Membrane Structure and Function

Question 1

Why are biological membranes bilayers?

Answer 1

Biological membranes are composed of lipids and proteins

The lipids are amphipathic meaning they have a hydrophilic head and hydrophobic tail.

Question 2

key facts of membranes

Answer 2

• membrane lipids are amphipathic
• major lipid classes are phospholipids, glycolipids and cholesterol.
• lipids spontaneously form bilayers in aqueous solutions
• phospholipid synthesis takes place on the cytosolic surface of the endoplasmic reticulum.
• distribution of newly formed lipid requires the enzymes flippase and floppase

Question 3

membrane fluidity

Answer 3

lateral movement of lipids in the membrane is rapid.

transverse movement is slow and requires the action of 3 enzymes.

Question 4

what factors affect fluidity?

Answer 4

• temperature
• fatty acid composition
• chain length
• degree and extent of saturation
• cholesterol content

Question 5

what happens at low temperatures of cholesterol?

Answer 5

-molecules pack tightly together. this reduces the interactions between the phospholipid fatty acid chains. this helps to maintain and increase the fluidity

Question 6

what happens at high temperatures of cholesterol?

Answer 6

molecules of the membrane will have more energy associated and motile. Cholesterol will act to decrease fluidity. this means organisms will be able to live in extreme conditions

Question 7

what is spur cell anemia?

Answer 7

cholesterol content is increased by 25%-65% leading to decreased membrane fluidity. Red blood cells get trapped as they pass through the capillaries some cells will rupture leading to anemia.

Question 8

The fluid mosaic model

Answer 8

it is:

• flexible
• self sealing
• selectively permeable
• they define boundaries
• they divide the internal spaces

Question 9

integral membrane proteins

Answer 9

• single/multi pass
• strong non covalent bonds
• trans membrane domain often an alpha helix
• can be predicted from the sequence

Question 10

Peripheral membrane proteins

Answer 10

it is located on both the extracellular cytosolic membrane and is associated by non covalent bonds

Question 11

lipid anchored membrane proteins

Answer 11

it is covalently linked to a lipid molecule such as glycosyl-phosphatidylinositol

2 major types of membrane proteins: integral and peripheral

• integral proteins interact extensively with the lipid bilayer
• peripheral proteins interact with integral proteins or lipid polar head groups

Question 12

Membrane Carbohydrates

Answer 12

Carbs are associated with both membrane lipids and proteins

Form 2-10% of the membrane weight

The majority of glycolipids and glycol proteins are externally facing

They are often involved in cell-cell interactions or cellular recognition

The blood group antigens are glycolipids

In red blood cells 8% of the weight is carbohydrate

Selectins are an important group of membrane glycoproteins.

Question 13

describe the role of cholesterol in membrane fluidity?

Answer 13

cholesterol acts as a bidirectional regulator of membrane fluidity because at high tempertaures, it stabilizes the membrane and raisesx its m.p. whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening

Question 14

describe the major molecular components of biological membranes and their distribution

Answer 14

biological membranes consist of a double sheet (bilayer) of lipid molecules. - phospholipid bilayer.