Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind - AQA - A-Level Biology - Question 4 - 2018 - Paper 3

When the first oxygen molecule binds to haemoglobin, it induces a conformational change in the protein structure. This change increases the affinity of haemoglobin for subsequent oxygen molecules, making it easier for them to bind. Essentially, the first binding turns the haemoglobin from a tense state to a relaxed state, facilitating the binding of additional oxygen molecules.