B. Diversity of Living Things · Diversity of Living Things

Protists: Animal-like, Plant-like and Fungus-like

B3.3 B2.2 B2.1 A1.2 A1.12About 75 minutes

Learning goals

I can describe the three informal groups of protists and give examples of each.
I can explain how protists move, feed and reproduce, using correct structure names.
I can draw and label a biological diagram of a protist following scientific conventions.
I can estimate the size of a microorganism from the field of view of a microscope.

Words to know

protist
A eukaryote that is not a plant, fungus or animal; most are unicellular.

protozoan
An animal-like (heterotrophic) protist, such as an amoeba or Paramecium.

pseudopod
"False foot": a temporary bulge of cytoplasm used for movement and engulfing food.

cilia (singular: cilium)
Many short, hair-like structures that beat together to move a cell or sweep in food.

flagellum (plural: flagella)
A long, whip-like structure that moves a cell.

contractile vacuole
An organelle that collects excess water and pumps it out of a freshwater protist so it does not burst.

algae (singular: alga)
Plant-like, photosynthetic protists, from single-celled diatoms to large kelps.

phytoplankton
Tiny photosynthetic organisms that drift in water, including diatoms, dinoflagellates and cyanobacteria.

A drop of pond water

Put a drop of water from a pond or a ditch under a microscope and a whole world appears: slipper-shaped cells racing past, blobs flowing and swallowing particles, green spindles spinning on a whip-like tail, and glassy boxes in every geometric shape. These are protists, one of the most varied kingdoms of all.

As you learned in the domains and kingdoms lesson, Protista is a group of convenience. To study it, biologists sort protists into three informal groups, based on how they get food: animal-like, plant-like and fungus-like.

Animal-like protists (protozoans)

Protozoans are heterotrophs that ingest food. They are often grouped by how they move:

Sarcodines move and feed with pseudopods. An Amoeba flows around a food particle and encloses it in a food vacuole.
Ciliates are covered in cilia. Paramecium sweeps food into its oral groove with cilia. It has a large macronucleus that runs everyday cell activities and a small micronucleus used in reproduction.
Zooflagellates move with one or more flagella. Giardia, which causes "beaver fever" when people drink untreated stream or lake water, is one. Trypanosoma, spread by tsetse flies in Africa, causes sleeping sickness.
Sporozoans (apicomplexans) have no structures for movement and are all parasites. Plasmodium, spread by Anopheles mosquitoes, causes malaria, one of the world's most serious diseases.

Freshwater protozoans have contractile vacuoles. Water constantly diffuses into the cell by osmosis, and the vacuole pumps it out so the cell does not burst.

A labelled biological drawing of Paramecium. Notice the drawing conventions: clean single lines, no shading, ruled label lines that do not cross each other, and labels lined up at the sides.

Plant-like protists (algae)

Algae are photosynthetic protists. They have chloroplasts but no roots, stems, leaves or vascular tissue.

Euglena has chloroplasts and can absorb food when there is no light, so it is a mixotroph. It swims with a flagellum and senses light with a red eyespot. A flexible pellicle lets it change shape.
Diatoms live in glass-like shells made of silica, in two halves that fit together like a box and lid.
Dinoflagellates have two flagella; some cause toxic red tides, and some glow (bioluminescence).
Green algae range from single cells (Chlamydomonas) to colonies (Volvox) and filaments (Spirogyra). Land plants evolved from green algae.
Red algae (nori used in sushi) and brown algae (kelp, which can be tens of metres long) are multicellular.

Marine phytoplankton, including diatoms, dinoflagellates and cyanobacteria, carry out roughly half of all photosynthesis on Earth, so they make a large share of the oxygen we breathe and form the base of ocean food webs.

Amoeba (left) moves and feeds with pseudopods. Euglena (right) swims with a flagellum, detects light with its eyespot, and photosynthesizes with its chloroplasts. Both have contractile vacuoles to pump out extra water.

Fungus-like protists

Slime moulds spend part of their life as single cells and part as a moving mass that feeds on bacteria in rotting wood and mulch. The bright yellow "dog vomit slime mould" sometimes appears on garden mulch in Ontario after rain. When food runs out, slime moulds form stalks that release spores, just like fungi.
Water moulds (oomycetes) look like fungi but have cellulose cell walls. Phytophthora infestans causes late blight, a destructive disease of potatoes and tomatoes.

A note on DNA evidence: these groups (and the protozoan groups above) are informal. DNA shows, for example, that water moulds are close relatives of diatoms and brown algae, that slime moulds are close relatives of amoebas, and that neither is closely related to true fungi. Euglena turns out to be more closely related to the parasite Trypanosoma than to green algae; its chloroplasts came from a green alga it engulfed.

Reproduction in protists is varied: many reproduce asexually by binary fission (amoeba) or multiple fission (Plasmodium), and many also reproduce sexually. Paramecium can pair up and swap micronuclei in a process called conjugation, which mixes their genes.

Group	Nutrition	Movement	Examples
Animal-like (protozoans)	heterotroph: ingests	pseudopods, cilia, flagella, or none	Amoeba, Paramecium, Giardia, Plasmodium
Plant-like (algae)	autotroph: photosynthesis (some mixotrophs)	flagella or none	Euglena, diatoms, dinoflagellates, Volvox, kelp
Fungus-like	heterotroph: absorbs	slime moulds creep; water moulds make swimming spores	slime moulds, water moulds

The three informal groups of protists.

How to draw a biological diagram

Use a sharp pencil and unlined paper. 2. Draw large (at least half a page), with clean single lines; no sketchy lines and no shading (use stippling dots only if needed). 3. Draw only what you see. 4. Put labels at the side, in a neat column, joined by ruled lines that do not cross. 5. Add a title (the organism's name), the magnification used, and an approximate actual size or scale.

