Protozoa and how DNA trumps external form

📚 Article Contents

What factors determine the classification of protists?

Modern classification relies on several factors, the most important of which are:

DNA & rRNA sequencing

To be precise and clear, it is: "similarity in the sequence of genetic letters," specifically in ribosomal RNA (rRNA), and searching for "rare shared genetic mutations or errors" (like the story of the cheated exam paper – two papers were found with a certain word written, for example, "biomogan" instead of "biology").

Scientists are not concerned with the complexity of DNA (I mean its size or length), because sometimes a very primitive organism is found to possess a huge amount of DNA with no known function (junk DNA).

Number of cell membranes

Scientists aren't concerned with the number of membranes surrounding the cell itself (because all cells have a single cell membrane). The key is the number of membranes surrounding the organelles inside the cell (plastids and mitochondria). Two membranes

indicate primary symbiosis (direct engulfment of bacteria).

Three or four membranes indicate secondary or tertiary symbiosis (engulfment of a cell that has engulfed bacteria).

The number of membranes here represents the "GPS" that guides scientists to the origin of the plastid within an organism throughout evolutionary history.

Protozoa

Mitochondrial norms and their complexity

The shape of the cristae (the folds inside the mitochondria) is not simple to complex; it serves as a "constant morphological signature" for the evolutionary group:

Is it tubular? (Tubular) The organism belongs to the amoeba and slime mold group.

Is it disc-shaped? (Discoid) It belongs to the excavata group (Euglena and Trypanosoma).

Is it flat? (Flat) It belongs to algae and plants.

In addition, the presence of mitochondria themselves is crucial: some organisms in modern classification (such as members of the excavata) possess mitochondria that are completely atrophied and modified, and do not use oxygen. This alone is a decisive taxonomic criterion.

Summary by Muhammad Abdul Ghani Abdul Hamid on the development of the "Modern Catalog":

DNA and rRNA code: (Blood and ancestral relationship?).

Cellular bullying history: (Number of plastid and mitochondrial membranes).

Mitochondrial fingerprint: (Shape of cristae and whether mitochondria are complete or atrophied).

Cell motor (cytoskeleton and flagellar roots): (Arrangement of microtubules that control cell movement).

Summary by Mohamed Abdelghani Abdelhamid – Dr. Mohgan: “Philosophy” of Modern Science in two words.

What are the criteria set by the Supergroup?

Scientifically and definitively, the concept can be summarized as follows: a supergroup is a major evolutionary clade comprising eukaryotic organisms, all descended from a single common ancestor (monophyletic).

For taxonomists to agree on placing a group of organisms within a single supergroup, these organisms must meet four strict scientific criteria representing the "evolutionary fingerprint" of this common ancestor.

Here is a precise scientific summary of these criteria:

Molecular Evidence

It is the cornerstone, and it depends on proving genetic homology (Sequence Homology).

Analysis of conserved genes : matching the sequence of nitrogenous bases in specific genes that do not change easily over time, the most important of which are ribosomal RNA (rRNA) genes, and genes that code for essential structural proteins (such as actin and tubulin).

Ultrastructural Signatures

Structural properties observed at the electron microscope level that are unaffected by the external environment include:

Mitochondrial crest morphology: Are the internal folds of the mitochondria tubular, disc-shaped, or flat? (Or are the mitochondria completely atrophied?).

Cytoskeleton architecture: The precise spatial arrangement of the microtubules that support the cell.

Flagellar root apparatus: The precise geometric details of how the flagellum is attached to the cytoskeleton within the cell.

Endosymbiotic Signatures

Tracing the cellular history of energy-producing organelles (plastids and mitochondria) to determine their origin:

Plastid lineage: Determining whether the cell acquired the plastid through primary symbiosis (engulfing cyanobacteria = two membranes) or secondary/tertiary symbiosis (engulfing algae = three to four membranes). This criterion establishes the lineage between autotrophs and some heterotrophs that subsequently lost their plastids.

Rare Genomic Changes (RGCs)

Scientifically known as synapomorphies , these are very unique genetic mutations or events that cannot occur twice by chance. Examples include gene fusions, where two separate genes combine to form a single gene that produces a protein with dual function. If this fused gene is found in several organisms, it is strong evidence that they belong to the same supergroup. Another example is gene insertions or deletions, where identical pieces of DNA are added or deleted at the exact same genetic locus. In summary, for an organism to be classified as a supergroup, it must match the group's DNA code, have the same mitochondrial and flagellar design, share the same symbiotic history, and inherit the same rare genetic mutations from its first common ancestor.

Why was it named (SAR) and why were they put together?

The reason for the name: The word (SAR) is not a complex Latin name, but simply an acronym for the first letters of the three groups that comprise it: Stramenopila, Alveolata, and Rhizaria.

