Mod1: Practical Skills Flashcards – OCR A Biology

Practical questions according to spec

What apparatus is used to measure mass accurately?

Electronic balance (to ±0.01 g or more precise depending on balance).

What apparatus is used to measure time accurately?

Stopwatch or data logger.

What apparatus is used to measure volume precisely?

Pipette or burette (more precise than measuring cylinder).

What apparatus is used to measure temperature?

Thermometer or temperature probe.

What apparatus is used to measure length in microscopy?

Eyepiece graticule calibrated with a stage micrometer.

How to calibrate an eyepiece graticule?

Using a stage micrometer to align measurements and convert the eyepiece scale to actual length

What apparatus is used to measure pH accurately?

pH probe (more precise than universal indicator).

Why should appropriate apparatus be chosen?

To increase accuracy and reduce percentage uncertainty.

What does a colorimeter measure?

Absorbance of light to determine concentration.

Why use a colorimeter instead of judging colour by eye?

Provides quantitative, objective measurement.

What does a potometer measure?

Rate of water uptake (used as an estimate of transpiration rate).

Why is a potometer not a direct measure of transpiration?

It measures water uptake, not actual water loss.

What is the reason for cutting the stem under water when setting up a potometer?

Prevents air entering the xylem (avoids air embolism).

Why is the stem inserted into the potometer while still under water?

Ensures no air enters the xylem

What is the purpose of sealing all joints with petroleum jelly?

Prevents air leaks which would invalidate results

Why should the apparatus be removed from water and dried carefully?

So you can see the bubble clearly

What is the purpose of introducing a single air bubble into the capillary tube?

This bubble acts as a marker to measure water uptake

How do you measure the rate of water uptake using a potometer?

Measure the distance the bubble moves over a set time

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What is a serial dilution?

Stepwise dilution of a solution to produce a range of known concentrations.

Why are serial dilutions used?

To create a range of concentrations for calibration or investigation.

Why must pipettes be rinsed between dilutions?

to avoid contamination and ensure accurate concentration.

Why calibrate an eyepiece graticule?

To convert arbitrary units into micrometres.

What happens to field of view when magnification increases?

Field of view decreases.

Why use low power first?

To locate specimen before switching to high power.

What are the key rules for biological drawings?

Clear lines, no shading, label with straight lines.

Why should drawings be large and detailed?

To show clear structural features.

What test identifies reducing sugars?

Benedict’s test (blue → green/yellow/orange/red on heating).

What test identifies starch?

Iodine (brown → blue-black).

What test identifies protein?

Biuret (blue → lilac).

What test identifies lipids?

Emulsion test (white cloudy suspension).

Test for NON-reducing sugars (e.g Sucrose)

1. Add dilute HCl

2. Heat (breaks glycosidic bond)

3. Neutralise with sodium hydrogencarbonate

4. Add Benedict’s + heat

If it now turns orange/brick-red → non-reducing sugar was present.

What does Acid-Hydrolysis do?

breaks glycosidic bonds to convert non-reducing sugars into reducing sugars

What does paper chromatography separate based on?

Differences in solubility and attraction to stationary phase.

What is an Rf value?

Distance moved by substance ÷ distance moved by solvent.

What does electrophoresis separate based on?

Size and charge of molecules in an electric field.

Why minimise harm to organisms in experiments?

Ethical considerations and legal requirements.

Why use the smallest sample size necessary?

To reduce harm and environmental impact.

Why flame the neck of a culture bottle?

To kill microorganisms and prevent contamination.

Why tape agar plate lids partially rather than fully seal?

To allow oxygen in but reduce contamination risk.

Why incubate plates at 25°C in schools?

To reduce growth of harmful human pathogens.

Accuracy

How close a measured value is to the true value

Precision

How close repeated measurements are to each other.

How can accuracy be improved?

Calibrating equipment

Using more precise apparatus

How can precision be improved?

Using equipment with smaller scale divisions

Reducing random error

Reliability

Consistency of results when repeated

How can reliability be improved?

Repeating the experiment, identifying any anomalies and calculating a mean

Validity

Whether the method measures what it is intended to measure.

How is validity improved?

controlling variables

Removing confounding factors

Reproducibility

Whether the same results can be obtained by different people or labs.

Repeatability

Whether the same person gets similar results using the same method.

What is random sampling?

Selecting sample locations by chance to reduce bias.

What is systematic sampling?

Sampling at regular intervals along a transect.

Why use a data logger?

To collect continuous, precise measurements.

Why use computer modelling?

To simulate biological processes and analyse patterns.