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AS Biology Cambridge Topic 2 Notes & exam resources for paper 1

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-AS Biology Cambridge Topic 2 full notes covering all points of the sylabus as you go on. contact us on: -contact us about any help you need FOR FREE -Includes diagrams & illustration for further understanding -includes handwritten worked examples for common Paper 1 misconceptions. -Most pa...

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Cambridge AS Biology topics 1,2,3 notes, common misconceptions & many mindmps and diagrams

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1. Interview - As biology cambridge topic 1 full notes and paper 1 solved common misconceptions
2. Interview - As biology cambridge topic 2 notes & exam resources for paper 1
3. Interview - As biology cambridge t-3 enzymes notes + classified p2 2022-2019
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Cambridge AS Biology notes

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T2 : Biological molecules
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Rodayna Elnaggar

,Rodayna Elnaggar (+201021214991)

2.1 Testing for biological molecules

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1. Benedict's test for reducing sugars
(contains parts for p3)
Reducing sugars carry a reduction reaction & can be oxidized
because they have a reducing end (or free functional group)

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Examples of reducing sugars:
glucose, galactose, fructose, lactose, maltose. (include all
monosaccharides)

Benedict's reagent:
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● is copper (ii) sulfate in alkaline solution
● Has a blue color
● Reducing sugars reduce the soluble blue copper sulfate → insoluble brick red copper oxide containing copper (i)
ions
● Positive test for reducing sugar = brick red precipitate
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Procedure:
1) add benedict's reagent to solution & heat in water bath.
2) Boil the mixture for 5 minutes.
3) positive if seen a color change through green → yellow → orange → red-brown (note: remember order of color
changes through ROYGBIV in reverse)
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This is a semi-quantitative test meaning:
● Only when excess reagent is used, the
intensity of the color is related to the
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concentration
● Green having the lowest conc of reducing
sugar
● Red having the highest concentration of
reducing sugar
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● The test is ‘semi-quantitative’ since an
indication of the concentration of
reducing sugar can be estimated by
degree of color change.

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,Rodayna Elnaggar (+201021214991)

Semi-quantitative benedict's test setup
1. Set up standard solutions with known concentrations and color degree
2. Use serial dilution for setting up standard solutions (discussed in detail later)
3. Add benedict reagent to unknown sample as normal.
4. Compare samples to standard.

Other methods for estimating reducing sugar concentrations:
1. Using color standards with known concentrations & comparing with the colors observed.
2. Measure the time taken for the first color change to occur; the solution with the fastest color change = one with

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the highest conc.
3. Use a colorimeter to measure small differences precisely.

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Serial dilutions (p3)
Created by taking a stock solution and decreasing its concentration by the same quantity each time.
Practical explained in following video:
https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:intermolecular-forces-and-properties/x2eef
969c74e0d802:solutions-and-mixtures/v/dilution

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2. Testing for non-reducing sugars
Examples of reducing sugars:
Some disaccharides such as sucrose.

If we add just benedicts to a non reducing sugar, we would get a
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●
negative result.
● Before we add benedict's solution to a non reducing sugar, we must
break the glycosidic bond first →
● Add HCL (hydrochloric acid) to hydrolyse the glycosidic bond by a
hydrolysis reaction
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● The resulting monosaccharides will then have an aldehyde or ketone
functional group that is able to reduce copper (ii) ions to copper (i) &
produce a positive result with benedict's solution.

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, Rodayna Elnaggar (+201021214991)
Procedure:
1. Add dilute hydrochloric acid to sample & boil.
2. Add sodium hydrogen carbonate to neutralize the sample → reason is benedict's only work in alkaline conditions!
3. Add benedict's reagent as normal & boil sample
4. If positive → color change will be observed.

Important note for paper 1:
If both a reducing and non-reducing sugar is present in sample:

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● If only benedict is added to the sample first (test for reducing sugars), then the test for non reducing sugars is
carried out, there will be a color change to darker color.
● example : after testing for reducing sugars the solution changed color to green, then after testing the same
sample for non reducing too, the color turned from green→ brick red.

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● This indicates that also non reducing sugars were present.

● OR the brick red precipitate will be even heavier & darker after testing for non reducing sugar

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3. Iodine test for starch
Procedure:
1. Add a drop of iodine solution to the sample
2. If starch is present sample changes color from yellow/brown to
blue-black
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Why does the test actually work?
● Starch is a spiral macromolecule
● Iodine molecule can fit exactly into the whole that runs down the
middle of the starch spiral
● The starch-iodine complex is what forms the blue-black color.
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4. Ethanol emulsion test for lipids
Procedure:
1. Shake and mix sample with ethanol
2. This allows lipids to dissolve since lipids are soluble in ethanol
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3. Add water to sample
4. If lipids are present – solution turns cloudy
5. If no lipid present → solution remains clear

Why does the solution turn cloudy if lipid is present?
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After adding water to the sample, lipids cannot remain dissolved in ethanol, so lipid molecules form droplets seen as
cloudy emulsion.

5. Biuret test for proteins
● Biuret reagent is made up of: a dilute solution of potassium hydroxide or sodium hydroxide AND dilute solution of
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copper (ii) sulfate

● Sodium potassium tartrate OR sodium citrate stops copper ions reacting with the hydroxide ions and forming a
precipitate.

Procedure:
1. Add biuret reagent so the sample (no heating needed)
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