Transport Processes in Cell Membranes

The plasma membrane of a cell serves as a selective barrier, crucial in maintaining homeostasis. It is composed primarily of a phospholipid bilayer with embedded proteins that facilitate various functions. These include transport, signal transduction, and cell recognition. When a substance moves across the membrane without the assistance of energy input and against its concentration gradient, a specific type of transport process is involved. Understanding cellular transport is fundamental to cellular biology.

Which of the following processes best describes the movement of substances across the plasma membrane that does not require energy and involves movement against a concentration gradient?