Final answer:
The correct answers are a) and b), with oxygen binding to hemoglobin displaying positive cooperativity and having a lower affinity when fewer oxygen molecules are bound, depicted by a sigmoidal oxygen dissociation curve.
Step-by-step explanation:
The correct answer to the student's question is d) Answers a and b are true. This is because oxygen binding to hemoglobin shows positive cooperativity, meaning as one molecule of oxygen binds to an heme group within hemoglobin, the affinity of the remaining heme groups for oxygen increases. Consequently, the fewer oxygen molecules that are bound to hemoglobin, the lower the affinity of hemoglobin for oxygen. Therefore, binding is not a linear function but a sigmoidal one, which illustrates a cooperative binding effect.
It's important to highlight that the oxygen dissociation curve of hemoglobin, which plots the partial pressure of oxygen against hemoglobin saturation, is sigmoidal or S-shaped. This shape is a result of cooperative binding, where each oxygen molecule that binds to a hemoglobin molecule increases the likelihood that additional oxygen molecules will bind to the remaining sites.