Introduction to Eukaryotic Cells

Endosymbiont Theory

The endosymbiont theory proposes that larger prokaryotic cells engulfed smaller prokaryotic cells, which then evolved into organelles such as mitochondria and chloroplasts. This theory is supported by the presence of double membranes and their own DNA in these organelles.

• Mitochondria and chloroplasts have 70S prokaryotic-like ribosomes and circular DNA.

• These organelles replicate independently of the host cell.

Differences Between Eukaryotes and Prokaryotes

Eukaryotic cells are structurally and functionally distinct from prokaryotic cells.

• Eukaryotes have membrane-bound organelles, including a nucleus, and are generally larger and more complex.

• Prokaryotes lack a nucleus and most organelles, and their DNA is not contained in a membrane-bound compartment.

DNA Organization in Prokaryotes vs. Eukaryotes

• Prokaryotes:

  • DNA is usually a single, circular chromosome located in the nucleoid region.

  • May contain plasmids (small, circular DNA molecules).

• Eukaryotes:

  • DNA is linear and organized into multiple chromosomes within a membrane-bound nucleus.

  • Usually have more chromosomes than prokaryotes.

Intracellular vs. Extracellular Parasites

• Intracellular parasites live inside host cells (e.g., some protozoa, viruses).

• Extracellular parasites live outside host cells (e.g., some bacteria, fungi).

Types of Fungi

• Yeasts: Unicellular fungi, reproduce by budding.

• Molds: Multicellular fungi, form hyphae and reproduce by spores.

Protozoa

Protozoa are unicellular eukaryotes, often found in moist environments. Some are intracellular parasites in humans.

• Move using cilia, flagella, or pseudopodia.

• Examples: Plasmodium (malaria), Giardia.

Cell Wall Differences

• Fungal cell walls contain chitin.

• Plant cell walls contain cellulose.

• Animal cells lack cell walls.

Eukaryotic vs. Prokaryotic Flagella

• Eukaryotic flagella are complex, covered by the plasma membrane, and have a 9+2 arrangement of microtubules.

• Prokaryotic flagella are simpler, made of flagellin, and rotate like a propeller.

Phagocytosis and Endocytosis

• Phagocytosis: Cell engulfs large particles or other cells (e.g., immune cells engulfing bacteria).

• Endocytosis: General process of taking substances into the cell by engulfing them in a vesicle.

Cell Surface Structures

• Glycocalyx: Sticky outer layer for protection and cell recognition.

• Cilia: Short, hair-like structures for movement or moving substances along the cell surface.

Forms of DNA in Eukaryotes

• Chromatin: DNA plus proteins (histones), less condensed, found during interphase.

• Chromosomes: Condensed form of DNA, visible during cell division.

Location of DNA in Eukaryotes

• Primarily in the nucleus.

• Also found in mitochondria and chloroplasts (supporting endosymbiont theory).

Endoplasmic Reticulum (ER) Stress Response

The ER stress response is triggered by the accumulation of misfolded proteins in the ER. This can lead to cell death and is implicated in diseases such as cancer and neurodegenerative disorders.

• ER stress response increases protein folding capacity and degrades misfolded proteins.

Functions of Eukaryotic Organelles

Summary Table: Prokaryotes vs. Eukaryotes

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology textbooks.