This activity allows students to explore cells in a new way. The set of protists used in this activity offer a wide range of motilities, sizes, and shapes. This activity also offers a great introduction to using different kinds of microscopes, and how to make observations and use those observations as evidence to make a scientific argument (that the observed cells are alive!).
Cells are alive. They share common characteristics. But they come in many different shapes and sizes.
- Deluxe Digital Microscopes
- Disecting Scope
- Demoslides (Carolina Biological e.g. item #131011)
- Small petri dishes
- Collection of protists, for example:
Daly Ralson Resource Center:
Deluxe Digital Microscopes (E769, E770, E771, E772, E773, E774, E775)
Disecting Scope (E069, E070, E071, E073, E074, E075, E075, E076, E077, E078, E079, E080, E082, E083, E084, E088)
Groups of 3-4 students
40 min total
5 min – Intro
20 min – Exploration (10 mins/station)
15 min – Discussion/share out
Prerequisites for students
None. A bit of experience with microscopes is helpful, but not necessary.
Learning goals/objectives for students
- Challenge students preconceptions about what a cell is by offering cells with differing motility, structure, shape, and size.
- Develop students understanding of cells as biological machines, especially in how they are able to sense and respond to their environment.
- Develop students ability to collect evidence through observation.
- Use evidence to support a scientific claim.
- Identify the proper tool (dissecting scope or compound microscope) for observing organisms of different sizes.
Content background for instructor
Protists are a diverse collection of primarily single-celled organisms. They generally have a nucleus and specialized organelles (they are eukaryotes, just like plants, animals, and fungi). Some multicellular protists exist, but they are very simple, consisting of a single cells type in a connected cluster. Protists gain nutrition from a variety of sources. Some are photosynthetic; some are heterotrophs (organisms that eat other organic material). Some are both (termed mixotrophs). Below is a bit of information about the five suggested protists for this activity.
Stentor: Stentor are large ciliates found in lakes (like those in Golden Gate Park). They attach to leaf litter with a “foot” at one end and stretch into a trumpet like shape that gives them their name (Stentor is the Greek god of trumpets). In this shape, the stentor use cilia to sweep bacteria into their gullets for food. They can become several millimeters in length, making them one of the largest single-celled organisms. They have a macronucleus shaped like beads on a string. To keep from getting eaten by passing fish, stentor contract into a ball when they feel disturbances. However, they can learn to ignore disturbances despite not having a brain or nervous system (see: https://www.youtube.com/watch?v=ooA0J6DWWTM). They also have amazing regenerative abilities. They are able to repair themselves after being divided in half, or smaller (see: https://www.ucsf.edu/news/2017/02/405816/science-focus-new-mysteries-genome-stentor).
Blepharisma: are single-celled ciliate that are usually red or pink in color. Like stentor, they use their cilia for food collection and movement. When a blepharisma traps food, it surrounds its food in vacuoles, which may be visible under the microscope. The pink color is a photosensitive pigment that detects light. Blepharisma will avoid light and seek out darker areas. Blepharisma are heterotrophs. They eat bacteria, algae, and other ciliates, sometimes even smaller members of their own species. If they are allowed to cannibalize other blepharisma, they grow into giant versions (almost a millimeter in length).
Paramecium: This small, fast-moving ciliate is widely used in scientific research. They use their cilia for speed an maneuverability, and have been clocked at speeds of four times their own body length per second. Paramecium are heterotrophs, feeding on bacteria, algae, and yeast. Paramecium “learning” has also been studied, with uncertain results. Paramecium seem to be able to be trained in certain experiments, but it is unclear whether this can be considered “learning” (see: https://www.biorxiv.org/content/10.1101/225250v2.full)
Spirostomum: Are long, worm-like ciliates. They also have macronuclei, formed in a string of beads in some species. They are extremely contractile, and, when started, can contract their body by more than 60% of its length within a few milliseconds (the fastest contraction known in any cell). Some species of spriostomum are sensitive to the presence of heavy metals and have therefore been used as an bioindicator of water purity by ecologists (see: https://www.sciencedirect.com/science/article/pii/S0048969705801287).
