Which statement about insulin's effect on glucose uptake into muscle cells is correct?

• A. Glucose enters the bloodstream
• B. Beta cells release insulin
• C. Insulin transports glucose to muscle cells
• D. Glycogen stored in the liver

Answer: (C)

Insulin promotes glucose uptake into muscle cells by signaling GLUT4 transporters to move to the cell surface. When insulin binds its receptor on muscle cells, a signaling cascade (involving IRS-1, PI3K, and Akt) causes GLUT4-containing vesicles to fuse with the plasma membrane. This increases the number of GLUT4 transporters on the cell surface, allowing glucose to enter muscle cells from the bloodstream via facilitated diffusion. In this sense, insulin enables glucose uptake into muscle, even though it isn’t physically carrying glucose itself.

The other statements don’t describe the uptake process: glucose entering the bloodstream is not about uptake into muscle; beta cells release insulin is about the source of insulin rather than its effect on muscle glucose entry; and glycogen stored in the liver refers to storage, not the mechanism of glucose entry into muscle cells.