Any organic acid having one or more -COOH groups. The intermediates of the TCA cycle are important di- and tricarboxylic acids, while acetic acid is a monocarboxylic acid. Carboxylic acids in metabolic pathways are often phosphate or thioesters of the acid, e.g. acetyl CoA is a thioester of acetic acid.
Some higher plant cells contain high concentrations of certain carboxylic acids, for example citrus fruits are rich in citric acid, and oxalic acid is present in high concentrations in rhubarb and tobacco leaves. In the Crassulaceae and some other succulents and semisuc-culents there is a large diurnal fluctuation in carboxylic acid levels. This is due to incorporation of carbon dioxide into malic acid in the dark (see crassulacean acid metabolism