Campbell Biology Chapter 28 Review: Protists

28.1 What protists are, and how endosymbiosis built them

pp. 575–579

On the AP exam? Yes

Topic 2.10 tests the evidence that mitochondria and chloroplasts were once free-living bacteria, and Topic 7.9 treats family trees as hypotheses; protist group names and secondary endosymbiosis won't be asked.

In the course: Topic 2.10 Origins of Cell Compartmentalization, Topic 2.7 Tonicity and Osmoregulation, Topic 7.7 Common Ancestry, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

"Protist" is an everyday label for any eukaryote that isn't a plant, an animal or a fungus. It's a leftover category, not a real family group: some protists are closer kin to animals or to plants than to other protists.
Protists make up most branches of the eukaryote family tree, and the majority of them are single cells. A single protist cell can still be remarkably complex, with specialized organelles standing in for organs; a contractile vacuole, for instance, works like a tiny kidney that bails out extra water.
Protists feed in every possible way. Photoautotrophs have plastids and make their own food, heterotrophs absorb or swallow food, and mixotrophs do both. Each of these lifestyles evolved many separate times.
Early in eukaryote history, a host cell took in an oxygen-using bacterium that became the mitochondrion. Almost every eukaryote still has mitochondria or shrunken leftovers of them.
Later, one line of eukaryotes kept a cyanobacterium, which became the first plastid (primary endosymbiosis). Its descendants include land plants as well as red and green algae. Their plastids have two membranes, and their plastid genes and membrane transport proteins match cyanobacterial ones.
In secondary endosymbiosis, a eukaryote swallowed a red or green alga and kept it as its plastid. Those plastids are wrapped in three or four membranes, and a few still hold a tiny leftover nucleus from the engulfed alga, called a nucleomorph.
The eukaryote family tree keeps being redrawn as DNA data pile up. The book's five "supergroups" are out of date: most researchers now join stramenopiles, alveolates and rhizarians into one clade (SAR), split the old "excavates" into several groups, and still debate where the tree's root is.

Key terms (12)

protist: Any eukaryote that isn't a plant, animal or fungus. It's a handy label, not a true clade, because protists don't share one common ancestor that excludes everything else.
photoautotroph: An organism that uses light energy to make its own food from carbon dioxide. Algae with plastids are photoautotrophs.
heterotroph: An organism that gets its carbon and energy by eating, absorbing or breaking down other living things or their remains.
mixotroph: An organism that can both photosynthesize and take in food from its surroundings, switching between the two or doing both at once.
endosymbiosis: One cell living inside another. Over time the inside partner can become a permanent organelle, which is how mitochondria and plastids arose.
endosymbiont: The cell that lives inside a host cell. Mitochondria and plastids are the descendants of ancient endosymbionts.
plastid: The family of organelles that includes chloroplasts. All plastids trace back to a cyanobacterium kept inside an ancient eukaryote.
primary endosymbiosis: When a eukaryote engulfs a bacterium (for plastids, a cyanobacterium) and keeps it as an organelle. The resulting plastid has two membranes.
secondary endosymbiosis: When a eukaryote engulfs an alga that already has a plastid and keeps it. The plastid ends up wrapped in three or four membranes.
nucleomorph: A tiny, shrunken nucleus left over from an engulfed alga, found between the membranes of some secondary plastids. It's strong evidence for secondary endosymbiosis.
contractile vacuole: An organelle that collects extra water that seeps into a freshwater cell and pumps it out, so the cell doesn't swell and burst.
paraphyletic group: A group that includes a common ancestor and some, but not all, of its descendants. "Protists" is one, since it leaves out plants, animals and fungi.

Check yourself: 28.1 What protists are, and how endosymbiosis built them

4 questions on 28.1 What protists are, and how endosymbiosis built them. Pick an answer to see if you got it, and why.

Question 1 of 4

In a dividing algal cell, new plastids are never built from scratch. Each one forms when an existing plastid pinches in two, using a ring of protein closely related to the one bacteria use when they divide. How does this observation bear on the endosymbiotic theory?

