There. that is INSULlN and in the surface of this cells there are insulin receptors on them and I've drawn a simplified version of them -- places where these magenta circles can attach can bind, and what happens is that in order for the glucose to be taken up by the cell insulin has to attach to this receptors which unlock the channels for glucose in order for the glucose to be taken up by the cell, insulin has to bind to the insulin receptors and once that happens then the glucose can be taken up by the cell now unfortunately things don't work always as planned.