Which physiological process involves the movement of substances across cell membranes?

The movement of substances across cell membranes is primarily characterized by the process of diffusion. Diffusion refers to the passive movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This is essential for many physiological functions, such as the exchange of oxygen and carbon dioxide in the lungs, nutrient absorption in the intestines, and the removal of waste products from cells.

In the context of cell membranes, semi-permeable membranes allow certain molecules to pass through while restricting others. Diffusion is crucial in maintaining homeostasis within cells, as it facilitates the necessary exchange of materials like ions, nutrients, and waste products.

While other processes listed, such as respiration, metabolism, and excretion, involve transport mechanisms or energy transformations, they do not specifically define the passive movement of substances at the cellular level in the way that diffusion does. Therefore, diffusion is recognized as the correct answer due to its fundamental role in cellular transport across membranes.