Hemoglobin is the oxygen carrying protein inside red blood cells, and it makes efficient aerobic life possible in humans and many animals. Each hemoglobin molecule can bind oxygen in the lungs, travel through the bloodstream, and release oxygen to tissues that need it for cellular respiration. This transport system matters because oxygen is only slightly soluble in plasma, so blood needs a specialized carrier to deliver enough oxygen to active cells.
The shape of the oxygen hemoglobin dissociation curve shows how hemoglobin loads oxygen strongly in the lungs but unloads it more easily in tissues.
Hemoglobin is a tetramer made of four protein subunits, and each subunit contains a heme group with an iron ion that can bind one O2 molecule. Oxygen binding is cooperative, meaning the first oxygen makes it easier for the next oxygen molecules to bind, producing an S shaped dissociation curve. In active tissues, higher CO2, higher acidity, and warmer temperature shift the curve to the right, which helps hemoglobin release oxygen.
This is called the Bohr effect, and it connects metabolism directly to oxygen delivery.
Key Facts
- One hemoglobin molecule can bind up to 4 O2 molecules.
- Hemoglobin is made of 4 subunits, each with one heme group containing Fe2+.
- Oxygen saturation = bound O2 sites / total O2 binding sites x 100%.
- Cooperative binding gives hemoglobin an S shaped oxygen dissociation curve.
- High pO2 in the lungs favors oxygen loading, while lower pO2 in tissues favors oxygen unloading.
- Bohr effect: increased CO2 and decreased pH shift the curve right, reducing hemoglobin's O2 affinity.
Vocabulary
- Hemoglobin
- Hemoglobin is a protein in red blood cells that binds oxygen and transports it through the bloodstream.
- Heme group
- A heme group is a ring shaped molecule in hemoglobin that contains an iron ion and binds one oxygen molecule.
- Cooperative binding
- Cooperative binding means that oxygen binding to one hemoglobin subunit changes the protein shape and makes additional oxygen binding easier.
- Oxygen dissociation curve
- The oxygen dissociation curve shows the relationship between oxygen partial pressure and the percentage of hemoglobin binding sites occupied by oxygen.
- Bohr effect
- The Bohr effect is the decrease in hemoglobin oxygen affinity caused by higher carbon dioxide and lower pH in metabolically active tissues.
Common Mistakes to Avoid
- Thinking hemoglobin carries only one oxygen molecule is wrong because each hemoglobin tetramer has four heme groups and can carry up to four O2 molecules.
- Confusing oxygen content with oxygen saturation is wrong because saturation is the percent of occupied binding sites, while content is the total amount of oxygen carried in blood.
- Assuming hemoglobin releases oxygen best in the lungs is wrong because high lung pO2 favors loading, while lower tissue pO2 plus the Bohr effect favors unloading.
- Treating the dissociation curve as a straight line is wrong because cooperative binding produces an S shaped curve with different sensitivity at different pO2 levels.
Practice Questions
- 1 A sample contains 2,000 hemoglobin molecules. What is the maximum number of O2 molecules that can be bound if every hemoglobin molecule is fully saturated?
- 2 A hemoglobin sample has 800 total oxygen binding sites, and 600 sites are occupied by oxygen. What is the oxygen saturation percentage?
- 3 During intense exercise, muscle cells produce more CO2 and acid. Explain how this changes the oxygen hemoglobin dissociation curve and why that helps the muscle.