Scientific Method and Experimental Design: Effects of Chemicals on Daphnia Heart Rate

Scientific Method: Daphnia Experimental Unit

Objectives

This unit introduces the scientific method and its application in biological experiments, specifically investigating the effects of chemicals on the heart rate of Daphnia. Students will learn to formulate hypotheses, design experiments, and prepare serial dilutions.

• Explain the process of the scientific method.

• Use the scientific method to formulate a hypothesis and design an experiment to investigate factors affecting heart rate.

• Prepare and dilute serial dilutions.

The Scientific Method

Steps of the Scientific Method

The scientific method is a systematic approach used in biology to investigate phenomena, acquire new knowledge, or correct and integrate previous knowledge. It involves several key steps:

• Observation: Identify and describe a phenomenon or problem.

• Background Research: Gather information related to the observation.

• Construct a Hypothesis: Formulate a testable statement predicting an outcome.

• Test with Experiments: Design and conduct reproducible experiments to test the hypothesis.

• Procedure Working? If not, troubleshoot and refine the procedure.

• Analyze Data and Draw Conclusions: Interpret results to determine if they support the hypothesis.

• Communicate Results: Share findings with others.

Key Terms:

• Hypothesis: A proposed explanation for a phenomenon, which can be tested.

• Independent Variable: The factor that is changed or manipulated in an experiment.

• Dependent Variable: The factor that is measured or observed in response to changes in the independent variable.

• Control: The standard for comparison in an experiment.

Lab Notebooks

General Guidelines

Maintaining a well-organized lab notebook is essential for documenting experimental procedures and results.

• Include a table of contents.

• Use ink and cross out mistakes with a single line.

• Write in the notebook first; do not rely solely on digital documents.

• Keep entries organized, legible, and sequential.

• Respond to prompts in the lab manual.

Specific Guidelines

• Header: Add a header for each unit before class.

• Purpose: Write the purpose before coming to class; it may be reviewed during quizzes.

• Procedure: Summarize the lab manual procedure.

• Data (Written): Include units, labels, and descriptions.

• Data (Graphic/Visual): Provide two printouts of any digital additions.

• Conclusions: Write a few sentences or a short paragraph summarizing findings.

Practicing the Scientific Method: Effects of Chemicals on Daphnia Heart Rate

Stimulants and Depressants

Stimulants and depressants are chemicals that affect an organism's nervous system. In this experiment, caffeine (a stimulant) and alcohol (a depressant) are used to study their effects on heart rate.

• Caffeine: Found in chocolate, tea, coffee, and some medications; increases heart rate.

• Alcohol: Depresses the nervous and cardiovascular systems; slows heart rate and may cause dizziness.

Why Use Daphnia?

Daphnia is a small aquatic organism with a translucent body, allowing easy visualization of its heart and other body parts under a light microscope. Its heart rate can be measured directly, making it an ideal model for physiological studies.

Preparing Solutions: Serial Dilutions

Serial Dilution Formula

Serial dilutions are used to prepare solutions of desired concentrations from a stock solution. The formula is:

$C_1V_1 = C_2V_2$

• $C_1$: Concentration of the stock solution

• $C_2$: Desired concentration of the final solution

• $V_1$: Volume of stock solution needed

• $V_2$: Final volume of the solution

Example Calculation: To make 15 mL of a 5% solution from a 30% stock:

$C_1 = 30\%$, $C_2 = 5\%$, $V_2 = 15\text{ mL}$

Solve for $V_1$:

$V_1 = \frac{C_2V_2}{C_1} = \frac{5 \times 15}{30} = 2.5\text{ mL}$

Exercise 1: Creating Dilutions of Simulated Ethanol

Prepare 500 μL of 2% and 6% simulated ethanol solutions from a 10% stock using the dilution formula. Record volumes and calculations in your lab notebook.

Concentration of Dilution	Volume of 10% stock needed	Volume of diH2O needed	Final volume of dilution
2%	100 μL	400 μL	500 μL
6%	300 μL	200 μL	500 μL

Additional info: Volumes inferred based on standard dilution calculations.

Micropipette Settings

Micropipettes are used to accurately measure and transfer small volumes of liquids. Settings for different pipettes are as follows:

Pipette	Setting	Volume
P20 (2-20 μL)	1-4-2	14.2 μL
P200 (20-200 μL)	1-6-9	169 μL
P1000 (100-1000 μL)	0-7-5	750 μL

Experimental Design: Measuring Daphnia Heart Rate

Exercise 2: Baseline Heart Rate Measurement

Establish baseline data for the normal heart rate of untreated Daphnia:

• Capture a Daphnia and place it on a depression slide.

• Locate the eye, antenna, and heart under the microscope.

• Allow the organism to rest for 1-2 minutes.

• Count the number of heartbeats in 15 seconds; repeat for three trials.

• Calculate the average heart rate.

Exercise 3: Effect of Caffeine or Ethanol on Heart Rate

Investigate how different concentrations of caffeine or ethanol affect Daphnia heart rate:

• Formulate a hypothesis regarding the effect of the chemical.

• Identify independent (chemical concentration) and dependent (heart rate) variables.

• Test at least three concentrations (caffeine: 0.2%-1%, ethanol: 1%-5%).

• Calculate a 20% change in heart rate: $\text{Target Heart Rate} = \text{Baseline} \times 1.2$

• Apply the test solution, wait five minutes, and measure heart rate three times for 15 seconds each.

• Allow recovery between tests by replacing the solution with water and resting the organism.

• Record and analyze data to determine the concentration that produces a 20% change.

Results and Discussion

Data Analysis

Plot average heart rate per minute versus solution concentration using graph paper or computer software. If the exact concentration for a 20% change is not found, estimate using the graph.

• Include results and discussion in your lab notebook as per unit instructions.

Summary Table: Experimental Variables

Variable	Description
Independent Variable	Chemical concentration (caffeine or ethanol)
Dependent Variable	Daphnia heart rate
Control	Untreated Daphnia (baseline heart rate)

Additional info: This guide expands on brief points to provide a self-contained overview suitable for exam preparation in General Biology.