PHYSARUM CHEMOTAXIS 1

CCW Cohort Teachers, click on the link below to access editable worksheets and presentation slides.

Lesson Overview

Summary

 This activity is designed as an open-ended exploration into physarum preferences by observing its chemotaxis. Students first observe that physarum will move towards oats, and then design experiments to determine its preference by observing its chemotactic behavior. This activity is a great way to introduce experiment design using hypotheses, controls, dependent and independent variables.

Big Idea(s)

Single-celled organisms can sense and respond to their environment.

Chemotaxis is movement towards/away from a chemical (towards food, away from toxins).

Vocabulary words

Physarum (slime mold)

Chemotaxis

Hypothesis

Dependent Variable

Independent Variable

Materials
  • Disecting Scopes
  • Dissecting Forceps
  • Slime mold growing kit (Carolina Biological, item # 155775)
  • Physarum live culture
  • 10 agar petri dishes
  • Scalpel
  • Autoclave bag
  • Physarum sclerotium (dried physarum)
  • Instructions
  • Petri dishes
  • Tinfoil
  • Humidifying Chamber (box with damp sponge inside)
  • Filter paper
  • pencils
  • An assortment of chemicals to test physarum chemotaxis (suggested below):
    • Glucose (5mM, 10mM, 100mM)
    • 5mM Sucrose
    • 5mM Lactose
    • Milk
    • 5mM Splenda
    • 5mM Stevia
    • 5mM NaCl
    • 5mM KCl
    • 5mM glutamine
    • White Vinegar
    • Apple Cider Vinegar

Daly Ralson Resource Center:

Disecting Scope (E069, E070, E071, E073, E074, E075, E075, E076, E077, E078, E079, E080, E082, E083, E084, E088)

Dissecting Forceps (E254, E304, E371)

Grouping

Groups of 3-4 students

Timing

2-3 days

Day 1

5 min – Intro

20 min – Set up physarum

Day 2

10 min – Check physarum

10 min – Intro to Chemotaxis

10 min – Experiment Planning

20 min – Experiment Set-up

10 min – Record data point 1

10 min – Record data point 2

Day 3

10 min – Record overnight data point (optional)

30 min – Group presentations (gallery walk)

Prerequisites for students

Students should have some experience using the dissecting scopes. Students should also have a bit knowledge hypotheses and variables before beginning.

Learning goals/objectives for students
  • Observe that cells (physarum slime mold) can sense and respond to its environment.
  • Develop a hypothesis from a scientific question about an organism’s behavior.
  • Design an experiment to test a hypothesis using dependent and independent variables.
  • Distinguish between dependent and independent variables.
  • Develop students ability to collect evidence through observation.
  • Use evidence to support a scientific claim.
Content background for instructor

Although physarum’s common name is “slime mold”, it is not a mold. It is actually classified as another protist. Physarum spends much of its life in the plasmodium stage (large yellow, web like structure). It searches for food and feeds in this stage. Although it is very large, it is technically one cell (one cell membrane surrounding the entire organism, but many, many nuclei). If it dries out, it will form a sclerotium (dormant stage of physarum) and wait until more favorable moist conditions return. If food runs out, it will begin the reproductive stage of its life cycle and construct spores. The spores are carried by the wind. When spores find favorable conditions, spores germinate and release ameboid single-celled swarm cells. The swarm cells will fuse together and create another plasmodium to start the cycle over again.

Slime molds are another example of a very smart protist. They can solve mazes, the traveling salesman problem, and can even teach other slime molds what they have learned (see: https://www.vox.com/science-and-health/2018/3/6/17072380/slime-mold-intelligence-hampshire-college). They do all of these things without a brain or nervous system.

To keep your physarum in the plasmodium stage, make sure it has plenty of food, and humid conditions in which to grow. Also, physarum does not like light. Keep cultures covered to prevent sporing. Physarum are very curious and will quickly explore their petri dish in search of food. To avoid exploring the same area twice, it leaves a chemical trail. Therefore, don’t be surprised if your physarum gets bored of its petri dish after a few days and escapes.

To keep your physarum healthy and contained, take a new cutting every few days and place it in a new agar petri dish with a few sterile oats. Cuttings of a thick yellow vein of physarum are easily transferred (and used in all experiments for this activity). Cut a 0.5 cm x 0.5 cm cube of agar with a vein growing across it (physarum will quickly heal after cutting) and place the cube physarum side down on a clean agar petri dish.

Getting ready

When ordering physarum from Carolina, they will ask for a delivery date. Make sure to have your physarum delivered 1 or 2 days before your activity. It is even advisable to have two cultures delivered, one 1 or 2 days before, and 1 the day of (just in case something goes wrong with the first culture). 1 culture is enough for about 20 students (5 groups).

Cut filter paper so that two strips lay flat, side-by-side, inside a petri dish. Be sure to prep all of your “chemotactic” solutions before hand in clean (or sterile) water. Place a few sterile oats in petri dishes at each station. Wipe down a set of tweezers with alcohol for each station.

