In most cases, in order for a herbicide to reach its site of action within a plant, it must cross several barriers. The herbicide must cross the cell wall, the cell membrane (plasma membrane) and even perhaps organellar membranes within the plant cell. The herbicide then accumulates at its site of action, causing phytotoxicity (negative effects on plant growth). A herbicide’s absorption into a cell and its accumulation within the cell are both controlled by the herbicide’s physiochemical characteristics, including its lipophilicity (ability to dissolve in lipids) and acidity properties. In addition, the permeability of plant cell membranes and electrochemical potential in the plant cell contribute to the herbicide’s absorption success. Absorption can be via passive (diffusion) or active mechanisms, with some herbicides able to accumulate in high concentrations. The major mechanisms for both passive and active absorption, as well as effects of herbicide accumulation are discussed in this lesson.