The reason for grouping (genetic characteristics): These morphologically diverse organisms were placed in a single group based on molecular evolution and DNA analysis. Genetics has shown that these organisms, despite their enormous morphological differences, descend from a single common ancestor, making the modern classification genetically accurate, even if morphologically confusing.

Modern accurate classification (Higher Groups System - Contemporary 2026)

With the tremendous development in molecular phylogenetics, scientists have proven that protists represent a paraphyletic group — that is, they do not descend from a single common ancestor that is unique to them. 

Consequently, the kingdom Protista has been scientifically abolished in modern academic classifications.

Instead of traditional kingdoms, the current world system classifies all eukaryotes (including what were formerly known as protists, as well as animals, plants, and fungi) into supergroups as follows:

The SAR Clade: This clade encompasses enormous diversity and is named using the first letters of its three subclades:

1. Straw flagellates (Stramenopiles): such as diatoms and brown algae.
2. Alveolata : Paramecium, Plasmodium, and Dinoflagellates.
3. Root protozoa (Rhizaria): such as Foraminifera and Actinomycetes.
4. Excavata : These are characterized by the presence of a "feeding" groove in their cell structure, and include single-celled organisms that possess flagella, such as Euglena and Trypanosoma.
5. Archaeoplastida : These include organisms that acquired chloroplasts long ago, and include green and red algae (as well as terrestrial plants) .
6. Amorphea / Uniconta : This group includes organisms that move with pseudopodia or possess a single flagellum, and includes: Amoebas (Amoebozoa): such as amoebas and slime molds.
7. Opisthokonta : (This is the group from which animals and fungi branch out) .

SAR Supergroup

This group is the greatest testament to the genius of modern classification ; it includes organisms that were completely scattered in the old classification among animals, plants, and fungi, but modern classifications have proven that they belong to a single evolutionary origin.

The organisms transferred to this group from the old classification are:

1. Examples of animal-like protists include Paramecium (ciliates) and Plasmodium (spores).
2. Examples of plant-like protists include brown algae (kelp), diatoms, and dinoflagellates.
3. Diatoms are "jewels of the sea" and creatures that live inside "glass boxes"!

In the old classification, diatoms were simply regarded as "rod-shaped or disc-shaped vegetative algae" because they perform photosynthesis. 

But when scientists examined its genes and cellular history, they discovered it belonged to a completely different family, concealing a complex evolutionary secret.

Here is a classification of diatoms and their overall characteristics in light of modern classification, focusing on the minute details that will ensure any illustration or explanation you create is of the highest scientific accuracy:

First: The modern classification of diatoms (Hierarchy)

Domain: Eukaryota.

Supergroup: SAR.

Subgroup: Straminopiles.

Class: Diatoms (Bacillariophyceae).

Second: Characteristics of diatoms in light of the governing principles of modern classification

1. The symbiotic history (the secret of the golden color and the four membranes)

This is the criterion that placed diatoms in the Species of Species (SAR). The plastids in diatoms are not green like those in plants, but rather tend towards a golden or yellowish-brown color because they contain the pigment fucoxanthin.

How did they acquire it? Through secondary endosymbiosis. The ancestor of the diatom was a heterotrophic organism that ingested a red alga (not a green one like Euglena).

The conclusive evidence: if you examine a diatom plastid under an electron microscope, you will find it surrounded by four complete membranes (evidence of the complex process of ingesting the red alga).

2. The fine cellular structure (glass box and hidden flagella)

How did diatoms get into the Stramenopiles clade, given that they mostly don't move with flagella?

Hairy flagella: The cellular hallmark of this clade is the presence of two distinct flagella: one covered in fine, straw-like hairs, and the other smooth. Adult diatoms lost these flagella to adapt to life inside a shell, but during sexual reproduction, the male gametes (diatom sperm) display these distinctive straw flagella, revealing their evolutionary origin!

Silica shield (frostule): The iconic feature of diatoms. Instead of a flexible cellulose cell wall, diatoms build a rigid wall of hydrated silica (silicon dioxide – practically glass) around themselves. This wall is perforated with very fine, geometrically patterned holes that allow the passage of gases and nutrients.

3. Molecular Phylogenetics

Ribosomal RNA (rRNA) analysis has demonstrated the close and surprising kinship of diatoms with giant brown algae (such as kelp) and with water molds, all of which fall into the same evolutionary branch (Stramenopiles) within the SAR group, despite the enormous variation in their sizes and phenotypes.

Third: The most important anatomical details 

When designing a scientific diagram of a diatom cell, these are the key anatomical elements that should be clearly highlighted:

Shell structure (box and cap): The silica shield (frustule) consists of two overlapping halves resembling a "shoebox" or a laboratory "Petri dish":

the upper shell (epithecine): the larger half that forms the cap;

the lower shell (hypotheca): the smaller half that forms the base and is nested within it.

Longitudinal slit (rabphe): Many diatoms (especially pennates) possess a longitudinal slit that runs through the shell. 

