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1

protists

Protists are a polyphyletic assortment of mostly unicellular (or they are multicellular without specialized tissues; this simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants, although some fungi and animals are also unicellular) organisms that account for the bulk of eukaryotic diversity. Much of this diversity is unseen, evident only from small subunit ribosomal RNA gene sequences extracted from environmental samples, used to redefine this group in modern taxonomy as diverse and often distantly related phyla. Furthermore, novel species and unexpected diversity from every major protist group continue to be identified from diverse environments. In current views of eukaryotic phylogeny, these protist groups are combined, together with metazoans and fungi, into six major eukaryotic clades that are thought to be descended from an ancestral diversification and radiation of the earliest eukaryotic organism(s) around 1–1.5 Ga years ago. There have been attempts to remove the kingdom from modern taxonomy but it is still very much in use. The group of protists is now considered to mean diverse phyla that are not closely related through evolution and have different life cycles, trophic levels, modes of locomotion and cellular structures. Besides their relatively simple levels of organization, the protists do not have much in common.

2

how close are the evolutionary ties amongst varying protist?

Genetic and morphological studies have also shown that some protists are more closely related to plants, fungi, or animals than they are to other protists. As a result, the kingdom in which all protists once were classified, Protista, has been abandoned, and various protist lineages are now recognized as kingdoms in their own right. Most biologists still use the term protist, but only as a convenient way to refer to eukaryotes that are not plants, animals, or fungi.

3

what is generally meant by the modern definition of protists

The kingdom in which all protists once were classified, Protista, has been abandoned, and various protist lineages are now recognized as kingdoms in their own right. Most biologists still use the term protist, but only as a convenient way to refer to eukaryotes that are not plants, animals, or fungi.

4

Aside from compartmentalisation and a nucleus, what is a major difference between prokaryotes and eukaryotes?

Eukaryotic cells have a well-developed cytoskeleton that extends throughout the cell. The cytoskeleton provides the structural support that enables eukaryotic cells to have asymmetric (irregular) forms, as well as to change in shape as they feed, move, or grow. In contrast, prokaryotic cells lack a well-developed cytoskeleton, thus limiting the extent to which they can maintain asymmetric forms or change shape over time.

(Shown: green for microtubules and reddish orange for microfilaments. A third component of the cytoskeleton, intermediate filaments, is not evident here. Blue color tags the DNA in the nucleus.)

5

how many eukaryortes are considered protists?

the organisms in most eukaryotic lineages are protists, and most protists are unicellular.

6

describe the difference between a protist and a metazoan

Many protists are very complex—the most elaborate of all cells. In multicellular organisms, essential biological functions are carried out by organs. Unicellular protists carry out the same essential functions, but they do so using subcellular organelles, not multicellular organs. The organelles that protists use include the nucleus, endoplasmic reticulum, Golgi apparatus, and lysosomes. Certain protists also rely on organelles not found in most other eukaryotic cells, such as contractile vacuoles that pump excess water from the protistan cell.

7

describe protistian nutrition

Some protists are photoautotrophs and contain chloroplasts. Some are heterotrophs, absorbing organic molecules or ingesting larger food particles. Still other protists, called mixotrophs, combine photosynthesis and heterotrophic nutrition. Photoautotrophy, heterotrophy, and mixotrophy have all arisen independently in many different protist lineages. 

8

what does nutrition say about the evolutionary lineage of a protistan?

nothing, photoautotrophy, heterotrophy, and mixotrophy have all arisen independently in many different protist lineages

9

what types of sexual cycles can protists complete?

All three basic types of sexual life cycles are represented among protists, along with some variations that do not quite fit any of these types. The common feature of all three cycles is the alternation of meiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the timing of these two key events. 

10

are more eukaryotes unicellular or multicelluar?

the organisms in most eukaryotic lineages are protists, and most protists are unicellular.

11

mixotroph

a combination of photoautotrophic and heterotrophic nutrition 

12

do all protists reproduce sexually?

No, some protists are only known to reproduce asexually; others can also reproduce sexually or at least employ the sexual processes of meiosis and fertilization. All three basic types of sexual life cycles are represented among protists, along with some variations that do not quite fit any of these types.

13

list two common examples of endosymbiosis

mitochondria, plastids (other answers possible)

14

plastid

The plastid (pl. plastids) is a major double-membrane organelle found, among others, in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. They often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's color. They have a common origin and possess a double-stranded DNA molecule, which is circular, like that of prokaryotes.

In normal intraspecific crossings (resulting in normal hybrids of one species), the inheritance of plastid DNA appears to be quite strictly 100% uniparental. In interspecific hybridisations, however, the inheritance of plastids appears to be more erratic. Although plastids inherit mainly maternally in interspecific hybridisations, there are many reports of hybrids of flowering plants that contain plastids of the father.

Plastids are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 billion years ago and enabled eukaryotes to carry out oxygenic photosynthesis. Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and muroplasts in the glaucophytes. The plastids differ both in their pigmentation and in their ultrastructure.

