3.4 Testing for carbohydrates Flashcards
What solution is used to test for carbohydrates (sugars)
Benedict’s solution (copper sulphate)
Benedict’s test steps
Add 2cm^3 of food sample (liquid form)
Add equal amount of Benedict’s reagent
Heat mixture gently in water bath for 5 mins
Results in benedict’s test for reducing sugars
Reducing sugars react with blue Cu2+ ions in benedict’s
form a red precipitate (brick red)
Method for benedict’s test for non-reducing sugars
Must first be hydrolysed into its monosaccharide components
- first test with benedicts
If there is no colour change with non-reducing sugars…
Add HCl
Heat in water bath for 5 mins
Add alkali sodium hydrogen carbonate and indicator until neutralised
Then reconduct benedict’s test
Example of reducing sugar
Glucose
Example of non-reducing sugar
Sucrose
Why does a red precipitate form with reducing sugars?
The reducing sugar reacts with the blue Cu2+ ions in the reagent which reduces them to Cu + ions which forms a red precipitate
Why must non-reducing sugars need to be hydrolysed before re-conducting the benedict’s test?
It must be broken down into its monosaccharide components
e.g. sucrose breaks down into glucose and fructose (both reducing sugars)
Test for starch and results
Iodine mixed with potassium iodide
If solution turns black = positive result
If solution remains yellow/brown = negative result
What are reagent strips?
Manufactured reagent strips can be used to test for the presence of reducing sugars using a coloured chart to determine the concentration of sugar
What is a colorimeter?
Measures the absorbance/transmission of light by a coloured solution
The more concentrated a solution is, the less light it will transmit which can be used to calculate its concentration from a plotted graph
Steps of colorimetry
1) Filter is placed in colorimeter
2) Calibrated using distilled water
3) Benedict’s test performed on a range of known glucose concentration
4) Solutions filtered to remove precipitate/ centrifuged
5) % transmission calculated by colorimeter
6) Calibration curve plotted
Stages 3-6 repeated with unknown glucose concentration
Why is it important to use a filter in colorimeters?
To maximise absorption
What is the name of the solution after centrifuging/ filtering?
Supernatant
What are biosensors used for?
Use biological molecules to determine the presence and concentration of molecules such as glucose
What happens during molecular recognition in biosensing?
a protein or single strand of DNA is immobilised to test strip which will interact/bind to the molecule being investigated
What happens during transduction in biosensing?
interaction causes a change in the transducer which detects this change and produces a response
e.g. release of dye or electric current
What happens during the display stage of biosensing?
produces visual signal e.g. particular colour or a reading on the test machine
Why are enzymes specialised for use in biosensors?
They have a specialised active site that binds to specific substrates and so can identify only desired molecules
How can iodine be used to prove that photosynthesis requires light?
Starch is a product of photosynthesis and the test shows the starch is produced when light is available but not in darkness
Why are reagent strips good for those with diabetes?
Quantitative
Estimates reducing sugar levels in the blood easily and simply
Simple method
