Biochemical entities

In understanding the kinds of processes by which toxic substances harm an organism, it is important to understand the concept of receptors.9 Here a receptor is taken to mean a biochemical entity that interacts with a toxicant to produce some sort of toxic effect. Generally receptors are macromolecules, such as proteins, nucleic acids, or phospholipids of cell membranes, inside or on the surface of cells. In the context of toxicant-receptor interactions, the substance that interacts with a receptor is called a ligand. Ligands are normally relatively small molecules. They may be endogenous, such as hormone molecules, but in discussions of toxicity are normally regarded as xenobiotic materials. 

The covalent bond, as mentioned earlier, is the most tenacious type of chemical bond since it involves the mutual sharing of orbital electrons. It is the type of bond that holds organic compounds such as proteins, carbohydrates, and lipids together. Fortunately, for these important biochemical entities, it normally does not lend itself to easy reversibility. However, it is not the typical drug-receptor bond type. If it were the typical bond formed between drugs and their receptors, all pharmacological effects would have an inordinately long duration of action. 

The normal functioning of the central nervous system (CNS) presupposes a well-balanced interaction between different biochemically and structurally linked neuronal systems. When one member of a neuronal circuit is altered in its structural or biochemical entity, an imbalance in the functional system results and a compensatory mechanism must be activated in order to maintain physiological equilibrium. 

Different techniques for manipulating cells in microfluidic devices have been proposed for a wide variety of applications. The purpose of this essay is to systematize the techniques by the physical principles and forces used for controlling cell position. The reader is encouraged to follow the links to chapters describing in more detail how the interaction between cells and mechanical and biochemical entities or with electric, magnetic, or optical fields can be implemented at microscale for the handling of cells for various applications (Table 1). 

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