Chemically, carotenoids consist of hydrocarbon chains that can be classified as xanthophylls and carotenes depending on the presence or absence of oxygen in the chain, respectively. Carotenoids are synthesised in plants in the plastids via the 1-deoxy-D-xylulose-5-phosphate from the isopentenyl diphosphate, the same precursor molecule used to synthesise cholesterol in animals. Isopentenyl diphosphate reacts with dimethylallyl diphosphate in a condensation reaction to form geranylgeranyl diphosphate (GGPP) which reacts with another molecule of GGPP to form phytoene. In turn phytoene is metabolised to phytofluene, ζ-carotene, neurosporene and then lycopene. Lycopene is a C40 carotenoid found primarily in tomatoes that confers red pigmentation on account of it absorbing light maximally at 472 nm. Carotenoids contribute to the red, yellow and orange colours found in plants and over 600 carentenoids have been identified. Other carotenoids are synthesised from lycopene via a series of metabolic steps.
In plants carotenoids such as β-carotene, vioxanthin, lutein and neoxanthin accumulate in green plant tissue where they act to absorb light in the 400 to 500 nm range. This increases the light range available for photosynthesis and thus harvests light for photosynthesis and allows the plant to grow in areas of low light or shade. In microorganisms carotenoids may perform similar functions to increase the use of available light for photosynthesis. As light exposure increases, carotenoids also function to protect the plants from overexposure. The antioxidant function of carotenoids is well researched, and their ability to quench singlet oxygen is well understood. Carotenoids can dissipate extra energy as heat by use of their conjugated double bonds. This duel role in light harvesting and as an antioxidant to protect from light and oxygen makes them ideal compounds for plants and other photosynthetic organisms.
The antioxidant function is also of interest in human nutrition because humans consume plants containing carotenoids and it is known they are absorbed. Carotenoids are fat soluble and so accumulate is hydrophobic areas of the body such as the cell membranes. Singlet oxygen can be generated from peroxidation of lipids in membranes and subsequently react with molecule such as DNA, lipids or proteins causing cellular damage. Carotenoids quench the electronically excited singlet oxygen (1O2) by transfer of its excitation to its own structure, thus returning oxygen to the ground state (3O2). Following excitation of the carotenoid structure, stabilisation occurs and the energy is then released as heat. Because of the way carotenoids act as antioxidants their structure does not need to be regenerated as with hydrogen donating reducing agents such as glutathione, vitamin C and vitamin E.
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