What type of transport moves substances with the concentration gradient without the use of energy?

The correct answer is passive transport, which refers to the movement of substances across cell membranes without the input of energy. This process relies on the concentration gradient, meaning substances move from areas of higher concentration to areas of lower concentration until equilibrium is achieved. This natural tendency of molecules to spread out is what drives passive transport mechanisms.

Passive transport includes various processes such as simple diffusion, where nonpolar and small polar molecules directly pass through the phospholipid bilayer, and facilitated diffusion, where specific transmembrane proteins assist larger or polar molecules across the membrane. While facilitated diffusion is an important subcategory of passive transport, it specifically involves the use of protein carriers or channels to increase the presence of certain molecules across the membrane.

In contrast, active transport requires energy, typically in the form of ATP, to move substances against their concentration gradient, which is not the case for passive transport. Endocytosis involves the engulfing of substances into the cell via membrane invagination, and it is also an energy-dependent process. Thus, passive transport is the correct term for movement along the concentration gradient without energy expenditure.