Euglena: As a flagellate, Euglena move by beating their flagella, a long whip-like appendage. Euglena are mixotrophs. They can absorb organism material from their surrounding for food, but they also have chloroplasts allowing them to undergo photosynthesis. Because of the chlorophyll contained in their chloroplasts, Euglena are tinted green. A small red dot close to the base of its flagella, called an eye spot, helps Euglena detect and move towards light.
Before the activity begins, make two samples of each protist, one in a demoslide and one in a small petri dish. Each group should get two protists to explore (total of four samples, two demoslide and two petri dishes). The demoslides should be used with the compound microscopes while the petri dishes are for the dissecting scopes.
Let the students know they will be exploring different single-celled organisms under the microscope. Let them know about the variety of organisms they are about to see and how they may need to use either the compound microscope or the dissecting scope to best see the organisms.
In this introduction, a quick orientation to microscope use is appropriate, in case you have students that are completely new to microscopes.
Each station has a digital microscope and/or a dissecting scope. The dissecting microscopes are low power – provide 10x magnification – so are good for observing larger specimens and things that move really quickly. The digital scopes magnify from 40x to 400x – allowing you to see the details on a specimen – though it may quickly move out of the field of view.
Pointers for using compound microscopes:
- To start, make sure the stage is all the way down, and the lowest power objective is selected.
- Place the specimen on the stage. Make sure it’s secure.
- Bring the stage up toward the objective until the specimen is in focus (Hint: if you can’t find the specimen, focus on the edge of the slide and then move laterally to find the specimen).
- If higher power magnification is desired, once you have the specimen focused in 40x, switch to a higher power objective. You should only need to adjust the fine focus slightly to refocus your specimen.
- To remove the specimen, go back to the lowest power objective, bring the stage all the way down, and remove the slide.
Pointers for using dissecting scopes:
- Dissecting scopes can only magnify an object to one magnification (10x) or sometimes two.
- Place the object on the stage and use the focus knob to focus (Hint: if you can’t find the specimen, focus on the edge of the petri dish then move the petri dish laterally to find the specimen).
- Sometimes transparent organisms are easier to see over a black background. The stage has a reversible center circle that can be flipped from white to black.
(20 mins, 10 mins per station)
- Have groups introduce themselves, school they attend, last movie they saw.
- Observe their two organisms
- Approximately 10 minutes
- Take notes:
- What do you notice?
- What do you wonder about?
- Which of the functions of life are you able to observe for each organism? (To be shared out)
- Remind students that they may need to use one or the other scope to properly see each organism.
- After 10 minutes have each group move to a new station with two new organisms.
Checking for student understanding
Common problems include: finding and focusing on the organism, not finding large organisms under the compound microscope (stentor, spirostomum, and blepharisma), misidentifying the organisms food as the organism (stentor have blue-green algae they attach too in their culture, blepharisma have small ciliates they eat for food in their culture, etc.).
Stentor will contract in response to distrubances. You can try banging on the table while a stentor is in view to see it contract. It must be attached with it’s “foot” and in its enlongated feeding form to contract.
Euglena will actually move away from light if its too bright. They will only move towards moderate light sources.
- Pick a reporter from each group at random. Have them report out one function of life they observed in their culture.
- Continue thorugh all groups twice (two functions per group) or until no group has any more observations to add to the list.
- Ask “How is a cell like a machine?” – 5’ to discuss at their tables, then ask each table to report on one idea. Possible responses include:
- Both use energy
- Both sense their environment
- Respond to changes in their environment
- Outputs – react to environment
- Have instructions to do job
- Summarize: “Cells are complex. One analogy is that they are machines – they use energy to complete many different tasks that are part of the larger function of “Life.” Over the next few days we’ll dig much deeper into this – and start exploring the analogy of cells as robots.
HS-LS1-3 From Molecules to Organisms: Structures and Processes
(Cells sense and respond to stimuli, often using negative or positive feedback to maintain homeostasis and for survival. These movements are easily visible under the microscope.)
Disciplinary Core Ideas
Science and Engineering Practices
Practice 1. Asking Questions and Defining Problems
Practice 2. Developing and Using Models
Practice 6. Constructing Explanations and Designing Solutions
Cause and Effect
Systems and System Models
Structure and Function