Gradients and potentials

Recall: solutions are homogenous mixtures of a solute in a solvent. Sometimes solutes are found at uneven concentrations within a solution. The concentration of a solute in a solution is the amount of solute per volume, usually measured in moles per litre or molarity (M) or as the percent mass of solute per volume (%). If the concentration of a solute is non-uniform within a solution, this results in a concentration gradient within that solution. A gradient refers to a difference in the distribution of matter within a system.

Concentration gradients are a form of chemical potential energy. This is because solutes tend to move from high concentration to low concentration. This phenomenon is known as diffusion and you will discuss it further in lab and lecture. Solutions move toward even mixing or equilibrium. Therefore, a concentration gradient within a solute represents a chemical potential where the molecules have the potential to move to an area of lower concentration within the solution. A potential refers to a difference in the distribution of energy within a system.

Recall that charged atoms are called ions. Ions form a chemical concentration gradient within a solution but they also bear charges. If the charge distribution within a solution is non-uniform, this results in an electrical gradient. Electrical gradients result in one part of the solution being more negatively charged and another part of the solution being more positively-charged. The resulting non-uniform distribution of charge is an electrical potential because negative charges tend to move toward areas of positive charge and vice versa. Ion gradients represent both a chemical concentration gradient AND an electrical gradient. The sum of these two gradients is collectively referred to as an electrochemical gradient and the difference in chemical and electrical energy within that system is called an electrochemical potential.

Electrochemical gradients and the resulting electrochemical potentials are biologically important. Cells actively maintain electrochemical gradients across their membranes to ensure that electrochemical signals can be sent from the brain, along nerves, to muscle and gland cells. Electrochemical gradients across cell membranes ensure that cells can generate energy. You will have a fuller understanding of these processes by the completion of this course.