What purpose does a protein serve once it leaves the cell through the cell membrane?

The protein goes through the nucleolus, ribosomes in the rough endoplasmic reticulum, vesicles, golgi apparatus, vesicles and then is transported somewhere in the cell or is transported out of the cell. Where can it go out of the cell and what does it do? I'm talking about a hemoglobin blood cell for example. Thanks!What purpose does a protein serve once it leaves the cell through the cell membrane?
Some proteins stay in the cell. Some proteins goes to cell membrane and become either integral /peripheral protein or become receptors. Some proteins leave the cell. Proteins can be receptors, enzymes, hormones, transporters....etc



Hemoglobin itself is protein. Red blood cell has hemoglobin, which contains Fe. Antibodies on the hemoglobin are also proteins.



I hope this helps.