7 Electric Forces in Biology

Electric forces are just as important in living organisms as they are in electric circuits. Every cell in your body depends on electrostatic interactions. Proteins recognize one another because of electric forces, ions move through cell membranes to generate nerve impulses, and DNA maintains its structure partly because of interactions between charged atoms. Although these processes occur on the molecular scale, they are governed by the same Coulomb force introduced in the previous chapters.

DNA is an excellent example of a biologically important charged molecule. Its sugar-phosphate backbone carries electric charge, and electrostatic interactions help determine how DNA folds and interacts with proteins. Figure 7.1 shows the double-helix structure of DNA.

The four nucleotide bases are adenine (A), thymine (T), cytosine (C), and guanine (G). Although the sequence of these bases varies from one DNA molecule to another, the pairing rules remain constant: adenine pairs with thymine, and cytosine pairs with guanine. These reliable pairings allow genetic information to be copied accurately during cell division.

Electrostatic forces are especially important at the molecular scale because interacting charges are extremely close together. Coulomb's law follows an inverse-square relationship:

As the distance r between charges increases, the electrostatic force decreases rapidly. Since atoms within molecules are typically separated by only fractions of a nanometer, electrostatic interactions can become quite strong.

DNA carries a substantial amount of charge along its backbone—roughly on the order of two elementary charges every 0.3 nm. The two strands of the double helix are separated by only about 1 nm, while individual atoms within each nucleotide are even closer together. At these tiny distances, electrostatic forces play a major role in determining molecular structure and function.

If biological molecules carry so much charge, why don't electrostatic forces dominate every interaction inside a cell? The answer is that living cells contain water and dissolved ions that greatly reduce long-range electric forces through a process called screening.

Polarity of Water Molecules

Water, [latex]\text{H}_{2}\text{O}[/latex], is the most abundant molecule inside living cells and is largely responsible for electrostatic screening. Water is a polar molecule, meaning its positive and negative charges are not distributed evenly.

Because oxygen attracts electrons more strongly than hydrogen, the electrons spend slightly more time near the oxygen atom. As a result, the oxygen end of the molecule carries a small partial negative charge, while the hydrogen atoms carry small partial positive charges.

This separation of charge causes each water molecule to behave like an electric dipole. The greater the separation of charge, the larger the molecule's dipole moment. Figure 7.2 illustrates the charge distribution in a polar water molecule.

Inside cells, millions of water molecules rotate and align themselves around nearby charged molecules such as DNA and proteins. This reduces the electric field extending away from the charged molecule, weakening the effective Coulomb interaction between biomolecules. In aqueous biological environments, electrostatic interactions are therefore much weaker than they would be in a vacuum and become effectively short-ranged.

Dissolved ions also contribute to screening. Important examples include [latex]\text{Na}^{+}[/latex], [latex]\text{K}^{+}[/latex], and [latex]\text{Cl}^{-}[/latex]. These ions are present both inside and outside cells, and differences in their concentrations across cell membranes are essential for nerve impulses, muscle contraction, and the electrical activity that coordinates the heartbeat.

Researchers continue to investigate how electrical interactions are influenced by cellular structures such as microtubules. These hollow protein tubes help organize cell division, transport materials throughout the cell, and provide structural support. Their possible role in intracellular electrical signaling remains an active area of research.