Ch. 23.1: Endosymbiosis

This set of flashcards covers key concepts from Chapter 23.1 on Endosymbiosis, including the definition of protists, the characteristics and origins of eukaryotes, the endosymbiotic theory proposed by Lynn Margulis, and evidence supporting the bacterial origins of mitochondria and plastids.

What are protists?

A diverse group of eukaryotic microorganisms that are not classified as animals, plants, or fungi, mostly microscopic and unicellular.

What is the endosymbiotic theory?

The theory that one cell engulfed another, and the engulfed cell survived, leading to mutual benefit and the evolution of eukaryotes.

When do the earliest eukaryote fossils date back to?

Approximately 2.1 billion years ago.

List three unifying characteristics of eukaryotes.

Cells with nuclei and nuclear envelopes, mitochondria, and a cytoskeleton of microtubules and microfilaments (others include flagella/cilia, chromosomes organized by histones, mitosis, sexual reproduction, cell walls).

From what type of bacterium are mitochondria proposed to have evolved?

Alphaproteobacteria.

What important metabolic process is found in prokaryotes but absent in eukaryotes?

Nitrogen fixation.

Who proposed the endosymbiotic theory in the 1960s?

Lynn Margulis.

Name two features mitochondria share with alphaproteobacteria, suggesting an endosymbiotic origin.

Their own circular genome distinct from the nuclear genome, and prokaryotic-like protein synthesis machinery (ribosomes, tRNAs).

How do mitochondria reproduce, supporting their endosymbiotic origin?

They divide independently via a process similar to binary fission in prokaryotes and only arise from pre-existing mitochondria.

What does the finding of reduced organelles with mitochondrial genes in anaerobic eukaryotes suggest?

Most anaerobic eukaryotes likely descended from lineages that once had mitochondria, rather than never acquiring them.

What are plastids, and what are photosynthetic plastids called?

Plastids are a group of related organelles in plant cells involved in storage; photosynthetic plastids are called chloroplasts.

What characteristic do chloroplasts from primary endosymbiosis share with cyanobacteria?

They possess thylakoids, circular DNA, ribosomes, and are enclosed by two membranes.

Besides gene transfer and replication, what is another parallel between plastids and mitochondria?

The endosymbionts (plastids) are unable to survive independently outside the host cell.

What is secondary endosymbiosis?

Secondary endosymbiosis occurs when an ancestral eukaryotic cell containing a photosynthetic endosymbiont is engulfed by another eukaryotic cell.

What two key events are proposed by the endosymbiotic theory for the evolution of eukaryotes?

Infoldings of the plasma membrane leading to a nucleus and ER; and the engulfment of aerobic bacteria (leading to mitochondria) followed by the engulfment of photosynthetic bacteria (leading to chloroplasts).