Eukaryotes

Overview of Eukaryotes

  • Definition: Eukaryotes are organisms with cells that have a true nucleus.

Evolutionary Relationship

  • Discussion of the evolutionary links between the three domains of life:

    • Bacteria: The simplest life forms, diverged first from the last common ancestor.

    • Archaea: Evolved after bacteria, sharing closer genetic ties to eukaryotes.

    • Eukaryotes: Evolved last and are more closely related to Archaea than to bacteria.

Types of Eukaryotes

  • Major groups of eukaryotes include:

    • Animals: Members of the kingdom Animalia.

    • Fungi: Eukaryotic organisms, including yeasts and molds.

    • Protozoa: Single-celled eukaryotes, often studied in microbiology.

  • Note: Plants are not a focus for this lecture due to their limited role in human disease.

Characteristics of Eukaryotic Cells

  • Size: Eukaryotic cells typically range from 10 to 20 micrometers, about 10 times larger than prokaryotic cells.

  • Examples of large eukaryotic cells include certain amoebas, which can be up to 1 millimeter.

Evolution of Eukaryotic Cells

  • Origin: Life evolved from a simple prokaryotic organism, referred to as the last common ancestor.

  • Significant developments in evolution of eukaryotes:

    • Nucleus Formation:

    • Involves infoldings of the plasma membrane that created a nuclear envelope around the DNA.

    • Endosymbiotic Events:

    • Uptake of aerobic bacteria led to the formation of mitochondria, providing ATP through cellular respiration.

    • Uptake of photosynthetic bacteria resulted in the formation of chloroplasts, enabling photosynthesis.

Organelles in Eukaryotic Cells

  • Definition of organelles: Membrane-bound compartments within the cytoplasm, each serving specific functions.

Common Structures in Prokaryotes and Eukaryotes

  • Both types of cells share certain structures including:

    • Cytoplasm: A dense fluid matrix where biochemical reactions occur.

    • Ribosomes: Responsible for protein synthesis.

    • Eukaryotic ribosomes (80S) differ from prokaryotic ribosomes (70S). The Svedberg unit (S) reflects size and shape.

    • Plasma Membrane: Composed of phospholipids and proteins, serving as a barrier between the cell's interior and exterior.

    • Glycocalyx: Some eukaryotes also possess a cell wall.

Differences in Eukaryotic Structures

  • Cytoplasm:

    • Site of biochemical reactions; location of organelles in eukaryotes.

  • Ribosomes:

    • Eukaryotic ribosomes include 80S ribosomes and are found free in the cytoplasm or bound to the endoplasmic reticulum.

    • Mitochondria and chloroplasts contain 70S ribosomes, similar to prokaryotes.

  • Plasma Membrane:

    • Contains sterols (e.g., cholesterol) for stability, especially in organisms lacking a cell wall.

    • Specialized lipids, such as sphingolipids, that are unique to eukaryotes.

Bulk Transport Mechanisms

  • Endocytosis: A process requiring energy to transport materials into the cell using the membrane.

    • Types of endocytosis:

    • Phagocytosis: Engulfment of large particles, crucial for immune response against pathogens.

    • Pinocytosis: Cell drinking; formation of cup-like invaginations capturing solutes.

    • Receptor-mediated endocytosis: Specific uptake triggered by receptors binding to particular molecules.

    • Each mechanism generates membrane-enclosed vesicles differing in uptake specificity.

Summary

  • Eukaryotes possess a true nucleus and more complex cellular structures than prokaryotes, evolving through significant endosymbiotic events. They also have distinct transport mechanisms that facilitate cellular interactions with their environment through endocytosis.