Difficulty: Intermediate | Prerequisites: Basic eukaryotic cell structure, microscopy techniques
This lab introduces the first two major supergroups of microbial eukaryotes you will encounter in the protista unit: Euglenozoans and Stramenopiles. These organisms sit at the base of eukaryotic diversity, and understanding their structures is essential before moving on to alveolates, rhizarians, and amoebozoans in later sections. You should already be comfortable identifying basic cell organelles (nucleus, chloroplast, vacuole) and using oil immersion on the compound microscope. If terms like "flagellum" or "photosynthesis" feel unfamiliar, review your introductory cell biology notes first.
Euglenozoans include kinetoplastids (like the parasite Trypanosoma, which causes sleeping sickness) and euglenids (like Euglena, a mixotroph with both animal-like and plant-like traits). Stramenopiles include diatoms (silica-shelled algae in pennate and centric forms) and brown algae (seaweeds such as Laminaria, Ectocarpus, and Sargassum with distinct body plans). The lab asks you to identify key structures under the microscope for each organism.
Kinetoplastid
A protist within the Euglenozoa that possesses a kinetoplast, a large mass of DNA inside a single mitochondrion. Think of it as a parasite group defined by having an unusually conspicuous blob of mitochondrial DNA.
Undulating membrane
A fin-like extension of the cell membrane in trypanosomes, formed where the flagellum is attached along the body surface. In simple terms, it is the wavy "sail" running along the parasite that helps it move through blood.
Flagellum (plural: flagella)
A long, whip-like appendage used for locomotion. Think of it as the organism's outboard motor.
Euglenid
A flagellated protist in the phylum Euglenozoa, typically photosynthetic but capable of heterotrophy. In simple terms, Euglena is the classic example: it has chloroplasts but can also absorb nutrients when light is unavailable.
Stigma (eye spot)
A light-sensitive, pigmented organelle near the base of the flagellum in Euglena that detects light direction. Think of it as a primitive "eye" that tells the cell which way the light is.
Photoreceptor
The actual light-sensing structure in Euglena, located near the stigma, that triggers flagellar responses to light. In simple terms, the stigma shades it, and the photoreceptor does the sensing.
Paramylon body
A carbohydrate storage granule unique to euglenids, composed of beta-1,3-glucan. Think of it as Euglena's version of a starch grain.
Contractile vacuole
An organelle that pumps excess water out of the cell to maintain osmotic balance. In simple terms, it is the cell's bilge pump.
Reservoir
A flask-shaped invagination at the anterior end of Euglena where the flagellum emerges and the contractile vacuole empties. Think of it as the docking bay at the front of the cell.
Diatom
A unicellular alga in the Stramenopiles with a cell wall (frustule) made of silica. In simple terms, diatoms are microscopic algae that live inside tiny glass boxes.
Pennate diatom
A diatom with bilateral (elongated, boat-shaped) symmetry.
Centric diatom
A diatom with radial (circular or wheel-like) symmetry.
Diatomaceous earth
A sedimentary deposit composed of fossilised diatom frustules. Think of it as ancient diatom shells compressed into a chalky powder, used in filters and abrasives.
Stipe
The stem-like structure of a brown alga that connects the holdfast to the blade. In simple terms, it is the "stalk" of a seaweed.
Blade
The broad, flat, leaf-like portion of a brown alga where most photosynthesis occurs.
Holdfast
The root-like anchoring structure at the base of a brown alga. It grips the substrate but does not absorb nutrients the way a true root does.
Air bladder (float / pneumatocyst)
A gas-filled structure in brown algae such as Sargassum that provides buoyancy, keeping the blades near the water surface for light.
Unilocular sporangium
A single-chambered reproductive structure in Ectocarpus that produces zoospores by meiosis.
Plurilocular sporangium
A many-chambered reproductive structure in Ectocarpus that produces zoospores by mitosis.
Filament
A thread-like chain of cells, the basic body form of Ectocarpus.
Viewed under oil immersion among red blood cells
Key structures to identify:
Flagellum – whip-like projection used for propulsion through blood
Undulating membrane – the wavy membrane running alongside the body where the flagellum is attached
Nucleus – the central, darkly staining organelle
Red blood cells – the surrounding host cells (much smaller than the parasite in cross-section)
Trypanosoma is the causative agent of African sleeping sickness, transmitted by the tsetse fly
The organism is extracellular: it lives freely in the host's bloodstream rather than inside cells
Prepared slide: identify the nucleus
At-home assignment (detailed drawing): identify all eight structures:
Flagella – one or two whip-like appendages at the anterior end, used for locomotion
Stigma (eye spot) – orange-red pigmented spot near the reservoir that shades the photoreceptor
Photoreceptor – the light-sensing structure adjacent to the stigma; together they enable phototaxis
Paramylon body – refractile storage granules scattered in the cytoplasm
Contractile vacuole – osmoregulatory organelle near the reservoir
Chloroplast – green, disc-shaped organelles where photosynthesis occurs
Reservoir – anterior invagination from which the flagellum emerges
Nucleus – large, centrally located
Wet mount with methyl cellulose: slows the organism so you can observe the stigma, chlorophyll (green colour), and flagellum if visible
Euglena is a mixotroph: photosynthetic in light, heterotrophic in dark. This dual nature historically confused taxonomists (zoologists saw it move with a flagellum and called it an animal; botanists saw it photosynthesize and called it a plant).
