Cellular Absorption of Herbicides

Biological membranes surround the contents of cells in all living organisms. In plants, cell membranes (also called plasma membranes) are located between the cell wall and cytoplasm (Figure 1).

Figure 1. Cell cross-section. Note the location of the plasma (cell) membrane.	Figure 2. Close-up view of the lipid bilayer of biological membranes, showing the orientation of the heads and tails. Notice the hydrophobic tails oriented towards the center of the membrane, and the hydrophilic heads towards the outer edges of the membrane.

Singer-Nicholson’s Fluid Mosaic Model describes that all biological membranes are made up of lipids containing fatty acid tails and proteins, arranged as a lipid bilayer (Figure 2). This bilayer is oriented such that the tails are in the center of the membrane layer and the heads face the outer edges of the membrane. The tails are hydrophobic, or highly soluble in organic solvents, but not in water.   In contrast, the tails are hydrophilic.  The lipid bilayer keeps all the cell’s contents (i.e. organelles, cytoplasm) contained, and also allows for movement of molecules into and out of the cell.

In healthy, intact membranes, hydrophobic or non-polar solutes penetrate through the membrane to the cytoplasm at rates positively related to their relative lipid solubility or lipophilicity (lipophilicity will be discussed in detail later). Conversely, hydrophilic molecules (highly soluble in water) and ions penetrate cell membranes at rates inversely related to their size. However, membrane integrity is reduced when cells are not active and metabolizing, causing membrane permeability to solutes to increase. Various aspects of herbicide transport across membranes will each be addressed next. These include passive absorption, ion trapping of weak acids and active absorption.