Solutions

Water is critical to the function of microorganisms and all other living things. Water is a polar compound, which means opposite sides of the molecules have slightly positive and slightly negative charges due to differences in electronegativities of the atoms. The polarity of water allows it to form hydrogen bonds with charged, ionic, or polar compounds and dissolve them in solution. A solution is a homogeneous mixture of two or more substances in which at least one substance (solute) is evenly dispersed throughout a dissolving medium (solvent). In contrast to a solution, a heterogeneous mixture is a combination of two or more substances that are unevenly distributed. A solution's concentration is the amount of solute dissolved in a given volume of solvent in a solution. Because it easily dissolves many solutes, water is considered the universal solvent, and it plays an important role in living cells. For instance, water is a large component of bacterial cells' cytoplasm, and it helps transport dissolved substances into and out of cells. A solution is referred to as aqueous if water is the solvent. Solutes, both inorganic and organic compounds, often become hydrated in water. A solute is hydrated when it is solvated by water molecules such that the solute and water have chemically interacted and formed bonds.

Aqueous solutions with an ionic solute can conduct electric current. Ionic compounds exist as cations (positive ions) and anions (negative ions) in solution. The ionic solute is hydrated when anions are attracted to the positively charged side of water molecules and the cations are attracted to the negatively charged side of water molecules. The solution conducts electric current because the ions are electrically charged. An electrolyte is an ionic solution that can be decomposed by electricity. Electrolytes include some minerals vital to microbial cellular function, such as potassium, magnesium, and calcium.

Solutions vary in their acidity or alkalinity. This property is measured on the pH scale. The pH scale ranges from 0 (most acidic) to 14 (most basic or alkaline). A solution's pH is a measure of the relative proportion of acidic hydrogen ions (H⁺) and basic hydroxide ions (OH^–). Acidic solutions (pH $<$ 7) have more hydrogen ions than hydroxide ions. the reverse is true for basic or alkaline solutions (ph $>$ 7). a neutral solution has a ph of 7, representing a balance (equal quantities) of the two types of ions. Neutralization is a chemical reaction in which an acid and a base in solution interact, producing a salt and water and changing the pH.

Many living things require a near-neutral internal pH for their survival. This is because cellular processes, such as cellular respiration in some bacteria, cannot occur without a neutral internal pH level. Most bacteria can maintain steady pH values in the cytoplasm across a wide range of environmental pH values. Some bacteria possess biological strategies that allow them to function successfully even when environmental pH is acidic or basic enough to stop or harm their cellular processes.

In order to maintain a certain internal pH range, organisms use buffers, which resist changes in pH. A buffer is a solution containing either a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers maintain a solution's pH despite additions of a strong acid or a strong base. Every buffer is an aqueous solution (solution containing water). A conjugate acid is the acid formed by the donation of an H⁺ from a base. Conversely, a conjugate base is the base formed by accepting the H⁺ from an acid. Calcium carbonate is a common buffer utilized in microbial cytoplasm to maintain the appropriate pH in the cell.