Final answer:
Without insulin, GLUT4 glucose transporters remain inside the cell and are not efficiently translocated to the cell membrane, leading to reduced glucose uptake and elevated blood glucose levels.
Step-by-step explanation:
Without insulin, the glucose transporter GLUT4 remains inside the cell in vesicles, rather than being translocated to the cell surface. Normally, insulin binds to its receptor on the cell, initiating a sequence of phosphorylation events through a tyrosine kinase receptor pathway. This leads to the movement of GLUT4-containing vesicles to the cell membrane where they fuse, allowing GLUT4 to facilitate the entry of glucose into the cell by facilitated diffusion. In the absence of insulin, this translocation does not occur efficiently, and thus glucose uptake into cells such as muscle and adipose tissue is significantly decreased, leading to higher levels of glucose in the bloodstream.