Biology Chapter 28: Eukaryotic Origins and Cell Structure

Eukaryotic Origins

• Eukaryotic cells differ from prokaryotes in having a cytoskeleton and compartmentalization (nucleus and organelles).
• Eukaryotes appeared in microfossils about 1.5 billion years ago.

Evolution of Nucleus and Organelles

• The nucleus and endoplasmic reticulum arose from infoldings of the prokaryotic cell membrane.
• Many organelles evolved via endosymbiosis between an ancestral eukaryote and a bacterial cell.

Endosymbiosis

• Mitochondria evolved from aerobic bacteria engulfed by larger bacteria.
• Chloroplasts evolved from photosynthetic bacteria engulfed by larger bacteria.
• Chloroplasts originated from a single line of cyanobacteria.
• Hosts are not monophyletic, and brown algae engulfed red algae that already had chloroplasts, resulting in secondary endosymbiosis.

Evidence for Endosymbiosis

• DNA inside mitochondria and chloroplasts is similar to bacterial DNA in size and character.
• Ribosomes inside mitochondria are similar to bacterial ribosomes.
• Chloroplasts and mitochondria replicate by binary fission, not mitosis.
• Mitosis evolved in eukaryotes.

Mitosis

• Prokaryotes carry genes on a single DNA molecule, while eukaryotes have multiple chromosomes.
• Mitosis and cytokinesis developed to separate chromosomes and other cell contents during cell division.
• Mitosis evolved over thousands of years.

Overview of Protists

• Protists are the most diverse of the four eukaryotic kingdoms.
• They are united by not being fungi, plants, or animals, but vary considerably in other aspects.
• Protists can be unicellular, colonial, or multicellular, and have all types of nutrition and symmetries.

Classification of Protists

• Protists are not monophyletic, but are present in all six eukaryotic supergroups: Excavata, Chromalveolata, Archaeplastida, Rhizaria, Amoebozoa, and Ophisthokonta.

Cell Surface in Protists

• Protists have a varied array of cell surfaces, including plasma membrane, extracellular matrix (ECM), and diatoms with silica shells.
• Cysts are dormant cells with resistant outer coverings used for disease transmission.

Locomotion in Protists

• There are three primary means of locomotion: flagella, cilia, and pseudopodia ("false feet").

Nutrition in Protists

• Autotrophs can be photosynthetic or chemoautotrophic.
• Heterotrophs include phagotrophs that ingest particulate food matter, and mixotrophs that are both phototrophic and heterotrophic.

Reproduction in Protists

• Asexual reproduction is typical, and includes mitosis, budding, and schizogony.
• Sexual reproduction involves meiosis, producing haploid gametes that unite to form a zygote, allowing for genetic recombination.