Reducing sugars – testing for mono
and some di.
Learning:
• To investigate various foods for reducing
sugars.
Doing:
• Magic 5
• Investigation of reducing sugars.
Magic 5
• Write 5 things you remember about
carbohydrates from last lesson.
Quick quiz
• Put your answers on the work sheet.
Disaccharides
• Glucose linked to glucose ---• Glucose linked to fructose ----• Glucose linked to galactose ------
Testing for reducing sugars
• All mono and some disaccharides are reducing
sugars.
• This means that the sugar can donate
electrons to another chemical (benedict's or
copper II sulphate).
• When reducing sugars are present they form a
brick red precipitate.
Reducing sugars
• All reducing sugars have an aldehyde group,
this is what allows the reduction (loss) of
electrons to the copper sulphate.
How to test for reducing sugars.
1) Add 2cm of the food sample to be tested in
to a test tube. It needs to be in liquid form so
get grinding!
2) Add an equal volume of benedict’s reagent.
3) Heat in a water bath for 5 minutes.
4) Write your results in a table.
Reducing sugar results.
Questions
1) How could you compare concentrations of
reducing sugars, other than colour change?
2) Explain why it is not possible to distinguish
between very concentrated samples?
Testing for non-reducing sugars
• If the sample has not changed colour then it is a
non-reducing sugar.
• Add the non reducing sample to 2cm3 of HCl and
heat for 5 mins (dilute HCl hydrolyses any
disaccharide present).
• Add sodium hydrogen carbonate to neutralise the
acid. Test with pH paper.
• Retest with benedict’s in a water bath.
• If a non reducing sugar is present it will now turn
orange red.
Digestion of carbohydrates
Learning:
• To explain how starch digestion is complete.
• To explain lactose intolerance.
Use the keywords to explain starch
digestion
• Teeth, large surface area.
• Salivary amylase, hydrolysis, maltose, mineral
salt, pH.
• Stomach, acid, denatures
• Small intestine, pancreatic juice.
• Pancreatic amylase, hydrolysis, maltose, alkaline
salts, pancreas, intestinal wall, pH
• Small intestine, epithelial lining, maltase, alpha
glucose.
Lactose intolerance
• Lactose is made up of glucose and galactose
and is hydrolysed by lactase.
• Babies produce large amounts of lactase as
their only food source is milk. As we get older
the lactase levels decrease, in some the levels
reduce to nothing – leading to lactose
intolerance.
• What do you think the symptoms of lactose
intolerance are?
Digestion
• 7 (a) Describe the role of the enzymes of the
digestive system in the complete breakdown
of starch. (5 marks)
• 7 (b) Describe the processes involved in the
absorption of the products of starch digestion.
(5 marks)