José A. Hernández Cortés. Scientist Researcher of CEBAS-CSIC. Group of Fruit Biotechnology.
As it was mentioned in a previous chapter (https://antioxidantsgroup.wordpress.com/2013/06/21/origin-of-the-oxygen-in-the-earths-atmosphere-a-necessity-to-live-a-threat-to-the-living-organisms/), the aerobic metabolism facilitated the evolution of the multicellular organisms. The life with oxygen provided benefits in terms of metabolic diversification, since there are more than 1000 metabolic reactions which use oxygen and which are specific to the aerobic metabolism (Raymond y Segre 2006; Halliwell 2006; Falkowski 2006).
The use of oxygen entails a greater energy production; however, the oxygen is a toxic molecule since the generation of reactive oxygen species (ROS), derived from its metabolism, cannot be avoided. In view of this situation, different strategies were developed to avoid the oxygen toxicity, from the occupation of anaerobic niches to develop systems which control the distribution and concentration of oxygen. In plants, the production of ROS poses a particular problem; on the one hand, by its inability to move to safer environments, and on the other hand, because the photosynthesis process, essential in plants, favours the ROS generation.
Evolution in the antioxidant systems in plants: the ascorbic acid or vitamin C
In a recent revision (Gest et al., 2013), it is emphasised the necessity of the evolution and development of antioxidant systems to control the ROS production. These authors mention that a good antioxidant must provide protection against the oxidation (loss of electrons) by the transfer of electrons to the oxidised molecules in order to reduce them and restore a non-toxic and non-oxidised environment. Moreover, a good antioxidant must not be toxic in its oxidised form and it must be able of recycling or regenerating itself to recover its electrons from another source. In this sense, a molecule which plays a paper of excellent antioxidant is the ascorbic acid, also known as vitamin C (Foyer y Noctor 2011).
This molecule can neutralize directly ROS such as hydroxyl radicals (.OH), hydrogen peroxide (H2O2). superoxide (O2.-), singlet oxygen (1O2), but it can also repair oxidised organic molecules, such as alpha-tocopherol o vitamin E. The oxidised ascorbate form, the monodehydroascorbate radical, is a very stable molecule which can be turned into another non-radical molecule, the dehydroascorbate (Smirnoff 2000). Ascorbate can be easily regenerated by the action of specific reductases and with the help of other molecules such as the NADH, the NADPH or the GSH on the so-called Ascorbate-Glutathione cycle (ASC-GSH) (see https://antioxidantsgroup.wordpress.com/2013/07/12/antioxidant-defense-mechanisms-ii-enzymatic-mechanisms/).
But ascorbate is not only an excellent antioxidant, it also fulfils some functions in plants. For example, it is the great protector of the photosynthetic apparatus; it participates in the flavonoid biosynthesis, including the anthocyanins, and the ethylene and gibberellins biosynthesis. These compounds play an important function in the development and fruit ripening. Other processes in which it participates include the growth and development, the progression of the cellular cycle, seeds viability, and the germination.
Ascorbate is not as broadly present as its antioxidant “brother”, the glutathione (GSH). In this sense, ascorbate is absent in bacteria; while they can use it as a carbon source, they cannot synthesise it (Yew y Gerlt 2002). Cyanobacteria, precursors of the chloroplasts, have very little ascorbate or else it is absent, whereas higher plants gathered more ascorbate in all its cellular compartments and in all its organs. The fruit is a source of vitamin C particularly important that you should to consume due to its huge benefits for health, since human beings cannot biosynthesize it.
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