Membranes and Cell Surface

Question 1

What are the outer (2) and inner (3) phospholipids of the plasma membrane?

Answer 1

Outer:
- phosphatidylcholine 
- sphingomyelin 
Inner: 
- phosphatidylethanolamine 
- phosphatidylserine (-ve) 
- phosphatidylinostol (-ve)

Question 2

Where are lipids predominantly synthesized? How are they distributed in the membrane?

Answer 2

In the ER and are distributed asymmetrically within the membrane.

Question 3

What affects the fluidity of the plasma membrane?

Answer 3

• the amount of saturated vs unsaturated fatty acids
• temperature
• cholesterol (reduces mobility and can buffer the effect of a temperature change)

Question 4

How do unsaturated fatty acids increase membrane fluidity?

Answer 4

They have carbon-carbon double bonds which introduce kinks in the fatty acid chains, which prevents phospholipids from packing tightly together.

Question 5

What are peripheral membrane proteins?

Answer 5

Proteins that are not inserted into the hydrophobic region of the membrane but are associated indirectly with membranes through other protein interactions or through lipids and glycolipids

Question 6

What are integral membrane proteins?

Answer 6

Proteins that have a portion within the membrane, usually extended on either side of the membrane with a series of hydrophobic amino acids remaining within the bilayer.

Question 7

How do membrane proteins and phospholipids diffuse?

Answer 7

Laterally within the membrane with some restrictions

Question 8

What are the restrictions on the mobility of plasma membrane proteins?

Answer 8

• cannot reverse orientation across a membrane (can’t move back and forth between leaflets)
• association with cytoskeletal elements may anchor membrane proteins to a defined location
• association with lipid rafts

Question 9

What are lipid rafts made up of? What are they involved in?

Answer 9

Made up of clusters of cholesterol, sphingomyelin, and glycolipids.
Involved in signalling, movement, endocytosis (through integral membrane proteins).

Question 10

What do lipid rafts create within the plasma membrane?

Answer 10

specialized functional domains

Question 11

What is separation of membrane domains maintained by?

Answer 11

In part by specialized junctions

Question 12

What is passive diffusion?

Answer 12

Unassisted, direct movement of membrane-permeable molecules across the membrane along a concentration gradient.

Question 13

What kinds of molecules are transported by passive diffusion?

Answer 13

Small non-polar molecules, small uncharged polar molecules, and hydrophobic molecules

Question 14

What is facilitated diffusion?

Answer 14

Assisted transport of molecules that are not soluble in the phospholipid bilayer

Question 15

What do carrier proteins do?

Answer 15

Bind specific molecules to be transported on one side of the membrane and then transport them to the opposite side of the membrane by a protein conformational change.

Question 16

What do channel proteins do?

Answer 16

Form open pores through the membrane, allowing the free diffusion of any molecule of the appropriate size and charge.

Question 17

How can glucose move in either direction across the membrane? What does this depend on?

Answer 17

The conformational changes on the glucose transporter are reversible, and the movement of glucose is dependent on concentration gradients.

Question 18

What are aquaporins? What do they allow for?

Answer 18

Aquaporins are multi-pass water channel proteins that are impermeable to charged ions or other small molecules.
Allow for more rapid movement of water across the plasma membrane.

Question 19

What are ion channels? What do they allow for?

Answer 19

Most ion channels are gated and open only in response to a specific signalling molecule or change in membrane polarity.
They allow for the movement of selected ions down an electrochemical gradient.

Question 20

What is active transport?

Answer 20

A process in which energy provided by a coupled reaction is used to drive the transport of molecules across the plasma membrane against a concentration gradient.

Question 21

What are ion pumps responsible for?

Answer 21

Responsible for maintaining gradients of ions across the plasma membrane

Question 22

What are the concentrations of Na+, K+, and Cl- inside vs outside of the cell?

Answer 22

Inside the cell: 
- low Na+ (10mM) and Cl- (4mM)
- high K+ (140mM)
Outside the cell: 
- low K+ (5mM)
- high Na+ (145mM) and Cl- (110mM)

Question 23

What is the sodium potassium pump?

Answer 23

An active transport mechanism that establishes the gradient of sodium and potassium ions across the plasma membrane of different cell types.

Question 24

How does active transport driven by ion gradients occur?

Answer 24

Some molecules are transported against their concentration gradients using energy from the coupled transport of a second molecule in the energetically favourable direction.

Question 25

Symport vs Antiport

Answer 25

symport: both molecules transported in the same direction across the membrane
antiport: molecules transported in opposite directions across the membrane

Question 26

What is endocytosis?

Answer 26

A process in which material to be internalized is surrounded by an area of the plasma membrane, which then buds off inside the cell to form a vesicle containing the ingested material.

Question 27

What is pinocytosis?

Answer 27

The uptake of fluids or macromolecules in small vesicles (non selective)

Question 28

What does receptor-mediated endocytosis rely on?

Answer 28

Specific cell surface receptors that recognize molecules/particles to be taken up

Question 29

What is phagocytosis?

Answer 29

The ingestion of large particles (such as bacteria)

Question 30

What is low density lipoprotein (LDL) particle used for?

Answer 30

To transport cholesterol through the bloodstream to target cells

Question 31

What is LDL taken up through?

Answer 31

By the target cell through receptor mediated endocytosis

Question 32

What recognizes LDL?

Answer 32

Recognized by receptors that are attached to clathrin, which begin to form an invaginated clathrin-coated pit.

Question 33

Where do LDL bound to receptors undergo endocytosis? What is the endocytosis facilitated by?

Answer 33

Undergo endocytosis at the cell membrane on clathrin-coated vesicles. The formation of this vesicle is facilitated by dynamin.

Question 34

Where is the clathrin-coated vesicle containing LDL targeted to?

Answer 34

The lysosome, where cholesterol is released.