Biochemistry - Membranes

Question 1

Fluid mosaic model of membranes + movement of components

Answer 1

• A biological membrane is made of a fluid phospholipid bilayer that has proteins embedded within it that have the ability to move freely
• Lipids vibrate, flex back + forth, spin around axis, move sideways, and exchange places within same half of bilayer (millions of times/second)
• Proteins move slower (due to larger size), those anchoring cytoskeleton to membrane do not move

Question 2

Why do phospholipids have the tendency to form bilayers in water?

Answer 2

Nonpolar hydrophobic fatty acids aggregate together and polar heads associate with water (this arrangement represents lowest energy state, more likely to occur)

Question 3

Glycolipids

Answer 3

Any membrane lipid bound to a carbohydrate

Question 4

Glycoprotein

Answer 4

Protein bound to a carbohydrate

Question 5

Major factors affecting membrane fluidity

Answer 5

• Composition of lipid molecules making up the membrane: the more unsaturated, the more space between phospholipids, the more fluid the membrane is
• Temperature: the higher the temperature, the more fluid the membrane (if temp low enough, lipids pack closely together, membrane forms viscous semisolid gel)

Question 6

Role of sterols in stabilizing membrane

Answer 6

• At high temps: help restrain movement of lipid molecules in membrane, reducing fluidity
• At lower temps: sterols occupy spaces between lipid molecules, preventing fatty acids from associating + forming a non-fluid gel

Question 7

Transport proteins

Answer 7

• Provide a means for molecules to enter cell
• Some proteins provide hydrophilic channel
• Shape shifting may allow some proteins to shuttle molecules from one side of membrane to the other

Question 8

Enzymatic activity proteins

Answer 8

• Some membrane proteins are enzymes (e.g. those associated with respiration + photosynthesis)
• Help speed up biological reactions

Question 9

Triggering Signal Proteins

Answer 9

• Membrane proteins may bind to specific chemicals (e.g. hormones)
• Binding to such chemicals triggers changes on inner membrane surface causing a cascade of events within the cell

Question 10

Attachment + Recognition Proteins

Answer 10

• Proteins exposed to internal + external membrane surfaces that act as attachment points for cytoskeleton elements and cell to cell recognition components
• Bond to extracellular matrix
• e.g. surface proteins recognize disease-causing microbes attempting invasion and trigger immune response

Question 11

Peripheral proteins

Answer 11

• Protein on surface of membrane
• Do not interact with hydrophobic membrane core
• Held to membrane surfaces by non-covalent bonds (hydrogen + ionic), usually by interacting with exposed integral proteins/directly with membrane lipids
• Hold some integral proteins in place
• e.g. proteins that link cytoskeleton together

Question 12

Integral proteins

Answer 12

• Protein that is embedded in lipid bilayer
• Have at least one region that interacts with hydrophobic membrane core
• Most are transmembrane proteins that span entire membrane bilayer + have regions exposed to aqueous environment on both sides of membrane