Membrane Transport

Movement of substances across the plasma membrane can be passive or active depending on whether the cell expends energy. As outlined in NCERT Class 11, Chapter 8 (Cell: The Unit of Life), polar molecules cannot pass freely through the hydrophobic core and often need carrier proteins; when such transport occurs against the concentration gradient it requires energy supplied by ATP and is termed active transport. The option naming simple diffusion is wrong because diffusion is always passive and proceeds only down a gradient, from higher to lower concentration. Facilitated diffusion does use carrier proteins but still moves solutes along the gradient without ATP, so it cannot push a solute uphill. Passive osmosis refers specifically to the movement of water across a selectively permeable membrane and does not describe the transport of a polar solute against its gradient. A familiar example of active transport is the sodium-potassium pump, which expels sodium ions and imports potassium ions across the cell membrane despite both moving toward regions where they are already more concentrated. Such pumps hydrolyse ATP and change their conformation to carry the ions, illustrating how energy input is unavoidable for uphill movement. A direction-based sanity check settles the answer: only active transport can move a solute from a region of lower concentration to one of higher concentration, and that uphill movement necessarily consumes metabolic energy in the form of ATP.