Membranes and Membrane Transport

Question 1

Membrane formation is driven and maintained by what?

Answer 1

the Hydrophobic effect

Question 2

What is the Fluid mosiac model?

Answer 2

Phospholipid bilayer acts like a 2 dimensional, fluid-like sea, with proteins and other macromolecues floating like icebergs within it

Question 3

At human body temperature, what is the role of cholesterol in the bilayer?

Answer 3

Cholesterol RESTRICTS movement of the phospholipids within the bilayer, reducing its fluidity and permeability to small molecules

Question 4

Describe the structure of cholesterol

Answer 4

1. Polar head group
2. Rigid steroid ring structure
3. nonpolar hydrocarbon tail

Question 5

What are three factors that increase membrane fluidity?

Answer 5

1. Short fatty acid chains
2. High unsaturation (one or more cis-double bonds)
3. High temperatures

Question 6

What class of molecule is this and where is it usually found?

Answer 6

• Esterified cholesterol
• Found in: Hydrophobic core of lipoprotein (such as chylomicrons)

Question 7

What class of molecule is this and where is it usually found?

Answer 7

1. Glycerophospholipid (composed of 2 fatty acids, glycerol, phosphate, and head group)
2. Found in: Cell membranes (polar head group is oriented to face aqueous environment)

Question 8

What class of plasma membrane proteins are these?

Answer 8

Integral membrane proteins

• Wind in and out of bilayer one or more times

Question 9

What type of plasma membrane proteins are these?

Answer 9

Lipid-anchored proteins

• Hydrophilic proteins which are associated with the plasma membrane via a covalently attached lipid molecule that tether the protein to the membrane
• Ex: Ras

Question 10

What type of plasma membrane protein are C and D?

Answer 10

Peripheral membrane Proteins

• Attached to the membrane by electrostatic or hydrogen bonds that noncovalently link them to integral membrane proteins or to phospholipid head groups

Question 11

Describe the Glycocalyx and its importance

Answer 11

• Carbohydrates covalently bind to lipids and proteins on extracellular surface of plasma membrane
• These sugar residues absorb water which cushions the plasma membrane
• Referred to as (Cell Coat)
• ABO blood groups are determined by specific carbohydrate sequences in the cell coat of RBC

Question 12

Movement of molecules across a membrane bilayer depends on what characteristics?

Answer 12

1. Size: small molecules move across more easily
2. Charge: only uncharged molecules can cross unassisted
3. Hydrophobicity: only hydrophobic molecules can cross without assistence
4. Solute concentration: Favored from areas of high concentration to areas of low concentration

Question 13

Facilitated Diffusion

Answer 13

• Transport of molecules down an energectically favorable concentration gradient (high to low)
• Does not require extra energy input (passive transport)

Question 14

Define the Electrochemical gradient

Answer 14

• Refers to transport of charged molecules
• Transport is influenced by concentration gradient AND the electrostatic environment (net charge)

Question 15

Membrane potential

Answer 15

• Difference in net charge across the membrane
• also known as Voltage Gradient

Question 16

What is the voltage gradient of most plasma membranes?

Answer 16

• Inside of cell is more negative than the outside
• Favors the entry of positively charged molecules

Question 17

Do channel porteins bind to solutes?

Answer 17

• NO!
• Channel proteins form hydrophilic tunnel through bilayer that allows certain solutes to diffuse down their concentration gradient (passive transport)
• Can be gated (opening and closing in response to binding of signaling molecules)

Question 18

Acetylcholine gated ion channel: purpose

Answer 18

• Binding of acetylcholine to channel allows NA+ to flow into the cell along its concentration gradient
• Influx of NA+ causes depolarization which signals muscle cells to contract

Question 19

Do carrier proteins bind to solutes?

Answer 19

• Yes! carrier proteins bind to solutes for transport
• Can be involved in active or passive transport

Question 20

Whats the difference between Paracellular and Transcellular transport?

Answer 20

• Paracellular: Passive; Between adjacent cells through tight junction
• Transcellular: Active and passive; Through cells

Question 21

GLUT2 carrier protein:

1. Location
2. What does it transport?
3. Type of Transport

Answer 21

1. Cell membrane between enterocyte and basal side of cell (Connective Tissue and Blood Supply)
2. Transports glucose
3. Passive transport: concentration of glucose in enterocyte is higher than that of extracellular space

Question 22

Primary Active Transport

Answer 22

• One step process that directly involves ATP to transport a solute against its concentration gradient

Question 23

What is the function of the NA+/K+ ATPase?

Answer 23

• Pumps out 3 molecules of Na+ for every 2 molecules of K+ it imports into the cell
• Leaves cell with a negative charge
• Increases the electrochemical gradient of Na+ (higher levels of Na+ outside of the cell)

Question 24

If the Na+/K+ ATPase releases Na+ to the basal side of the cell, how does the apical side of the cell get a high concentration of Na+?

Answer 24

Na+ at the basal side leaks back through tight junction to the apical side resulting in high levels of extracellular Na+ at apical surface

Question 25

The Na+ dependent Glucose transporter is an example of Primary or Secondary Active Transport?

Answer 25

Secondary Active transport

• ATP is not used by the glucose transporter

Question 26

Via what transporter does glucose from the diet get into enterocytes in the intestinal lumen?

Answer 26

Na+ dependent Glucose Transporter

• Transporter binds and transports one glucose molecule and two Na+ ions into the cell
• Na+ flows down ints concentration gradient and “drags” glucose molecule with it in a direction that is not favored by the glucose gradient