This slit is not decorative; diatoms secrete a mucous gel from it. As this gel flows, it rubs against the surface they live on, giving them a smooth, "gliding" motion without the need for flagella or pseudopodia.

The plastids should be stained gold or yellowish-brown for scientific identification, and they take on different shapes (striates, discs, or lobes) depending on the diatom species.

Oil droplets: a very important detail! Diatoms do not store their excess food as starch, but rather as droplets of oil or a carbohydrate compound called chrysolaminarin . 

These oil droplets serve a dual purpose : storing energy and reducing the cell's specific gravity, allowing it to float near the water's surface (where sunlight is present) instead of sinking due to its heavy glassy shell.

An important environmental note: when discussing diatoms, it's never omitted to mention that they are the "hidden lungs of the Earth," responsible for producing approximately 20% to 25% of the oxygen we breathe on our planet—more than many of the world's forests combined!

Another example of fungus-like protozoa is water molds.

Why were they grouped together? (The governing pillars):

Genetic and molecular analysis revealed a striking similarity in the ribosomal RNA (rRNA) sequences among these phenotypically diverse organisms.

Endosymbiosis events: Autotrophic organisms in this group (such as brown algae, diatoms, and dinoflagellates) share the characteristic of having originated through secondary endosymbiosis, where their common ancestor ingested a single-celled red alga. Consequently, their chloroplasts are surrounded by three or four membranes.

Microcellular structure: Members of the Alveolates phylum (such as Paramecium, Plasmodium, and dinoflagellates) possess distinctive membrane vacuoles directly beneath the cell membrane called alveoli. Members of the Stramenopiles phylum (such as diatoms, brown algae, and water molds) possess unique flagella covered with fine, straw-like hairs during certain life stages.

2. Archeplastida group

This group includes organisms that possess a direct and pure production line for photosynthesis.

Organisms transferred to it from the old classification:

from plant-like protists: green algae (such as Spirogyra and Chlamydomonas), and red algae (as well as true land plants that were merged with them).

Why were they grouped together? (The governing pillars):

Endosymbiotic events: This group is the only one representing primary endosymbiosis; its common ancestor directly engulfed a cyanobacterium and transformed into a chloroplast surrounded by only two membranes.

Genetic analysis and fine structure: Its cells possess cellulose cell walls, are genetically identical in chlorophyll synthesis genes, and have flat cristae (mitochondria).

3. Excavata Group

It brought together single-celled organisms that were previously classified into disparate groups based on their mode of nutrition (some herbivorous, others parasitic).

Organisms transferred to it from the old classification:

from animal-like protists: Trypanosomas (parasitic flagellates).

Trypanosoma is the "evil twin" of Euglena!

If Euglena is the peaceful organism that makes its food from the sun, then Trypanosoma is a vicious and deadly parasite (causing African sleeping sickness). But, in a remarkable feat of modern classification and genetic analysis, it has been proven beyond any doubt that this blood-borne parasite is a direct cousin of Euglena and belongs to the same evolutionary branch.

Classification of trypanosomas and their overall characteristics 

First: The modern classification of trypanosomas (Hierarchy)

Domain: Eukaryota.

Supergroup: Excavata.

Phylum: Euglenozoa.

Class: Kinetoplastea - here it differs from Euglena.

Second: Characteristics of trypanosomas in light of the governing pillars

1. Molecular fingerprinting and the wonder of Kinetoplast

This is the gold standard that has placed trypanosomas in their own class. Cytological and genetic analysis has revealed a very unique structure called the kinetoplast.

What is the kinetoplast? It is a massive, dense mass of DNA found within a single giant mitochondria that spans the entire length of the cell. This genetic mass is always located near the base of the flagellum and controls the mitochondria's function of producing the energy required for the flagellum's violent movement through the viscous blood.

2. Ultrastructure

When examining trypanosomas under an electron microscope, the hallmarks of the Excavata group are clearly visible, along with the parasite's adaptations:

Discoid mitochondrial crests: Just like Euglena, it possesses internally folded, disc-shaped mitochondria, a definitive indicator of their kinship.

Undulating membrane: Unlike other organisms, the flagellum in trypanosomas does not emerge freely from the front of the cell. Instead, it emerges from the rear and extends, attached to the cell wall along the body by a membrane fold. When the flagellum moves, it causes this membrane to quiver and undulate, giving the organism a powerful "twisting" or spiraling motion that allows it to penetrate the viscous blood plasma and cerebrospinal fluid of the host.

Cytoskeleton (flexible shell): It possesses a network of microtubules directly beneath the plasma membrane, giving the cell its streamlined, flexible, spindle-shaped form.

3. Symbiotic history and feeding pattern (abandonment of photosynthesis)

Complete absence of plastids: Unlike Euglena, the ancestor of Trypanosoma did not ingest any green algae (or perhaps it did and lost them very early in its evolutionary history).

