Cells 3 - Cell Membranes

Question 1

What are the properties required for biological membranes?

Answer 1

They must be selectively permeable (permeable to nutrients and waste products, impermiable to macromolecules, and biochemical intermediates). They must allow transfer of information.

Question 2

Explain the formation of phospholipid bilayers in an aqueous environment.

Answer 2

Lipid micelles form in water (droplets with mydropholic tails pointing inwards), while the lipid bilayer forms two layers thick. Called liposomes.

Question 3

What does amphiphillic mean?

Answer 3

Contain both hydrophobic and hydrophillic groups. Hydrophilic groups contain anionic and cationic groups, and may be net anionic or neutral.

Question 4

What are lipid rafts?

Answer 4

More densely packed areas of the membrane made of phospholipids with saturated straight hydrocarbon chains.

Question 5

What is cholesterol and what is its function?

Answer 5

Cholesterol is a steroid which binds to phospholipid molecules to decrease the permeability, and affect interactions with the cytoskeleton.

Question 6

How is the membrane asymmetrical?

Answer 6

Glycolipids are on the extracellular side (leaflet) and have a similar structure to sphingomyelin. Negative charges are inside the cell.

Question 7

How much can phospholipids in the membrane move?

Answer 7

Phospholipids can move laterally and can switch sides on the membrane.

Question 8

What is the fluid mosaic model?

Answer 8

A model to describe the structure of the cell membrane - proteins float in a sea of lipids in either leaflet of the bilayer, or span both leaflets.

Question 9

What is the general structure of membrane proteins, and hw do they affect fluidity?

Answer 9

• Proteins have a-helical conformation in the core of the bilayer, with hydrophobic amino acids facing the hydrophobic core.
• Proteins increase fluidity as they disrupt lipid packing.

Question 10

How are membrane proteins constrained from lateral movement?

Answer 10

The acin cytoskeleton forms fences.

Question 11

How does the proportion of lipids in membranes differ?

Answer 11

Myelin sheath has the most, plasma membrane next and then mitochondrial inner membrane has the least.

Question 12

What are the teo types of coupled transporters?

Answer 12

• Symporters (eg. Amino acids with Na+ as it moves down the concentration gradient)
• Antiporters (eg. movement in the opposite direction, such as sodium lotassium pump)

Question 13

Describe the structure of the sodium potassium pump.

Answer 13

There are two polypeptide chains - an alpha chain which spans the membrane 10 times to form a hydrophilic pore, and a beta chain which acts as a controller.

Question 14

Describe the process that occurs in the sodium potassium pump.

Answer 14

Phosphorylation and hydrolysis of an aspartyl residue drives the mechanism. This changes the conformational energy state of the pump, so 3Na+ is moved out and 2K+ are pumped in.

Question 15

What would happen if the sodium potassium pump stopped working?

Answer 15

Sodium would diffuse into the cell, lowering the water potential. Water would follow resulting in cells bursting.

Question 16

What isthe purpose of the action of the sodium potassium pump?

Answer 16

Ionic gradients are generated, and a charge gradient is generated.

Question 17

When is the nernst equation used?

Answer 17

To calculate the voltage of an electrochemical cell.

Question 18

Describe the structure of the potassium channel.

Answer 18

There are four subunits with a pore in the middle.

Question 19

Describe the use of a coupled transporter in the kidney early proximal lumen.

Answer 19

Glucose cant diffuse through the membrane without a transporter. Glucose is symported with Na+, which moves out through a pump and then diffuses back through a glucose transporter alongside glucose.

Question 20

What are the three bulk transport mechanisms?

Answer 20

• Pinocytosis is englufment of extracellular solute and small molecules
• Phagocytosis is englufment of extracellular objects, paired with actin rearrangement.
• Exocytosis is movement of proteins and other molecules from intracellular vesicles into the extracellular space.