Biological Molecules

• Carbohydrates contain carbon (C), hydrogen (H) and oxygen (O).
• Carbon atoms readily form bonds with other carbon atoms. This allows a sequence of carbon atoms which forms a 'backbone' where other atoms can be attached.
• Monomer: one of many small molecules that combine to form a larger one known as a polymer.

  Examples: monosaccharides, amino acids, nucleotides.
• Polymer: a large molecule made up of repeating small molecules (monomers).

  Examples: proteins, nucleic acids, starch.
• Monosaccharides are the simplest carbohydrates and are made up of one sugar unit.

  Examples: glucose, galactose, fructose.
• Reducing sugars: a sugar that can donate electrons to (or reduce) another chemical (in the test, this is Benedict's reagent).

  Examples: all monosaccharides and some disaccharides (e.g. maltose).
• Benedict's reagent is an alkaline solution of copper (II) sulfate.

  When a reducing sugar is heated with Benedict's reagent it forms an insoluble red precipitate of copper (I) oxide.
• Test for Reducing Sugars

  • Add 2cm3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up in water.
  • Add an equal volume of Benedict's reagent.
  • Heat the mixture in a gently boiling water bath for five minutes.
  • If there's a reducing sugar present, the solution will turn orange-brown. The closer to red a solution is, the higher the concentration of reducing sugar present.
• When combined in pairs, monosaccharides form disaccharides.

  For example:

  • Glucose + Glucose --> Maltose
  • Glucose + Fructose --> Sucrose
  • Glucose + Galactose --> Lactose
• Monosaccharides join by condensation reactions (where a molecule of water is removed) to form disaccharides. The bond formed is called a glycosidic bond.
• Hydrolysis: addition of water that causes breakdown. Breaks the glycosidic bond in a disaccharide, releasing the two monosaccharides.
• Non-reducing sugars do not change the colour of Benedict's reagent when heated with it. Some disaccharides, such as sucrose, are non-reducing sugars. To detect a non-reducing sugar, it must first be hydrolysed into its monosaccharide components by hydrolysis.
• Test for Non-Reducing Sugars

  1. Sample must be in liquid form

  2. Add 2cm3 sample to 2cm3 Benedict's reagent in a test tube and filter

  3. Place in a gently boiling water bath for 5 minutes

  4. No colour change=no reducing sugar present

  5. Add 2cm3 sample to 2cm3 dilute hydrochloric acid in a test tube and place in a gently boiling water bath for 5 minutes

  6. Slowly add sodium hydrogencarbonate to neutralise the acid

  7. Test with pH paper to check that the solution is alkaline

  8. Re-test: if a non-reducing sugar was originally present, there will now be a colour change due to the reducing sugars produced from hydrolysis of the non-reducing sugar
• Polysaccharides: polymers, formed by combining together many monosaccharide molecules by glycosidic bonds formed in condensation reactions.
• Test for Starch

  • Room temperature
  • Place 2cm3 of the sample being tested into a test tube (or add two drops into a depression on a spotting tile).
  • Add two drops of iodine solution and shake or stir.
  • The presence of starch is indicated by a blue-black colouration.
• Starch is the major energy source in most diets and forms an important component of food. It is made up of branched or unbranched chains of alpha glucose monosaccharides linked by glycosidic bonds formed during condensation reactions.
• The main role of starch is energy storage in plants- never found in animal cells.
• Starch is insoluble and therefore doesn't affect the water potential, so water is not drawn into the cells by osmosis.
• Starch is large and insoluble, so doesn't diffuse out of cells.
• Starch has unbranched chains of alpha glucose which are wound into a tight coil, making the molecule very compact, so a lot of it can be stored in a small space.
• When starch is hydrolysed, it forms alpha glucose, which is both easily transported and readily used in respiration.
• The branched form of starch has many ends, each of which can be acted on by enzymes at the same time, meaning glucose monomers are released very rapidly.
• Glycogen is found in animals and bacteria, but never in plant cells.
• Glycogen in very similar in structure to starch but has shorter chains and is more highly branched. It is sometimes called 'animal starch' as it is the major carbohydrate storage product in animals.
• In animals, glycogen is stored as small granules mainly in the muscles and liver. However, the mass of carbohydrate stored is relatively small as fat is the main storage molecule in animals.
• Glycogen is insoluble and therefore does not affect the water potential of cells, meaning water doesn't tend to be drawn into cells by osmosis.
• Glycogen is insoluble and so does not diffuse out of cells, making it a suitably-structured storage molecule.
• Glycogen is compact, so a lot of it can be stored in a small space.
• Glycogen is more highly-branched than starch, meaning there are more ends for enzymes to act upon in order to break it down. This means that it can be broken down more quickly into its glucose monomers, which are used in respiration. This is useful in animals as they have a higher metabolic rate than plants because they are more active.
• Cellulose is made of beta glucose monomers, unlike starch and glycogen, which are made up of alpha glucose monomers.
• Cellulose has straight, unbranched chains of beta glucose which run parallel to each other. This allows hydrogen bonds between chains which, although individually adding very little, make a considerable contribution to strengthening cellulose due to the sheer number of them.
• In cellulose chains, adjacent glucose molecules are linked by 1-4 glycosidic bonds and rotated 180 degrees. This is what allows hydrogen bonds to be formed.
• Cellulose is a major component of cell walls in plants and provides rigidity to the plant cell. The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis by exerting an inwards pressure which stops any further influx of water. This means living plant cells are turgid and push against each other, making non-woody parts of the plant semi-rigid. This is especially important in maintaining the turgidity of stems and leaves so they can provide the maximum surface area for photosynthesis.
• Cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres, all of which provides more strength.