Experiments

Experiments in Biology are systematic investigations designed to test scientific ideas, theories, or hypotheses by manipulating specific factors and observing their effects. They are a fundamental part of practical skills in Biology and are assessed both through hands-on work and written examinations.

Purpose of Experiments Experiments are carried out to:

• Test a theory or prediction (hypothesis): Scientists start by making a prediction or hypothesis—a specific, testable statement based on theory—about what will happen in the experiment. The experiment then provides evidence to support or disprove this prediction. For example, a theory that photosynthesis requires light can lead to a prediction that the rate of photosynthesis will increase with light intensity, which can then be tested by an experiment.

• Investigate relationships: Many experiments aim to investigate how one factor affects another.

• Draw valid conclusions: By carefully controlling variables, experiments allow for the drawing of valid conclusions.

Key Stages of an Experiment

1. Planning an Experiment:

   • Defining the problem: This involves constructing a hypothesis, which should be quantifiable, testable, falsifiable, and graphically representable.

   • Identifying variables:

     • Independent variable: The factor that is changed or selected by the experimenter.

     • Dependent variable: The factor that is affected by changes in the independent variable, and which is measured.

     • Controlled/Standardised variables: All other variables that could affect the dependent variable must be kept constant to ensure that only the independent variable is influencing the results.

   • Controls: Negative controls are crucial to check that only the independent variable is affecting the dependent variable. They are not expected to have any effect on the experiment. For instance, in a photosynthesis experiment, a negative control would involve carrying out the experiment in the dark, where no photosynthesis is expected.

   • Repeats and Sample Size: Taking several repeat measurements reduces the effect of random error, making results more precise and reliable. A large sample size also increases the reliability and validity of results.

   • Risk Assessment: An important part of planning is identifying potential hazards and explaining how to minimize them, such as wearing goggles or lab coats, and disposing of waste safely.

2. Carrying Out an Experiment:

   • This involves using the correct apparatus and techniques, making accurate and relevant observations, and collecting valid, accurate, and precise data.

   • Aseptic Techniques: These are vital when investigating the effect of antimicrobial substances on microbial growth to prevent contamination of cultures.

   • Ethical Issues: Experiments involving living organisms (e.g., insects) must be carried out safely and ethically, treating them with respect and keeping them away from harm.

3. Processing and Presenting Data:

   • Recording Data: Data should be recorded in suitable tables with proper headings, units, and appropriate levels of accuracy and significant figures.

   • Presenting Data: Quantitative results are typically displayed in tables and graphs. Line graphs are used for continuous variables, while histograms or bar charts are used for categorical or continuous data with ranges. Axes must be fully labeled with units, and scales should use equal intervals.

   • Calculations: This may involve calculating means, ranges, standard deviations, ratios, or interpreting statistical tests like the Chi-squared test or Hardy-Weinberg principle.

4. Concluding and Evaluating:

   • Drawing Conclusions: Conclusions should be valid, supported by valid data, and directly answer the question or hypothesis of the investigation. They should not overstate what the results show, nor confuse correlation with causation.

   • Evaluating Methods and Results: This involves assessing reliability (repeatability, reproducibility), validity, and precision. It also includes identifying sources of error (systematic errors, random errors, anomalous results) and suggesting improvements, such as using more sensitive apparatus or taking more frequent measurements.

Examples of Experimental Investigations The sources highlight numerous practical investigations:

• Enzyme activity: Investigating factors like temperature, pH, enzyme concentration, and substrate concentration on the rate of enzyme-controlled reactions. This includes experiments on catalase, amylase, rennin, and pepsin.

• Cell study and microscopy: Preparing temporary mount microscope slides, observing mitosis in root tip cells, and analyzing cell components through cell fractionation.

• Transport across membranes: Investigations into diffusion (e.g., using Visking tubing or agar blocks) and osmosis (e.g., water potential of plant tissue like potato strips or beetroot cells).

• Microbial growth: Investigating the effect of antimicrobial substances (like antibiotics) on microbial growth using aseptic techniques.

• Respiration and Photosynthesis: Experiments on factors affecting respiration (e.g., in yeast using respirometers or redox indicators) and photosynthesis (e.g., light intensity, CO2 concentration, temperature, using redox indicators or aquatic plants).

• Mass Transport in Plants: Ringing experiments and radioactive tracer experiments to investigate transport in plants.

• Dissections: Performing dissections of gaseous exchange or mass transport systems in animals or plants.

• Populations: Assessing abundance and distribution of organisms using methods like quadrats, transects, and mark-release-recapture.

Assessment of Experimental Skills Practical skills, including experimental design and evaluation, are a significant part of A-level Biology exams (at least 15% in AQA A-level Biology written exams). Questions may involve commenting on experimental design, suggesting improvements, analyzing data, and drawing conclusions from given results. Students are expected to understand the apparatus and techniques used in required practicals.