Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of these notes, basic understanding of amoeboid movement
Amoebozoans are the final supergroup in this protista lab series. They are united by their use of pseudopodia ("false feet") for movement and feeding, though the group is otherwise quite diverse, ranging from plasmodial slime molds that form visible, branching networks to single-celled amoebae to parasitic species that cause serious human disease. You should already be familiar with the euglenozoan, stramenopile, alveolate, rhizarian, and algal material from Parts 1 and 2. This section is shorter but contains several favourite exam topics, particularly the reclassification of slime molds and the distinction between cyst and trophozoite stages.
Amoebozoans move and feed using pseudopodia. The lab covers three types: plasmodial slime molds (Physarum, which forms a visible, branching plasmodium with bidirectional cytoplasmic streaming), tubulinids (Amoeba proteus, the classic amoeba with pseudopodia, ectoplasm, and endoplasm), and Entamoeba histolytica (a parasitic amoeba viewed in cyst and trophozoite stages). Key exam points include why slime molds were reclassified from fungi to protists and how to distinguish the two Entamoeba life stages.
Plasmodium (in slime molds)
A large, multinucleate mass of cytoplasm formed by a plasmodial slime mold. This is a single "supercell" with many nuclei but no internal cell boundaries. Do not confuse this with Plasmodium the malaria parasite; same word, completely different organism.
Cytoplasmic streaming
The flow of cytoplasm within a cell or plasmodium. In Physarum, this streaming is bidirectional, meaning it reverses direction periodically.
Pseudopodia (singular: pseudopodium)
Temporary extensions of the cell membrane and cytoplasm used for locomotion and engulfing food by phagocytosis. Think of them as the cell reaching out a "foot" to pull itself forward or to wrap around prey.
Ectoplasm
The clear, gel-like outer layer of cytoplasm in an amoeba. It is more rigid and sits just beneath the cell membrane.
Endoplasm
The granular, fluid inner cytoplasm of an amoeba. It contains organelles and flows during pseudopod formation.
Cell membrane (plasma membrane)
The outer boundary of Amoeba proteus. Unlike many other protists, amoebae lack a rigid cell wall or pellicle, which is why they can change shape so freely.
Food vacuole
A membrane-bound compartment in an amoeba where ingested food particles are digested.
Trophozoite
The active, feeding, motile stage of a parasitic protist. In Entamoeba histolytica, the trophozoite has a single visible nucleus and one or more pseudopods.
Cyst
The dormant, resistant stage of a parasitic protist, surrounded by a protective wall. In Entamoeba histolytica, the cyst contains multiple nuclei (typically four in the mature form) and is the infective stage transmitted through contaminated water or food.
Entamoeba histolytica
A parasitic amoeba that causes amoebic dysentery in humans. It exists in two forms: the trophozoite (active, in the gut) and the cyst (dormant, passed in faeces, survives outside the body).
Observed under a stereoscopic (dissecting) microscope, not the compound scope
Examine the mature plasmodium for:
Colour, size, and shape – typically yellow, fan-shaped or branching
Branching veins – channels through which cytoplasm flows
Cytoplasmic streaming – visible as movement within the veins; note that it is bidirectional (the flow periodically reverses direction)
The plasmodium is a single, multinucleate cell (a "supercell") that creeps across surfaces engulfing food
Slime molds were once classified as fungi because they produce spore-bearing reproductive structures. They are now classified as protists (amoebozoans) because they have a motile, amoeboid stage in their life cycle that true fungi lack.
