Introduction
Carbohydrates are essential biomolecules that serve as a primary source of energy, structural components, and signaling molecules in living organisms. They are composed of carbon, hydrogen, and oxygen, typically following the empirical formula \( (CH2O)n \).
Structure of Carbohydrates
Monosaccharides
Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They are the building blocks of more complex carbohydrates.
- Glucose: An aldohexose and the most important energy source in cells.
- Fructose: A ketohexose found in fruits and honey.
- Galactose: An aldohexose that is part of lactose and other polysaccharides.
Disaccharides
Disaccharides are composed of two monosaccharide units linked by a glycosidic bond.
- Sucrose: Composed of glucose and fructose; common table sugar.
- Lactose: Composed of glucose and galactose; found in milk.
- Maltose: Composed of two glucose molecules; a product of starch digestion.
Polysaccharides
Polysaccharides consist of long chains of monosaccharide units and can be either linear or branched.
- Starch: A storage form of glucose in plants, composed of amylose and amylopectin.
- Glycogen: A highly branched storage form of glucose in animals, primarily in liver and muscle cells.
- Cellulose: A structural component of plant cell walls, composed of beta-glucose units.
Metabolism of Carbohydrates
Carbohydrate metabolism is crucial for energy production and involves several key pathways.
Glycolysis
Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH in the process. It occurs in the cytoplasm of cells and is anaerobic.
Key Steps:
- Glucose to Glucose-6-phosphate: Catalyzed by hexokinase; ATP is consumed.
- Fructose-6-phosphate to Fructose-1,6-bisphosphate: Catalyzed by phosphofructokinase; a major regulatory step.
- Fructose-1,6-bisphosphate to Glyceraldehyde-3-phosphate and Dihydroxyacetone phosphate: Split into two 3-carbon molecules.
- Glyceraldehyde-3-phosphate to 1,3-Bisphosphoglycerate: Produces NADH.
- 1,3-Bisphosphoglycerate to 3-Phosphoglycerate: Generates ATP.
- 2-Phosphoglycerate to Phosphoenolpyruvate (PEP).
- Phosphoenolpyruvate to Pyruvate: Catalyzed by pyruvate kinase; ATP is produced.
Pyruvate Metabolism
- Aerobic Conditions: Pyruvate enters the mitochondria and is converted into acetyl-CoA, entering the citric acid cycle.
- Anaerobic Conditions: Pyruvate is converted into lactate in animals (lactic acid fermentation) or ethanol and CO2 in yeast (alcoholic fermentation).
Key Products:
- 2 Pyruvate molecules
- 2 ATP (net gain)
- 2 NADH
Functions of Carbohydrates
- Energy Production: Carbohydrates are the primary source of energy in most organisms. Glucose is the main fuel for cellular respiration.
- Energy Storage: Glycogen in animals and starch in plants serve as energy reserves.
- Structural Role: Cellulose provides structural support in plant cell walls, and chitin is an important component of the exoskeletons of arthropods.
- Cell Signaling and Recognition: Carbohydrates are involved in cell-cell communication and recognition through glycoproteins and glycolipids on the cell surface.
Summary
Carbohydrates are vital for energy storage, structural integrity, and cellular communication. Their metabolism through pathways like glycolysis is central to cellular energy production.
References
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2015). Biochemistry. W.H. Freeman and Company.
- Voet, D., & Voet, J. G. (2011). Biochemistry. John Wiley & Sons.
- Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry. W.H. Freeman and Company.