Q5 - Molecular Crowding of Hemoglobin and Ion Diffusion in Red Blood Cells

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Plasma Membrane Structure & Transport Human Circulatory System

Cytoplasm is generally occupied by very high concentrations of biomolecules and condensed organelles. This property is called “molecular crowding”, which affects the rate of intra-cytoplasmic diffusion and enzymatic reactions. Mammalian red blood cells (RBCs, Figure 1) are a typical case that demonstrate molecular crowding. The concentration of hemoglobin molecules (molecular mass, 64,000 g/mol) inside RBCs is called “mean corpuscular hemoglobin concentration (MCHC)”. It is as high as about 320 mg/mL in humans. From this concentration, we can estimate the mean cytoplasmic volume in a RBC occupied by a single molecule of hemoglobin. If hemoglobin molecule has a density as usual protein molecules of about 1.35 g/mL, we can also estimate how large is the molecular volume of hemoglobin.

Figure 1. Figure 1.

Figure 2. Figure 2.

Figure 3. Figure 3.

Using the information and data, determine which of the statements are true or which are false.

Q5-1. Choose the closest number for the percentage of the total cytoplasmic volume in RBCs occupied by hemoglobin molecules. Use the Avogadro constant, 6.02x10^23, for the calculation if needed.

Q5-2.A. Alpaca RBCs, which have a hemoglobin concentration about 1.5 times higher than that of humans, have an internal ion diffusion rate of less than 50% of that of human RBCs.

Q5-2.B. Ion diffusion rate is low inside human RBCs at low osmolarity, due to the reduced volume of RBCs.

Q5-2.C. There is a proportional relationship between the concentration of hemoglobin and the rate of ion diffusion in RBCs.

Q5-2.D. Alpaca RBCs have been evolutionally optimized to increase hemoglobin concentration and transport large amounts of oxygen, while promoting O2 and CO2 diffusion.

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Question reproduced from IBO 2020, Theoretical Exam 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)