Osmotic parasitic nutrition: Trypanosoma is an obligate heterotroph. It lacks a cell mouth and does not perform phagocytosis like an amoeba. Instead, it lives swimming in the blood of its host (human or animal) and absorbs glucose and other readily available nutrients directly through its cell membrane.

4. Genetic Deception (Antigenic Variation)

One of the amazing genetic events that scientists study in trypanosomas is their ability to constantly change their surface protein coat (VSG). 

Once the human immune system recognizes the parasite's outer protein shape and begins to attack it, the trypanosoma "turns on" a new gene and completely changes the protein shape, making the immune system start from scratch time after time, which explains why it is so difficult to find a vaccine for sleeping sickness.

Third: The most important anatomical details

External spindle shape : A streamlined shape pointed at both ends, resembling a willow leaf or sickle.

Flagellum and undulating membrane: Protrudes as a wavy border extending along one side of the cell and continues as a short "free flagellum" at the front.

Kinetoplast: A distinctive dark spot located at the back of the cell (at the origin of the flagellum). Important note for students: Do not confuse it with the nucleus.

Nucleus: Large and usually centrally located.

Absence of contractile vacuole (in the hematopoietic phase): Unlike Euglena and Amoeba, which live in freshwater and require a pump to remove excess water, Trypanosomas live in blood plasma (an isotonic environment), so water does not flow into them, and they do not need a contractile vacuole in this phase.

Euglena (Euglenoidae) is a plant-like protist.

Why were they grouped together? (The governing pillars):

Fine cellular structure (mitochondrial crests and skeleton): Most individuals possess an excavated feeding groove on the side of their body, which they use to capture food. Importantly, Euglena and Trypanosoma both belong to a fine cellular clade called Euglenozoa, characterized by mitochondria with disc-shaped crests and a unique arrangement of microtubules in the roots of their flagella.

Genetic analysis: This has demonstrated a strong genetic relationship between Euglena and Trypanosoma, showing that Euglena acquired its chloroplasts later through endosymbiotic interaction independently, but its cellular origin and structural genes link it to Trypanosoma.

4. Amoebozoa group

This group dismantled the long-held belief that every creature that moves with pseudopodia was a protozoan, and every gelatinous mass was a fungus.

Organisms transferred to this group from the old classification:

1. Animal-like protozoa: free-living amoebas (some sarcoptics).
2. One example of fungi-like protozoa is slime molds.

Why were they grouped together? (The governing pillars):

Fine cellular structure: These organisms move and feed using broad lobe-like pseudopodia, and their mitochondria are characterized by tubular cristae.

Genetic analysis and rare events: Molecular analyses have shown that slime mold is not a true fungus at all, but rather a genetically amoeboid aggregation that undergoes stages in its life cycle where it behaves as a completely independent amoebae, placing it in the same evolutionary branch as free-living amoebae.

Trypanosoma

Paramecium is the "microscopic submarine" and the king of organized movement!

In older curricula, Paramecium was the star of the "Animal Protists" section under the class Ciliates, simply because it moves with cilia and ingests its food. But when its genes and fine cellular structure were examined, modern classification revealed a startling surprise: genetically, Paramecium is a cousin of Plasmodium (the virulent parasite that causes malaria) and Dinoflagellates (marine algae)!

Here is the classification of Paramecium and its overall characteristics .

First: The modern classification of Paramecium (Hierarchy)

Domain: Eukaryota.

Supergroup: SAR.

Subgroup: Alveolata.

Phylum: Ciliophora.

Second: Paramecium characteristics in light of the governing principles of modern classification

1. The fine cellular structure (the secret behind the naming of alveoli - Alveolata)

Why were Paramecium and Plasmodium placed in the same branch? 

The crucial criterion here was not the cilia, but a subtle cellular structure visible only with an electron microscope:

the alveoli. Directly beneath the plasma membrane of the paramecium lies a series of flattened, closed membrane-bound vacuoles or sacs called alveoli. These alveoli support the membrane and form the strong, flexible pelicule that gives the paramecium its characteristic "shoe-sole" shape. This cellular signature (alveoli) is present in all organisms within this evolutionary branch.

2. Genetic fingerprinting (unique nucleosome pairing)

Paramecium possesses a very rare genetic phenomenon unique to ciliates: nuclear dimorphism. Unlike other cells, it does not have a single nucleus, but rather two types of nuclei to control this highly complex cell:

the macronucleus, which is bean-shaped or kidney-shaped. This is the "control" nucleus, containing numerous copies of active genes (polyploid), and is responsible for managing all the cell's daily functions (nutrition, respiration, and excretion).

The micronucleus is small and spherical, containing a single original diploid copy of DNA. This nucleus is "dormant" at all times, and its only role is in sexual reproduction (conjugation), where it divides and exchanges genetic material with another paramecium to regenerate the cell.

