Carbohydrates (Monosaccharides)

CARBOHYDRATES MONOSACCHARIDES Carbohydrates are a class of organic biomolecules composed of carbon (C), hydrogen (H) and oxygen (O). These atoms are typically present in a standard ra,o of (CH2O)n. Carbohydrates are composed of recurring monomers called monosaccharides. These monosaccharides tend to adopt a more energe,cally favourable configura,on by forming a cyclic ring structure. Most monosaccharides contain either five carbons (pentose sugars) or six carbons (hexose sugars). An example of a pentose sugar is ribose – a core component of RNA nucleo,des and ATP. DNA nucleo,des have a modified version missing an oxygen atom (deoxyribose). An example of a hexose sugar is glucose – a monosaccharide that is used preferen,ally by cells as a source of stored chemical energy. Glucose monomers exists as two dis,nct isomers (a-glucose and b-glucose) that differ in structure according to the orienta,on of hydroxyl group (–OH) aUached to the 1’–carbon atom. HOCH2 OH O HOCH2 CH2OH OH O CH2OH OH OH H OH deoxyribose ribose OH OH HO OH O OH O HO OH OH ⍺–glucose β–glucose POLYSACCHARIDES Monosaccharides are covalently joined together by condensa,on polymerisa,on to form polysaccharides. The monosaccharides are connected by glycosidic linkages and water is released as a by-product. The type of polysaccharide formed is determined by the monosaccharides involved and their bonding arrangements. CH2OH CH2OH CH2OH O + HO OH OH O O OH CH2OH O OH OH HO OH OH + OH O HO OH OH OH H2O ENERGY STORAGE One of the primary func,ons of carbohydrates is to act as a short-term energy storage molecule within the cell. Monosaccharides (primarily glucose) can be oxidised via cell respira,on to release the energy stored within their chemical bonds, which can in turn be used to synthesise ATP (the energy currency of the cell). Monosaccharides are suited to this func,on because they are small, polar molecules which are hydrophilic and easy to transport between cells. Organisms will store these glucose monomers as compact polymers that are insoluble due to their size, but can be rapidly digested (via hydrolysis) to mobilise the energy stores of the cell. Monomers of a-glucose are combined via 1’–4’ glycosidic linkages to form long chains, that are then branched via addi,onal 1’–6’ linkages. Plants store these a-glucose sugars as either starch (plants) or glycogen (animals). Starch molecules can either adopt a helical structure (amylose) or a branched structure (amylopec.n). Glycogen is branched, with the 1’–6’ linkages occurring more regularly than in amylopec,n. Starch (amylose) Starch (amylopec,n) Glycogen STRUCTURE Carbohydrates can also func,on as a structural component within a cell. Polymers of b-glucose will form linear strands due to the alterna,ng arrangement of the monomeric subunits (every second b-glucose sugar is inverted in the chain). These linear strands can be cross-linked with hydrogen bonds to form a mechanically stable polysaccharide called cellulose. Cellulose is a principal component of plant cell walls. CH2OH O OH OH OH O CH2OH OH OH OH OH HO O O O O O OH OH CH2OH OH CH2OH OH O O O CH2OH OH O OH CH2OH RECOGNITION Proteins may be complexed to carbohydrates to form glycoproteins. Glycoproteins are commonly employed by the cell as surface markers (anEgens). Human red blood cells can be categorised according to the type of glycoprotein that is present on the cell surface (A or B). These glycoproteins func,on as iden,fica,on tags to allow the immune cells to recognise the blood cells as ‘self'. Blood transfusions with different blood types will be unsuccessful as the immune system aUacks foreign cells). The excep,on is AB blood – these individuals will be compa,ble with all blood types as they possess both the A and the B glycoproteins. A B O AB