Hemoglobin, the protein that carries oxygen through the bloodstream, is a large, complex molecule built mostly from amino acids folded into a specific protein shape. But the actual chemical work of binding and releasing oxygen isn't done by the amino acid chain itself — it's done by a single iron atom, held at the centre of a smaller, ring-shaped organic molecule called heme, one of which sits embedded in each of hemoglobin's four protein subunits. Bioinorganic chemistry is the field that studies exactly this kind of arrangement: a metal atom, not normally thought of as part of "organic" biology at all, performing an essential chemical function inside a living system.
A metal ion held in place by a specifically shaped organic ring
Heme's ring structure, called a porphyrin, holds its central iron atom in place through bonds to four nitrogen atoms arranged symmetrically around it, with the iron additionally free to bind a fifth and sixth position above and below the ring's plane. It's this fifth position that oxygen actually binds to when hemoglobin picks up oxygen in the lungs, with the porphyrin ring's rigid structure holding the iron atom in exactly the geometric arrangement needed for that binding to happen efficiently and, crucially, reversibly — the iron needs to bind oxygen firmly enough to pick it up in the oxygen-rich environment of the lungs, but loosely enough to release it again once hemoglobin reaches oxygen-poor tissue elsewhere in the body that actually needs it.
The same basic architecture, doing different jobs in different proteins
Heme-containing proteins aren't limited to hemoglobin; a related but chemically distinct set of proteins, including myoglobin, which stores oxygen within muscle tissue itself, and the cytochromes, which shuttle electrons during cellular respiration, all rely on the same basic architecture — an iron atom held at the centre of a porphyrin ring — doing meaningfully different jobs depending on the specific protein environment surrounding that heme group and the exact chemical demands of the task at hand. This pattern, of a metal ion doing essential chemical work while held in a precise position by a surrounding organic structure, recurs throughout biology well beyond just iron and heme — other metals, including copper, zinc and magnesium, play comparably essential and chemically specific roles in a wide range of other biological molecules, which is a large part of why bioinorganic chemistry treats "biological molecule" and "metal-free organic molecule" as two categories that overlap far less cleanly than the everyday distinction between organic and inorganic chemistry might otherwise suggest.
What we're still unsure about
Heme's basic structure, hemoglobin's four-subunit architecture, and the central role iron plays in binding and releasing oxygen are all extremely well-established biochemistry, confirmed through extensive structural and functional studies. What remains an active area of ongoing biochemical research is the precise, complete detail of how binding oxygen at one of hemoglobin's four heme groups subtly reshapes the whole protein in a way that changes how readily the other three heme groups then bind or release their own oxygen — a real, well-documented phenomenon called cooperative binding that's central to how efficiently hemoglobin actually works in the body, but whose exact mechanical and structural details, down to the full underlying physical explanation, are still being refined by researchers rather than being a fully closed question.
This sits inside Bioinorganic Chemistry, one of seven topics in Inorganic Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.