Structure of experimental tasks 23 and 24
Biology Unified State Exam tasks 23 and 24 share a single experimental description but test different skills. Task 23 requires identifying the experimental variables and describing the control setup. Task 24 requires analyzing the result: what the experimenter observed and its biological explanation.
To get full points, the answer must follow a fixed scheme with mandatory clichés. Omitting any structural element leads to point loss even if the core essence of the answer is correct.
Dependent and independent variables
Independent variable — the one set by the experimenter (the initial condition). Dependent variable — the one that changes in response to the independent variable, i.e., the recorded consequence.
In the experiment with Mucor mold placed in Petri dishes at temperatures of 2–6 °C, 10–15 °C, and 24–26 °C:
- independent variable — ambient temperature;
- dependent variable — growth rate of Mucor mold.
In the experiment with germinating wheat seeds in test tubes with water at different temperatures:
- independent variable — ambient temperature;
- dependent variable — degree of limewater turbidity (an indicator of respiration rate).
Negative control and its setup
Negative control — an experimental group that is not exposed to the studied factor but is kept under conditions identical to the experimental group. It is necessary to exclude the influence of extraneous factors and prove that the observed effect is caused precisely by the studied variable.
For the mold experiment, the negative control is formulated as follows: *Mucor* spores are placed in Petri dishes where temperature conditions remain unchanged throughout the experiment. All other parameters (mold species, experiment duration, substrate moisture) must be kept constant.
For the seed experiment: seeds with roots are placed in vessels with a constant temperature throughout the entire experiment, while other parameters (seed species, test tube volume, volume of limewater) are left unchanged.
The answer concludes with a mandatory cliché: "This control allows us to establish whether…" — followed by the formulation of the tested hypothesis:
- "…whether a temperature of 24–26 °C is indeed more favorable for mold growth";
- "…whether temperature truly affects the activity of the seed respiration process".
Analysis of the results of the *Mucor* mold experiment
Task 24 describes what the experimenter observed. Since the temperature conditions varied, the fungal growth activity also changed:
- the growth rate of *Mucor* mold at 24–26 °C is significantly higher than at 10–15 °C;
- at a temperature of 2–6 °C, mold growth was practically not observed.
Biological explanation: this is the organism's response to environmental changes. A temperature of 24–26 °C is more favorable for *Mucor* vital activity, therefore enzymatic activity and mycelium growth rate increase. At low temperatures, enzyme activity drops sharply, and fungal growth stops.
Analysis of the germinating seed respiration experiment
In the wheat experiment, limewater is used as an indicator: its turbidity indicates the formation of carbon dioxide released by seeds during respiration.
- Germinating seed respiration requires oxygen, which is consumed in the oxidation of organic substances.
- Limewater turbidity is caused by carbon dioxide (CO₂) — the end product of cellular respiration, which forms insoluble calcium carbonate.
- Moist germinating seeds respire intensively, releasing a significant amount of heat, which causes them to warm up. Part of the energy released during oxidation is dissipated as heat.
The higher the temperature (within the optimum range), the more active respiration is, and the more turbid the limewater becomes in the corresponding test tube.
Algorithm for structuring the answer
- State the dependent variable.
- State the independent variable.
- Describe the negative control setup (the same experimental setup without the studied exposure).
- List the parameters that remain constant.
- Provide the cliché: "This control allows us to establish whether…" with the formulation of the hypothesis.
- In Task 24 — describe the observed result and provide a biological explanation through the organism's properties and environmental conditions.
Key points
- The independent variable is set by the experimenter, while the dependent variable is measured as a consequence.
- Negative control — a group without exposure under otherwise identical conditions.
- Mandatory cliché: "This control allows us to establish whether…".
- The temperature optimum for *Mucor* mold growth is 24–26 °C; at 2–6 °C, growth is suppressed.
- Limewater turns turbid under the action of carbon dioxide — a product of respiration.
- Germinating seeds require oxygen; intensive respiration is accompanied by heat release.
- Changes in growth rate and respiration intensity are the organism's response to environmental conditions.
- Omitting any structural element (variables, control, cliché, explanation) leads to point loss.