| MadSci Network: Cell Biology |
Our science text explained that hemoglobin cells "spit-out" their nucleus in order to be able to change shape and fit through small spaces. Does the nucleus become part of the outside structure of the cell? If not, what regulates the activities of the cell?
Hemoglobin is carried in the blood by Red Blood Cells (RBC's, the proper name is Erythrocytes), so named because all of the hemoglobin gives them a red color, which is also why blood is red. Hemoglobin is an enzyme made from four protein subunits which interact with each other to form a single, functional unit. Each hemoglobin subunit binds to an atom of iron which allows it to carry oxygen through the bloodstream. Hemoglobin is also capable of absorbing protons to buffer the RBC's, allowing them to carry carbon dioxide through the bloodstream.
Since hemoglobin is able to carry out all of the major functions of the RBC's, there is no need for other enzymes to be regulated through the life of the cell. So, to save space, mammalian RBC's have developed the ability to expel their nuclei, mitochondria, ribosomes, and other organelles before entering the bloodstream. These expelled components are immediately engulfed and destroyed by macrophages (white blood cells which act as the vacuum cleaners of the body).
The result of this is that RBC's cannot produce more proteins after they begin to circulate, but that's alright, since all they need to function is hemoglobin - there is no other activity to regulate. RBC's can function in the human bloodstream for about 120 days, after which the worn-out RBC's are destroyed by macrophages in the liver and spleen.
As an aside, all other vertebrates, beside mammals, have RBC's with nuclei. In fact, camels are the only mammals with nucleated RBC's (a good thing to know for trivia night).