Question 2 of 4

A newly described marine protist has plastids and fixes CO₂ when it is lit. When researchers add bacteria tagged with a fluorescent dye, glowing bacteria show up inside the protist's food vacuoles within minutes. Which term best describes how this protist gets nutrients?

Question 3 of 4

A protist's plastid is wrapped in four membranes. Transport proteins in the two innermost membranes are related to cyanobacterial proteins, and between the second and third membranes sits a tiny nucleus whose genes most resemble the nuclear genes of red algae. Which history best explains this structure?

Question 4 of 4

Two research teams build trees of the same protist lineages. One team uses 12 genes and places group K next to the green algae; the other uses 250 genes and places group K next to the brown algae. What is the most reasonable way to treat these results?

0 of 4 answered

28.2 Excavates: shrunken mitochondria and odd flagella

pp. 580–581

On the AP exam? Not tested

The current course doesn't cover these protist groups. The ideas you can reuse are anaerobic energy pathways (Topic 3.5) and reading family trees (Topic 7.9).

In the course: Topic 2.10 Origins of Cell Compartmentalization, Topic 3.5 Cellular Respiration, Topic 6.5 Regulation of Gene Expression, Topic 7.7 Common Ancestry, Topic 7.9 Phylogeny, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

The book groups diplomonads, parabasalids and euglenozoans as "excavates," named for a feeding groove some of them have. Newer studies don't support excavates as one clade, so these lineages now sit in separate groups (diplomonads and parabasalids in Metamonada, euglenozoans in Discoba).
Diplomonads and parabasalids live where oxygen is scarce, and their mitochondria are shrunken versions that can't run an electron transport chain. Giardia, a diplomonad, causes diarrhea when people swallow its cysts in water fouled by sewage or animal waste. Trichomonas vaginalis, a parabasalid, is a sexually transmitted parasite.
A diplomonad's mitosome makes no ATP from oxygen, so the cell relies on anaerobic pathways like glycolysis. A parabasalid's hydrogenosome does make some ATP without oxygen and gives off hydrogen gas.
These remnant organelles show the ancestors once had full mitochondria that were later cut down. In 2016 researchers even found a relative, Monocercomonoides, that has lost its mitochondrion entirely.
Euglenozoans have an extra spiral or crystal-like rod running inside their flagella, beside the standard 9 + 2 microtubule arrangement. Kinetoplastids keep a large clump of DNA, the kinetoplast, inside their single big mitochondrion.
Trypanosomes are kinetoplastids that cause sleeping sickness (spread by tsetse flies) and Chagas disease (spread by bloodsucking "kissing" bugs). They dodge the immune system by switching, again and again, which surface coat protein they make.
Euglenids such as Euglena swim with a flagellum and use a pigmented eyespot and a light sensor to steer toward good light. Many are mixotrophs, and their green plastids came from an engulfed green alga.

Key terms (12)

excavate: The book's name for a supergroup of protists, some with a feeding groove. DNA studies no longer support it as one clade.
diplomonad: A protist with two nuclei, several flagella and reduced mitochondria that lives in low-oxygen places. Giardia is one.
mitosome: A diplomonad's shrunken mitochondrion. It has no electron transport chain, so it can't use oxygen to make ATP.
parabasalid: A protist with reduced mitochondria and several flagella that lives in low-oxygen places; some are parasites, and others help termites digest wood.
hydrogenosome: A parabasalid's reduced mitochondrion. It makes a little ATP without oxygen and releases hydrogen gas.
euglenozoan: A protist whose flagella contain an extra spiral or crystal-like rod. The group includes kinetoplastids and euglenids.
kinetoplastid: A euglenozoan with one large mitochondrion holding a dense DNA mass. Trypanosomes belong here.
kinetoplast: A dense, structured clump of DNA (many linked DNA circles) inside the single large mitochondrion of a kinetoplastid.
trypanosome: A parasitic kinetoplastid that causes sleeping sickness or Chagas disease and keeps changing its surface coat to escape the immune system.
euglenid: A euglenozoan whose one or two flagella sprout from a small pouch at the front of the cell. Many, like Euglena, can photosynthesize or absorb food.
eyespot: A red-orange pigment spot that casts a shadow on a nearby light sensor, so the sensor's reading changes as the cell turns. It tells a swimming cell where the light is.
pellicle: A layer of interlocking protein strips just inside a euglenid's membrane. It stiffens the cell yet lets it bend, since euglenids have no cell wall.