3. Symbiotic history and feeding patterns

The current paramecium is a heterotrophic organism and does not possess plastids. 

However, scientists believe, based on genetic analysis of the SAR group, that the earliest ancestor of Paramecium may have possessed a chloroplast (of red algae origin) and then lost it later during evolution because it did not need it after mastering predation and phagocytosis. 

Environmental note: There is a specific species called Paramecium bursaria that engages in a modern ecological symbiosis, where it ingests live green algae and allows them to live inside its cytoplasm to provide it with food in exchange for protection. However, this is a modern ecological relationship and not of cellular origin.

Third: The most important anatomical details of the Paramecium 

Paramecium is considered one of the most complex single cells on Earth. To create an accurate scientific diagram, these structures must be clearly highlighted:

Cilia: These cover the entire body and are shorter and denser than flagella. They are used for movement in water (like boat oars) and to direct food-laden water currents toward the mouth.

The cellular digestive system (very complex compared to amoebas):

The oral groove : a slanted lateral depression on the cell surface, covered with long, strong cilia for sucking in bacteria.

The cytospore or cytostome: an opening at the end of the oral groove.

The cytopharynx: a funnel-shaped passage that descends from the mouth into the cytoplasm.

The food vacuole: formed at the end of the cytopharynx. Once full, it detaches and embarks on a circular journey within the cytoplasm (a current-like movement called cyclosis) to be digested.

The cytoproct/anal pore: a fixed opening at a specific location on the cell wall (unlike amoebas, which excrete waste from anywhere). The food vacuole merges with it to expel solid waste.

The contractile vacuoles : Paramecium possesses two contractile vacuoles (one at the anterior end and one at the posterior end).

A very important detail for drawing: do not draw it as a simple circle like the amoeba! The contractile vacuole in Paramecium has a "star" shape; it consists of a circular central reservoir, surrounded by radiating canals that collect water from the cytoplasm and pour it into the central reservoir before it contracts and expels the water to regulate the osmotic pressure.

Paramecium

Trichocysts: Tiny defensive organelles resembling "poisoned arrows" or spears, arranged in regular rows beneath the cortex (between the vesicles). When a paramecium is attacked by a predator, it releases these sticky, irritating threads to immobilize or repel the enemy.

Protozoa

Dinoflagellates, or "fire algae" 

They are the "sleight of hand and firecrackers" of the microbiological world!

The name you mentioned, " Diflagellate fire algae ," is a very accurate description of their appearance and behavior. 

In the past , they were classified as pure plants (algae) because many of them perform photosynthesis. But with the advent of modern classification, scientists discovered that these strange organisms conceal a shocking kinship with creatures we never imagined.

Here is the modern classification of dinoflagellates. 

First: The modern classification of dinoflagellates (Hierarchy)

: Domain: Eukaryota;

Supergroup: SAR;

Subgroup: Alveolata;

Phylum: Dinoflagellates.

(A surprising classification: by placing them in the Alveolata subphylum, genetics proved that these "algae" are direct cousins ​​of Paramecium and Plasmodium, the malaria-causing parasite!)

Secondly: Its characteristics in light of the governing principles of modern classification

1. The fine cellular structure (secret of the shield and vesicles)

How did it become related to Paramecium? The answer lies in the cellular signature of the Alveolata branch:

Membranous vesicles (alveoli) : These possess flattened vacuoles directly beneath the cell membrane. However, dinoflagellates ingeniously utilized these vesicles by depositing hard plates of cellulose within them, creating a cellulose shield (thecal plates) that encircles the hive like the armor of ancient knights.

Grooves and flagella (the secret of the name): The shield is not a single piece, but rather geometrically divided by deep grooves from which two flagella emerge (hence the name "diphaenus"):

Transverse flagellum : This flagellum wraps around the hive in a circular band or groove called the cingulum. This ribbon-like flagellum's movement causes the hive to rotate and twist on its axis like a spinning top.

Longitudinal flagellum: This flagellum hangs from a vertical groove called the sulcus, acting like a boat's rudder and propelling the hive forward.

(The movement of the two flagella together makes the creature swim in a unique spiraling, spindle-like motion in the water).

2. Genetic fingerprinting (a crazy core that doesn't follow the rules!)

Dinoflagellates possess a unique nucleus known as a dinokaryon, which presents a nightmare for classical geneticists because it breaks the rules of eukaryotes:

the chromosomes within them remain permanently condensed and coiled (even during periods of non-division).

They almost entirely lack the histone proteins that other eukaryotes use to coil their DNA! This genetic anomaly is considered a rare occurrence that characterizes this group.

3. Symbiotic History (The Mystery of Polystellite)

Dinoflagellates are the biggest "plast thieves" in the kingdom of protists:

the vast majority of them acquired their photosynthetic ability through secondary endosymmetry by ingesting a red alga. Therefore, their chloroplasts are surrounded by three membranes and contain the pigment pyridinin, which gives them their red or burnt-brown color.

