Protists: the eukaryotes that are not plants, animals or fungi
Protists are diverse eukaryotes informally grouped outside animals, land plants and fungi. They include feeding, photosynthetic and mixed lifestyles. Their cell structures and genes provide evidence of bacterial organelle origins, later organelle loss and uptake of photosynthetic eukaryotes.
What a learner can do afterwards
- Explain the bacterial origins of mitochondria and plastids using genetic and cellular evidence, recognising host dependence and later organelle loss.
- Describe contrasting protist lifestyles and the cell structures used for feeding and photosynthesis.
- Explain why the informal category protist is not a clade containing an ancestor and all its descendants.
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Here, protist is an informal name for eukaryotes outside animals, land plants and fungi, including algae. Many protists are single-celled; others form colonies or multicellular bodies. They do not form one clade. A clade contains an ancestor and all its descendants. Some protists are closer relatives of animals or fungi than of other protists. Excluding those descendants prevents the category from forming a clade.
Endosymbiosis is a relationship in which one organism lives inside another. Mitochondria descended from bacterial partners related to alphaproteobacteria. Plastids descended from cyanobacteria; photosynthetic plastids are chloroplasts. Over many generations, these partners became integrated organelles, and many genes moved to the host nucleus. Details of the initial mitochondrial partnership remain uncertain.
Gene comparisons link mitochondria to alphaproteobacteria and plastids to cyanobacteria. Typical mitochondria and chloroplasts retain DNA and bacterial-like ribosomes. They grow and divide from existing organelles, but depend on host proteins and regulation. Mitochondria and primary plastids have two surrounding membranes. Some plastids acquired through later endosymbiosis have additional membranes.
Comparisons across eukaryotic lineages support a last common ancestor with mitochondria. Some descendants later reduced or lost them. Plastids arose later in particular branches. Present distribution contributes evidence when combined with genetic relationships. Distribution alone cannot prove acquisition order: loss can remove an ancestral organelle. An organism lacking mitochondria need not represent a stage before their acquisition.
An amoeba uses pseudopodia, temporary cell projections, to surround prey. The enclosed prey enters a food vacuole, where digestion occurs. Photosynthetic protists use chloroplasts to capture light energy and make organic molecules. Some protists combine photosynthesis with feeding, called mixotrophy. The balance depends on species and conditions; it is not a rule that all protists switch feeding methods when light changes.
Primary plastid endosymbiosis involved a host acquiring a cyanobacterium. In secondary endosymbiosis, a eukaryotic host acquired an alga that already had a plastid. Some secondary plastids retain extra membranes and a remnant algal nucleus. Together, these features support uptake of a eukaryotic alga. Later losses mean that not every secondary plastid retains the same membrane count or a nuclear remnant.
Protists span different lineages. Genes and cell structures reveal bacterial organelle ancestry, host dependence, later losses and uptake of algae.
Teaching text, questions and flashcards written for this dataset. OpenStax Biology 2e chapter 23 was consulted for organelle ancestry and protist diversity.
Readings and flashcards written for this dataset after factual study of the supplied biology records. Linked video: How we think complex cells evolved - Adam Jacobson, TED-Ed. The video remains the publisher's work.
More to read
Tracing an organelle's bacterial ancestry
Finding a structure inside a cell does not tell us how it originated. An ancestry claim needs comparisons with possible relatives.
Mitochondrial genes and other features connect mitochondrial ancestry with alphaproteobacteria. Plastid ancestry is connected with cyanobacteria. These comparisons support bacterial partners becoming established inside host cells and gradually becoming organelles.
Typical mitochondria and chloroplasts retain some DNA and bacterial-like ribosomes. Existing organelles grow and divide to produce more organelles. That division does not make them independent organisms. Many ancestral genes moved to the host nucleus, and host-produced proteins now perform essential functions within the organelles.
Membranes add evidence, but their numbers vary with history. Mitochondria and primary plastids have two surrounding membranes. Some other plastids have extra membranes associated with later uptake of an alga. Membrane structure is therefore interpreted alongside genetic evidence, rather than used as a universal two-membrane test.
Feeding and photosynthesis among protists
Calling an organism a protist leaves many questions unanswered. It does not specify its food source, body size or method of movement. In this lesson, the label includes algae and other eukaryotes outside animals, land plants and fungi.
For an amoeba, feeding involves changing the shape of the cell. Pseudopodia extend around prey until it becomes enclosed. Digestion then takes place in a food vacuole. The projections capture the food; the vacuole contains it during digestion.
A photosynthetic protist uses chloroplasts to capture light energy and make organic molecules. Some protists also obtain food by feeding. Combining photosynthesis with feeding is called mixotrophy. The contribution of each method varies with species and conditions.
Protist does not mean a single-celled body without specialised structures. Many protists are single-celled, but colonial and multicellular forms also exist. Their different lifestyles and bodies occur across several evolutionary lineages.
When absence results from loss
Imagine comparing related organisms when one lacks an organelle that the others possess. At least two histories need consideration. Its ancestors might never have had the organelle, or its lineage might have lost it later.
Comparisons across eukaryotic lineages support mitochondria in their last common ancestor. Some descendants retain reduced mitochondrial organelles, and some have lost them. A present-day organism without mitochondria is therefore not automatically a surviving example of life before mitochondrial acquisition.
Plastids have a different distribution. Their origins lie later in particular eukaryotic branches, and subsequent losses and uptake of algae have changed where they occur. Distribution helps reconstruct this history, but genetic relationships are needed to interpret it.
Fossils around two billion years old offer evidence of early eukaryotes. They do not provide a complete record of internal organelles or an exact date for every acquisition. An evolutionary explanation must distinguish the observations from the history inferred from them.
Evidence for acquiring an alga
A host can acquire a photosynthetic partner in different ways. Primary plastid endosymbiosis involved a cyanobacterium. Secondary endosymbiosis involved a eukaryotic alga that already contained a plastid.
These histories make different predictions. Uptake of an alga can leave traces of the algal cell around its plastid. Some secondary plastids retain additional membranes. Some also retain a small remnant of the algal nucleus. A nuclear remnant helps distinguish an algal partner from a cyanobacterium, which has no nucleus.
Later evolution can remove structures. A secondary plastid need not retain every original membrane or any nuclear remnant. Researchers compare surviving structures with genes to test the proposed history.
These observations also show why photosynthesis cannot define one protist branch. Photosynthetic partners have been acquired in different lineages, and some descendants have lost photosynthetic structures. Classification must consider ancestry rather than group every sunlight-using organism together.
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