Cell Membrane Structure

Question 1

What is the function of the cell membrane?

Answer 1

* Define the boundaries of the cell but allow import and export

• Retain metabolites and ions within the cell
• Sense external signalsand transmit information into the cell
• Provide compartmentalization within the cell
• Store energy as a proton gradient and support synthesis of ATP
• Produce and transmit nerve signals

Question 2

What is the fluid mosaic model?

How is the membrane kept from bursting considering this fluid nature?

Answer 2

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.

Cytoskeleton keeps membrane from pulling apart when stretched or under stress/tension given its fluidity

Question 3

Comment on the composition of membranes in different organelles in the same cell.

Answer 3

Membrane bound organelles may contain similar components (i.e. all have phospholipids, proteins, cholesterol, etc.) but the amount of each component differs based on the organelle and often the contents of the membrane vary between organelles as well.

Question 4

What does amphipathic mean?

Answer 4

Amphiphilic - has both hydrophobic and hydrophilic ends

Question 5

What is the most common phospholipid in the cell membrane?

Answer 5

Phosphatidylcholine

Question 6

What is PIP? Why is it an important glycerophospholipid?

Answer 6

Phosphatidylinositol 4,5-bisphosphate (PIP)

Major signaling molecule, very negatively charged at physiologic pH

Question 7

What is a micelle?

Answer 7

Single layer of membrane lipids, hydrophobic core

Question 8

What is a vesicle?

Answer 8

Spherical bilayer of amphipathic lipids, central cavity is aqueous and can carry charged molecules, useful b/c readily fuse with membrane so act as carriers

Question 9

Describe the motion of lipids within the bilayer?

Answer 9

They can flip between one face of bilayer and they can diffuse laterally within the bilayer to a new position up/downstream of current location

Question 10

What factors contribute to determining how rigid a cell membrane is?

Answer 10

composition

• Amount of cholesterol
• Saturated vs. Unsaturated - greater degree of saturation = more rigid

temperature

• Higher temperature = more fluid

Question 11

What is the function of cholesterol in cellular membranes?

Answer 11

• Makes the membrane more rigid –> better barrier
• Decreases permeability of water soluble molecules

Question 12

Glycolipids

• What are they
• Where are they located
• What is their backbone?
• What is their function?

Answer 12

• Lipids with sugar group attached to backbone
• Located exclusively on the extracellular side of cell membrane
• Sphingosine backbone
• Involved in cell recognition processes

Question 13

What is the difference between glycoprotein and proteoclycans?

Where are proteins glycosylated?

Answer 13

Glycoprotein = protein with oligosaccharide attached

Proteoglycan = protein with polysaccharide attached (longer/bigger/more branched sugar group)

ER and Golgi

Question 14

What is the glycocalyx?

What is its function?

Answer 14

• Carbohydrate rich coating on cell surface.
• Function:
  
  • Protects against mechanical and chemical damage.
  • Regulates cell/cell interactions
  • Integrates the cell and its environment b/c glycoproteins/proteoglycans incorporate into ECM

Question 15

How do you extract a protein from the lipid bilayer?

Answer 15

Use detergent - detergent has a polar head and non-polar tail so it will break apart the cell membrane and associate with the components of the membrane such that polar head interacts with polar areas and non polar tail interacts with non polar areas. Will break apart both proteins and lipids.

Question 16

What does membrane asymmetry mean?

Answer 16

That there can be a difference in composition between one leaf of the lipid bilayer and the other

Question 17

Describe the 3 enzymes that are involved in moving lipids in the membrane.

Answer 17

Question 18

How does the membrane ensure that, despite its fluid nature, some components are able to stay relatively stationary? (3)

Answer 18

• Lipid Rafts
• Confinement
  
  • Tight junctions (some receptors and important transmembrane proteins are localized in parts of the cell membrane that are in close contact with another cell so they can’t move freely)
  • Cortical cytoskeleton (bands of actin traverse the extracellular side of the cell and block membrane components from diffusing oustide if the actin banding
• Restriction
  
  • Self-assembly (lipids like to bind to one another so they will aggregate in membrane)
  • Outside/Inside tethering (some components of membrane will want to bind to components inside or outside cell which will dictate location preference)
  • Cell-cell interactions (proteins will assemble in an area where they can bind to complement proteins in another cell)

Question 19

What is a lipid raft?

Answer 19

Contain clusters of glycolipid with longer than usual tails and cholesterol that introduce localized areas of order and association within the membrane, allows for some components of the membrane to stay anchored together despite the fluidity of the membrane, transmembrane proteins like to be inside the lipid rafts

Question 20

What is surfactant in the lungs?

Answer 20

A lipid rich substance that reduces surface tension in the alveoli of the lungs. They coat the interior of the alveoli and prevent them from collapsing in on themselves upon inspiration/expiration. They do this by actively exchanging lipids with the cell membrane.