Glucose molecules are transported across cell membranes by facilitated diffusion or active transport. Most of the time, the former process takes place, as it requires no energy from the cell. Active transport indirectly requires energy from the hydrolysis of ATP.
When glucose molecules move into the cell by facilitated diffusion, the concentration gradient plays an essential part. Glucose only enters the cell by diffusion if its intracellular concentration is lower than the extracellular one. To move across the cell membrane without requiring any energy, glucose needs a protein that acts like a carrier. Because there is a limited number of transporting proteins, this process reaches a maximum rate of transport that cannot be improved, even if the concentrations on either side of the membrane continue to present a significant difference.
Glucose enters the cell against the concentration gradient, by active transport. In fact, glucose takes part in a process of cotransport, along with an ion such as Na+. When the sodium ion binds to the receptor, the binding of glucose is also stimulated, despite its concentration gradient. The receptor closes on the exterior side and opens on the inside of the cell, releasing the sodium ion along with the glucose molecule. The passive and active glucose transport systems work independently, and drugs that inhibit one of the processes do not influence the other.