Chapter 21

Protists

I. Characteristics of Protists A. Diversity 1. Characteristics

• Various ways of metabolizing food a. photosynthetic b. ingest food c. absorb food

• Some move by flagella or cilia• Found almost everywhere there is water• Many have mechanisms responding to stimuli in their

environment. Many have structures called eyespots.

2. The First Eukaryotes• Evolved about 1.5 billion years ago• Two important eukaryotic features evolved among

protists a. sexual reproduction b. multicellularity

B. Reproduction 1. Asexual reproduction in

unicellular protists• A mature haploid (n) cell

divides by mitosis• Haploid cells called

zoospores are produced and remain within the wall of the parent cell

• Zoospores then break out of the parent cell

2. Sexual reproduction in unicellular protists• A haploid (n) cell divides by mitosis to produce

haploid gametes• A pair of gametes fuse• The pair sheds their cell walls and fuse into a

diploid (2n) zygote with a think protective wall called a zygospore

• In favorable environmental conditions meiosis within the zygospore produces haploid (n) cells that break out of the zygospore

3. Sexual reproduction in multicellular protists a. Alteration of generations

• Characterized by two distinct multicellular processes

i. sporophyte generation - diploid, spore-producing phase ii. Gametophyte generation - haploid, gamete-producing phase

• Step 1: Adult sporophyte’s (2n) reproductive cells, sporangia, produce haploid (n) spores by meiosis

• Step: 2 Spores grown into multicellular haploid gametophytes

• Step 3: The mature gametophyte produces haploid gametes

• Step 4 Haploid gametes fuse into diploid zygotes• Step 5: Diploid zygotes divide by mitosis to form

new diploid sporophytes

mitosis

Diploid (2n)Sporophyte Generation

Haploid (n)Gametophyte generation

b. Conjugation• Two protists temporarily unite to exchange

nuclear material• The nucleus of one cell passes through the bridge

made by the fusion into the adjacent cell

II. Amoebas and Forams• Unicellular heterotrophsA. Amoebas• Protists that move by using flexible, cytoplasmic

extensions called pseudopodia• In a process called ameboid movement, a

pseudopodium bulges form the cell, stretches outward, and anchors itself to a nearby surface

• Live in both fresh and salt water• Reproduce by fission, dividing into two cells

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B. Foraminifera• Typically live in marine sand or attach themselves

to other organisms or rocks• Characterized by their porous shells, or tests,

made of calcium carbonate

III. Algae• Strict phototrophsA. Green Algae• Freshwater unicellular

and multicellular organisms

• Contain the same pigments found in the chloroplasts of plants

• most have sexual and asexual reproductive stages

B. Red Algae• Multicellular organisms found in warm ocean

waters - contributes to the great coral reefs• Red pigment is efficient at absorbing the light

that penetrates deep waters• Life cycle involves alternation of generations

C. Brown Algae• Multicellular organisms found mostly in marine

environments• Kelp is among the largest organisms on Earth• Reproduce by alternation of generations

IV. Diatoms• Photosynthetic• Unicellular• Unique double shells made of silica• Found in oceans and lakes• Important producers in the food chain

• Diploid (2n) individuals that reproduce asexually - the two halves of the shell separate, and each half

regenerates another matching half - tend to get smaller and smaller with each

generation - when an individual gets too small it slips out of its

shell, grows to full size, and regenerates a new shell

V. Flagellates• Move using flagellaA. Dinoflagellates• Unicellular phototrophs• Most are marine and make up plankton• Have a protective coat made of cellulose

• Most have two flagella 1. one encircles the body like a

belt 2. the other beats perpendicular

to it causing it to spin through water like a top

• Reproduce asexually by mitosis• Some produce powerful toxins

that are associated with the poisonous “red tides”

B. Euglenoids• Live in freshwater• Unicellular, asexual• 1/3 contain chloroplasts and are photosynthetic• Others lack chloroplasts, ingest their food, and are

heterotrophic• A light-sensitive organ called the eyespot helps

orient the organisms movements towards light

C. Kinetoplasts• Unicellular• Heterotrophs• Most reproduce asexually• Some live symbiotically in the guts of termites,

where they provide the enzymes that digest wood• Others cause diseases such as African sleeping

sickness

D. Ciliates• Most complex of the protists• Have large numbers of cilia used for movement• Unicellular• Heterotrophs• Most form vacuoles for ingesting nutrients and

regulating their water balance

• Most have two types of nuclei 1. micronuclei - contain normal chromosomes that divide by

mitosis 2. macronuclei - contain small pieces of DNA derived from the

micronuclei• Most reproduce by mitosis, with the body splitting in

half• Engage in sexual reproduction through conjugation

VI. Protistan Molds• Heterotrophs• Cell walls contain different carbohydrates than

fungal cell walls

A. Cellular slime molds• Individual organisms behave as separate amoebas

in soil• In the event of environmental stress, they gather

together and form multicellular colonies called slugs

- each slug develops a base, a stalk, and a swollen tip that develops spores

- each spore releases and becomes a new individual

B. Plasmodial Slime Molds• A group of organisms that move along as a mass

of cytoplasm, or plasmodium• Engulf bacteria and other organic material as they

move• Contains many nuclei that are not separated by

cell walls

• Under stress, the plasmodium divides into smaller mounds

- each mound produces a stalk tipped with a capsule in which haploid spores develop

- in favorable conditions, the spores germinate and become haploid cells

- the haploid cells fuse into diploid zygotes that undergo mitosis and form a new plasmodium

C. Oomycetes• Water molds, white rusts, an downy mildews in

fresh water• Either parasites or feed on dead organic matter

VII. Sporozoans• Parasitic in hosts• Form spores during their reproduction cycle• Nonmotile• Unicellular• Life cycle involves both asexual and sexual

reproduction - sexual reproduction between a male and female

gamete results in a zygote with a think-walled structure that makes it resistant to unfavorable conditions

VIII. Protists and Health A. Beneficial protists

• Single largest group of photosynthesizers on the planet

- help to support food chains• Live in the digestive tracts of humans and

animals - cattle could not digest the cellulose in the hay

and grass they eat without the aid of protists

B. Malaria• 3 million people die from malaria each year - die of anemia, kidney failure, or brain damage1. Malaria Life Cycle• Spread by the bite of certain mosquitoes

• Three stages a. sporozoite

• A mosquito injects saliva containing about 1,000 protists into a human

• These sporozoites infect the liver where they divide and reproduce

b. Merozoite• The second generation cells from the liver infect

red blood cells and divide• Red blood cells rupture, releasing the merozoites

and toxic substances that prevents the blood from clotting

c. Gametes• Some merozoites develop into gametes• When eaten by a mosquito, male and female

gametes fuse into a zygote• The zygotes divide in the mosquito and form

sporozoites