<Li> Alpha - fetoprotein </Li> <Li> Erythropoietin (EPO) </Li> <Table> Some functions served by glycoproteins <Tr> <Th> Function </Th> <Th> Glycoproteins </Th> </Tr> <Tr> <Td> Structural molecule </Td> <Td> Collagens </Td> </Tr> <Tr> <Td> Lubricant and protective agent </Td> <Td> Mucins </Td> </Tr> <Tr> <Td> Transport molecule </Td> <Td> Transferrin, ceruloplasmin </Td> </Tr> <Tr> <Td> Immunologic molecule </Td> <Td> Immunoglobulins, histocompatibility antigens </Td> </Tr> <Tr> <Td> Hormone </Td> <Td> Human chorionic gonadotropin (HCG), thyroid - stimulating hormone (TSH) </Td> </Tr> <Tr> <Td> Enzyme </Td> <Td> Various, e.g., alkaline phosphatase, patatin </Td> </Tr> <Tr> <Td> Cell attachment - recognition site </Td> <Td> Various proteins involved in cell--cell (e.g., sperm--oocyte), virus--cell, bacterium--cell, and hormone--cell interactions </Td> </Tr> <Tr> <Td> Antifreeze protein </Td> <Td> Certain plasma proteins of coldwater fish </Td> </Tr> <Tr> <Td> Interact with specific carbohydrates </Td> <Td> Lectins, selectins (cell adhesion lectins), antibodies </Td> </Tr> <Tr> <Td> Receptor </Td> <Td> Various proteins involved in hormone and drug action </Td> </Tr> <Tr> <Td> Affect folding of certain proteins </Td> <Td> Calnexin, calreticulin </Td> </Tr> <Tr> <Td> Regulation of development </Td> <Td> Notch and its analogs, key proteins in development </Td> </Tr> <Tr> <Td> Hemostasis (and thrombosis) </Td> <Td> Specific glycoproteins on the surface membranes of platelets </Td> </Tr> </Table> <Tr> <Th> Function </Th> <Th> Glycoproteins </Th> </Tr>

What is the function of the glycoprotein molecule in the cell membrane
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