrane, typically with the inside of the membrane being more negative compared to the outside.

Answer 21

membrane potential

Question 22

Membrane potential influence the transport of ions by favoring the entry of __ charged ions into the cell and opposes the entry of __ charged ions.
choices: positively; negatively

Answer 22

• positively
• negatively

Question 23

It is the movement of solutes across a membrane against their electrochemical gradient, mediated by transporters using energy from sources like ion gradients or ATP hydrolysis.

Answer 23

Active transport

Question 24

Can channels mediate active transport?

Answer 24

No, channels only mediate passive transport.

Question 25

It allows cells to maintain essential concentration differences for ions and molecules, which is crucial for processes like nutrient uptake, waste removal, and regulation of intracellular environments.

Answer 25

Active transport