3.1Unit 3: Practical Skills in Biology I
Practical Skills in Biology I — IAL Edexcel Modular Revision Notes & Topical Questions
5 min read 11 parts Lesson 1 of 2 2 quizzes
Lesson summary
3.1 Practical Skills in Biology I — IAL EDEXCEL MODULAR notes: Plan safe investigations and carry out the AS core practicals on sugars, vitamin C, membrane permeability and enzyme rate.Testable question — the investigation changes one independent variable, such as temperature or …
- Control variables
- Measurement plan
- Eye and skin protection
- Biological samples and blades
- Negative result
- Estimating concentration
Part 1 of 9Planning a fair investigation
Testable question
- Testable question the investigation changes one independent variable, such as temperature or concentration.
Dependent variable
- Dependent variable it is the measurement that gives evidence of the effect.
Control variables
- Control variables they are kept constant so any difference can be attributed to the independent variable.
Control
- Control a comparison sample, for example without the substance being tested.
Repeats
- Repeats repeat readings and calculate a mean, and identify an anomalous result rather than quietly deleting it.
Measurement plan
- Measurement plan state the range, intervals, units and precision before collecting data.
Part 2 of 9Risk assessment
1Eye and skin protection
- Eye and skin protection it is needed with hot water, glassware, Benedict's reagent or DCPIP.
2Heating
- Heating use a water bath where controlled heating is needed.
3Biological samples and blades
- Biological samples and blades handle them carefully.
4
Each hazard is matched to a precaution.
Part 3 of 9Practical 1: reducing sugars and Benedict's test
1
Add excess blue Benedict's reagent to a measured sample.
2
Heat in a boiling water bath for a few minutes.
3
A reducing sugar converts copper(II) ions to insoluble copper(I) oxide.
4
Green, yellow, orange then brick-red precipitate show increasing amounts of reducing sugar.
Negative result
- Negative result the solution stays blue.
Estimating concentration
- Estimating concentration prepare standards of known concentration by serial dilution and treat each with the same reagent volume, heating time and temperature.
- A colorimeter and calibration graph give a more objective estimate than colour by eye.
- Repeat each standard and the unknown.
Part 4 of 9Practical 1: non-reducing sugars and starch
Non-reducing sugars
- Non-reducing sugars if the sample is Benedict's-negative, boil a fresh portion with dilute hydrochloric acid to hydrolyse glycosidic bonds.
- Neutralise with sodium hydrogencarbonate until slightly alkaline, then repeat Benedict's test.
- A new positive result shows a non-reducing sugar was present.
Starch
- Starch iodine solution changes from orange-brown to blue-black.
- Starch concentration can be estimated from standards and a colorimeter calibration curve.
Part 5 of 9Practical 2: vitamin C and DCPIP
- Principle
- DCPIP is blue and becomes colourless when reduced by vitamin C (ascorbic acid).
1
Place a fixed volume of DCPIP in a tube.
2
Add vitamin C solution drop by drop, mixing in a consistent way, until the blue colour disappears.
3
Record the volume or number of drops needed, repeat and calculate the mean.
4
Repeat with a series of known vitamin C concentrations and plot a calibration curve of mean volume against concentration.
5
Test a food or drink sample under the same conditions and read its concentration from the graph.
- Controlled variables
- DCPIP volume, drop size, mixing time and sample preparation.
Part 6 of 9Practical 3: membrane permeability
- Principle
- beetroot cells contain a red-purple pigment, and more pigment in the surrounding water indicates greater membrane permeability.
Practical 3: membrane permeability
Cut equal-sized beetroot pieces with a cork borer and rinse away pigment released by cutting.
Place each piece in the same volume of water for the same time, varying one condition such as temperature.
Remove the beetroot and measure absorbance of the liquid with a colorimeter.
Result
- Result higher absorbance means more pigment has leaked out.
- Rising temperature can disrupt the phospholipid bilayer and denature membrane proteins, increasing permeability.
Controlled variables
- Controlled variables beetroot size, water volume and exposure time, with each temperature repeated and means compared.
Part 7 of 9Practical 4: enzyme reaction rate
- Principle
- catalase breaks down hydrogen peroxide into water and oxygen.
1
Collect oxygen over water in an inverted measuring cylinder.
2
Record the volume of oxygen at fixed time intervals.
3
Change one factor, such as catalase concentration, keeping hydrogen peroxide volume and concentration, buffer, temperature and total volume constant.
4
Repeat each condition and calculate the mean oxygen volume at each time point.
Rate
- Rate the initial gradient of an oxygen volume–time graph estimates the initial reaction rate.
Enzyme concentration
- Enzyme concentration more enzyme gives more active sites while substrate remains, and at high enzyme concentration substrate becomes limiting.
Temperature and pH
- Temperature and pH higher temperature increases collision frequency, but excess heat denatures the active site, and extreme pH also changes its shape.
Part 8 of 9Recording and presenting results
Recording and presenting results
Tables
- Tables units go in the column headings and means are given to suitable significant figures for the equipment.
Graphs
- Graphs the independent variable is on the horizontal axis and the dependent variable on the vertical axis, with axes labelled with variable and unit.
Conclusions
- Conclusions describe the trend with data values, then explain it using the biology.
Repeatability and validity
- Repeatability and validity repeats improve confidence in a pattern but cannot repair an uncontrolled variable.
Improvements
- Improvements a specific limitation is matched with an improvement, such as using a colorimeter instead of judging colour by eye.
Part 9 of 9More from the specification
More from the specification
Reducing sugar aldehyde carbonyl group
- Reducing sugar aldehyde carbonyl group the free aldehyde group on glucose reduces alkaline copper(II) ions in Benedict's reagent to insoluble red copper(I) oxide.
Condensation Synthesis of Disaccharides
- Condensation between two alpha-glucose monosaccharide molecules forms a 1,4-glycosidic bond, releasing a water molecule to produce the disaccharide maltose.
Triglyceride Structure and Energy Density
- A triglyceride molecule is assembled from one glycerol backbone esterified to three fatty acid tails, yielding highly reduced, hydrophobic macromolecules ideal for compact long-term metabolic energy storage.
Saturated versus Unsaturated Fatty Acid Chains
- Saturated fatty acids contain only single carbon-carbon covalent bonds,
- forming straight chains that pack densely into solid fats linked to elevated cardiovascular risk,
- whereas unsaturated chains contain kinks from double bonds.
Covalent bond
- A bond in which atoms share a pair of electrons.
Molecule
- Two or more atoms bonded together.
Glucose
- A sugar that stores energy.
Condensation
- Joining of monomers with loss of water.
Key terms
| Term | Meaning |
|---|---|
| Independent variable | The variable deliberately changed in an investigation. |
| Dependent variable | The variable measured to show the effect of the independent variable. |
| Calibration curve | A graph of known concentrations used to estimate an unknown. |
| Reducing sugar | A sugar that gives a positive Benedict's test. |
| DCPIP | A blue dye decolourised by vitamin C. |
| Colorimeter | An instrument that measures light absorbed by a coloured solution. |
Avoid these mistakes
1
Changing more than one variable at once — only the independent variable should change.
2
Concluding that a blue solution after heating proves no sugar of any kind — non-reducing sugars give a negative Benedict's result until hydrolysed.
3
Forgetting to rinse beetroot pieces — pigment released by cutting gives false high readings.
4
Using a single reading as the result — repeat and use the mean, and identify anomalies.
Marked practiceQuizzes for this lesson
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