Water, Ions, Membranes, Pumps, and Transporters

Abstract

The use of electrical energy for the purpose of generating and integrating meaningful signals is a core property of the nervous system. This biological electricity derives from the fundamental properties of ions in the water-based solutions inside and outside of living cells. Cells actively control their intracellular composition and therefore also control their intrinsic electrical properties. This chapter covers the basis for biological electrical signals in regard to the physiochemical properties of ions in solution, the proteins that control the ionic composition of intracellular solutions, and the passive electrical properties of cell membranes.

Glossary

Amphipathic

Molecules such as detergents that can mix with both water and oily liquids. Usually there are two parts to an amphipathic molecule: one part is a fat or other type of hydrocarbon, while the other part is charged or polar.

Association/Dissociation

Association is the coming together of parts of atoms and molecules usually involving the formation of attractions other than covalent bonds. Dissociation is the coming apart of atoms or molecules usually involving the dissolution of attractions other than covalent bonds. In most cases the terms are used to refer to reversible processes which occur in aqueous solutions, such as the dissolving of salts or the binding of drugs to protein receptors. Interactions with water molecules may provide energy to substitute for the original attractive intermolecular forces when molecules dissolve or dissociate.

Aqueous

Water-based.

Capacitance

The electrical property of being able to store charge. A capacitor has two surfaces positioned close enough together that an electrical field on one side may be sensed by charged particles on the other side. If charge builds up on one side of a capacitor particles of like charge are repelled from the other side, and an electrical force (voltage) is created. Biological membranes function as capacitors and “membrane potentials” are measurements based on the charge of that capacitor.

Carbohydrates

Organic molecules such as sugars and starches that contain varying combinations of carbon hydrogen, and oxygen. Oxygen-containing groups can usually interact with water, making most carbohydrates water soluble, at least to some degree.

Dipole

A molecule may contain covalent bonds with uneven distribution of charge such that the atoms involved have either a partial positive or negative charge. If the molecule has an overall structure that allows the partial charges to be aligned then it will respond as a dipole to electromagnetic fields and charged particles with positive and negative poles.

Hydration

Because water molecules are functional dipoles they will orient themselves around ions in solution so that the partial charges of the water which are complementary to the charge of the ion form a group of surrounding water molecules. This cluster of water molecules is known as a hydration shell and can shield the ion’s charge from other particles in solution.

Hydrocarbon

Organic molecules containing one or more carbon and hydrogen atoms. Each carbon atom makes four covalent bonds with hydrogens or other carbon atoms. Some of the carbon-carbon connections may be double bonds so that the carbons are not “saturated” with hydrogen.

Hydrophilic

The character of molecules that interact with water molecules through ionic or polar interactions. Small hydrophilic molecules dissolve readily in water. Larger complex molecules such as proteins may have subdomains which are more hydrophilic than others. If these molecules are in solution or contact with water they may be most stable in conformations that maximize the contact between the most hydrophilic domains and the surrounding water.

Hydrophobic

The character of molecules that cannot interact with water molecules through ionic or polar interactions. Hydrophobic molecules do not dissolve readily in water. Large complex molecules such as proteins may have subdomains which are more hydrophobic than others. If these molecules are in solution or contact with water they may be most stable in conformations that minimize the contact between the most hydrophobic domains and the surrounding water.

Ion

A charged atom or molecule that is stable in aqueous solution. Ions are formed when uncharged molecules that contain ionic bonds are put into solution.

Mole

A standard number of particles either atoms, ions, or molecules, that corresponds to the number of particles which have the same mass in grams as the original particles have in molecular mass units. This number has been determined to be 6.02 × 10²³, a value which is referred to as Avogadro’s number. A 1 M solution of a substance has one “gram molecular weight” of the substance dissolved per 1 L of water.

Phospholipid

Amphipathic molecule containing a hydrophilic phosphate group and a hydrophobic lipid fat-like hydrocarbon.

Physiological Conditions

Conditions that emulate or reproduce those experienced by a living cell or organism. Scientists often design experiments that depart from physiological conditions in order to test specific ideas or hypotheses. For example they may use an antagonist of one receptor in order to better observe the activity of another type of receptor. Although such experiments may be of scientific value, it is important that the results can be related back to what occurs under physiologically relevant conditions.

Polar

Having a full or partial charge or being able to react with other charged molecules in an aqueous environment.

Solute

An atom molecule, or particle dissolved in a solution.

Receptors

Large biological molecules usually proteins, that bind to and react with small molecules such as hormones or neurotransmitters in such a way as to change the activity state of the receptor molecule. The activity states of the receptor may regulate ionic conductances or any of a variety of intracellular processes.