Check yourself: 28.2 Excavates: shrunken mitochondria and odd flagella

4 questions on 28.2 Excavates: shrunken mitochondria and odd flagella. Pick an answer to see if you got it, and why.

Question 1 of 4

A gut protist lives where there is almost no oxygen. Its small double-membrane organelles contain no cytochromes and no ATP synthase. If cyanide, which blocks the last step of the electron transport chain, is added to its culture, what will most likely happen?

Question 2 of 4

A gut-dwelling protist has no mitochondrion and no remnant of one. Its closest relatives on the family tree all have mitosomes, and its nucleus still carries a few genes that are found only in organisms with mitochondria. What is the best conclusion?

Question 3 of 4

Blood samples from a patient infected with a trypanosome are checked every week. Parasite numbers rise, crash, then rise again in repeated waves, and each new wave carries a different surface coat protein. Which explanation best fits this pattern?

Question 4 of 4

A swimming flagellate has a light sensor near the base of its flagellum and a pigmented eyespot that shades the sensor from one side. Normally it swims toward a lamp. A mutant makes the light sensor but no eyespot pigment. What will the mutant most likely do in a dish lit from one side?

0 of 4 answered

28.3 Alveolates and stramenopiles

pp. 582–589

On the AP exam? Not tested

These groups aren't in the current course. Useful links: alternation of generations rests on meiosis (Topic 5.1), oomycetes and fungi show convergent evolution (Topic 7.9), and diatoms move carbon through ecosystems (Topic 8.2).

In the course: Topic 5.1 Meiosis, Topic 7.2 Natural Selection, Topic 7.9 Phylogeny, Topic 8.2 Energy Flow Through Ecosystems (notes, videos and more questions)

Key points

The book groups alveolates and stramenopiles as "chromalveolates," said to share one ancient engulfed red alga. That grouping has been dropped. DNA now joins alveolates, stramenopiles and rhizarians (28.4) in the SAR clade, and their red-algal plastids may have been passed along through more than one endosymbiosis.
Alveolates have small membrane sacs, called alveoli, just under the cell membrane. They include dinoflagellates, apicomplexans and ciliates.
Dinoflagellates whirl as they swim, driven by two flagella set in grooves between the cellulose plates that cover the cell. They're a big part of plankton; population explosions (blooms) can color water red and release toxins that build up in shellfish. Some live inside reef corals and feed them.
Apicomplexans are animal parasites with a tip of organelles (the apical complex) for invading host cells, and most keep a nonphotosynthetic plastid. Plasmodium causes malaria, moving between Anopheles mosquitoes and humans (first the liver, then red blood cells). The WHO estimated about 260 million cases and nearly 600,000 deaths in 2023, and two malaria vaccines are now recommended for children.
Ciliates move and feed with cilia and have two kinds of nuclei: a big macronucleus with many gene copies that runs daily life, and small micronuclei that are swapped during conjugation. Conjugation mixes genes but doesn't make new cells; ciliates reproduce by binary fission.
Stramenopiles have a "hairy" flagellum, usually paired with a smooth one. Diatoms, with glassy silica walls in two halves, are leading ocean producers, and dead diatoms that sink carry carbon to the deep sea. Golden algae are mostly plankton. Brown algae, including kelps, are the largest algae, with a holdfast, stipe and blades but no true roots, stems or leaves.
Some brown algae have alternation of generations: a many-celled diploid sporophyte makes haploid spores by meiosis, and the spores grow into many-celled haploid gametophytes that make gametes. Oomycetes, such as water molds and Phytophthora, look like fungi but have cellulose walls; the resemblance is convergent evolution.