Surprisingly, some species lost their chloroplasts and reverted to predatory behavior, then underwent tertiary endosymmetry by ingesting diatoms or other green algae to regain their photosynthetic ability!

Third: Exceptional environmental phenomena (why were they called fiery?)

Red Tide: During certain seasons and with increased nutrient availability, dinoflagellates proliferate explosively, coloring seawater red or brown (due to chloroplast pigments). The problem is that some species secrete potent neurotoxins (such as brevetoxins) that kill fish in enormous quantities. If a human consumes shellfish that have fed on these toxic protozoa, they can suffer paralysis that may be fatal.

Bioluminescence: This is responsible for the phenomenon of "glowing seas" or "blue glowing water" at night on some beaches. When exposed to wave action, a chemical reaction occurs in specialized organelles (scintillons) that combine the enzyme luciferase with the protein luciferin, releasing dazzling blue light flashes. This glow is believed to be a defense mechanism to temporarily blind predators or reveal their location to larger predators.

Fourth: The most important anatomical details 

The cellulose shield (theca) : Should be drawn as overlapping, polygonal geometric plates covering the organism's surface (resembling the skin of a pineapple or the surface of a football).

Grooves: The transverse equatorial groove (cingulum) that divides the cell into upper and lower halves.

The longitudinal groove (sulcus) extending downwards.

Flagella : A flagellum coiled within the transverse groove (often not drawn freely, but rather attached to the groove), and a free flagellum hanging downwards from the longitudinal groove.

Plastids : Should be reddish-brown or golden to emphasize the nature of the pigments.

Rotating nucleus : The nucleus should show clear, condensed lines within it to represent the permanently condensed chromosomes (dinokaryon).

diflagellate fire algae

The majority of phytoplankton in seas and oceans acquires a red color due to the presence of a red pigment alongside chlorophyll, causing red tides in the seas.

Euglena

It was placed in a separate phylum because it represents the "missing link" or the amazing combination between plants (chloroplasts and photosynthesis) and animals. Its means of locomotion is flagella, and it does not contain a true cell wall in some stages.

Single-celled organisms containing chloroplasts, which is why they perform photosynthesis

. Euglena is an organism belonging to the Excavata group (closer to trypanosoma), possessing a disc-shaped mitochondrial signature and a flexible shell, and acquired its ability to photosynthesize later in evolutionary history by ingesting a green alga in a secondary endosymbiotic event.

Euglenoida algae.

Euglenoida algae.

Formerly part of the kingdom Protista, phylum Euglenoidae, the classification has changed.

First: The modern classification of Euglena (Hierarchy)

Domain: Eukaryota.

Supergroup: Excavata.

Phylum: Euglenozoa.

Class: Euglenoida.

Second: Euglena characteristics in light of the four pillars of modern classification

1. Molecular and genetic fingerprinting (shocking kinship)

Genetic analysis of DNA and ribosomal RNA (rRNA) has shown that the harmless Euglena green is a close relative of the dangerous and destructive parasite Trypanosoma (the causative agent of sleeping sickness). Outwardly, there is no resemblance, but genetically they belong to the same ancestor (Euglenozoa); Euglena and Trypanosoma share a very unique gene sequence and identical structural proteins in their flagella.

2. Ultrastructure

When Euglena is examined under an electron microscope, the distinctive cellular signatures of its supergroup are revealed:

Discoidal mitochondrial crests: The internal folds of the mitochondria in Euglena take the form of solid discs, a hallmark of the Discoba subclade within the Excavata subclade.

Flexible pellet: Unlike plants, Euglena lacks a cellulose cell wall. Instead, beneath its plasma membrane lie helical protein bands supported by microtubules. This pellet provides rigidity for protection while simultaneously offering exceptional flexibility, allowing it to change shape during movement—a characteristic movement known scientifically as "Euglenoid movement."

Complex flagella: Euglena possesses two flagella originating from an internal sac (reservoir); a long flagellum protrudes outside the body and is used for locomotion, while a very short flagellum remains inside the reservoir, invisible from the outside. The flagellum is characterized by the presence of a specialized internal protein rod alongside the microtubules, called the paraxonemal rod.

3. Symbiotic history (the secret of the stolen plastid)

The chloroplasts in Euglena are conclusive evidence of secondary endosymbiosis.

How did this happen historically? The original ancestor of Euglena was a heterotroph (an animal-like organism that did not produce its own food). This ancestor ingested a single-celled green alga (from the Archipelago phylum). Instead of digesting it, the alga lived inside it and transformed into a chloroplast.

The decisive scientific evidence: the chloroplast in Euglena is surrounded by three membranes (not two like plants) and contains chlorophyll a and b, just like plants and green algae, thus proving, both molecularly and cellularly, that it "stole" this organelle through secondary endosymbiosis.

