6.1.4Unit 6: Practical Skills in Biology II
Core Practical 13: Rate of Growth of Microorganisms — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Setting Up the Culture
1
Disinfect the work area, and prepare a conical flask containing a glucose solution as the liquid culture medium.
2
Add a measured mass of dried yeast using aseptic technique, working close to a Bunsen flame.
3
Seal the flask with cotton wool so that air can enter but contaminants cannot, then swirl to mix.
4
Place the flask on a magnetic stirrer, loosely covered with foil, and incubate at about 20 °C.
5
Use a safe temperature for school work, rather than 37 °C, to avoid encouraging pathogens.
Part 2 of 6Measuring Turbidity
Turbidity is mostly a measure of the number of living cells in suspension.
Blank
- Blank Fill a cuvette with plain glucose solution to set the colorimeter to zero before each series of readings.
Sampling
- Sampling Use a dropping pipette to transfer a sample of the yeast suspension to a cuvette at set intervals.
Reading
- Reading Record absorbance against time, for example every 30 minutes at first and then every two or three hours.
Graph
- Graph Plot absorbance against time over the period, which can be about 12 hours, to show the growth curve.
Part 3 of 6Extending and Checking
Extending and Checking
Change a variable
- Change a variable Repeat with a different temperature or glucose concentration to see how it alters growth.
Haemocytometer
- Haemocytometer Counting cells directly provides a check on the turbidity readings.
Field of view
- Field of view Using graph paper copied onto acetate, the field of view under ×40 is 0.01 of the area seen under ×4.
Stain
- Stain Methylene blue stains yeast so cells can be counted, and a serial dilution is needed if many cells overlap.
Part 4 of 6Evaluation
Limitation
- Limitation Dead cells and other suspended particles also scatter light, so turbidity can overestimate living cells.
Reliability
- Reliability Repeat the whole investigation and calculate means of the absorbance values.
Controls
- Controls Keep volumes, the mass of yeast, stirring and temperature the same, apart from the variable being tested.
Safety
- Safety Wear eye protection, take care near the Bunsen burner and destroy the cultures safely afterwards.
Part 5 of 6Worked Example
Question
- The absorbance of a yeast culture rose from 0.10 at the start to 0.74 after 8 hours.
- Calculate the mean rate of increase in absorbance.
- Find the change in absorbance0.74 − 0.10 = 0.64.
- Divide by the time in hours0.64 ÷ 8.
- State the unit as absorbance units per hour.
Answer 0.08 absorbance units per hour.
Part 6 of 6Exam Checklist & Tips
6.1.4 – Key Points for the Exam
- Turbidity The cloudiness of a liquid caused by suspended particles such as cells.
- Colorimeter An instrument that measures how much light passes through a liquid.
- Absorbance A measure of how much light a sample absorbs or scatters.
- Aseptic technique Methods that prevent contamination of a culture.
Common Mistakes
- Not zeroing the colorimeter with a blank cuvette before taking readings.
- Assuming turbidity measures only living cells, when dead cells also scatter light.
- Changing two variables, such as temperature and glucose concentration, in one run.
- Incubating at 37 °C in a school laboratory.
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