Red cell metabolism and membrane

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Red cell metabolism and membrane, part of Hematology. The first 5 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   NADH-cytochrome b5 reductase keeps hemoglobin iron in the ferrous state. The NADH it uses comes mainly from:

   1. AThe hexose monophosphate shunt
   2. BThe Rapoport-Luebering shunt
   3. CThe Embden-Meyerhof glycolytic pathway
   4. DMitochondrial oxidative phosphorylation

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   Answer: C · The Embden-Meyerhof glycolytic pathway

   In glycolysis, glyceraldehyde-3-phosphate dehydrogenase makes NADH, which cytochrome b5 reductase uses to reduce methemoglobin back to hemoglobin. This pathway removes most of the methemoglobin formed each day, keeping it to a small fraction of the total. The hexose monophosphate shunt makes NADPH, which mainly keeps glutathione reduced. Mature red cells have no mitochondria.

   • A: The hexose monophosphate shunt makes NADPH, not NADH.
   • B: The Rapoport-Luebering shunt makes 2,3-BPG and produces no NADH.
   • D: Mature red cells have no mitochondria.

2. Question 2 · Hard

   An EDTA sample is left at room temperature over a long weekend and run 72 hours after collection. Compared with the result on the day of collection, the MCV has risen and the MCHC has fallen. The main reason is:

   1. AWater evaporates from the tube and concentrates the cells
   2. BExcess EDTA shrinks the red cells by drawing water out
   3. CRed cells lose hemoglobin into the plasma by hemolysis
   4. DATP runs out, the Na+/K+ pump fails and cells swell

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   Answer: D · ATP runs out, the Na+/K+ pump fails and cells swell

   Red cells depend on glycolysis for ATP, which powers the Na+/K+ pump that keeps sodium and water out. In a stored sample glucose is used up and ATP falls, so sodium and water enter and the cells swell; the MCV rises and the calculated MCHC falls. Samples should be analyzed within the time limit the laboratory has validated, or refrigerated if a delay is expected. Excess EDTA from an underfilled tube has the opposite effect, shrinking the cells.

   • A: Evaporation would concentrate the sample without changing cell size.
   • B: Excess EDTA shrinks cells and would lower the MCV, not raise it.
   • C: Hemolysis would lower the red cell count and, if anything, raise the MCHC.

3. Question 3 · Medium

   A red cell about 8 µm across can pass through splenic openings much narrower than itself and then recover its shape. This mainly depends on:

   1. ASpare membrane surface and a flexible skeleton
   2. BA high internal hemoglobin viscosity
   3. CA rigid skeleton that holds a fixed shape
   4. DA small nucleus that compresses on entry

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   Answer: A · Spare membrane surface and a flexible skeleton

   Deformability comes from three properties: a biconcave shape with more surface area than its volume needs, a flexible spectrin skeleton, and a low internal viscosity. These let the cell fold and stretch through narrow spaces and spring back. Loss of membrane (spherocytes), a stiff skeleton, or high viscosity from dehydration or Hb S polymer reduces deformability, and the spleen traps and removes such cells. Mature red cells have no nucleus.

   • B: High internal viscosity makes cells stiffer and less deformable.
   • C: A rigid skeleton would stop the cell from deforming.
   • D: Mature red cells have no nucleus.

4. Question 4 · Easy

   Under normal conditions, about what share of the glucose used by red cells goes through the hexose monophosphate (pentose phosphate) shunt?

   1. ANone at all
   2. BAbout 90%
   3. CAbout 10%
   4. DAbout 50%

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   Answer: C · About 10%

   About 90% of red cell glucose is broken down by anaerobic glycolysis (the Embden-Meyerhof pathway), which supplies ATP. About 10% passes through the hexose monophosphate shunt, which makes NADPH to keep glutathione in its reduced form. Under oxidant stress, flow through the shunt can rise many times over. Red cells with G6PD deficiency cannot raise this flow, so oxidant drugs damage their hemoglobin.

   • A: The shunt always runs at a low rate to keep glutathione reduced.
   • B: About 90% is the share used by anaerobic glycolysis.
   • D: About 50% overstates the shunt; glycolysis takes most of the glucose.

5. Question 5 · Easy

   Which protein forms the main mesh of the red cell membrane skeleton beneath the lipid bilayer?

   1. AGlycophorin A
   2. BAnkyrin
   3. CSpectrin
   4. DBand 3

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   Answer: C · Spectrin

   Alpha and beta spectrin chains form long, flexible heterodimers that join head to head into tetramers, creating a mesh on the inner surface of the membrane. The mesh is linked to the bilayer through ankyrin and protein 4.2 to band 3, and through protein 4.1 and actin junctional complexes to glycophorin C. It gives the cell its strength and its ability to deform and recover. Defects in these proteins cause hereditary spherocytosis and elliptocytosis.

   • A: Glycophorin A is an integral membrane protein that carries the MN antigens.
   • B: Ankyrin links spectrin to band 3 but does not form the mesh.
   • D: Band 3 is the integral anion exchanger that anchors the skeleton.