Final answer:
Insulin helps the movement of sugar from the bloodstream to the cells, primarily muscle and fat cells, by increasing the number of glucose transporters in cell membranes, which facilitates glucose uptake for energy and storage, leading to a reduction in blood glucose levels.
Step-by-step explanation:
The action of insulin is crucial for maintaining the balance of glucose levels in the bloodstream. The correct answer to the question, "Which of the following is an action of insulin?" is D. It helps the movement of sugar from the bloodstream to the cell. Insulin facilitates the entry of glucose into cells, particularly muscle cells and adipocytes, by increasing the number of glucose transporter proteins in their membranes. This process allows glucose to be taken up by the cells and used for energy or stored for future use, which is known as a hypoglycemic effect.
Insulin is produced by the beta cells of the pancreas when the blood glucose levels rise, and it acts by binding to insulin receptors on the cell membranes. Once insulin has engaged with its receptors, it triggers the incorporation of glucose transport proteins into the cell membrane, enabling glucose to enter the cell by facilitated diffusion. This process lowers blood glucose levels, bringing them back to homeostasis. As a consequence, and contrary to what is stated in option B, insulin does not increase circulating glucose levels, and options A and C are also incorrect as insulin does not assist with sugar transfer from the digestive tract nor block the uptake of sugar by cells