membrane ultrastructure

Question 1

phospholipids

Answer 1

• Fatty acid tails – non-polar, hydrophobic, saturated and unsaturated bonds
• Phospholipid head – polar (charged), hydrophilic

Question 2

membrane permeability

Answer 2

• Phospholipid bilayer – fluidity modified by cholesterol and temperature
• Freely permeable – water (aquaporins), gases (CO2, N2, O2), small uncharged polar molecules (urea, ethanol)
• Impermeable to – ions (Na+, K+, Cl-, Ca2+ etc)

Question 3

why are membranes and membrane proteins needed?

Answer 3

• Cell polarisation
• Compartmentalisation
• Ionic gradients (diffusion – Nernst potential, membrane potential)
• Tightly regulated
• Disease disrupts this (kidney failure)

Question 4

RER

Answer 4

site of protein synthesis

Question 5

smooth ER

Answer 5

site of lipid synthesis

Question 6

diffusion potential

Answer 6

(aka Nernst or Equilibrium potential)
• Diffusion potential across a membrane is a voltage that exactly opposes the net diffusion of a particular ion through that membrane: No net diffusion of that ion

↑[ion] →↑tendency for ion to diffuse in one direction
• Therefore larger Nernst potential required to prevent further net diffusion
Logarithmic ratio: [ion]E:[ion]I

Question 7

epithelia

Answer 7

• Require polarisation of plasma membrane – apical vs basolateral surfaces
• Permits cell-specific function – secretion/absorption
• Strongly adhere to neighbours – tight junction
• Three examples: parietal cell (gastric pits), intestinal epithelium, nephron

Question 8

the nephron

Answer 8

• Morphology and permeability of tubular epithelial cells changes along the tubule
• Reflects specific function of each aspect of tubule

Question 9

cell membrane receptors

Answer 9

• Signal transduction

* Internalise extra-cellular signal … first message into second message

Question 10

calcium concentrations - tightly regulated

Answer 10

Total serum calcium measured clinically (ionised [Ca2+] unionised [Ca])
Serum Calcium:
• 45% Free ionised Ca2+
 Biologically active
 Change Ca2+ (active): Ca (inactive) ratio with no change in total calcium
Acidosis
less Ca2+ bound to plasma proteins (H+ ions buffered by albumin)
Alkalosis
more Ca2+ bound to plasma proteins (fewer H+ ions on protein)
 Alkalotic patients more susceptible to hypocalcaemic tetany
 Due to increased neuronal membrane Na+ permeability
 55% bound, not biologically active, 45% bound to albumin, 10% anions – phosphate; lactate active form