Biology Chapter 21

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Chapter 22: Protists

22-1: Characteristics of Protists

Protists Are a Varied and Ancient Group of Organisms

Protists are the most diverse of all organisms
- most are unicellular
- microscopic
- few are complex and multicellular


- eukaryotes
- some photosynthesis
- some ingest their food
- some absorb their food
- some have flagella or cilia
o used for locomotion or getting food
- found almost everywhere there is water
- damp soil and sand
- some are parasitic
- most have mechanisms for monitoring and responding to their environment - some have eyespots
o small organelles containing light-sensitive pigments that detect changes in light

The first eukaryotes

The first eukaryotes are thought to have evolved about 1.5 bya through the process of Endosymbiosis. This kingdom gave rise to the three kingdoms of multicellular organisms – fungi, plants and animals.

There are two important features that evolved among the protists 1. sexual reproduction
2. multicellularity

Many protists reproduce only asexually – mitosis. Some use meiosis and sexual reproduction only in times of stress and then others reproduce sexually most of the time.

What unites protists?

The kingdom Protista contains all eukaryotes that don’t fit in animals, plants and fungi.

See table on page 479 for distinguishing one phyla of protists from another. Generally protist that are heterotrophic are called protozoan and photosynthetic protists are called algae.

Protists Reproduce Sexually and Asexually

Typically unicellular protists reproduce by forming complex colonies. Chlamydomonas species reproduce sexually and asexually. The single-celled protist is haploid and reproduces asexually by absorbing its tail and dividing by mitosis. Mitosis produces two to eight haploid cells called zoospores. Many zoospores break out of the parent cell and grow to become mature haploid cells.

Sexual reproduction among unicellular protists

During environmental stress certain protists reproduce sexually - haploid cells divide by mitosis producing haploid gametes - a pair of gametes fuse to form a pair
- this pair sheds their cell wall and fuse into a diploid zygote o zygospore (thick protective wall) can withstand unfavorable conditions

Sexual reproduction among multicellular protists

There are a variety of ways in which protists reproduce sexually: 1. alternation of generations – two distinct multicellular phases a. sporophyte generation – diploid (spore producing phase) i. reproductive cells called sporangia produce haploid spores by meiosis b. gametophyte generation – haploid (gamete producing phase) i. produce gametophyte haploid gametes that fuse divide through mitosis 2. conjugation- the temporary union of two protists to exchange nuclear material a. two filaments align side by side

b. cell walls form a bridge between the cells
c. the nucleus of one then passes through the tube into the other cell d. the two nuclei form a resting spore
e. producing an new haploid

22-2: Protists Diversity

Protists Can Move by Using Cytoplasmic Extension

Amoebas and forams are unicellular heterotrophs that have a unique form of movement.

Ameobas – phylum Rhizopoda
- move by using flexible, cytoplasmic extensions – pseudopodia - extremely flexible, no cell wall
- pseudopodia surrounds and engulfs food particle
- live in freshwater and salt water
- reproduce by binary fission – dividing into two new cells - most free-living, some parasitic

Forams – phylum Foraminifera
- marine protists
- live in sand or attach themselves to other organisms or...
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