Editing Acid (section)

==Biological occurrence==
[[Image:Aminoacid.png|thumb|left|Basic structure of an [[amino acid]]]]Many biologically important molecules are acids. [[Nucleic acid]]s, which contain acidic [[phosphate|phosphate groups]], include [[DNA]] and [[RNA]]. Nucleic acids contain the genetic code that determines many of an organism's characteristics, and is passed from parents to offspring. DNA contains the chemical blueprint for the synthesis of [[protein]]s, which are made up of [[amino acid]] subunits. [[Cell membrane]]s contain [[fatty acid]] [[ester]]s such as [[phospholipids]].

An α-amino acid has a central carbon (the α or [[alpha and beta carbon|''alpha'' carbon]]) that is covalently bonded to a [[carboxyl]] group (thus they are [[carboxylic acid]]s), an [[amine|amino]] group, a hydrogen atom and a variable group. The variable group, also called the R group or side chain, determines the identity and many of the properties of a specific amino acid. In [[glycine]], the simplest amino acid, the R group is a hydrogen atom, but in all other amino acids it is contains one or more carbon atoms bonded to hydrogens, and may contain other elements such as sulfur, oxygen or nitrogen. With the exception of glycine, naturally occurring amino acids are [[Chirality (chemistry)|chiral]] and almost invariably occur in the [[Chirality (chemistry)#By configuration: D- and L-|<small>L</small>-configuration]]. [[Peptidoglycan]], found in some bacterial [[cell wall]]s contains some <small>D</small>-amino acids. At physiological pH, typically around 7, free amino acids exist in a charged form, where the acidic carboxyl group (-COOH) loses a proton (-COO<sup>−</sup>) and the basic amine group (-NH<sub>2</sub>) gains a proton (-NH{{su|b=3|p=+}}). The entire molecule has a net neutral charge and is a [[zwitterion]], with the exception of amino acids with basic or acidic side chains. [[Aspartic acid]], for example, possesses one protonated amine and two deprotonated carboxyl groups, for a net charge of −1 at physiological pH.

Fatty acids and fatty acid derivatives are another group of carboxylic acids that play a significant role in biology. These contain long hydrocarbon chains and a carboxylic acid group on one end. The cell membrane of nearly all organisms is primarily made up of a [[phospholipid bilayer]], a [[micelle]] of hydrophobic fatty acid esters with polar, hydrophilic [[phosphate]] "head" groups. Membranes contain additional components, some of which can participate in acid–base reactions.

In humans and many other animals, [[hydrochloric acid]] is a part of the [[gastric acid]] secreted within the [[stomach]] to help hydrolyze [[protein]]s and [[polysaccharide]]s, as well as converting the inactive pro-enzyme, [[pepsinogen]] into the [[digestive enzyme|enzyme]], [[pepsin]]. Some organisms produce acids for defense; for example, ants produce [[formic acid]].

Acid–base equilibrium plays a critical role in regulating [[mammal]]ian breathing. [[molecular oxygen|Oxygen]] gas (O<sub>2</sub>) drives [[cellular respiration]], the process by which animals release the chemical [[potential energy]] stored in food, producing [[carbon dioxide]] (CO<sub>2</sub>) as a byproduct. Oxygen and carbon dioxide are exchanged in the [[lungs]], and the body responds to changing energy demands by adjusting the rate of [[ventilation (physiology)|ventilation]]. For example, during periods of exertion the body rapidly breaks down stored [[carbohydrate]]s and fat, releasing CO<sub>2</sub> into the blood stream. In aqueous solutions such as blood CO<sub>2</sub> exists in equilibrium with [[carbonic acid]] and [[bicarbonate]] ion.
: {{chem2|CO2 + H2O <-> H2CO3 <-> H+ + HCO3−}}
It is the decrease in pH that signals the brain to breathe faster and deeper, expelling the excess CO<sub>2</sub> and resupplying the cells with O<sub>2</sub>.

[[Image:Aspirin-skeletal.svg|thumb|right|[[Aspirin]] (acetylsalicylic acid) is a [[carboxylic acid]].]]

[[Cell membrane]]s are generally impermeable to charged or large, polar molecules because of the [[lipophilicity|lipophilic]] fatty acyl chains comprising their interior. Many biologically important molecules, including a number of pharmaceutical agents, are organic weak acids that can cross the membrane in their protonated, uncharged form but not in their charged form (i.e., as the conjugate base). For this reason the activity of many drugs can be enhanced or inhibited by the use of antacids or acidic foods. The charged form, however, is often more soluble in blood and [[cytosol]], both aqueous environments. When the extracellular environment is more acidic than the neutral pH within the cell, certain acids will exist in their neutral form and will be membrane soluble, allowing them to cross the phospholipid bilayer. Acids that lose a proton at the [[intracellular pH]] will exist in their soluble, charged form and are thus able to diffuse through the cytosol to their target. [[Ibuprofen]], [[aspirin]] and [[penicillin]] are examples of drugs that are weak acids.