system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we need Red Blood Cells containing hemoglobin, instead of just free-floating hemoglobin in the bloodstream?

From what I understand, RBCs lack mitochondria, nuclei and many other organelles. They are essentially just sacs of hemoglobin. So why do we even need the extra membrane layer? Why waste energy making an entire cell? Wouldn't it be more efficient for the body to just release hemoglobin protein directly into the blood?
gpt
The shape of the RBC is what makes it effective as a oxygen carrier. It has a biconcave, discoid shape which allows it to flow through small blood vessels easily, like capillaries, and be able to reach all tissues of the body.

The problem with free hemoglobin in the blood is that it sticks everywhere. It gets lodged into tissues and enzymes will break it down. For example, free hemoglobin molecules get lodged into kidney cells and can clog up the microscopic tubules which can lead to kidney failure.

The other issue is that the ability of hemoglobin to do its job is actually regulated by enzymes within the RBC. So the RBC is not just a "sac of hemoglobin," it also contains machinery to optimize hemoglobin to do its job. 

Hemoglobin's purpose is not just to carry oxygen. In the lungs, hemoglobin needs to be able to pick up oxygen, and in the rest of the tissues, it needs to know that to release the oxygen. One of the major mechanisms by which this occurs is due to the heat and pH of its environment. 

For example, using your muscles requires oxygen. When your muscles are in use, they generate heat and carbon dioxide. Carbon dioxide diffuses into the RBC where there is an enzyme called carbonic anhydrase that combines carbon dioxide and a water molecule, and converts them into it into a molecule called bicarbonate and a proton. The proton binds to hemoglobin, which reduces its affinity for oxygen and allows hemoglobin to release the oxygen to where oxygen is needed.

Furthermore, RBCs will then carry that bicarbonate molecule until it reaches the lungs. At that point, the carbonic anhydrase enzyme does the same reaction but in reverse: converting bicarbonate and hydrogen into water and carbon dioxide. This is how carbon dioxide gets to the lungs where we can exhale it.

So you see that RBCs have several functions: 1) to carry hemoglobin molecules safely through the body so that the hemoglobin does not get lodged and get deposited into tissues where it can cause damage. 2) to contain enzymes which are essential for hemoglobin to bind and release oxygen in the appropriate locations. And 3) to use those same enzymes to deliver carbon dioxide to the lungs where it can be exhaled.

There is much more to this discussion, but these are the main functions of RBCs.