Sugars and Carbohydrates

• What is the definition of a monomer?

  The smaller units from which larger molecules are made.
• What is the definition of a polymer?

  Molecules made from a large number of monomers joined together.
• What is a hexose sugar?

  A hexose sugar is a monosaccharide that consists of 6 carbon atoms.
• What is the definition of a disaccharide?

  Two monosaccharides bonded together by a condensation reaction
• What is the definition of a condensation reaction?

  Formation of a chemical bond between two molecules and involves the elimination of a molecule of water.
• What is a glycosidic bond?

  The bond between two monosaccharides.
• What is the definition of a hydrolysis reaction?

  A reaction that breaks the chemical bond between two molecules and involves the use of a water molecule.
• What is the definition of a structural isomer?

  Molecules that have the same molecular formula, but have different arrangement of atoms. Therefore they have different properties and behaviours even though they have the same number of atoms of each element.
• What are monosaccharides and what are the common ones?

  They are the monomers from which larger carbohydrates are made. The common ones are glucose, fructose and galactose.
• What are the three disaccharides you need to know and what two monosaccharides are they made up of?

  Maltose - Two alpha-glucose molecules

  Sucrose - alpha-glucose and fructose

  Lactose - alpha-glucose and galactose
• What is the structure of alpha-glucose?

  Both OH- ions are on the bottom.

  
• What is the structure of beta-glucose ?

  The OH- ions are on opposite sides, one on the bottom on on top.

  
• What is the definition of reducing sugars?

  Sugars that donate electrons to another chemical e.g. glucose, fructose, galactose, lactose, maltose.
• What is the definition of non-reducing sugars?

  Sugars that do not donate electrons to other chemicals, sucrose.
• What do you have to do before testing non-reducing sugars?

  They must be hydrolysed into it's monosaccharide components by hydrolysis, adding an acid.
• Why does a colour change occur if reducing sugars are present in a benedict's test?

  Benedict's reagent is an alkaline solution of copper(II) sulphate, that is a blue colour. The reducing sugars donate an electron meaning copper(I) oxide is formed, which is an insoluble red precipitate.
• What is the chemical test for starch?

  1. add 2cm$^3$ of the sample to a test tube.
  2. add a few drops of iodine/potassium iodide solution to the sample, gently shake.
  3. positive result=blue/black colour
  4. negative result=orange/brown colour
• What are qualitative tests?

  Tests that help identify the presence(or absence) of an ion or compound in a solution. Usually a colour change is observed.
• What are quantitative tests?

  Tests that analyse the amount of an ion or compound or chemical that is in a sample.
• What is the test for reducing sugars?

  1. add 2cm$^3$ of the sample to a test tube in liquid form, if not grind up using pestle and mortar and add distilled water then filter.
  2. add 2cm$^3$ of benedict's reagent
  3. heat mixture in gently boiling water at 80degrees Celsius for 5 minutes. This is to ensure there is enough energy for the reaction to begin.
  4. positive: green - very low conc., yellow - low conc., brown/orange - medium conc., brick/red - high conc.
  5. negative - blue
• What is the test for non-reducing sugars?


  1. First test for reducing sugars, if negative continue.
  2. add 2cm$^3$ of the sample to a test tube.
  3. add 2cm$^3$ of hydrochloric acid to the test tube.
  4. Heat in a water bath at 100 degrees for 5 minutes.
  5. neutralise using sodium hydrogen carbonate, then test pH using red litmus paper
  6. add 2cm$^3$ of benedict's reagent to the test tube
  7. gently heat at 80 degrees for 5 minutes in a water bath
  8. The results will be the same as those for reducing sugars.
• Why is the benedict's test semi-quantitative?

  The colour change shows how low the concentration of sugar is in the sample but does not give an exact numerical value.
• Why are food tests important in biology?

  They are important to identify nutrients and aid in understanding nutritional content.
• What are the limitations of a benedict's test?

  It is semi-quantitative, produces no numerical-values, it can be difficult to describe the colour change.
• How to make a known concentration, e.g. 2 mmol dm$^{-3}$?


  1.Determine the total volume for each concentration, 2cm$^3$.

  2. The standard molarity is the highest concentration.

  3. we need 2/10 x 100 = 20% glucose.

  4. 2/100 x 20 = 0.4cm$^3$ of glucose

  5. the rest of the volume is water 2 - 0.4 = 1.6cm$^3$
• What are polysaccharides and what are some examples?

  Polysaccharides are polymers formed by combining many monosaccharide molecules, they are insoluble, so are suitable for storage and structural support to plant cells. Examples include: starch, glycogen and cellulose.
• What is the structure and function of glycogen?

  Structure: alpha glucose form 1-4 and 1-6 bonds which form branches for a larger surface area, it coils slightly for storage. It has more 1-6 bonds then starch.

  Function: it’s a major carbohydrate store in animal cells, stored as small granules in the muscle and liver.
• How do the structure and function of glycogen relate to eachother?

  Insoluble - does not allow water into cell by osmosis

  Large - does not diffuse out of cells

  Compact - stores a lot in a small space

  Highly branched - larger surface area so can be acted on by more enzymes so more rapidly hydrolysed for respiration, as animals have a higher metabolic rate therefore also have a higher respiratory rate so require more glucose.
• What is the structure and function of starch?

  Structure: alpha glucose chains that are made up of amylose and amylopectin. Amylose is made up of 1-4 bonds only and is linear, amylopectin is branched and made up of 1-4 and 1-6 bonds. It coils for storage.

  Function: energy storage molecule, found in plants, stored as small grains that occur in seeds, it is an important component of food.
• How do the structure and function of starch relate to eachother?

  large - does not diffuse out of a cell

  insoluble - does not allow water into cell by osmosis so does not affect the water potential

  compact - lots can be stored in a small space

  helical - highly branched making them helical so are compact for storage in cell
• What is the structure and function of cellulose?

  Structure: made of beta-glucose that form long, straight, unbranched chains by 1-4 bonds, parallel strands form microfibrils, hydrogen bonds link parallel strands together making the structure turgid.

  Function: provides strength and structural support to plant cell walls
• How do the structure and function of cellulose relate to eachother?

  inverted- every other beta-glucose is inverted

  hydrogen bonds- they are weak on there own but in large numbers make the structure strong

  turgid structure- prevents cell from bursting as water enters by exerting an inward pressure which stops an influx of water

  microfibrils- provide maximum surface area for photosynthesis