How do cells use carbohydrates for energy?

Carbohydrates are organic compounds that are defined by having reactive aldehyde and ketone functional groups and multiple hydroxyl groups. Mostly made up of carbon, hydrogen and oxygen with the empirical formula Cm(H2O)n; some carbohydrates also contains sulfur and nitrogen, phosphorus. Biologists believe that carbohydrates make up a higher percentage of biomass than any other biomolecules class. Like other major class of biomolecules, carbohydrates are found in all forms for life and serve many different functions including:

• Carbohydrates can perform a structural function, providing scaffolding for bacterial and plant cell walls, connective tissues such as cartilage in animals and exoskeleton shells in arthropods.

• The monosaccharides ribose and deoxyribose are components of nucleic acid. They serve both a structural and chemical role in RNA and DNA

• Carbohydrates are covalently bound to proteins and lipids on cell surfaces to act as markers for molecular recognition in signal transduction.

But carbohydrates are probably best known for their role in energy metabolism.

The first step of carbohydrate metabolism is glycolysis. The pathways name literally meaning splitting sweet substances; is believed to the universal extraction method of the energy available in carbohydrates by all plants and animals. Because molecular oxygen is not require in any of the reaction steps, the glycolytic pathway is used by both anaerobic (no oxygen) and aerobic (needs oxygen) organisms. Glucose (a six carbon sugar) is the preferred monosaccharide for glycolysis. The pathway consists of ten enzymes-catalyzed reactions that begin with the hexose substrate that is split into two molecules of a three-carbon sugar. Glycolysis yields two molecules of ATP, two molecules of pyruvic acid and two "high energy" electron carrying molecules of NADH.

The two molecules of pyruvic acid also known as pyruvate can be further metabolized to produce more energy. Pyruvate metabolism depends on the amount of oxygen that is available. In organism that are anaerobic or aerobic cells where oxygen levels are low, other molecules besides oxygen must serve as electron acceptors to oxidize NADH to NAD+  so metabolism can continue.  The extraction of energy from carbohydrates and other organic substrates without using oxygen as an electron acceptor is called fermentation.  There are two kinds of fermentation lactate fermentation and ethanol fermentation.

When enough oxygen is available, Pyruvate  is  converted to acetyl-CoA. In animals, the conversion of pyruvate to acetyl-CoA is irreversible. Acetyl-CoA is used for lipid synthesis or as the substrate for the TCA cycle. The conversion of pyruvate to acetyl-CoA in aerobic organisms is a necessary step, since glycolytic pathway extracts a relatively small amount of energy from glucose; most of the energy originally present in glucose is still present in the pyruvate product of glycolysis. In order to extract the remaining energy from pyruvate, the compound must enter the TCA cycle. The conversion of pyruvate to acetyl-CoA removes the fully oxidized carbon while extracting some energy, and prepares the molecule for the remaining process. This conversation occurs in the Pyruvate Dehydrogenase.