Respiration

Abstract

Energy required for growth as well as for functions in all living beings is met through ATP generated during respiration. Additionally, in green plants, light energy is conserved as ATP and NADPH, which are subsequently utilized for CO₂ assimilation. Carbohydrates thus synthesized are consumed as the primary source of energy both in plants and animals. Though in animals, lipids may also be consumed, carbohydrates remain the main energy source in plants. Plants store carbohydrates largely as starch since it is osmotically inactive. All carbon accumulation in plants is the result of photosynthesis, which remains the primary source of energy. Additionally, it is also the source for biosynthesis of various other biomolecules. In cells, hexose monophosphate pool constitutes an important component of carbohydrate metabolism. Besides being the source of energy, sugars play important role in regulating expression of many genes. In Arabidopsis, rice, and maize, an increase in soluble sugars represses expression of some photosynthetic genes, such as Rubisco small subunit, carbonic anhydrase, sedoheptulose bisphosphatase, Chla/Chlb-binding protein, and a starch-degrading enzyme, alpha-amylase. Hexose monophosphate pool maintained in cells consists of glucose 6-phosphate, glucose 1-phosphate, and fructose 6-phosphate. Sucrose translocated from source is consumed during respiration at the sinks.