Transport mechanisms are processes that move substances across cell membranes. These mechanisms can be classified into two main types: passive transport and active transport. Passive transport does not require energy and occurs when molecules move from an area of higher concentration to an area of lower concentration, such as through diffusion or osmosis. In contrast, active transport requires energy, usually in the form of ATP, to move substances against their concentration gradient. This means that molecules are transported from areas of lower concentration to areas of higher concentration, often involving specific proteins or pumps in the cell membrane.