Hemoglobin
Hemoglobin was the compound in red blood cells that gave them their red color and served to convey oxygen to the body tissues. 
  
Human hemoglobin was iron-based, while Vulcan hemoglobin was copper-based and green in color. (; ; )

In 2268, Spock suggested he ought to set a trap for the dikironium cloud creature, given the low level of hemoglobin in his blood. ()

In 2268, after Leonard McCoy had received treatment for his xenopolycythemia, Spock reported that the doctor's hemoglobin count was back to normal. () 

Terran leeches consumed hemoglobin. In 2370, Doctor Beverly Crusher compared the interphasic organism's consumption of cellular peptides to leeches consu,ming hemoglobin. ()

In 2374, a shrunken Julian Bashir dissuaded a shrunken Miles O'Brien from leaving the miniature , as the oxygen molecules outside were nearly 2,000 times too big for his hemoglobin to take in. ()

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Physiology
Biochemical compounds