15

which evolved first, mitochondrial or plastidic eukaryotic endosymbiosis?

evidence suggests that mitochondrial endosymbiosis evolved before plastidic endosymbiosis, thus a defining moment in the origin of eukaryotes occurred when a host cell (likely an archea or archeal divergent "protoeukaryote") engulfed a bacterium that would later become an organelle found in all eukaryotes—the mitochondrion

16

where did mitochondria originate from and how many times did this occur?

Mitochondria arose from an alpha proteobacterium. Results from mtDNA sequence analyses also indicate that the mitochondria of protists, animals, fungi and plants descended from a single common ancestor, thus suggesting that mitochondria arose only once over the course of evolution. Similar analyses show that plastids arose once from an engulfed cyanobacterium.

17

where did plastids originate from and how many times did this occur?

Studies of plastid-bearing eukaryotes suggest that plastids evolved from a single gram-negative cyanobacterium that was engulfed by an ancestral heterotrophic eukaryote (primary endosymbiosis). That ancestor then diversified into red algae and green algae, some of which were subsequently engulfed by other eukaryotes (secondary endosymbiosis). Primary endosymbiosis occurred once, secondary endosybiosis occurred multiple times.

18

explain the diversity of plastids produced by endosymbiosis

Studies of plastid-bearing eukaryotes suggest that plastids evolved from a single gram-negative cyanobacterium that was engulfed by an ancestral heterotrophic eukaryote (primary endosymbiosis). That ancestor then diversified into red algae and green algae, some of which were subsequently engulfed by other eukaryotes (secondary endosymbiosis). Primary endosymbiosis occurred once, secondary endosybiosis occurred multiple times.

19

how many times did secondary endosymbiosis of plastids occur?

multiple times

20

how many times did primary endosymbiosis of plastids occur?

once

21

chlorarachniophytes

[archaeplastidia>chloarachniophytes] have a single, large plastid (seconary endosymbiont) that contains a nucelomorph, the vestigial nucleous of a green algae (primary endosymbiont).

22

nucleomorph

Protists known as [archaeplastidia>chlorarachniophytes] likely evolved when a heterotrophic eukaryote engulfed a green alga. Evidence for this process can be found within the engulfed cell, which contains a tiny vestigial nucleus, called a nucleomorph. Genes from the nucleomorph are still transcribed, and their DNA sequences indicate that the engulfed cell was a green alga. 

23

excavata

Originally proposed based on morphological studies of the cytoskeleton, some members of this diverse group have an “excavated” feeding groove on one side of the cell body. The supergroup and its three subgroups are all individually monophyletic:

[Excavata>Diplomonads]

[Excavata>Parabasalids]

[Excavata>euglenozoans>kinetoplastids] / [Excavata>euglenozoans>euglenids]

24

categorise the phylogeny of kinetoplastids

[Excavata>euglenozoans>kinetoplastids]

25

categorise the phylogeny of euglenids

[Excavata>euglenozoans>euglenids]

26

diplomonads

[Excavata>diplomonads] : monophyletic subgroup of the supergroup excavata, lack plastids and have reduced mitochondria that lack functional electron transport chains and hence cannot use oxygen to help extract energy from carbohydrates and other organic molecules, termed mitosomes. Most diplomonads are found in anaerobic environments. Best known example is Giardia intestinalis, parasitic to intestine of mammals.

27

parabasalids

[Excavata>parabasalids]: monophyletic subgroup of the supergroup excavata, lack plastids and have reduced mitochondria which lack functional electron transport chains and hence cannot use oxygen to help extract energy from carbohydrates and other organic molecules, termed hydrogenosomes, that generate hydrogen gas as a byproduct. Most parabasalids are found in anaerobic environments. Best example is Trichomonas vaginalis, parasitic to sexual organs of humans.

28

kinetoplastids

[Excavata>euglenozoans>kinetoplastids]: a monophyletic clade within subgroup euglenozoans (which are excavates that have a special spiral or crystalline rod of unknown function inside their flagella) have a single, large mitochondrion that contains an organized mass of DNA called a kinetoplast. Kinetoplastids in the genus Trypanosoma (shown) infect humans and cause African sleeping sickness.

29

euglenozoans

[Excavata>euglenozoans]: monophyletic subgroup of the supergroup excavata, euglenozoans belong to a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs, mixotrophs, and parasites. The main morphological feature that distinguishes protists in this clade is the presence of a rod with either a spiral or a crystalline structure inside each of their flagella. The two best-studied groups of euglenozoans are the [Excavata>euglenozoans>kinetoplastids] and the [Excavata>euglenozoans>euglenids].

(shown a flagellum cross section: the rod lies alongside the 9 + 2 ring of microtubules found in all eukaryotic flagella)

30

euglenids

[Excavata>euglenozoans>euglenids], a monophyletic clade within subgroup euglenozoans (which are excavates that have a special spiral or crystalline rod of unknown function inside their flagella) have a pocket at one end of the cell from which one or two flagella emerge, many are mixotrophs, others use phagocytosis.