Carbohydrates

• Key molecules required to build structures enabling organisms to function:

  • Carbohydrates
  • Proteins
  • Lipids
  • Nucleic Acids
  • Water
• Monomers are smaller units from which larger molecules are made, while polymers are molecules made from a large number of monomers joined together in a chain
• Carbohydrates, proteins, lipids, and nucleic acids contain carbon (C) and hydrogen (H), making them organic compounds
• Carbon atoms can form four covalent bonds, can bond with oxygen, nitrogen, and sulfur, and can form straight chains, branched chains, or rings
• Carbon compounds can form small single subunits (monomers) that bond with many repeating subunits to form large molecules (polymers) through polymerisation
• Carbohydrates are one of the main carbon-based compounds in living organisms, containing C, H, and O in a 2:1 ratio
• The three types of carbohydrates are monosaccharides, disaccharides, and polysaccharides
• A covalent bond is the sharing of two or more electrons between two atoms, forming stable bonds that require high energy to break
• Condensation (dehydration synthesis) occurs when monomers combine to form polymers or macromolecules, with the removal of water
• Hydrolysis breaks covalent bonds in polymers when water is added
• Sugars can be classified as reducing or non-reducing based on their ability to donate electrons
• Glucose is a common monosaccharide with the molecular formula C6H12O6, existing in two forms: alpha (α) glucose and beta (β) glucose
• Different polysaccharides are formed from the two isomers of glucose
• To make monosaccharides more suitable for transport, storage, and to have less influence on a cell’s osmolarity, they are bonded together to form disaccharides and polysaccharides
• Every glycosidic bond results in one water molecule being removed, thus glycosidic bonds are formed by condensation
• Disaccharides and polysaccharides are formed when two hydroxyl (-OH) groups on different saccharides interact to form a strong covalent bond called the glycosidic bond
• Each glycosidic bond is catalyzed by enzymes specific to which OH groups are interacting, resulting in different types of glycosidic bonds forming (e.g., maltose has an α-1,4 glycosidic bond and sucrose has an α-1,2 glycosidic bond)
• The glycosidic bond is broken when water is added in a hydrolysis reaction, catalyzed by enzymes different from those present in condensation reactions
• Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration
• Paper chromatography is a technique used to separate a mixture into its individual components based on differences in solubility, using a mobile phase and a stationary phase
• In paper chromatography, the mobile phase is the solvent, and the stationary phase is the chromatography paper; larger molecules move slower than smaller ones, causing the mixture to separate into different spots or bands on the paper
• Paper chromatography can be used to separate a mixture of monosaccharides by staining the sample, placing it on chromatography paper, adding known standard solutions of different monosaccharides, and identifying the unknown monosaccharides by comparing their chromatograms
• Monosaccharides can join together via condensation reactions to form disaccharides, releasing a water molecule in the process; the new chemical bond that forms between two monosaccharides is known as a glycosidic bond
• The new chemical bond that forms between two monosaccharides is known as a glycosidic bond
• To calculate the chemical formula of a disaccharide, you add all the carbons, hydrogens, and oxygens in both monomers then subtract 2× H and 1× O (for the water molecule lost)
• Common examples of disaccharides include:

  • Maltose (the sugar formed in the production and breakdown of starch)
  • Sucrose (the main sugar produced in plants)
  • Lactose (a sugar found only in milk)
• The disaccharide maltose is formed from two α-glucose monomers (sub-units)
• The disaccharide sucrose is formed from α-glucose and fructose monomers (sub-units)
• Starch and glycogen are polysaccharides formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form chains
• Starch is the storage polysaccharide of plants, stored as granules in plastids like chloroplasts
• Starch is constructed from two different polysaccharides:

  • Amylose (unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules)
  • Amylopectin (branched molecule with 1,4 and 1,6 glycosidic bonds between glucose molecules)
• Glycogen is the storage polysaccharide of animals and fungi, highly branched and not coiled
• Cellulose is a polysaccharide consisting of long chains of β-glucose joined together by 1,4 glycosidic bonds
• Cellulose is used as the main structural component of cell walls due to its strength from the many hydrogen bonds found between the parallel chains of microfibrils
• Cellulose fibers are freely permeable, allowing water and solutes to leave or reach the cell surface membrane
• Benedict's test for reducing sugars:

  • Benedict's reagent is a blue solution containing copper (II) sulfate ions
  • In the presence of a reducing sugar, copper (I) oxide forms as a precipitate
  • A positive test result is a color change from blue (no reducing sugar) to green, yellow, orange (low to medium concentration), to brown/brick-red (high concentration)
• Test for non-reducing sugars:

  • Add dilute hydrochloric acid to the sample and heat in a water bath
  • Neutralize with sodium hydrogencarbonate
  • Use an indicator to identify neutralization, then add more sodium hydrogencarbonate for slightly alkaline conditions for the Benedict's test
  • Carry out the Benedict's test: if a color change occurs, a reducing sugar is present
• Test for starch:

  • Add iodine in potassium iodide solution to the sample
  • If starch is present, a blue-black color complex forms
• Finding the concentration of glucose using Benedict's solution:

  • Semi-quantitative test to determine the concentration of reducing sugar
  • Use standard solutions with known concentrations of a reducing sugar for comparison
  • Carry out the test by adding Benedict's solution to each sample and heating in a water bath
• Serial dilutions:

  • Created by taking a series of dilutions of a stock solution
  • Concentration decreases by the same quantity between each test tube
  • Used for comparing unknown concentrations against standards