W7.2_Cell Membrane

Question 1

Describe the fluid mosaic model of cell membrane. Explain the properties and function of phospholipids and cholesterol in cell membrane.

Answer 1

• Fluid mosaic model: shows the fluid combination of lipids/proteins/carbohydrates in cell membrane
• Phospholipids: long, non-polar, hydrophobic fatty acid/lipid tails + polar/charged, hydrophilic head (phosphate/choline/glycerol)
• Cholesterol: polar head groups + steroid ring structure + non-polar hydrocarbon tail
• Makes phospholipid pack more tightly, regulates fluidity/flexibility of membrane

Question 2

What are the two main types of proteins in cell membrane? Explain the different functions of proteins in cell membrane.

Answer 2

• Proteins: integral (permanently embedded) vs peripheral (penetrate just one surface)
• Transport (with protein channels (facilitated), with protein pumps (active))
• Cell recognition (antigen)
• Receptors (peptide-based hormones ex. insulin)
• Intracellular joinings (tight junctions)
• Anchorage (cytoskeleton/extracellular matrix attachments)
• Enzymatic activity (metabolic pathways ex. ETC)

Question 3

Explain how large molecules move across cell membrane through use of examples.

Answer 3

• Large molecules: vesiculation
• Endocytosis (movement in): ex. pinocytosis (drinking), phagocytosis (eating)
• Exocytosis (movement out): ex. constitutive (continuous), regulated (triggered by receptors)

Question 4

Explain the different mechanisms of transport of small molecules across cell membrane.

Answer 4

• Simple diffusion (gases, small uncharged non-polar/some polar molecules)
• Facilitated diffusion (rate faster than simple diffusion, exhibits saturation kinetics)
• Selective at specific binding sites, susceptible to competitive/non-competitive inhibition
• Channel proteins: form water-filled pores, allow ions
• Carrier proteins: bind a specific solute and undergo conformational change
• Active transport (by carrier proteins, require ATP, against concentration gradient)
• Ion pumps: P class transporters, couple phosphorylation and conformational change to pump ions (other classes: A/B/C)
• Drug can exert its effect by blocking membrane transport (ex. ion)

Question 5

Explain the mechanism of multi-drug transporters by using chloroquine as an example.

Answer 5

• Multi-drug (MDR) transporters: pump drugs out of cells
• MDR is over-expressed in cancer cells -> simultaneously resistant to cytotoxic drugs
• MDR over-expressed in Plasmodium falciparum (causes malaria) -> resistance to anti-malarial drugs (ex. chloroquine)