Key terms (15)

SAR clade: A large, well-supported clade of eukaryotes made of stramenopiles, alveolates and rhizarians. It replaced the book's "chromalveolates" and separate "Rhizaria" supergroups.
alveolate: A protist with small membrane sacs (alveoli) just under its cell membrane. Dinoflagellates, apicomplexans and ciliates are alveolates.
dinoflagellate: A mostly single-celled alveolate with cellulose plates and two flagella in grooves that make it spin. Many are plankton, and some cause toxic blooms.
algal bloom: A sudden population explosion of algae or other phytoplankton. A "red tide" is a bloom, often of dinoflagellates, that tints the water.
apicomplexan: A parasitic alveolate with a cluster of organelles at one end for breaking into host cells. Plasmodium, which causes malaria, is one.
apicoplast: A plastid in many apicomplexans that no longer photosynthesizes but still runs other needed pathways. It probably came from a red alga.
ciliate: An alveolate that moves and feeds using many cilia and has both a macronucleus and micronuclei.
conjugation (in ciliates): Sex without reproduction: two ciliates pair up and trade haploid micronuclei. It mixes genes but doesn't produce new cells.
stramenopile: A protist with one flagellum covered in fine hairs, usually paired with a smooth one. Brown algae, diatoms, oomycetes and golden algae all belong here.
diatom: A single-celled alga with a glassy silica wall in two overlapping halves. Diatoms rank among the top producers in oceans and lakes.
thallus: The body of a large alga. It may look like a plant, but it isn't built from true roots, stems or leaves; a kelp's has an anchoring holdfast, a stalk (stipe) and flat blades.
alternation of generations: A life cycle in which a many-celled diploid stage (sporophyte) and a many-celled haploid stage (gametophyte) take turns.
sporophyte: The diploid, many-celled stage in alternation of generations. It makes haploid spores by meiosis.
gametophyte: The haploid, many-celled stage in alternation of generations. It makes gametes by mitosis.
oomycete: A fungus-like stramenopile, such as a water mold, that grows as threads but has cellulose cell walls. Some, like Phytophthora, are serious plant pathogens.

Check yourself: 28.3 Alveolates and stramenopiles

4 questions on 28.3 Alveolates and stramenopiles. Pick an answer to see if you got it, and why.

Question 1 of 4

In a ciliate, a researcher disrupts every copy of gene G in the macronucleus. The micronuclei still carry normal copies of G. The cell then conjugates with a normal partner, and each cell builds a new macronucleus from its micronuclear DNA. Which outcome is most likely?

Question 2 of 4

A person already has many Plasmodium parasites in their red blood cells. Which drug would best stop this person from passing malaria to mosquitoes that bite them, even if it didn't ease their symptoms?

Question 3 of 4

A seaweed has a many-celled diploid stage and a many-celled haploid stage that look almost the same. Cells of the diploid stage have 44 chromosomes. Which choice correctly gives the chromosome number in the spores the diploid stage releases and in the gametes the haploid stage releases?

Question 4 of 4Calculator allowed

Researchers track two ocean blooms that fixed the same amount of carbon at the surface. Sediment traps then measure how much of that carbon reaches 500 m depth (invented data). Bloom type | Carbon fixed at surface (g/m²) | Carbon reaching 500 m (g/m²) Diatoms | 12.0 | 2.4 Small flagellates without silica walls | 12.0 | 0.6 Which conclusion do the data best support?

0 of 4 answered

28.4 Rhizarians: amoebas with threadlike pseudopodia

pp. 589–590

On the AP exam? Not tested

Rhizarians aren't in the current course. Paulinella is a fresh example for the endosymbiosis story in Topic 2.10, and fossil forams help date rock layers, a kind of evidence used in Topic 7.6.