Viewed on a prepared slide; the task is to distinguish pennate (elongated, bilaterally symmetrical) from centric (round, radially symmetrical) forms
The diatom cell wall is called a frustule and is composed of silica (SiO2), which gives it a glass-like quality
Frustules fit together like a Petri dish: an epitheca (lid) overlaps a hypotheca (base)
Diatomaceous earth wet mount: fossilised frustules appear as tiny, varied geometric shapes, often with a honeycomb pattern visible under the microscope
Diatoms are among the most important primary producers on Earth, responsible for roughly 20% of global oxygen production
Laminaria (kelp)
A large, leaf-like ("leaf-like" morphology) brown alga
Key structures:
Stipe – the stem-like support
Blade – the broad, photosynthetic surface
Holdfast – the anchoring base
These structures are analogous to the stem, leaf, and root of a land plant, but they evolved independently. Brown algae are not plants.
Ectocarpus
A filamentous brown alga viewed as a wet mount
Key structures:
Filaments – branching chains of cells
Unilocular sporangium – single chamber, produces spores via meiosis (part of sexual reproduction)
Plurilocular sporangium – multi-chambered, produces spores via mitosis (asexual reproduction)
Distinguishing the two sporangia types is a common exam question
Sargassum
A preserved specimen with a more complex, branching body plan
Key structures:
Blade – flattened photosynthetic portions
Stipe – stem-like connections
Air bladder (float) – gas-filled spheres that provide buoyancy
Sargassum is the namesake of the Sargasso Sea, where massive floating mats accumulate
Students often think Euglena is a plant because it has chloroplasts. It is not. It is a protist (euglenid) that acquired chloroplasts through secondary endosymbiosis.
Students confuse the stigma with the photoreceptor. The stigma is the pigmented spot that shades the photoreceptor; the photoreceptor is the structure that senses light. They work as a pair.
Students sometimes assume the holdfast of a brown alga absorbs water and nutrients like a root. It does not. It is purely for attachment.
Students mix up unilocular and plurilocular sporangia. Remember: uni = one chamber = meiosis (sexual); pluri = many chambers = mitosis (asexual).
⚠️ Be able to label all eight structures of Euglena from a diagram, especially the stigma, photoreceptor, paramylon body, and reservoir.
⚠️ Know the difference between pennate and centric diatoms, and be able to identify each from a sketch or micrograph.
⚠️ Distinguish unilocular from plurilocular sporangia in Ectocarpus, including which type of cell division each uses.
⚠️ Understand why Euglena was historically claimed by both zoologists and botanists. This is a favourite short-answer prompt.
⚠️ Be ready to label stipe, blade, holdfast, and air bladder on a brown algae diagram.
True or false: The stigma in Euglena is the structure that directly detects light. (False. The photoreceptor detects light; the stigma shades it.)
Fill in the blank: Diatom cell walls are called ______ and are made of ______. (Frustules; silica.)
True or false: The holdfast of Laminaria absorbs water and minerals from the substrate. (False. It only anchors the organism.)
Fill in the blank: A unilocular sporangium produces spores by ______, while a plurilocular sporangium produces spores by ______. (Meiosis; mitosis.)
True or false: Trypanosoma brucei rhodesiense lives inside red blood cells. (False. It is extracellular, living freely in the bloodstream.)
Q: Historically, what characteristics of Euglena led zoologists to believe it was an animal, and botanists to believe it was a plant?
A: Zoologists classified it as an animal because it moves using a whip-like flagellum. Botanists classified it as a plant because it contains chloroplasts and can photosynthesize. It is now placed among the euglenids within the Euglenozoa, a protist supergroup.
Q: Name the structures you would label on a prepared slide of Trypanosoma brucei rhodesiense and state one function for each.
A: Flagellum (locomotion through blood), undulating membrane (aids movement and helps the organism evade the immune system by creating a wave-like motion), nucleus (contains genetic material), and surrounding red blood cells (host cells, not part of the parasite).
Q: Explain the difference between unilocular and plurilocular sporangia in Ectocarpus.
A: A unilocular sporangium is a single-chambered structure that produces zoospores through meiosis, contributing to the sexual phase of the life cycle. A plurilocular sporangium has many small chambers and produces zoospores through mitosis, contributing to asexual reproduction.
Q: Why are diatoms ecologically significant?
A: Diatoms are major primary producers in aquatic ecosystems, responsible for approximately 20% of global photosynthetic oxygen. Their silica frustules also accumulate as diatomaceous earth, which has commercial applications in filtration and as an abrasive.
Q: Compare the body plans of Laminaria and Sargassum. What structure does Sargassum have that Laminaria does not?
A: Both have a stipe and blade. Sargassum additionally has air bladders (floats/pneumatocysts) that provide buoyancy, allowing it to float at the ocean surface. Laminaria is typically anchored to the substrate by a holdfast without floats.
This material connects directly to the alveolates section (Part 2 of these notes), where you will encounter dinoflagellates and apicomplexans, two more groups of protists with distinct cell structures and ecological roles. The concept of endosymbiosis, relevant to how Euglena acquired its chloroplasts, also appears in discussions of mitochondrial and chloroplast origins in introductory cell biology. Brown algae body plans (stipe, blade, holdfast) offer a useful comparison to land plant anatomy when studying convergent evolution.
Euglenozoa, Euglenophyta, kinetoplastid, Trypanosoma, African sleeping sickness, tsetse fly, Euglena, euglenid, mixotroph, stigma, eye spot, photoreceptor, paramylon, contractile vacuole, reservoir, pellicle, Stramenopiles, Heterokonta, diatom, frustule, silica, pennate, centric, diatomaceous earth, brown algae, Phaeophyta, Laminaria, kelp, Ectocarpus, Sargassum, stipe, blade, holdfast, air bladder, pneumatocyst, unilocular sporangium, plurilocular sporangium, filament, protist lab, microbial eukaryotes, UF Cell Biology