BIOL 112 Lecture 18

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Reminder: Exam 2 will be Wednesday, October 17, 2012

Eukaryotes

Oldest recorded Eukaryotic cells are 2.7 billion years old.

• Evidence is from fossilized cholesterol in rocks.

Characteristics of Eukaryotic Cells:

1. Membrane-bound nucleus
2. Endoplasmic reticulum & Golgi apparatus
3. Mitochondria and maybe chloroplasts (obtained through endosymbiosis)

(1) & (2) obtained through plasma cell membrane invagination (enfolding)

Multicellular eukaryotes appear 1.2 billion years ago

Evolution of Multicellular Organisms

1. single-celled organisms
2. cooperative colonies of single-celled organisms (cells still survive autonomous)
3. simple multicellular organism: functional specialization of cells (cells are no longer independent but interdependent)
4. complex multicellular organism: tissues made of specialized cells (and different specialized cells within tissues)

Complex, large, and diverse multicellular forms appeared about 540 million years ago.

Why the delay?

global ice age (snowball earth) that thawed about 550 million years ago

harsh conditions limited diversification

thawing led to Cambrian explosion—a major adaptive radiation event

Multicellularity arose independently throughout domain Eukarya: namely plants, animals, and fungi

Protists

According to Whittaker, Eukaryotes that aren't plants, animals, or fungi. (still used today).

Very diverse as result of polyphyletic phylogeny (came from many ancestors):

• single-celled (e.g. diatoms and certain algae)
• multi-celled (e.g. seaweed)
• variable habitats (aquatic: fresh or marine, terretsrial)
• autotrophs, heterotrophs, and mixotrophs
• parasitic (e.g. trypanosoma → sleeping sickness; Plasmodium vivax → malaria) or free-living

Second-degree Endosymbiosis

Photosynthetic cell engulfed by a protist for a total of 4 membranes surrounding a chloroplast (called a plastid)

Second (degenerate; vestigial) nucleus introduced by 2° endosymbiont

Plastids present across multiple supergroups (but not all), so some groups lost their plastid endosymbiont (these organisms usually have parasitic lifestyles).