In the course: Topic 2.1 Cell Structure and Function, Topic 2.10 Origins of Cell Compartmentalization, Topic 7.6 Evidence of Evolution, Topic 7.9 Phylogeny, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Rhizarians were grouped by DNA, not looks. They vary hugely in shape, but their gene sequences mark them as one clade, which is now placed inside the SAR clade with stramenopiles and alveolates.
"Amoeba" describes a way of life, not a family. Moving and feeding with pseudopodia, which are bulging extensions of cytoplasm, shows up in many unrelated lineages. Most rhizarian amoebas have thin, threadlike pseudopodia.
Radiolarians are mostly ocean plankton with detailed silica skeletons. Their stiff pseudopodia, braced by microtubules, trap small prey, and their skeletons pile up on the seafloor as thick ooze.
Forams build porous shells (tests) hardened with calcium carbonate, push pseudopodia out through the holes, and often host photosynthetic algae inside. Their shells fossilize well, so geologists use them to match and date rock layers and to estimate past ocean conditions.
Cercozoans are common in water and soil; most are predators or parasites, and some are major bacteria-eaters. Chlorarachniophytes are mixotrophic cercozoans whose plastids came from an engulfed green alga, with four membranes and a nucleomorph.
Paulinella photosynthesizes with a chromatophore that came from a cyanobacterium unrelated to the one behind every other plastid, roughly 100 million years ago. That makes it a second, far more recent primary endosymbiosis, so scientists can study an organelle that is still early in the process of forming.

Key terms (10)

rhizarian: A member of a clade of mostly amoeba-like protists, many with threadlike pseudopodia, that was recognized from DNA rather than body shape.
pseudopodium: A temporary bulge of cytoplasm that a cell pushes out to crawl or to surround food. Plural: pseudopodia.
amoeba: Any protist that moves and feeds with pseudopodia. It's a body style found in many unrelated groups, not one clade.
radiolarian: A mostly marine rhizarian with a delicate silica skeleton and stiff, ray-like pseudopodia used to catch prey.
foram (foraminiferan): A rhizarian with a porous shell hardened by calcium carbonate. Its fossils are widely used to date and match rock layers.
test: The hard, porous shell of a foram, with pseudopodia reaching out through its holes.
cercozoan: A large rhizarian group of amoebas and flagellates with threadlike pseudopodia, common in soil and water. Most eat bacteria or other cells.
chlorarachniophyte: A mixotrophic cercozoan whose plastid came from an engulfed green alga and still holds a tiny leftover nucleus.
chromatophore: The photosynthetic organelle of Paulinella. It came from a different cyanobacterium, much more recently than ordinary plastids.
molecular systematics: Working out how organisms are related by comparing DNA, RNA or protein sequences.

Check yourself: 28.4 Rhizarians: amoebas with threadlike pseudopodia

4 questions on 28.4 Rhizarians: amoebas with threadlike pseudopodia. Pick an answer to see if you got it, and why.

Question 1 of 4

For many years, every protist that crawls and feeds with pseudopodia was placed in a single group of amoebas. Which finding most directly shows that this group does not reflect shared ancestry?

Question 2 of 4

Two cliffs 3,000 km apart each contain a rock layer packed with the fossil shells of one distinctive foram species. Other evidence shows that this species existed for only about 1 million years. What can a geologist reasonably conclude?

Question 3 of 4

A radiolarian's long, stiff pseudopodia are braced by bundles of microtubules, and prey sticks to them. A researcher adds a drug that makes microtubules fall apart. What is the most likely immediate effect?

Question 4 of 4

Researchers compare genes from the photosynthetic organelles of several organisms. The organelle of the amoeba Paulinella groups with one family of free-living cyanobacteria, while the plastids of land plants and of red and green algae group together with a different, distantly related cyanobacterial lineage. What is the best conclusion?

0 of 4 answered

28.5 Red algae, green algae and the plant connection

pp. 590–592

On the AP exam? Background

Algae aren't a topic of their own, but this section backs up Topic 2.10 (one ancient endosymbiosis gave plastids to this whole clade) and Topic 3.4 (pigments absorb certain colors of light).

