AICE Biology Lab Review:

Biological Molecules & Enzymes

B. Biological Molecules(a) carry out tests for reducing and non-reducing sugars (including semi-quantitative use of the Benedict’s test), the iodine in potassium iodide solution test for starch, the emulsion test for lipids and the biuret test for proteins;

Benedict’s Test: Reducing & Non Reducing Sugars, Round 1

RESULTS: Aqua-blue = negative.Green to Yellow to orange = positive. Note: to detect the simple sugar sucrose, you must do round 2 test

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

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

Benedict’s 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 Benedict’s testing (Acid Hydrolysis)

Procedure: • Perform round 1 of Benedict’s 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 Benedict’s Testing to see if you have sucrose if

Starch test is Negative.

Sample Procedure for Benedict’s 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???

Benedict’s 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?

Benedict’s 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?

Testing for Starch IKI (Iodine in Potassium Iodide)

RESULTS: Yellow-orange = negative.Purple-black = positive.

Left to right: • IKI only, • starch solution,• starch solution +

IKI.

Emulsion Testing for LipidsSample 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.

Testing for Polypeptides (proteins) Biuret’s Reagent

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

BrR.

RESULTS: Denim-blue = negative.

Lavender = positive.

Identify each sampleFood tests are carried out on four unknown chemicals. The

chart below shows the results of each test.

Solution Benedict’s Test

Acid hydrolysis then Benedict’s

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)

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)

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?

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.

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?

A. 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.