Introduction to Eukaryotes

Eukaryotes Introduction

• Eukaryotes have a true nucleus and membrane-bound organelles
• Key characteristics of eukaryotes include:
  • Nucleus
  • Cytoskeleton
  • Membrane-bound organelles (ER, Golgi, lysosomes, vacuoles, peroxisomes, mitochondria, chloroplasts)
  • Asexual reproduction (various methods)
  • Cell walls (in some)
  • Capsules (in some)
  • Binary fission (in some)
  • Cilia or flagella (in some)
  • Linear DNA associated with proteins
  • Larger ribosomes (80S)
  • Larger cell size (>10 μm diameter)
  • Mitosis and meiosis
  • Multicellular forms (in some)

Eukaryote Objectives

• Identify common characteristics of eukaryotes.
• Explain the endosymbiotic theory.
• Describe how multicellular forms may have evolved from protists.

Prokaryote/Eukaryote Comparison

• Prokaryotes lack a nucleus and membrane-bound organelles.
• Eukaryotes have a true nucleus and membrane-bound organelles.
• Both can have cell walls, capsules, binary fission, and be unicellular.

Eukaryote Evolution

• Eukaryotes are thought to have evolved from prokaryotes
• The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by a larger cell
• Evidence for endosymbiosis includes:
  • Mitochondria and chloroplasts have their own DNA
  • Mitochondria and chloroplasts have their own ribosomes
  • Mitochondria and chloroplasts divide by binary fission

Kingdom Protista

• Protists are a diverse group of eukaryotic organisms.
• Protists can be plant-like, animal-like, or fungus-like.
• Plant-like protists are photosynthetic (e.g., algae).
• Animal-like protists are heterotrophic (e.g., protozoa).
• Fungus-like protists are decomposers (e.g., slime molds).
• Protists arose from prokaryotes, and their descendants gave rise to all plants, fungi, and animals as well as modern protists.
• Classification is difficult due to the wide diversity of protists.

Classification of Protists

• Protists are classified by their mode of nutrition (plant-like, animal-like, or fungus-like).
• This system is artificial (non-phylogenetic), and the best approach is the traditional system of single kingdom Protista

Plastid Evolution

• Plastids (e.g., chloroplasts) are produced through secondary endosymbiosis.
• Mitochondria arose first then plastids.
• Organisms with plastids (chloroplasts) evolved from photosynthetic prokaryotes engulfed by heterotrophic eukaryotes.