4. Mixed feeding pattern (Mixotrophy)

Based on its symbiotic history, Euglena possesses remarkable nutritional flexibility:

in the presence of light, it becomes autotrophic and performs photosynthesis using chloroplasts.

In the absence of light (darkness), the chloroplasts temporarily atrophy, and Euglena transforms into a heterotrophic organism that absorbs decaying organic matter from the surrounding environment through its cell membrane or engulfs small bacteria.

Third: Other specialized organelles in Euglena

Paramylon: Excess carbohydrates are not stored as starch like in plants, but as a unique carbohydrate compound called paramylon, which is stored in the cytoplasm as extrachloroplast granules.

Stigma and paraflagellate: A red pigment spot (containing carotenoids) is located near the base of the flagellum. It is not a true eye, but rather acts as an umbrella, blocking light from a sensitive photoreceptor located at the base of the flagellum. This alignment helps Euglena precisely determine the direction of light to move towards it for photosynthesis (positive phototropism).

Contractile vacuole: Because it lives in freshwater (a low-concentration environment), water constantly enters its cell by osmosis. The contractile vacuole acts as a centrifugal pump, collecting excess water and expelling it from the cell to maintain osmotic balance and prevent the cell from bursting.

Amoeba 

It is the classic example we always use to explain the idea of ​​a "primitive animal," but in light of modern classification, the story has changed and become more nuanced. In the past, any creature with "false feet" was immediately thrown into a single category called (carnivores or rhizoids).

 Amoeba

But modern genetic analysis exposed this error, proving that the "false feet" shape is merely an environmental adaptation (Convergent Evolution) that evolved independently in organisms that are not related to each other!

Modern classification of amoebas, specifically free-living amoebas such as Amoeba proteus)

First: Modern classification of amoebas (Hierarchy)

Domain: Eukaryota.

Supergroup: Amoebozoa.

Phylum: Tubulina - named for the shape of their tube feet.

Genus: Amoeba.

Second: Characteristics of amoebas in light of the governing principles of modern classification

1. Genetic and molecular fingerprinting (the big surprise)

Genetic analysis of DNA and rRNA completely separated the amoeba from other organisms with pseudopodia (such as foraminifera and actinobacteria, which were transferred to the SAR group). The free-living amoeba was placed in the Amoebozoa group because its genes proved to be the sister species of slime molds! Slime molds, which were previously thought to be fungi, possess the same genetic code as the amoeba and undergo life stages in which they behave completely like solitary amoebas.

2. Ultrastructure

When examining the internal structure of the amoeba for scientific purposes, the hallmarks of its supergroup are clearly evident:

Tubular mitochondrial crests: Unlike Euglena (disc-shaped) and plants (flat), the folds within the amoeba's mitochondria take the form of branching microtubules. This cellular signature is crucial in distinguishing Amoeba members.

Lobopodia: Amoebas move and capture food using broad, thick pseudopodia that resemble lobes or expanding tubes. This is quite different from the thin, thread-like pseudopodia found in other groups.

Actin-myosin network: Amoebas do not rely on flagella or cilia, but rather possess a complex and dynamic network of actin and myosin filaments directly beneath the cell membrane. These filaments contract and expand to change the cell's shape and facilitate cytoplasmic flow.

3. Symbiotic history and feeding patterns

Absence of plastids (no photosynthesis): Amoebas have not undergone any primary or secondary endosymbiotic events to acquire plastids. They are entirely heterotrophic.

Phagocytosis: They rely on "cell predation." Their prey (bacteria or smaller protists) is surrounded by lobe pseudopodia and then engulfed in a vesicle known as the food vacuole. This vacuole then fuses with lysosomes that release their enzymes to digest the prey, and waste products are expelled from the cell via exocytosis.

Third: The most important anatomical details 

• Ectoplasm and Endoplasm: The cytoplasm in amoebas is not homogeneous.

1. Ectoplasm (outer layer): transparent, thin, and more solid (similar to gelatin).
2. Endoplasm (inner layer): Granular, fluid, and contains organelles (Sol).

Locomotion: Amoebic movement relies primarily on the continuous transformation of the cytoplasm from a liquid to a gel state and vice versa (sol-gel transformation) to propel the pseudopodia forward.

Contractile Vacuole: This appears as a large, transparent circle capable of expanding and contracting. Amoebas live in freshwater environments, meaning water constantly flows into them. This vacuole acts as a "survival pump," collecting excess water and rupturing at the membrane to expel it; without it, the cell would burst.

Nucleus: Large, often disc-shaped or oval, and containing a dense chromatin substance. The nucleus controls a very large cell (compared to bacteria) and is constantly changing shape.

Plasmalemma : The cell membrane is very flexible and allows for gas exchange (oxygen and carbon dioxide) via simple diffusion, as amoebas lack specialized respiratory organelles.