Worked example

Let's estimate the size of a protist

At 100× total magnification, the diameter of a microscope's field of view is 1.8 mm. A Paramecium stretches about one-eighth of the way across the field. Estimate its length in micrometres.

Convert the field diameter to µm: 1.8 mm × 1000 = 1800 µm.
Length ≈ field diameter × fraction of field = 1800 µm × 1/8 = 225 µm.
Precision: the "one-eighth" is an estimate, so report about 200 µm (one significant digit). That matches the known size of Paramecium (roughly 0.2 to 0.3 mm).

About 200 µm (0.2 mm).

Worked example

Let's classify protists from observations

Classify each organism and name how it moves: (a) a green cell with chloroplasts, an eyespot and one flagellum; (b) a cell that extends finger-like bulges and wraps around bacteria; (c) a yellow, slimy mass on bark mulch that later forms spore stalks; (d) a slipper-shaped cell covered with tiny beating hairs.

Ask two questions: How does it get food? (photosynthesis, ingest, absorb) and How does it move?
(a) Photosynthetic → plant-like, moves by a flagellum (Euglena).
(b) Ingests food → animal-like, moves by pseudopods (an amoeba).
(c) Feeds by absorbing/engulfing in decaying material and forms spores → fungus-like (a slime mould); it creeps slowly.
(d) Ingests food → animal-like, moves by cilia (Paramecium).

(a) plant-like, flagellum; (b) animal-like, pseudopods; (c) fungus-like slime mould; (d) animal-like, cilia.

Just for fun

A funny-but-true fact: in 2000, Japanese researchers put a slime mould, Physarum polycephalum, in a maze with food at two exits. It grew along the shortest route between them. It has no brain, yet it solved the maze.

Hands-on activity

Grow and observe a hay infusion

You need: a clean jar, a handful of dry grass or hay (or dead leaves), pond, puddle or rain water (or tap water left out for a day), a dropper or straw, and a microscope if you can get one (a clip-on phone microscope or a school or library microscope works). Wash your hands after handling the water; never drink it.

Put the grass in the jar and cover it with water. Leave it uncovered (or loosely covered) in a warm place out of direct sun.
Every two days for about 10 days, smell it (carefully), note its cloudiness, and if possible take one drop from near the surface and one from near the bottom to look at.
Under the microscope, look for organisms that swim fast (ciliates), creep and change shape (amoebas) or are green (algae). Record movement, colour and approximate size using the field of view.
Make one biological drawing of the organism you see best, following the drawing rules.
Record how the community changes over the days (this is a mini succession).

What to look for: bacteria appear first (cloudy water), then small flagellates and ciliates that eat them, often followed by larger ciliates such as Paramecium. Without a microscope, you may still see tiny moving specks with a strong magnifier; compare your observations with labelled online images from a reliable source. Pour the infusion down the toilet when finished and wash the jar with hot, soapy water.

Your turn

Name the three informal groups of protists and how each gets food.

How does an amoeba move and feed?

What is the job of a contractile vacuole, and why do freshwater protists need one?

Why is Euglena hard to classify as plant-like or animal-like?

Which protist causes malaria and how is it spread?

Give two reasons phytoplankton are important.

A Paramecium has two kinds of nuclei. Name them and their jobs.

More practice: warm-up

What does pseudopod mean?

What are diatom shells made of?

Name a multicellular protist.

What structure does Euglena use to detect light?

Which protozoan causes "beaver fever"?

More practice: on your own

The field of view at 400× is 0.45 mm. A diatom spans about one-third of the field. Estimate its size in µm.

Find two errors in this description of a student's drawing: "Drew a shaded amoeba in pen with labels written inside the cell."

Why are slime moulds placed with protists and not fungi, even though they make spores?

Explain why campers in Ontario are told to boil or filter stream water even when it looks clean.

Why does a marine (saltwater) protist usually not need a contractile vacuole?

Challenge

Land plants evolved from green algae. Name two features green algae share with plants that support this.

A student sees an organism move across the whole 1.8 mm field of view at 100× in 6 seconds. Calculate its speed in µm/s and mm/min.

Explain why biologists find it more useful to group protists by nutrition and movement in a course like this, even though these groups are not clades.

Check your understanding

Quiz

Paramecium moves using:

Which protist group makes its own food by photosynthesis?

What does a contractile vacuole do?

Which is a rule for biological drawings?

The field of view is 2.0 mm. An organism spans one-quarter of it. Its length is about:

Plasmodium, which causes malaria, is a:

For parents and teachers

Big idea (B3.3, B2.2): protists are a diverse kingdom of mostly single-celled eukaryotes, sorted informally into animal-like, plant-like and fungus-like groups. Students learn the structures each uses to move and feed, practise proper biological drawings, and estimate size from the field of view (A1.2, A1.12, A1.13).

How to help: The hay infusion is easy and safe if you wash hands and dispose of it properly. If you have no microscope, a school, library or inexpensive clip-on phone microscope is enough; otherwise use labelled images from a museum or university website.

Common difficulties:

Confusing cilia (many, short) with flagella (few, long).
Sketchy or shaded drawings. Insist on single clean lines and ruled labels.
Calling all algae plants. In this system, algae are protists.

Safety: never drink pond or stream water; wash hands after handling it.

Sources: protist features from standard biology texts; slime-mould maze study by Nakagaki, Yamada and Tóth (2000, Nature); phytoplankton share of global photosynthesis from NASA and NOAA educational materials.