Membranes

Question 1

Describe the structure and function of the plasma membrane (PM)

Answer 1

Structure: The PM is composed of a lipid bilayer and membrane proteins. Because of the hydrophilic head group and hydrophobic tail, the lipids in the PM are amphiphilic.

Function: the PM surrounds the cell and forms a barrier between the interior and exterior of the cell.

Question 2

Describe the basic structure of fatty acids

Answer 2

Fatty acids consist of a long-chain hydrocarbon group (nonpolar tail)
and a carboxylic acid group at one end (polar head group)

Question 3

What role do fatty acids play in the organism?

Answer 3

1. Components of membrane lipids
2. Components of lipoproteins
3. Precursor to signaling compounds
4. Major components of stored fat in animals and plants

Question 4

Describe the role of saturated vs unsaturated fatty acids in the plasma membrane

Answer 4

• Saturated FA pack closer → Membrane is more rigid, extended
• Double bonds in Unsaturated FA make packing more difficult,
  chains are more fluid

Question 5

Prokaryotes have the most fluid membranes.

Why would that be important for the organism?

Answer 5

Prokaryotes need to exhibit permeability to nutrients in their environment.

Question 6

What is the role of sphingolipids in the cell/organism?

Answer 6

• Sphingolipids play important roles in signal transduction and cell recognition
• They are particularly abundant in the nervous system
• Sphingomyelins occur in cell membranes in the nervous system; They are part of the myelin layer that protects nerve cells

Question 7

What is the role of glycolipids in the cell/organism?

Answer 7

• Glycolipids contribute to lipid raft formation
• They are also important in cell-cell recognition and antigenicity
• More than 60 structures exist that provide unique cell surfaces that define for example blood types
• Glucocerebrosides and gangliosides are found in the membranes of nerve and brain cells

Question 8

Describe the role of cholesterol in the plasma membrane

Answer 8

• Cholesterol inserts into bilayer membranes
• The presence of cholesterol in the lipid bilayer interferes with the close packing of FA tails and thus inhibits transition to the highly ordered crystalline state
• Cholesterol also adds localized rigidity, preventing the liquid crystal state

Question 9

Describe the fluid-mosaic model of membranes

Answer 9

• The cell membrane is made up of a mosaic of different proteins, lipids, and sugars including phospholipids, cholesterol, and carbohydrates
• Within the bilayer, phospholipids diffuse laterally and spin in the plane of the membrane
• There is constant motion on the membrane, lateral movement in two dimensions, along the membrane, with little diffusion across the bilayer w/o the help of specific proteins

Question 10

What features describe transmembrane proteins?

Answer 10

• Contain hydrophobic regions embedded in the lipid bilayer
• Usually span the complete lipid bilayer
• Contain one or more transmembrane helices

Question 11

Describe peripheral membrane proteins

Answer 11

• Peripheral membrane proteins are non-covalently bonded to the polar head groups of the lipid bilayer and/or to an integral membrane protein
• Associated with the membrane through charge-charge or hydrogen-bond interactions

Question 12

Describe lipid-anchored membrane proteins

Answer 12

• Lipid-anchored membrane proteins are tethered to the membrane through a covalent bond to a lipid anchor (palmitate or myristate; Cys-isoprene linkage; glycosylphosphatidyl-inositol /GPI anchor)