In the course: Topic 2.10 Origins of Cell Compartmentalization, Topic 3.4 Photosynthesis, Topic 5.1 Meiosis, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

Over 1 billion years ago, a protist kept a cyanobacterium as a plastid. Its descendants are land plants, red algae, green algae and a small group called glaucophytes. Together they form Archaeplastida, a clade marked by that original primary endosymbiosis.
Red algae owe their color to phycoerythrin, an accessory pigment that absorbs the blue and green light that reaches deep water; some species live more than 200 m down. Along warm tropical shores, they outnumber the other big seaweeds.
Most red algae are many-celled seaweeds, including nori. Unlike most algae, they never make flagellated cells, so water currents must carry their gametes together. Many have alternation of generations.
Green algae have chloroplasts much like those of plants: both chlorophyll a and chlorophyll b, with starch stored inside. They form two groups: chlorophytes and charophytes. Land plants evolved from within the charophytes, so some biologists include green algae in a wider plant group, Viridiplantae.
Chlorophytes turn up in ponds, oceans, wet soil, on snowfields and inside other organisms. Bigger bodies arose more than once: some form colonies of linked cells, some (like sea lettuce) are truly many-celled with specialized cells, and some are one enormous cell holding many nuclei.
Single-celled Chlamydomonas spends most of its life haploid. Gametes of two mating types fuse into a diploid zygote, which waits out hard times in a tough coat and then divides by meiosis to make new haploid cells.

Key terms (12)

Archaeplastida: The clade of land plants, red algae, green algae and glaucophytes, all descended from the protist that first kept a cyanobacterium as a plastid.
red algae: Mostly many-celled seaweeds colored by phycoerythrin. They never make swimming, flagellated cells, and they include nori.
phycoerythrin: A red accessory pigment in red algae that absorbs blue and green light, the colors that reach deepest into the sea.
accessory pigment: A pigment that captures light colors chlorophyll a misses and passes the energy on, widening the range of light a cell can use.
green algae: Algae with plant-like chloroplasts (chlorophylls a and b). The group includes chlorophytes and charophytes.
chlorophyte: A large group of green algae living in fresh water, the sea, soil and snow. Chlamydomonas, Volvox and Ulva are chlorophytes.
charophyte: The group of green algae that land plants arose from. Its members are plants' nearest living algal relatives.
Viridiplantae: A proposed wider "green plant" group made of green algae plus land plants. It forms a clade, which green algae alone do not.
colony (in algae): A group of similar cells that stick together and coordinate, without the full division of labor of a truly many-celled body.
multinucleate cell: One cell with many nuclei because its nuclei divided again and again without the cytoplasm dividing.
mating type: One of two or more compatible forms of a single-celled organism. Two gametes can fuse only if they belong to different mating types.
seaweed: An everyday name for large, many-celled marine algae. Seaweeds can be red, green or brown algae, which aren't closely related.

Check yourself: 28.5 Red algae, green algae and the plant connection

4 questions on 28.5 Red algae, green algae and the plant connection. Pick an answer to see if you got it, and why.

Question 1 of 4

Two red algae are grown under equal amounts of green light or red light, and their oxygen output is measured (invented data). Alga | O₂ output under green light | O₂ output under red light X | 2 | 18 Y | 15 | 9 Green and blue light reach much deeper into seawater than red light does. Which alga was most likely collected from deep water, and why?

Question 2 of 4

In a single-celled green alga, every cell is haploid except the zygote, which soon divides by meiosis. A new recessive mutation arises in one haploid cell. What is most likely true about this mutation?

Question 3 of 4

A red seaweed with no flagellated stage at any point in its life grows in a sheltered bay. A new breakwater sharply slows the water currents in the bay. Which effect on the seaweed is most likely?

Question 4 of 4

DNA shows that charophytes are more closely related to land plants than to chlorophytes. A group called "green algae" includes chlorophytes and charophytes but leaves out land plants. How should this group be described?

0 of 4 answered

28.6 Amoebozoans and our own branch of the tree

pp. 593–596

On the AP exam? Background

The exam won't name these groups, but slime molds are good examples for cell signaling (Topic 4.1), and the animal–fungus–choanoflagellate tree is good practice for Topics 7.7 and 7.9.