Lesson Implementation/Outline

Introduction

(5 mins)

Introduce students to physarum. Let them know a few facts (It’s one large cell composed of many cells fused together. It’s curious, and will search for food by exploring its petri dish).

Activity (Day 1)

Physarum Set-up (20 mins):

For this activity, each group needs one petri dish filled with non-nutrient agar. Ask each group to take a few oats and place them in their petri dish using the supplied tweezers.

Give each group a small cutting of physarum. Use the provided scalpel to cut a 0.5 cm x 0.5 cm square of agar with a thick yellow vein growing across it. Place that cutting physarum-side down on the students’ petri dish.

Students should label their plate, cover with tin foil, and label the tin foil. Store the physarum cultures overnight in a humidifying chamber (plastic box with damp sponge or equivalent).

 

Activity (Day 2)

Check Physarum Cultures (10 mins):

Have students check their cultures. Some part of their physarum should have reached the oats that were left in the petri dish. Encourage them to make observations using the dissecting scopes.

Chemotaxis Introduction (10 mins):

Have students brainstorm in groups what chemotaxis is (especially if they have been exposed to the term phototaxis). Chemotaxis is movement toward or away from a chemical. Why might chemotaxis be an important function in organism survival? (Movement toward food for nutrition, movement away from toxins for survival).

Present experimental set-up (chemicals available to test). Let the students know they can compare two chemicals side-by-side. If you have the materials, let them know they can do two sets of side-by-side tests (potentially four chemicals).

Move through the sides to demostrate the example set up. Slides contain an example question, hypothesis, variables, and experimental set up. Their handout include appropriate sentence stems.

Experimental Planning (10 Mins):

Give every student a copy of the physarum experiment handout. Allow student groups time to formulate their hypothesis and experimental set-up before they start handeling materials. If students are stuck, common experiments include glucose vs. sucrose (mono vs disaccarhide), lactose vs. milk, salt vs. sugar, white vs. apple cider vinegar, glucose or sucrose vs. artificial sweetener (stevia or splenda).

Experiment Set-up (20 mins):

Encourage each group to label their filter paper strips with pencil marking before dipping them into the chemotactic solutions. Once all groups have dipped their filter paper strips in their choosen chemotactic solutions, offer cuttings of physarum. It is a good idea to place 3 cuttings on each pair of filter paper. These 3 cuttings represent 3 trials per experiment. The cube of physarum should be placed physarum-side down so that half of the cube is over one filter paper strip and the other half is over the other strip.

Once the student group has its physarum cuttings over their filter paper, students should take note of the experimental set up. All physarum experiments should be labeled with the group name, covered in tin foil, labeled again, and placed in the humidifying chamber.

Check physarum growth every few hours if you can, or check the next day. Physarum growth path should be marked down in science notebooks or on the physarum worksheet. Encourage them to explore the physarum growth under the dissecting scopes.

Activity (Day 3)

Record Overnight Datapoint (10 mins):

(Optional, you can get some data just by allowing physarum to grow for about 4 hours, but the growth will be more dramatic and more obvious if left overnight).

Have the students check their physarum growth and drawn the path of growth in their science notebooks or on the physarum worksheet. Encourage them to explore the physarum growth under the dissecting scopes. 

Wrap-up/Closure

(30 mins)

Have student groups create their own science poster using chart markers and chart board paper (10 mins). Have them include the following information:

  • Question
  • Hypothesis
  • Experimental Set-up (including dependent and independent variables)
  • Results (draw the physarums path of growth)
  • Conclusions and Future Directions

Once each group is finished with the poster, have one student (randomly selected per group) stand next the the poster as an abassador for the group and explain their experiment. All other students should visit the other posters and ask the student abassador about their group’s experiment. After 10 mins, select a new student (randomly selected per group) to stand as an abassador for their group while the original abassador moves around the room to learn about each groups poster.

Extensions/Reflections

Extension

Programming a chemotaxis-like behavior in the robot is a natural extension to this activity.  See “Programming Chemotaxis”.

NGSS

Performance Expectations

HS-LS1-3 From Molecules to Organisms: Structures and Processes

(Physarum move toward food using chemotaxis, which uses positive feedback to move towards potential food or away from toxic substances.)

HS-LS1-6 From Molecules to Organisms: Structures and Processes

(As physarum attach and digest oats, it grows larger because it is coverting the sugar molecules into proteins and other larger molecules to increase its size.)

Disciplinary Core Ideas

HS LS1.A Structure and Function

HS LS1.C Organization for Matter and Energy Flow in Organisms

Science and Engineering Practices

Practice 1. Asking Questions and Defining Problems

Practice 3. Planning and Carrying Out Investigations

Practice 4. Analyzing and Interpreting Data

Practice 6. Constructing Explanations and Designing Solutions

Practice 7. Engaging in Argument from Evidence

Practice 8. Obtaining, Evaluating, and Communicating Information

Cross-Cutting Concepts

Cause and Effect

Stabiligy and Change