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Free MCAT-style practice, 1A

MCAT Hemoglobin and Myoglobin Practice Questions

By StudyStore · Published 2026-11-03 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on myoglobin versus hemoglobin oxygen binding, cooperativity, P50, the Bohr effect, 2,3-BPG, fetal hemoglobin and CO, with full explanations.

Quick concept primer

Myoglobin is a single polypeptide with one heme, so it binds one O2 and gives a hyperbolic saturation curve with a very low P50 (about 3 torr). That suits a storage protein in muscle: it stays loaded until tissue oxygen is nearly gone. Hemoglobin is an α2β2 tetramer with four hemes. Oxygen binding at one subunit shifts the tetramer from the low-affinity T state toward the high-affinity R state, so the curve is sigmoidal (P50 about 26 torr) and a small drop in pO2 releases a large amount of oxygen.

Acid, CO2, heat and 2,3-BPG stabilize the T state and shift the hemoglobin curve right, which favors unloading in active tissue. Fetal hemoglobin binds 2,3-BPG weakly and sits to the left. The 1A Amino Acids, Proteins and Enzymes pack adds flashcards and question sets on protein structure and function like this, and the free sampler lets you try it first.

Traps: a right shift means lower affinity and a higher P50, and carbon monoxide shifts the remaining sites left rather than right.

Free notes for this topic: Oxygen-Hemoglobin Dissociation Curve: Right vs Left Shifts

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

A purified sample of a muscle oxygen storage protein has a single binding site and a P50 of 3 torr. Assuming simple noncooperative binding, what fraction of the protein carries oxygen at a pO2 of 27 torr?

  1. 0.50
  2. 0.75
  3. 0.90
  4. 0.99
Show answer and explanation

Answer: C. 0.90

For one binding site the fraction bound is pO2 / (pO2 + P50) = 27 / (27 + 3) = 0.90. A value of 0.50 applies only when pO2 equals the P50, and 0.99 would need a pO2 roughly 100 times the P50, so those choices misapply the hyperbolic relationship.

Question 2

A student models oxygen unloading in a sprinting leg using these values.

QuantityValue
Hemoglobin concentration15 g/dL
O2 carried by fully saturated hemoglobin1.34 mL per g
Arterial saturation98%
Venous saturation leaving the muscle40%

About how much oxygen does hemoglobin release to the muscle per deciliter of blood?

  1. 11.7 mL
  2. 8.0 mL
  3. 19.7 mL
  4. 5.4 mL
Show answer and explanation

Answer: A. 11.7 mL

Full capacity is 15 × 1.34 = 20.1 mL per dL, and the saturation falls by 98 minus 40 = 58 points, so 20.1 × 0.58 = about 11.7 mL is released. 19.7 mL is the total arterial content, and 8.0 mL is what is still bound in the venous blood, not what was delivered.

Question 3

Each of the following would shift the hemoglobin oxygen saturation curve to the right EXCEPT:

  1. a rise in red cell 2,3-bisphosphoglycerate
  2. a rise in the partial pressure of CO2
  3. a rise in blood temperature
  4. a rise in blood pH
Show answer and explanation

Answer: D. a rise in blood pH

A higher pH means fewer protons, which removes the stabilization of the T state and raises oxygen affinity, a left shift. 2,3-BPG, CO2 and heat all favor the low-affinity T state, raise the P50 and shift the curve right, which helps tissues that are working hard.

Question 4

Which structural fact best explains why myoglobin gives a hyperbolic binding curve while hemoglobin gives a sigmoidal one?

  1. Myoglobin holds its heme iron in the Fe3+ state, which binds O2 one molecule at a time.
  2. Myoglobin is one polypeptide with one heme, so binding at one site cannot change the affinity of another site.
  3. Myoglobin lacks the histidine that coordinates the heme iron, so it cannot change conformation.
  4. Myoglobin has a lower oxygen affinity than hemoglobin, which flattens the curve.
Show answer and explanation

Answer: B. Myoglobin is one polypeptide with one heme, so binding at one site cannot change the affinity of another site.

Cooperativity needs several sites that communicate through subunit interfaces, and myoglobin has only one site. Both proteins bind O2 with ferrous Fe2+ iron held by a proximal histidine, and myoglobin actually has the higher affinity, so the other choices state false premises.

Question 5

A patient inhales carbon monoxide, and CO occupies one heme of many hemoglobin tetramers. What happens to oxygen bound at the other hemes of those tetramers?

  1. It is released more easily because CO shifts the curve to the right.
  2. It is unaffected because CO only oxidizes the iron it binds.
  3. It is held more tightly, which reduces unloading in the tissues.
  4. It is displaced because CO binds the 2,3-BPG pocket.
Show answer and explanation

Answer: C. It is held more tightly, which reduces unloading in the tissues.

A CO-bound subunit pushes the tetramer toward the R state, so the remaining hemes gain affinity and the curve shifts left, cutting delivery even beyond the lost carrying capacity. CO binds the heme iron itself, does not oxidize it, and does not act at the 2,3-BPG site.

Question 6

Which feature allows fetal blood to take up oxygen from maternal blood in the placenta?

  1. Fetal hemoglobin has γ chains that bind 2,3-BPG weakly, giving it a lower P50 than adult hemoglobin.
  2. Fetal hemoglobin is a monomer, so it binds oxygen with a hyperbolic curve.
  3. Fetal hemoglobin binds 2,3-BPG more tightly, which raises its affinity for oxygen.
  4. Fetal blood has a higher pO2 than maternal blood, so oxygen diffuses toward it.
Show answer and explanation

Answer: A. Fetal hemoglobin has γ chains that bind 2,3-BPG weakly, giving it a lower P50 than adult hemoglobin.

Fetal hemoglobin is an α2γ2 tetramer, and its γ chains bind 2,3-BPG poorly, so less of it is held in the low-affinity T state and its curve lies left of the maternal curve. It is still a tetramer, tighter 2,3-BPG binding would lower affinity, and fetal pO2 is lower, not higher.

Question 7

A hemoglobin variant follows the Hill equation, Y / (1 − Y) = (pO2 / P50)n, with n = 3 and P50 = 26 torr. What is its fractional saturation Y at a pO2 of 52 torr?

  1. 0.67
  2. 0.75
  3. 0.80
  4. 0.89
Show answer and explanation

Answer: D. 0.89

The pressure ratio is 52 / 26 = 2, and 23 = 8, so Y / (1 − Y) = 8 and Y = 8 / 9, about 0.89. 0.67 is the answer for n = 1 and 0.80 is the answer for n = 2, so those choices ignore the stated cooperativity.

Question 8

Where does 2,3-bisphosphoglycerate act on adult hemoglobin, and with what effect?

  1. It binds the heme iron and blocks oxygen from the sixth coordination position.
  2. It binds the central cavity between the β subunits and stabilizes the T state.
  3. It binds the central cavity and stabilizes the R state, raising affinity.
  4. It binds each α subunit separately, adding four molecules per tetramer.
Show answer and explanation

Answer: B. It binds the central cavity between the β subunits and stabilizes the T state.

One 2,3-BPG binds per tetramer in the positively charged cavity between the two β chains, a pocket that is open in deoxyhemoglobin. That stabilizes the T state and lowers affinity. It does not touch the iron, it does not favor the R state, and it does not bind four per tetramer.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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