AICE Biology Lab Review: Biological Molecules

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AICE Biology Lab Review Biological Molecules
Enzymes
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B. Biological Molecules

• (a) carry out tests for reducing and non-reducing
  sugars (including semi-quantitative use of the
  Benedicts test), the iodine in potassium iodide
  solution test for starch, the emulsion test for
  lipids and the biuret test for proteins

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Benedicts Test Reducing Non Reducing Sugars,
Round 1
Testing for simple sugars Benedict's Reagent,
round 1
Procedure Add Benedicts reagent to sample, place
in waterbath until just boiling.

• Left to right
• Benedict's reagent (BnR),
• potato extract BnR,
• onion extract BnR,
• 5 glucose BnR.

RESULTS Aqua-blue negative. Green to Yellow
to orange positive. Note to detect the simple
sugar sucrose, you must do round 2 test
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Benedicts Test Reducing Non Reducing Sugars,
Round 2

• A Negative result in round one DOES NOT mean an
  absence of carbohydrates!!

Sucrose is a non-reducing Sugar can only be
detected by doing round 2 of Benedicts testing
(Acid Hydrolysis)

• Procedure
• Perform round 1 of Benedicts Testing. Negative
  result indicates either no carbohydrate OR
  Non-reducing Sugars (Sucrose or starch).
• How would you test for the presence of starch???
• Perform Round 2 of Benedicts Testing to see if
  you have sucrose if Starch test is Negative.

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Sample Procedure for Benedicts Test for Non
Reducing Sugar

• In a test tube place 5 cm3 solution, add 3 cm3
  Benedict's reagent to the solution in the test
  tube and place the tube in the boiling water bath
  for five minutes.
• Add 1 cm3 dilute hydrochloric acid to the
  solution solution in your test tube. Note the
  time and place in the water bath
• After 10 minutes, remove the tube from the water
  bath cool it under the tap. Neutralize the acid
  by adding solid sodium bicarbonate, a little at a
  time, until the addition of one portion produces
  no fizzing.
• With a dropping pipette place 3 cm3 Benedict's
  solution in test tube return to the water bath
  and heat for five minutes.
• A color change indicates that there was Non
  reducing sugar present. How would you verify
  that your solution was sucrose not starch???

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Benedicts Testing Sample Question 1

• Solutions of four food substances are tested for
  sugars. The table shows the colours of the
  solutions after testing.

Which food is a non-reducing sugar?
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Benedicts Testing Sample Question 2

• Four sugar solutions were tested with a standard
  Benedicts solution. The table shows the colour of
  the solutions after testing.

What is the best interpretation of the results?
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Testing for Starch IKI (Iodine in Potassium
Iodide)

• Left to right
• IKI only,
• starch solution,
• starch solution IKI.

RESULTS Yellow-orange negative. Purple-black
positive.
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Emulsion Testing for Lipids

• Sample Procedure
• Add 2cm3 fat or oil to a test tube containing
  2cm3 of absolute ethanol. Dissolve the lipid by
  shaking vigorously. Add an equal amount of cold
  water.
• Observation A cloudy white suspension.

• Basis of test
• Lipids are immiscible with water. Adding water to
  a solution of the lipid in alcohol results in
  emulsion of tiny droplets in the water which
  reflect light and give a white , opalescent
  appearance.

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Testing for Polypeptides (proteins) Biurets
Reagent

• Left to right
• Biuret's reagent (BrR),
• water BrR,
• egg albumin solution,
• egg albumin solution BrR.

RESULTS Denim-blue negative. Lavender
positive.
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Identify each sample

• Food tests are carried out on four unknown
  chemicals. The chart below shows the results of
  each test.

Solution Benedicts Test Acid hydrolysis then Benedicts IKI Biuret Emulsion
A X X X X
B X X X X
C X X
D X X X
(KEY positive result , X negative result)
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C. Enzymes

• (c) follow the time course of an enzyme-catalysed
  reaction by measuring rates of formation of
  products (for example, using catalase) or rates
  of disappearance of substrate (for example, using
  amylase)

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Enzymes
(d) investigate and explain the effects of
temperature, pH, enzyme concentration and
substrate concentration on the rate of
enzyme-catalysed reactions, and explain these
effects
Practice Question 1 The curve X shows the
activity of an enzyme at 20oC. Curves A to D show
the effect of different conditions on the
activity of the enzyme.
Which curve shows the effect of increasing the
temperature by 10o C and adding extra substrate?
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Enzymes Practice Question 2

• The graphs show the effects of temperature and
  pH on enzyme activity.

Which statement explains the enzyme activity at
the point shown? A. At P, hydrogen bonds are
formed between enzyme substrate. B. At Q, the
kinetic energy of enzyme and substrate is
highest. C. At R, peptide bonds in the enzyme
begin to break. D. At S, the substrate is
completely denatured.
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Enzymes Practice Question 3

• The graph shows the effect of substrate
  concentration on the rate of an enzyme-controlled
  reaction. The enzyme concentration is constant.

Which statement about the graph is correct?

• Between W and X, the number of enzyme molecules
  is limiting.
• B. Between X and Y, the number of enzyme
  molecules is limiting.
• C. Between X and Y, the number of substrate
  molecules is limiting.
• D. Between X and Y, the product concentration
  remains the same.