The piecemeal presentation of biology found in most introductory textbooks doesn’t reflect modern thinking about how folks learn, and frankly doesn’t make sense on the face of it. The average textbook starts teaching biology with a summary of all the chemistry that will be needed within the book (despite the fact that some of it [looking at you, pH] won’t appear for another dozen or more chapters, then summarizes all the biomolecules and their roles–a summary of things not yet taught. There must be a better way; read on to see one model using hemoglobin.
(I’m releasing this because it’s pretty dense already, but hope to come back and add more links, info)