In the course: Topic 4.1 Cell Communication, Topic 4.5 Cell Cycle, Topic 5.1 Meiosis, Topic 7.7 Common Ancestry, Topic 7.9 Phylogeny (notes, videos and more questions)

Key points

The book's "unikonts" are now usually called Amorphea. The group holds two well-supported clades: amoebozoans, and opisthokonts, which are animals, fungi and their single-celled relatives.
Where the root of the whole eukaryote tree sits is still unsettled. Studies using different genes and methods put it in different places, and finding it would tell us what the first eukaryotes were like.
Amoebozoans push out blunt pseudopodia shaped like lobes or tubes. They include slime molds, gymnamoebas (free-living hunters of bacteria and other protists) and Entamoeba; one species, E. histolytica, spreads through dirty water or food and is the cause of amebic dysentery.
A plasmodial slime mold feeds as a plasmodium: one huge mass of cytoplasm with many nuclei, made by mitosis without cytokinesis. Cytoplasm streams back and forth through it, spreading nutrients. When food or water runs out, it builds stalked fruiting bodies where meiosis makes spores.
Cellular slime molds such as Dictyostelium live as separate haploid amoebas. When food runs out, starving cells release cyclic AMP as a signal, and neighbors crawl toward it, forming a many-celled slug and then a fruiting body whose stalk cells die so that spore cells can spread.
Slime molds' fungus-like fruiting bodies are a case of convergence; DNA puts slime molds with amoebas. Choanoflagellates, the closest relatives of animals, and nucleariids, the closest relatives of fungi, show why "protist" can't be a clade: any clade that holds them must also hold animals or fungi.

Key terms (13)

Amorphea (unikonts): The large eukaryote group made of amoebozoans and opisthokonts, plus a few small related lineages. The book calls it Unikonta.
amoebozoan: A protist that crawls with blunt, lobe-like or tube-like pseudopodia. Slime molds, gymnamoebas and Entamoeba are amoebozoans.
opisthokont: A member of the clade holding animals, fungi and their closest single-celled relatives. Their swimming cells push with a single flagellum at the back.
slime mold: An amoebozoan that forms fruiting bodies like a fungus but is related to amoebas, not to fungi.
plasmodium (slime mold): The feeding stage of a plasmodial slime mold: one big mass of cytoplasm with many nuclei and no walls between them. Not the malaria parasite Plasmodium.
cytoplasmic streaming: The flowing of cytoplasm inside a cell, which moves materials around faster than diffusion could, especially in very large cells.
cellular slime mold: A slime mold that lives as separate amoebas but gathers into a many-celled slug and fruiting body when food runs out.
fruiting body: A stalked structure that holds spores up high so they can spread.
cyclic AMP (cAMP): A small signaling molecule. Starving cellular slime mold cells release it, and other cells crawl toward it to gather.
gymnamoeba: A free-living amoebozoan in soil or water that hunts bacteria and other protists.
Entamoeba: A genus of parasitic amoebas. One species, E. histolytica, spreads through contaminated water and food and is the cause of amebic dysentery.
choanoflagellate: A single-celled or colonial protist with a collar of tiny projections around one flagellum. It's the closest living relative of animals.
nucleariid: An amoeba-like protist that is the closest living relative of fungi.

Check yourself: 28.6 Amoebozoans and our own branch of the tree

4 questions on 28.6 Amoebozoans and our own branch of the tree. Pick an answer to see if you got it, and why.

Question 1 of 4

In a cellular slime mold, starving cells release cAMP and crawl toward higher cAMP levels, detected by a receptor on their surface. A mutant cell has a normal receptor but can't make cAMP. If a few mutant cells are mixed into a large group of starving normal cells, what will the mutants most likely do?

Question 2 of 4

A plasmodial slime mold starts from a single cell with one nucleus. Its nuclei then go through 12 rounds of mitosis, all dividing together, with no cytokinesis at all. What does it end up with?

Question 3 of 4

A tree built from many genes shows this pattern: animals and choanoflagellates are each other's closest relatives; fungi and nucleariids are each other's closest relatives; those two pairs together form one clade; and amoebozoans branch off just outside that clade. Which statement does the tree support?

