National League for Nursing (NLN PAX) Practice Exam

Sugar molecules enter a cell against a concentration gradient primarily through the process known as active transport. This process requires energy, typically in the form of ATP, because it involves moving the sugar molecules from an area of lower concentration outside the cell to an area of higher concentration within the cell.

Active transport utilizes specific carrier proteins found in the cell membrane that change shape to transport the sugar molecules across the membrane. This mechanism is vital for cells to accumulate necessary nutrients, such as glucose, even when they are less concentrated outside the cell than inside.

In contrast, passive diffusion and facilitated diffusion do not require energy and operate along the concentration gradient, meaning substances move from a high concentration to a low concentration, which does not apply in this case. Osmosis specifically refers to the movement of water molecules across a semipermeable membrane and is not involved in the transport of sugar molecules. Therefore, active transport is the correct mechanism for sugar molecules entering a cell against their concentration gradient.