1. Cells take up extracellular molecules by either receptor-mediated endocytosis
ID: 212531 • Letter: 1
Question
1. Cells take up extracellular molecules by either receptor-mediated endocytosis or by constitutive endocytosis (fluid-phase endocytosis). A study compared the efficiencies of these two pathways by incubating cells with either labeled epithelial growth factor (EGF) to measure receptor-mediated endocytosis OR horseradish peroxidase (HRP) to measure constitutive endocytosis. The uptake of HRP was linear. The uptake of EGF was linear at first but reached a plateau at higher concentrations. Why would constitutive endocytosis exhibit a linear curve but receptor-mediated endocytosis demonstrate a plateau?
Explanation / Answer
constitutive endocytosis is a process where the extracellular molecules are directly taked up by the cell wothout the use of receptors on the cells. So, increasing the concentration of these molecules would increase their intake by the cell.
On the other hand, receptor mediated endocytosis requires the binding of extracellular molecule with a receptor first which then is taken inside by the cell in form of a vesicle. Since the number of receptors and their ecpression is limited, increasing the concentration of extracellular molecules above a certain point will have no effect on its intake due to the limited number of receptors present on the surface of the cell,
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