In order for a poison to produce toxicity, a sufficient quantity of that chemical must be absorbed into the body. Because the chemical must pass through a number of cell membranes before it can enter the blood, the ability of the chemical to cross these lipid-rich membranes determines whether it will be absorbed, and that ability depends on the chemical’s lipid solubility.

The cell membrane, the most external layer of all animal cells, is composed of two layers of lipid molecules (the lipid bilayer). The lipid molecules each have a hydrophilic (water-loving, or polar) end and a hydrophobic (water-hating, or nonpolar) end. Because they are surrounded by an aqueous environment, lipid molecules of the cell membrane arrange themselves so as to expose their hydrophilic ends and protect their hydrophobic ends. Suspended randomly among the lipid molecules are proteins, some of which extend from the exterior surface of the cell membrane to the interior surface.

A chemical tends to dissolve more readily in a solvent of similar polarity. Nonpolar chemicals are considered lipophilic (lipid-loving), and polar chemicals are hydrophilic (water-loving). Lipid-soluble, nonpolar molecules pass readily through the membrane because they dissolve in the hydrophobic, nonpolar portion of the lipid bilayer. Although permeable to water (a polar molecule), the nonpolar lipid bilayer of cell membranes is impermeable to many other polar molecules, such as charged ions or those that contain many polar side chains. Polar molecules pass through lipid membranes via specific transport systems.

The four types of chemical transport systems through cell membranes are diffusion, facilitated diffusion, active transport, and pinocytosis.

As mentioned above, lipophilic, nonpolar chemicals dissolve in the lipid bilayer. Simultaneously, some of the molecules are leaving the lipid bilayer. The net result is that chemicals cross the membrane until the concentrations of chemical molecules on both sides of the membrane are equal and there is no net flow of molecules across the cell membrane (diffusion). Therefore, chemicals diffuse across the membrane only when a concentration gradient exists across the cell membrane. Diffusion is considered to be passive transport because no external energy is used. Polar molecules, such as water and small water-soluble molecules (e.g., urea, chloride ions, sodium ions, and potassium ions), can diffuse across membranes through the water-filled channels created by membrane proteins. Large polar water-soluble chemicals, such as sugars, however, do not diffuse through the membrane.

Certain relatively large water-soluble molecules cross the cell membrane using carriers. Carriers are membrane proteins that complement the structural features of the molecules transported. They bind to the chemicals in order to move them across the cell membrane. Energy is consumed because the transport proceeds against the concentration gradient.

Active transport systems move chemicals essential to cellular functions through the membrane into the cell. Such essential chemicals include calcium ions, amino acids, carbohydrates, and vitamins. Because the structures of poisons usually are not similar to those of chemicals essential to cells, few poisons are absorbed by active transport. Active transport, however, is important in the elimination of organic acids, bases, and foreign compounds by the kidneys and liver.

Molecules of similar structure compete with one another in binding with the carrier molecule. Thus, the transport of one chemical can be inhibited by another chemical of similar structure, a phenomenon called competitive inhibition. The chemical being transported also competes with itself for a carrier molecule, so that only a limited amount of the chemical can be transported by the carrier protein during a specific time.

Transport systems that use carrier molecules but which do not require energy to proceed are called facilitated diffusion. A chemical first binds to the carrier protein in the cell membrane and then diffuses through the membrane. Because no energy is used, facilitated transport into the cell cannot proceed if the concentration of that chemical is greater inside the cell membrane than outside. The involvement of carriers means that the process is also subject to competitive inhibition and saturation.

Large molecules, such as proteins and solid particles, are often transported by pinocytosis. The cell membrane engulfs a particle or protein molecule outside the cell, and brings it into the cell. Although inefficient, pinocytosis operates in the slow absorption of proteins and particles in the intestine and respiratory tract.

Conditions of exposure

Figure 1 summarizes the conditions of exposure to toxicants.