Many animals are adapted to their environment by possessing different…

The idea

Different species, and different life stages, have globin chains with different primary structures. Primary structure determines tertiary and quaternary structure, so the environment around the haem groups differs, and so does the affinity for oxygen. The rule to learn is a rule about problems. An organism that struggles to load oxygen, because it lives where the partial pressure of oxygen is low, needs haemoglobin with a high affinity and a curve shifted to the left: a llama at altitude, a lugworm in oxygen-poor mud, and fetal haemoglobin, which must take oxygen from maternal blood across the placenta. An organism that struggles to unload oxygen fast enough, because it is very active or small with a high metabolic rate, benefits from a low affinity and a curve shifted to the right.

What earns the marks

• Reading from the graph that one haemoglobin is more saturated than another at the same partial pressure of oxygen.
• Stating that the curve further to the left shows the higher affinity for oxygen.
• Linking a high affinity to loading oxygen where the partial pressure of oxygen is low.
• Linking a low affinity, or a right-shifted curve, to readier unloading in tissues with a high rate of respiration.
• Relating the difference between the two haemoglobins to a different amino acid sequence and so a different tertiary or quaternary structure.

Common mistakes and how to put them right

• Saying an organism in low oxygen needs haemoglobin that carries more oxygen, rather than one with a higher affinity.

• Reversing left and right when describing affinity.

• Describing fetal haemoglobin as stronger instead of as having a higher affinity than maternal haemoglobin.

• Arguing entirely from the organism's habitat without quoting a value from the graph.

• Saying different species have different haem groups, when it is the globin chains that differ.

Exam tips

• Decide first whether the organism has a loading problem or an unloading problem; the direction of the shift follows from that.
• Support the statement with a pair of saturations read at one partial pressure of oxygen.
• In suggest questions any sensible argument consistent with the curve you were given is credited, so commit to one.