Protists, like plants, animals, and fungi, are eukaryotes; they belong to the category of Eukarya, one of the three domains of life. Eukaryotic cells, unlike prokaryotic cells, have a nucleus as well as other membrane-enclosed organelles such as mitochondria and the Golgi apparatus.

Such organelles offer specialized sites for specific cellular activities, making eukaryotic cell structure and organization more complicated than prokaryotic cell structure and organization.

Eukaryotic cells have a well-developed cytoskeleton that runs the length of the cell. The cytoskeleton provides structural support for eukaryotic cells, allowing them to be asymmetric.

We'll look at the diversity of eukaryotes throughouts the rest of this unit, starting with protists in this chapter.

Keep in mind that the organisms in most eukaryotic lineages are protists, and the majority of protists are unicellular.

As a result, life is not what most of us imagine it to be. The big, multicellular creatures we are most familiar with (plants, animals, and fungi) are only the tops of a few branches on the vast tree of life.

• The term excavation refers to some members of a supergroup that have an “excavated” groove on one side of the cell body.

• Two major clades (the parabasalids and diplomonads) have highly reduced mitochondria; members of a third clade (the euglenozoans) have flagella that differ in structure from those of other organisms.

• Excavates include parasites such as Giardia, as well as many predatory and photosynthetic species, as shown in the image attached.

Giardia intestinalis is a parasitic diplomonad. This diplomonad (colorized SEM) lives in the intestines of animals and lacks the Excavata's distinctive surface groove.

It can infect individuals if they consume water contaminated with Giardia cyst-containing feces. Drinking such water, even from an apparently clean stream, might result in severe diarrhea.

The parasite is killed by boiling the water.

These two species of protists lack plastids and have severely reduced mitochondria (it was previously assumed that they lacked mitochondria entirely). The majority of diplomonads and parabasalids live in anaerobic conditions.

Mitosomes are shrunken mitochondria found in diplomonads. Because these organelles lack functioning electron transport chains, they are unable to utilise oxygen to assist in the extraction of energy from carbohydrates and other organic compounds.

Diplomanads obtain their energy through anaerobic metabolic processes. Many diplomonads are parasites, including the notorious Giardia intestinalis, which lives in mammalian intestines.

Diplomanads feature two equal-sized nuclei and many flagella. Remember that eukaryotic flagella are cytoplasmic extensions made up of bundles of microtubules coated by the cell's plasma membrane. They are not the same as prokaryotic flagella, which are filaments made up of globular proteins linked to the cell surface.

Parabasalids also have decreased mitochondria, known as hydrogenosomes, which generate some energy anaerobically while emitting hydrogen gas as a byproduct. Trichomonas vaginalis, a sexually transmitted parasite that infects around 5 million individuals each year, is the most well-known parabasalid. T. vaginalis is on the move.

The majority of eukaryotes are single-celled creatures. Many protist groups, as well as plants, animals, and fungi, are included in the domain Eukarya.

Protists and other eukaryotes, unlike prokaryotes, contain a nucleus and other membrane-enclosed organelles, as well as a cytoskeleton that allows them to have asymmetric shapes and change shape when they eat, move, or grow.

Protists have a wide range of life cycles and structural and functional diversity. The majority are unicellular. Photoautotrophs, heterotrophs, and mixotrophs are all types of protists.

Current evidence suggests that eukaryotes evolved by endosymbiosis when an archaeal host (or a host closely related to archaeas) consumed an alpha proteobacterium, which evolved into the mitochondrion, an organelle found in all eukaryotes.

Plastids are considered to be the offspring of cyanobacteria that were devoured by early eukaryotic cells. The plastid-bearing lineage gave rise to red algae and green algae. Other protist groups developed as a result of secondary endosymbiotic occurrences in which red or green algae were swallowed.

According to one theory, eukaryotes are divided into four supergroups, each of which is a monophyletic clade: Excavata, SAR, Archaeplastida, and Unikonta.

Protists are important members of ecological communities. Protists engage in a variety of mutualistic and parasitic interactions that influence their symbiotic partners as well as many other members of the community.

Protists that produce photosynthetically are among the most significant producers in aquatic ecosystems. Factors affecting photosynthetic protists influence many other species in the community since they are at the bottom of the food chain.