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

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

Hypothesis is an prediction made after performing some background research.

Physarum (slime mold)

Chemotaxis

Hypothesis

Virtual Activity Worksheet

Two sets of Physarum Data (sugar/artificial sweetner, milk/lactose)

Groups of 3-4 students

3.5 hours

45 min – Preactivity (could be given as homework)

30 min – Intro and demo

1 hr 45 min – Group experiment/create poster

30 min – Group presentations (gallery walk)

It’s helpful for students to have a little bit of knowledge of what a hypothesis before starting, but not necessary.

• Observe that cells (physarum slime mold) can sense and respond to its environment.
• Develop a scientific question and hypothesis from background knowledge of the organism’s behavior and diet.
• Develop students ability to collect evidence through observation.
• Use evidence to support a scientific claim.

 Chemotaxis is movement of an organism in response to a chemical stimuli.  Positive chemotaxis is movement toward (usually food), while negative chemotaxis is movement away (usually toxins). This behavior is important to the survival all organisms, i.e. bacteria use chemotaxis to move toward higher concentrations of glucose (food) and away from any harmful toxins. The process of chemotaxis requires the organism sense a chemical and respond with movement appropriately. Hunting prey by scent is a type of chemotaxis used by large organisms. Similarly moving away from unpleasant odors is negative chemotaxis for large organisms.

This lesson focuses on an organism called physarum polycephalum. 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 an 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.

Heather Barnett is an artist that uses physarum as her medium.  She started an online network for other slime mold enthusiasts (http://slimoco.ning.com). Her TED talk can be found here: https://www.ted.com/talks/heather_barnett_what_humans_can_learn_from_semi_intelligent_slime?language=zh

The experiment photographs for this activity were real experiments performed in May of 2021 by a UCSF researcher. 

Three separate experiments were preformed, comparing three sets of two chemicals: water and sugar (dissolved in water); lactose(dissolved in water) and milk; sugar and stevia (both dissolved in water). Where applicable, the concentrations of each component were kept constant (i.e. lactose concentration in water similar to that found in lactose, sugar concentration matched to stevia concentration).

For each experiment, the researcher dipped one filter paper strip into each solution. The two filter paper strips were placed side-by-side in a petri dish. The physarum (growing in a petri dish with agar) was transferred to the filter paper strips by cutting a square out of the agar with a small piece of physarum on it (physarum generally heals very quickly from cuts. multiple cuts to the same petri dish create clones of the original organism). Each agar square was placed physarum-side-down such that half of the square is in contact with one filter paper strip and the other half in contact with the other filter paper strip (Three squares were used per filter paper strip. Each can be thought of as experimental replicates or “trials”). After only a few hours, physarum started to grow on to the filter paper. The filter paper that had more growth is presumed to contain the chemical that is most “attractive” to physarum. 

(45 mins)

Distribute the preactivity (“How Organisms Find Food” Virtual Worksheet) to students. Can also be given as a homework assignment.  Students work individually on this activity to brainstorm ways that organisms use their senses to find food in their environment.  They are then asked to think about how a single celled organism (like a white blood cell) can track and find bacteria without senses (or a nervous system).  This is just to get them to start wondering.  They are not expected to get the right answer yet. The question is designed to get them thinking.

They are also introduced to physarum through a series of videos.  The worksheet asks them to note the kinds of foods (and other substances) that physarum likes and dislikes.  This will be useful information for them to collect before they make a hypothesis in the chemotaxis activity.

Introduction (20 mins):

Included in the worksheets and slides folder is a short presentation/introduction to the activity.  This introduction serves to review how organisms find food, explain how cells find food through chemotaxis (including the white blood cell from the individual student assignment above), and what chemotaxis is. 

Demo and Activity Instruction (10 mins):

Next the presentation walks the class through a demo experiment with a setup identical to the experiments in the chemotaxis virual experiment activity (below). This demo explains the experimental setup (sugar vs. water), provides before and after photos of the experiment, and guides students through questions to help guide their thinking as to how they would quantify the experimental results.

The Chemotaxis Virtual Worksheet is designed for groups of 3-4 students to collaborate on the same worksheet.  Google docs works great for this, as long as all students have google accounts.

Students will use this worksheet to:

1. Preform background research on physarum and its food sources
2. Select an experiment (either “sugar vs. artificial sweetener” or “milk vs. lactose)
3. Develop a hypothesis
4. Analyze data
5. Create a “poster” organizing all of their findings so they can be presented to their peers.

The worksheet contains:

• Question and hypothesis page – on this page students write down their background information (developed from the next few pages on the worksheet), select an experiment (they select by moving “sugar vs artificial sweetener” or “milk vs. lactose” block to the “selected experiment here” space), select a scientific question (they have two prewritten questions to chose from inside moveable blocks), and complete a hypothesis using a sentence stem.
• Background information pages – these pages contain information about physarum, bacteria (one of physarums food sources), carbohydrates, artificial sweeteners, and milk composition.  Students should read these pages to help them fill in their background information and inform their hypothesis.  For example, by learning that artificial sweetener can slow growth in some bacteria, and that physarum eat bacteria, students may come to the conclusion that physarum will chemotax toward sugar over artificial sweetener (example hypothesis: We hypothesize that physarum will chemotax towards “sugar over artificial sweetener” because “stevia kills some bacteria and stops bacteria from communicating. Also based on the previous experiment physarum likes sugar a lot.”)
• Data page – Once students select an experiment, the before and after photos can be pasted on this page.  We suggest waiting to place the data until after they have written their hypothesis to discourage writing the hypothesis after they know the result (a wrong hypothesis is still a good hypothesis). They can do data analysis any way they choose.  Some examples to offer are: measuring how far away from the cube each bit of physarum has grown; measuring (approximating) the area that each physarum growth front occupies on the filter paper; a simple count of the number of physarum “squares” that grew on the filter paper (i.e. for water vs. sugar, the sugar filter paper had growth from all 3 squares vs only 1 square for the water.)
  
• Poster Template page – Once students have analyzed their data, they can use this page to build their poster. It includes sentence stems for writing help on a few sections. After completing all the above pages, they should be able to take what they have generated from the Question and hypothesis page and the data page to fill in a majority of the poster. Be sure to encourage them to be as clear as possible so that other students can read their poster and understand their thinking.

(30 mins)

Collect all student posters and paste as backgrounds in a Google “Jamboard” (https://support.google.com/jamboard/answer/7424836?hl=en).  This will allow all students to make comments using sticky notes on all of the posters.  Ask student to add one comment and one question for each groups poster. 

Alternatively, you can create a presentation jigsaw where (if you had groups of 4 students) you can have one student per group in one of 4 rooms (i.e. for 20 students, 1 group representative for each of the 5 groups would be in a breakout room together and each group representative would present their groups poster). This way all students get a chance to present and hear about the results from all other groups.

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

Structure and Function

Matter and Energy in Organisms and Ecosystems

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.A Structure and Function

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

Practice 1. Asking Questions and Defining Problems

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

Cause and Effect

Stabiligy and Change