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Photosynthesis is the Biosphere's Metabolic Foundation: A Review

 

On a global scale, photosynthesis makes about 160 billion metric tons of carbohydrate per year. No other chemical process on Earth is more productive or is as imprtant to life. Light reactions capture solar energy and use it to produce ATP and to transfer electrons from water to NADP+ to form NADPH. The Calvin cycle uses ATP and NADPH to fix CO2 and produce sugar. Photosynthesis transforms light energy to chemical bond energy in sugar molecules. Sugars made in chloroplasts supply the entire plant with chemical energy and carbon skeletons to synthesize organic molecules. Nonphotosynthetic parts of a plant depend on organic molecules exported from leaves in veins. The disaccharide sucrose in the transport form of carohydrate in most plants. Sucrose is the raw material for cellular respiration and many anabolic pathways in nonphotosynthetic cells. Much of the sugar is glucose- the monomer linked to form cellulose, the main constituent of plant cell walls. Most plants make more organic material than needed for respiratory fuel and for precursors of biosynthesis. Plants consume about 50% of the photosynthate as fuel for cellular respiration. Extra sugars are synthesized into starch and stored in storage cells of roots, tubers, seed, and fruits. Heterotrophs also consume parts of plants as food.