Hemoglobin has tetrahedral structure containing four heme groups that surround a globin group.
Heme is ringlike organic compound also known as a porphyrin to which an iron atom is attached. There are four iron atoms that bind four oxygen atoms and transport them to tissues.
Food chains I hope this is correct
Buildup of organisms means that the species number is pretty high.
Gradual change means that it was a slow, calm time to make the change.
I hope this helps!!
Answer:
The movement of ions or molecules across a cell membrane into a region of higher concentration