Endosymbiotic Theory

Two papers published recently, one in the journal Science and another in the Cell, have generated new interest in the endosymbiotic theory, which proposes that certain organelles in present-day eukaryotic cells originated as prokaryotic microorganisms.

The Endosymbiotic Theory:

1. Eukaryotic Organelles as Former Prokaryotes:

   1. The endosymbiotic theory suggests that some organelles in today's eukaryotic cells were once independent prokaryotic microbes.

2. The First Eukaryotic Cell:

   1. The earliest eukaryotic cell is believed to have been an amoeba-like organism that acquired nutrients through phagocytosis and contained a nucleus formed by the invagination of a portion of the cytoplasmic membrane around the chromosomes.

3. Symbiotic Relationships with Prokaryotes:

   1. Certain amoeba-like organisms engulfed prokaryotic cells, which survived within the host and established symbiotic relationships.

4. Mitochondria and Chloroplasts:

   1. Mitochondria evolved from ingested aerobic bacteria, while chloroplasts evolved from ingested photosynthetic bacteria.
   2. These organelles gradually lost their cell walls and most of their DNA, as it was no longer beneficial within the host cell.

5. Interdependence and Permanent Relationships:

   1. The endosymbiotic theory explains how a larger host cell and ingested bacteria can become dependent on each other for survival, forming a lasting relationship.

6. Evolutionary Specialization:

   1. Over millions of years, mitochondria and chloroplasts have become highly specialized and can no longer exist outside the host cell.

Prokaryotes vs. Eukaryotes:

1. Classification of Living Things:

   1. All living organisms can be categorised into 3 primary domains: Bacteria, Archaea, and Eukarya.

2. Prokaryotic Cells:

   1. Bacteria and Archaea are domains composed of primarily single-celled organisms known as prokaryotes.
   2. Prokaryotic cells are the smallest, simplest, and most ancient type of cell.

3. Eukaryotes and Eukaryotic Cells:

   1. Organisms in the Eukarya domain possess more complex eukaryotic cells.
   2. Eukaryotes include animals, plants, fungi, and protists, and they can be unicellular or multicellular.

4. Nucleus and Genetic Material:

   1. The primary distinction between eukaryotic and prokaryotic cells is the presence of a distinct nucleus in eukaryotic cells that contains the cell's genetic material, while prokaryotic cells lack a nucleus and have free-floating genetic material.

5. Evolution of Eukaryotes:

   1. Eukaryotes developed approximately 2.7 billion years ago, following a period of 1 to 1.5 billion years of prokaryotic evolution.

6. Additional Organelles in Eukaryotic Cells:

   1. Eukaryotic cells have several membrane-bound organelles not found in prokaryotic cells, including mitochondria, the endoplasmic reticulum, the Golgi complex, and chloroplasts in plant cells.

7. Shared Features of Prokaryotic and Eukaryotic Cells:

   1. Despite their differences, prokaryotic and eukaryotic cells share certain common features, such as DNA, the cell membrane, cytoplasm, and ribosomes.

Feature	Prokaryotes	Eukaryotes
Size	Smaller (typically 0.1 - 5 micrometers)	Larger (typically 10 - 100 micrometers)
Complexity	Simpler structure	More complex structure
Nucleus	No true nucleus. Genetic material (DNA) in a region called the nucleoid	True nucleus with a membrane enclosing the DNA
Organelles	Lack membrane-bound organelles	Contain membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, etc.)
DNA Structure	Single, circular chromosome	Multiple linear chromosomes
Cell Division	Binary fission (simple division)	Mitosis and meiosis (more complex processes)
Examples	Bacteria, Archaea	Animals, plants, fungi, protists

Similarities Between Prokaryotes and Eukaryotes

1. Both have DNA: The fundamental genetic material is the same.
2. Cell membrane: Both are enclosed by a plasma membrane that controls the flow of substances in and out of the cell.
3. Cytoplasm: Both have a gel-like cytoplasm where cellular processes occur.
4. Ribosomes: Both contain ribosomes, the sites of protein synthesis.

Phagocytosis Explained:

1. Phagocytosis, or "cell eating", is the process by which a cell ingests and digests a particle.
2. The term phagocytosis is derived from the Greek words phago-, meaning "devouring," and -cyte, meaning "cell."
3. Phagocytic cells in the immune system eat foreign invaders such as bacteria and remove cell debris.
4. Some single-celled organisms, like amoebas, use phagocytosis to consume nutrients.

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