carbohydrates ( mono and disaccharides ) Flashcards
what elements do carbohydrates contain?
carbon, hydrogen and oxygen
2 hydrogen : 1 oxygen
what 3 groups can carbohydrates be classifies into?
monosaccharides, disaccharides and polysaccharides
what are some properties of monosaccharides and disaccharides?
small, soluble molecules that are easy to transport and sweet to taste
what are monosaccharides?
single sugars that are the basic molecular units (monomers) of carbohydrates
what are monosaccharides mainly used in?
respiration to provide energy and growth during the formation of larger carbohydrates
what are examples of monosaccharides?
glucose, fructose and galactose which all have the same formula C6H12O6 (hexoses) but different structure
are monosaccharides reducing sugars?
yes - so they give a positive result for benedict’s test
draw the structure of alpha glucose
see diagram
how is a disaccharide formed?
a disaccharide is formed when when two monosaccharides are joined together by a glycosidic bond in a condensation reaction
what is a condensation reaction?
condensation reaction is a reaction in which smaller molecules join together to form a larger molecule with the release of water
what are examples of disaccharides?
sucrose, maltose, lactose
how are these disaccharides formed?
glucose + glucose → maltose + water
glucose + fructose → sucrose + water
glucose + galactose → lactose + water
C6H12O6 + C6H12O6 → C12H22O11 + H2O
how is a disaccharide broken down?
a disaccharide can be broken down into it’s monosaccharides during a hydrolysis reaction
what is a hydrolysis reaction?
hydrolysis is the breakdown (lysis) of a larger molecule into smaller molecules by the addition of water
how are disaccharides hydrolysed?
by boiling with acid eg dilute HCl or by heating with an enzyme at optimum temperature
are disaccharides reducing sugars?
yes - except sucrose
describe how you test for reducing sugars
heat in a water bath with benedict’s solution. a brick red colour indicates a positive result. if the benedict’s solution remains blue, so reducing sugar is present.
LONG ANSWER : benedict’s test. add a small amount of the sample to a test tube with 2cm^3 of benedict’s solution. heat in a water bath for 5 minutes. a brick red or orange colour (produced by copper (I) oxide) is a positive result. if the benedict’s solution remains blue, no reducing sugar is present.
how do you standardise the benedict’s test?
use the SAME VOLUME of benedict’s solution, the SAME VOLUME of the test solutions and the samples must be heated for the SAME AMOUNT OF TIME
how do you visually tell how much reducing sugar is present and what is the issue with this?
an intense brick red colour indicates a large amount of reducing sugar was present whereas a yellow or green colour shows a small amount was present. the issue with this is that intensity of colour is subjective.
what is a better way to measure how much reducing sugar is present?
use a colorimeter - ??
how do you test for non reducing sugars?
carry out benedict’s test on a small sample to confirm a negative result. hydrolyse another sample by boiling it with dilute acid eg HCl. when cooled, add dilute sodium hydroxide solution (NaOH) to neutralise the acid. add benedict’s solution and heat in a water bath for 5 minutes. a positive brick red colour indicates a non reducing sugar was originally present.
