Citric acid is a colorless organic tri-carboxylic acid that is commonly found in citrus fruits such as lemons, limes, and oranges. In biochemical intermediary metabolism, citric acid is heavily involved in the tricarboxylic acid or Krebs cycle that occurs in all plants and animals as well as most bacteria.

The citric acid cycle is confined to the mitochrondia and was discovered in 1937 by British biochemist Sir Hans Adolf Krebs. During metabolism, carbon-containing food molecules are degraded into small two-carbon units known as acetyl groups that combine with oxaloacetate to form citric acid. The citric acid molecule is then oxidized to provide energy to synthesize adenosine triphosphate (ATP) and guanosine triphosphate (GTP). As the cycle is completed, the original molecule of oxaloacetate is regenerated and can be used to start the citric cycle once again. Commercially, citric acid is used as a flavor chemical for pharmaceutical and food products such as sodas and candy. It also enjoys widespread use as a metal cleaner and to reduce the pH of canned foods.

Transmitted oblique illumination was obtained with a Hoffman-style substage condenser fitted with an offset aperture slit, which provides illumination from an oblique angle with respect to the optical axis of the QX3 microscope. Digital images were recorded with the QX3 interactive software operating from the Live View menu. In the case of birefringent specimens, a polarizer was added beneath the Hoffman condenser and an analyzer was placed in front of the QX3 lens cover. In some instances, a full-wave retardation plate was inserted between the specimen and the analyzer before capturing images.

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