Plasmodium

Plasmodium is the "silent killer" and the biggest shocker in the history of modern taxonomy!

In older curricula, it was simply classified in the "spore" group as a parasitic protozoan because it lacked cilia and flagella. But when scientists deciphered its genetic code and examined it with an electron microscope, they discovered an astonishing fact: this vicious blood parasite that causes malaria is actually a plant that has lost its ability to photosynthesize, and genetically, it is a cousin of Paramecium!

Modern classification of Plasmodium 

First: Modern classification of Plasmodium (Hierarchy)

Domain: Eukaryota.

Supergroup: SAR.

Subgroup: Alveolata.

Phylum: Apicomplexa.

Secondly: Its characteristics in light of the governing principles of modern classification

1. Symbiotic history (plant plastid shock)

This is the most important and exciting criterion that he places in the (SAR) group:

the apicoplast: the Plasmodium has a unique cellular organelle surrounded by four membranes. 

This organelle is originally a "chloroplast" that the ancestor of Plasmodium acquired through secondary endosymmetry by ingesting a red alga!

How does a plant live in the blood? Over time, and due to its parasitic life inside the pitch-black darkness of the human body, Plasmodium lost its photosynthetic genes, and this plastid (apicolast) turned into a chemical factory to produce the fatty acids necessary for its survival.

 In fact, you'll find that many modern malaria drugs are essentially "herbicides" designed to kill these plant chloroplasts without harming human cells.

 Plasmodium 

2. The fine cellular structure (the secret of vesicles and penetration weapons)

Plasmodium's inclusion in the Alveolata phylum alongside Paramecium is not arbitrary, but rather due to its structure:

Membranous vesicles (alveoli) : Like Paramecium and dinoflagellates, Plasmodium possesses flattened membranous sacs directly beneath its cell membrane to support its shape.

The apical complex : This is the reason for the phylum's name, "Alveolata." It is a biological arsenal located at the "ape" or front of the parasite (specifically in the infective sporozoite or merozoite stage), which the parasite uses as a chemical weapon to penetrate and invade liver and red blood cells.

3. Movement without flagella or cilia (gliding)

The adult parasite lacks flagella for swimming in blood. Instead, it uses a sophisticated cellular motor of actin and myosin filaments located directly beneath its cell membrane. This motor enables it to perform a powerful spiral "gliding motility," allowing it to navigate between host tissue cells. (The only flagellum present throughout its entire life cycle is in the male microgametes within the mosquito's abdomen.)

Anatomical observation of the life cycle: Plasmodium is a highly complex organism that lives between two hosts (humans as an intermediate host and female Anopheles mosquitoes as the primary host), undergoing radical changes in its morphology and cellular composition to adapt to each stage.

Third: The most important anatomical details (infectious phase - sporozoite)

The infectious stage that enters the human bloodstream via a mosquito bite (sporozoite):

Apical complex (at the pointed anterior end):

Polar rings: Protein rings at the apex that anchor microtubules.

Rhoptries: Pear-shaped organelles (often depicted as two large ones). They secrete proteins and enzymes that break down the red blood cell membrane, creating a vacuole for the parasite to enter.

Micronemes : Small tubular or oval vesicles that secrete sticky substances that help the parasite adhere to the human cell before puncturing it.

Apicoplast : Drawn as a round or oval organelle near the nucleus, it should be shown to be surrounded by several membranes (4 membranes).

Nucleus: Central and containing chromatin.

Mitochondria : Usually possess one large, tubular mitochondria to provide energy for movement.

Absence of the contractile vacuole: Because it is an endoparasite living in an osmotically balanced environment (blood or cytoplasm), it does not need an excess water pump.

The organism (example)	Its traditional old classification (based on appearance/movement)	Its modern classification (gene- and cell-based supergroup)	The crucial pillar in transportation
amoeba	Animal (meat)	Amoebozoa	Lobular feet + tubular crests + genes
colloidal mold	Fungal (sapromer)	Amoebozoa	Amoebic genetic phases + shared genes
Paramecium	Animal (ciliary)	SAR (Alveolates)	gaps existAlveoli under the membrane+ rRNA
Plasmodium	Animal (parasitic sporozoans)	SAR (Alveolates)	gaps existAlveoli + Genes
Diatoms and brown algae	Plant (algae)	SAR (Stramenopiles)	Straw flagella + secondary endosymmetry of red algae
Water mold	Fungal (saprophytic/parasitic)	SAR (Stramenopiles)	Straw flagella in its mobile stages (not a fungus)
green and red algae	Plant (algae)	Archaeplastida	Primary endosymbiotic cyanobacteria (only two membranes)
Euglena	Plant (Euglenites)	Excavata	Engraved groove + disc-shaped mitochondrial crests
Trypanosoma	Animal (flagellates)	Excavata	Engraved groove + disc-shaped mitochondrial crests

 

the reviewer

biomohgan.org