Biology · Lesson 28

Practical 5: food tests, enzymes, osmosis, transpiration, photosynthesis and respiration

WAECNECO

What the syllabus asks

WAEC: food tests, enzyme action (amylase, catalase), osmosis and plasmolysis (potato cylinders, isotonic point), transpiration (cobalt chloride, potometer), photosynthesis and respiration experiments, results tables and graphs.

NECO: food tests, enzyme action (amylase, catalase), osmosis and plasmolysis (potato cylinders, isotonic point), transpiration (cobalt chloride, potometer), photosynthesis and respiration experiments, results tables and graphs.

What you will learn

Carry out and interpret food tests.
Describe enzyme experiments with suitable controls.
Explain osmosis and plasmolysis experiments.
Calculate percentage change in mass and find the isotonic point.
Measure transpiration and calculate rates from a potometer.
Describe photosynthesis and respiration experiments and their controls.
Present results in tables and graphs.

Key points

Food tests: iodine, Benedict’s, acid hydrolysis, Biuret, emulsion/grease spot.
Enzymes: optimum temperature and pH; boiled enzyme as control.
Osmosis: % change in mass; isotonic point by interpolation.
Transpiration: cobalt chloride paper, weighing, potometer (πr²d).
Photosynthesis: destarch; light, chlorophyll, CO₂ experiments; starch test.
Respiration: CO₂ (lime water), heat (vacuum flasks), O₂ (respirometer).
Tables with units; graphs with labelled axes and good scale.

Start here: Experiments on the practical paper

Besides specimens, the biology practical paper tests experimental skills: food tests, enzyme action, osmosis, transpiration, photosynthesis and respiration. In a school practical you may perform a short test; in the alternative to practical (and often in the school paper too) you are given a description, a diagram or a table of results and must interpret it.

What the examiners test

Procedure: what you did, in order, with quantities and conditions.
Observation: colours and changes exactly as seen (“blue-black”, “brick-red precipitate”, “bubble moved 4.8 cm”).
Inference/conclusion: what the observation shows (“starch present”).
Control: an identical set-up lacking only the factor being tested, so that any difference is due to that factor.
Precautions and sources of error.
Calculations: percentage change, rate, magnification, mean.

The test–observation–inference table

Food tests are written in a three-column table, like chemistry salt analysis.

Format of a food-test answer
Test	Observation	Inference
To 2 cm³ of specimen solution add 3 drops of iodine solution	blue-black colour	starch present
To 2 cm³ of solution add 2 cm³ Benedict’s solution and heat in a boiling water bath for 5 minutes	blue → green → yellow → brick-red precipitate	reducing sugar present

Write the actual colour change, including the starting colour if asked, and never write a conclusion in the observation column.

Variables

In any experiment identify the independent variable (what you change), the dependent variable (what you measure) and the controlled variables (what you keep the same). Examiners frequently ask: “State two variables that should be kept constant.”

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Practice questions

1. Iodine solution turns blue-black with

starch
glucose
protein
fat

Show answer

A: starch
Starch test.

2. Benedict’s solution on heating gives a brick-red precipitate with

starch
fat
protein
reducing sugar

Show answer

D: reducing sugar
e.g. glucose.

3. Millon’s reagent tests for

fat
sugar
protein
starch

Show answer

C: protein
White then brick-red.

4. Sudan III tests for

reducing sugar
fats and oils
protein
starch

Show answer

B: fats and oils
Red layer.

5. A violet colour with the biuret test shows

protein
starch
fat
vitamin C

Show answer

A: protein
Peptide bonds.

Key words

Control: Set-up identical except for the factor tested.
Denaturation: Permanent change in an enzyme’s shape by heat or extreme pH.
Osmosis: Movement of water through a selectively permeable membrane from dilute to concentrated solution.
Plasmolysis: Shrinking of cytoplasm away from the cell wall due to water loss.
Isotonic: Having the same water potential/concentration as the cell sap.
Potometer: Apparatus measuring the rate of water uptake by a shoot.
Destarching: Keeping a plant in darkness to remove leaf starch.
Emulsion: Fine suspension of oil droplets in water.

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