Final answer:
Binding a second or third oxygen molecule to hemoglobin is easier than binding the first due to the change in conformation of the molecule, whereas binding the fourth is more difficult. A large binding energy per nucleon implies it is harder to remove a nucleon from a nucleus, and greater dissociation energy means it is more difficult to break a solid apart.
Step-by-step explanation:
In the context of hemoglobin (Hb) and oxygen binding, it is easier to bind a second and third oxygen molecule to Hb than the first due to positive cooperativity. When oxygen binds to the hemoglobin molecule, it changes its shape, enhancing its ability to pick up additional oxygen molecules. However, binding the fourth oxygen molecule is harder because the hemoglobin molecule is nearly saturated at this point. The behavior of oxygen binding to hemoglobin can be represented on an oxygen dissociation curve, which shows a sigmoidal or S-shape. This illustrates that as the partial pressure of oxygen increases, hemoglobin's saturation with oxygen also increases, but up to a point where it becomes more difficult to bind additional oxygen.
When considering nuclear binding in physics, a large binding energy per nucleon makes it harder to strip a nucleon from a nucleus. Similarly, a larger dissociation energy in chemistry indicates that more energy is required to break the solid apart, therefore, making it difficult. Both these examples stress that if binding or the energy associated with the cohesion of particles is higher, then the effort to separate them is also higher.