1.2Chapter 1: Classification and Life
Investigating living organisms: yeast — Alhussan Biology - Stage 7 Revision Notes & Topical Questions
5 min read 7 parts Lesson 2 of 11 1 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.
Lesson summary
Biology 1.2 Investigating living organisms: yeast — Alhussan International Academy — Biology (Stages 7–9) notes: Plan an investigation to show that yeast is living and to find how temperature affects its respiration, choosing equipment and controlling variables.Yeast — a microsco…
- Investigation questions
- Reproduction
- Test for carbon dioxide
- Best choice
- Independent variable
- Control variables
Part 1 of 5Yeast as a living organism
Yeast
- Yeast a microscopic single-celled fungus;
- dried yeast looks lifeless but grows, reproduces and respires when mixed with warm water and sugar.
Investigation questions
- Investigation questions is yeast a living organism, and does yeast grow and respire faster if the temperature increases?
Respiration
- Respiration yeast respires and, with little oxygen, ferments: glucose → ethanol + carbon dioxide (+ energy).
- The carbon dioxide forms bubbles, which are the observable evidence.
Reproduction
- Reproduction yeast reproduces by budding, where a small new cell grows from the side of the parent.
Part 2 of 5Choosing the equipment
Test for carbon dioxide
- Test for carbon dioxide limewater turns cloudy when carbon dioxide is bubbled through it.
Three set-ups were evaluated:
- a flask of yeast, water and sugar with a gas syringe;
- a flask with a bung and delivery tube leading into limewater in a test tube;
- a test tube with a balloon or bubble layer.
Best choice
- Best choice the limewater set-up shows the identity of the gas, which matters more than its quantity for showing respiration.
Result
- Result the limewater turns cloudy, so yeast with food and air respires and produces carbon dioxide, and the first conclusion is that yeast is living.
Part 3 of 5Controlling the variables
Independent variable
- Independent variable the temperature of the mixture, for example 20, 30, 40, 50 and 60 °C.
Dependent variable
- Dependent variable a measure of respiration, such as the volume of gas, the bubble depth or bubbles per minute.
Control variables
- Control variables the amount of sugar, yeast and water, the size of container and the time, kept the same each time to make a fair test.
Living organisms
- Living organisms yeasts are slightly different from each other, so an investigation with living organisms can never be completely fair.
- Repeating readings and using a mean helps.
Part 4 of 5Methods and their problems
Balloon method
- Balloon method a balloon over each test tube collects and measures the gas.
Bubble depth
- Bubble depth measuring the depth of the bubble layer in a test tube.
Counting bubbles
- Counting bubbles counting bubbles escaping into limewater each minute is unreliable, because bubbles do not come out in a steady flow.
Timing limewater
- Timing limewater timing how long limewater takes to turn cloudy fails at some temperatures, as the limewater did not go cloudy.
Gas syringe
- Gas syringe flasks with gas syringes measure the gas produced at each temperature and gave results similar to the balloon and bubble-depth methods.
Choosing a method
- Choosing a method a method must give reliable measurements, which is why nutrients tests need a method that measures the gas.
Part 5 of 5Results of the temperature investigation
Optimum
- Optimum yeast produces gas fastest at about 35 to 40 °C, because its enzymes work best near this temperature.
Low temperatures
- Low temperatures reactions are slow.
High temperatures
- High temperatures enzymes are denatured and yeast is killed, so no gas is made.
Graph
- Graph plot temperature on the x-axis and the gas measure on the y-axis;
- the curve rises to a peak then falls sharply.
Safety
- Safety handle hot water carefully and wash hands after touching yeast.
Planning rule
- Planning rule decide which variable to change, choose what to observe or measure, identify variables to control,
- and remember that living organisms mean the test is not completely fair.
Key terms
| Term | Meaning |
|---|---|
| Yeast | A single-celled fungus used in baking and brewing. |
| Fermentation | Anaerobic respiration in yeast making ethanol and carbon dioxide. |
| Budding | Reproduction in yeast by growth of a small new cell on the parent. |
| Independent variable | The variable you change. |
| Control variable | A variable kept the same to make a test fair. |
| Limewater | A solution that turns cloudy with carbon dioxide. |
Avoid these mistakes
1
Thinking dried yeast is dead — it is dormant and revives with warmth, water and sugar.
2
Changing more than one variable at once — this makes the test unfair.
3
Saying the bubbles are oxygen — fermentation gives carbon dioxide.
4
Forgetting to repeat readings and calculate a mean — living organisms vary.
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