Question 4 of 4

In a sample of garden soil, an organism feeds as separate amoebas, each with its own membrane. When food runs out, thousands of them gather into a slug that crawls, then forms a stalk topped with spores. The spores are genetically identical to the amoebas that made them. How is this organism best classified?

0 of 4 answered

28.7 Protists as partners, parasites and producers

pp. 596–597

On the AP exam? Yes

The ideas here are tested in Topics 8.2 (producers and energy flow), 8.5 (symbiosis and community diversity) and 8.7 (eutrophication). You just won't need protist names.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 8.5 Community Ecology, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Wherever there's moisture, there are protists: in seas and ponds, in wet soil, and inside the bodies of other organisms. Some float in open water as plankton; others live on the bottom, stuck to rocks or gliding over sediment.
Many protists are symbionts. Mutualists include dinoflagellates inside reef-building corals, which share sugars from photosynthesis, and gut protists that let termites digest wood.
Parasitic protists can reshape whole communities and economies. Malaria holds back health and income across the tropics, Phytophthora water molds kill crops and forest trees, and toxic dinoflagellate blooms can wipe out fish.
Photosynthetic protists and cyanobacteria are the main producers in water, forming the base of aquatic food webs. Ocean phytoplankton carry out close to half of Earth's net primary production.
Algal growth in water is usually capped by whichever nutrient runs short first, often nitrogen, phosphorus or (in parts of the open ocean) iron. Fertilizer runoff can trigger huge blooms; when the bloom dies, decomposers use up the dissolved oxygen and leave low-oxygen dead zones.
Warmer surface water floats on top of colder water and mixes less with it, so fewer nutrients reach the sunlit layer. Where seas have warmed, phytoplankton growth has often dropped, which can ripple up to fisheries and slow the ocean's uptake of CO₂.

Key terms (11)

producer: An organism that makes organic molecules from CO₂ using light or chemical energy. Every food web is built on producers.
phytoplankton: Tiny photosynthetic organisms, such as diatoms, dinoflagellates and cyanobacteria, that drift near the water's surface.
symbiosis: A close, long-term relationship between two species. It can help both (mutualism), help one and harm the other (parasitism), or help one and leave the other unaffected (commensalism).
mutualism: A symbiosis that helps both partners, like algae inside corals trading sugars for shelter and nutrients.
parasitism: A symbiosis in which one partner (the parasite) benefits by feeding on or living off the other (the host), which is harmed.
food web: The network of feeding links in a community, showing who eats whom and how energy moves.
net primary productivity: The energy producers capture and store as new biomass, after subtracting what they use for their own respiration.
limiting nutrient: The nutrient in shortest supply compared with need, so adding it boosts growth and adding others does little.
eutrophication: A buildup of nutrients, often from fertilizer or sewage, that fuels huge algal growth in a body of water.
dead zone: An area of water so low in oxygen that most animals can't survive, often after a large bloom dies and decomposes.
upwelling: Cold, nutrient-rich deep water rising toward the surface, which feeds phytoplankton growth.

Check yourself: 28.7 Protists as partners, parasites and producers

4 questions on 28.7 Protists as partners, parasites and producers. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

In a lake, phytoplankton store 2,000 kcal/m² of energy per year as new biomass. Zooplankton eat the phytoplankton, small fish eat the zooplankton, and large fish eat the small fish. If about 10% of the energy passes from each level to the next, about how much energy reaches the large fish each year?

Question 2 of 4

Researchers fill large tanks with lake water and add nutrients, then measure chlorophyll (a sign of phytoplankton) after two weeks (invented data). Treatment | Chlorophyll (µg/L) None (control) | 4.0 Nitrogen added | 4.3 Phosphorus added | 11.8 Nitrogen and phosphorus added | 25.6 Which conclusion do the data best support?

Question 3 of 4

Fertilizer runoff causes a huge algal bloom in a bay. A few weeks later the bloom dies off, and dissolved oxygen near the bottom drops so low that many fish and crabs die. What is the most direct cause of the oxygen drop?

Question 4 of 4

0 of 4 answered