Viewed on a prepared slide (oil immersion)
Key structures to identify:
Nucleus – typically one, visible as a dark or refractile body
Pseudopodia – the lobed, temporary extensions the cell uses for movement and feeding
Endoplasm – the granular, flowing inner cytoplasm ("inside the cell")
Ectoplasm – the clear, gel-like outer layer ("outside layer of the cell")
Cell membrane – the flexible outer boundary (no cell wall)
Food vacuole – visible as dark, rounded inclusions where food is being digested
Wet mount: take the sample from the bottom of the specimen jar; do not use methyl cellulose (it interferes with pseudopod formation)
Viewed on prepared slides using oil immersion, one showing the cyst stage and one showing the trophozoite stage
Cyst:
Round, compact
Contains multiple nuclei (up to four in the mature cyst)
Surrounded by a resistant cyst wall
This is the infective, transmissible stage
Trophozoite:
Irregular, amoeboid shape
Contains one visible nucleus
Has pseudopods
This is the active, feeding stage in the host's intestine
Entamoeba histolytica causes amoebic dysentery and is transmitted through the faecal-oral route via contaminated water or food
Students often confuse the slime mold "plasmodium" with the malaria parasite Plasmodium. They are entirely unrelated. Context tells you which is which: slime mold = a mass of cytoplasm; malaria = a genus of apicomplexan parasites.
Students sometimes think amoebae have a cell wall. Amoeba proteus does not. Its cell membrane is the outermost boundary, which is why it can change shape.
Students assume the cyst stage of Entamoeba is dead or inactive. The cyst is dormant but very much alive: it is the resistant, transmissible stage that survives outside the host.
⚠️ The question "Why were slime molds reclassified from fungi to protists?" appears on exams frequently. Answer: they have a motile, amoeboid stage; true fungi do not.
⚠️ Be able to distinguish ectoplasm from endoplasm in an Amoeba diagram.
⚠️ Know that cytoplasmic streaming in Physarum is bidirectional.
⚠️ Be able to compare the cyst and trophozoite stages of Entamoeba histolytica: number of nuclei, shape, function, and which is infective.
⚠️ Remember the three modes of locomotion across all protist groups: flagella, cilia, and pseudopodia, with an example organism for each.
True or false: Physarum plasmodium exhibits unidirectional cytoplasmic streaming. (False. It is bidirectional.)
Fill in the blank: The clear outer layer of cytoplasm in Amoeba proteus is called the ______, and the granular inner layer is the ______. (Ectoplasm; endoplasm.)
True or false: The trophozoite is the transmissible stage of Entamoeba histolytica. (False. The cyst is the transmissible stage.)
Fill in the blank: Slime molds were reclassified from fungi to protists because they exhibit a ______ stage. (Motile / amoeboid.)
Q: Explain why slime molds were once classified as fungi but are now classified as protists.
A: Slime molds were classified as fungi because they produce spore-bearing reproductive structures resembling those of fungi. They are now classified as protists (amoebozoans) because they exhibit a motile, amoeboid stage during their life cycle. True fungi never have a motile stage.
Q: Compare the cyst and trophozoite stages of Entamoeba histolytica. Which is the infective stage?
A: The trophozoite is the active feeding stage found in the host's intestine; it has an irregular shape, one nucleus, and pseudopods. The cyst is the dormant stage, round with a resistant wall and multiple nuclei (up to four). The cyst is the infective stage, transmitted through contaminated water or food.
Q: What structures would you label on a diagram of Amoeba proteus?
A: Nucleus, pseudopodia, endoplasm, ectoplasm, cell membrane, and food vacuole.
Q: Describe the cytoplasmic streaming observed in Physarum. Is it unidirectional or bidirectional?
A: Cytoplasmic streaming in Physarum is bidirectional. The flow of cytoplasm through the branching veins of the plasmodium periodically reverses direction.
The reclassification of slime molds connects to broader themes in systematics and phylogenetics: morphological similarity (spore structures) can be misleading, and molecular evidence often overturns traditional groupings. The cyst/trophozoite distinction in Entamoeba parallels similar life-stage concepts in Plasmodium (sporozoite, merozoite, gametocyte) and is relevant to parasitology and public health. Pseudopodial movement in amoebae is mechanistically related to cell crawling in human immune cells (neutrophils, macrophages), making this material relevant to immunology as well.
Amoebozoa, amoebozoans, plasmodial slime mold, Physarum, plasmodium, cytoplasmic streaming, bidirectional streaming, Tubulinea, tubulinid, Amoeba proteus, pseudopodia, pseudopod, ectoplasm, endoplasm, cell membrane, food vacuole, phagocytosis, Entamoeba histolytica, amoebic dysentery, trophozoite, cyst, faecal-oral transmission, protist lab, microbial eukaryotes, UF Cell Biology