21.1 General Biology of Protists
A. Protists are classified in the domain Eukarya (they have eukaryotic cells) and the kingdom Protista.
1. The endosymbiotic hypothesis suggests how the eukaryotic cells arose.
a. It proposes that aerobic bacteria became mitochondria.
b. Cyanobacteria became chloroplasts after being taken up by eukaryotic cells.
c. Giardia lamblia has two nuclei but no mitochondria, suggesting that a nucleated cell preceded the acquisition of mitochondria.
2. Although many protists are unicellular, they are highly complex.
a. Amoeboids and ciliates possess unique organelles, such as contractile vacuoles.
3. Most protists are free-living; some are parasitic, some (e.g., slime molds) are saprophytic (feed on decaying plant material), and some are mixotrophic (combining autotrophic and heterotrophic nutrition modes).
4. Some protists are photoautotrophic; some are heterotrophic.
5. Most protists use asexual reproduction, but sexual reproduction occurs in some species.
a. Formation of spores allows free-living and parasitic protists to survive hostile environments.
b. A cyst is a dormant cell with a resistant outer covering; the cyst allows a free-living species to overwinter and helps certain parasitic species survive the host’s digestive juices.
6. Some protists are of great medical importance because they cause disease; others are ecologically important.
7. Aquatic plankton serve as food for heterotrophic protists and animals.
8. Photosynthetic plankton produce much of the oxygen in the atmosphere.
9. Many protists enter symbiotic relationships; coral reefs rely on symbiotic photosynthetic protists.
B. Evolution and Diversity of Protists
1. Classification of protests has been based on modes of nutrition. a. Algae are autotrophic, similar to land plants b. Protozoans and slime molds are heterotrophic by ingestion, similar to animals c. Water molds are heterotrophic by absorption, similar to fungi
2. Protozoans include photosynthetic, heterotrophic, and mixotrophic organisms and have some form of locomotion (flagella, pseudopods, cilia).
3. Currently, the most widely accepted method of categorizing protests is by assigning them to supergroups, which is a major eukarotic group. There are six protist supergroups.
22.2 Diversity of Protists
A. Supergroup Archaeplastids 1. Archaeoplasts include land plants, green and red algae. 2. Green Algae a. Green algae contain both chlorophyll a and b. b. They live in the ocean, in fresh water habitats, snowbanks and on moist land. c. The majority of green algae are unicellular, but filamentous (Spirogyra), colonial (Volvox), and multicellular forms exist (Ulva). d. Green algae are not always green; some have pigments that give them an orange, red, or rust color. e. Plants are considered to be most closely related to the green algae. f. Green algae are divided into chlorophytes and charophytes.
1. Chlamydomonas is a minute, unicellular green alga less than 25 m long.
2. It has a cell wall and a single, large, cup‑shaped chloroplast with a pyrenoid for starch synthesis.
3. The chloroplast contains a light‑sensitive eyespot (stigma) that directs the cell to light for photosynthesis.
4. Two long whiplike flagella project from the anterior end to propel the cell toward light.
5. When growth conditions are favorable, Chlamydomonas reproduces asexually with flagellated spores called zoospores.
6. When growth conditions are unfavorable, Chlamydomonas reproduces sexually.
a. Gametes from two different mating types join to form a zygote.
b. A heavy wall forms around the zygote; a resistant zygospore survives until conditions are favorable. 7. Volvox a. Volvox is a Colonial Green Algae. A colony is a loose association of independent cells. b. Volvox is a hollow sphere with thousands of cells arranged in a single layer. c. Volvox cells resemble Chlamydomonas cells; a colony arises