8.4

What is the main difference between prokaryotes and eukaryotes?

Prokaryotes lack a nucleus and organelles, while eukaryotes have a nucleus, organelles, cytoskeleton, and compartmentalized structures.

Who proposed the endosymbiotic theory?

Lynn Margulis proposed that some organelles in eukaryotes were originally free-living bacteria engulfed by a host cell.

Which organelles evolved from bacteria according to endosymbiosis?

Mitochondria evolved from aerobic bacteria, and plastids/chloroplasts evolved from cyanobacteria.

How do mitochondria support endosymbiotic theory?

They divide independently, have circular DNA, and have 30S and 50S ribosomes, similar to bacteria.

What is the role of mitochondria?

Mitochondria generate ATP (energy), participate in thermogenesis, hydrogen production, and sulfur metabolism depending on the organism.

What are mitosomes?

Reduced mitochondria in anaerobic eukaryotes (e.g., Giardia) used for sulfur metabolism instead of ATP production.

What are hydrogenosomes?

Organelles in anaerobic eukaryotes (e.g., Trichomonas) that produce hydrogen gas as a byproduct.

Why is cell compartmentalization important?

It separates biochemical processes, preventing incompatible reactions, and allows specialized functions like lysosomes digesting molecules safely.

What is the origin of plastids/chloroplasts?

Plastids originated from cyanobacterial-like prokaryotes through primary endosymbiosis.

What is primary endosymbiosis?

A single-celled eukaryote engulfs a cyanobacterium, forming a plastid with 2 membranes (e.g., green algae, plants).

What is secondary endosymbiosis?

A eukaryote with a primary plastid is engulfed by another eukaryote, forming plastids with 3–4 membranes (e.g., diatoms, dinoflagellates).

Name the main structures of a chloroplast.

Outer membrane, inner membrane, stroma, thylakoids, grana, lumen.

What is the function of thylakoids and grana?

Thylakoids contain chlorophyll for light reactions; grana are stacks of thylakoids that increase surface area for light absorption.

What evidence supports endosymbiosis in mitochondria and plastids?

Independent division, circular DNA, bacterial-type ribosomes (30S + 50S), and bacterial-like protein sequences.

How did the first eukaryotic cell arise?

An anaerobic cell lost its cell wall, folded its membrane to form nucleus and organelles, and engulfed an aerobic bacterium, which became mitochondria.

What benefit did the host cell gain from engulfing aerobic bacteria?

Increased energy production (ATP) and protection, leading to a mutually dependent relationship.

What are the key features of eukaryotic cells?

Nucleus with membrane and pores, mitochondria, cytoskeleton (actin, microtubules), chromosomes, flagella/cilia, and mitosis.