Endosymbiotic theory: A Theory of how Eukaryotes were born Turner College & Career High School  2018

It’s good to be friendly with your neighbors, right? Individuals and communities do better if they help each other out. Cooperation isn’t just important for humans; without a bit of interaction with neighbors, life as we know it would not exist.

The earliest living neighbors on our planet were all single-celled, prokaryotes. Some of the neighboring prokaryotes joined and began living together as one organism, one inside the other. This symbiotic relationship was so successful that it led to the evolution of many of the life forms on our planet, including humans.

How did the eukaryotes become so complicated How did the eukaryotes become so complicated? And where did these battery-like organelles come from?

Cells that Changed the Earth Some of the oldest cells on Earth are single-cell organisms called bacteria. Fossil records indicate that bacteria once covered young Earth and began making their own food using carbon dioxide in the atmosphere and energy they harvested from the sun. This process (called photosynthesis) produced enough oxygen to change Earth's atmosphere. Soon afterward, new oxygen-breathing life forms came onto the scene, the stage was set for some amazing things to happen.

Bacteria are single-celled organisms with a circular DNA molecule and no organelles. Regina List-Grace

Endosymbiotic theory Eukaryotic cells may have evolved when multiple cells joined together into one. There is compelling evidence that mitochondria and chloroplasts were once primitive bacterial cells. This evidence is described in the Endosymbiotic theory. Symbiosis occurs when two different species benefit from living and working together.

Endosymbiotic theory During the 1950s and 60s, scientists found that mitochondria inside plant cells had their own DNA. It was different from the rest of the plant cell DNA. When scientists looked closer at the genes in the mitochondrial DNA, they found that the genes were more like those from prokaryotes. This tells us that this organelle is closely related to prokaryotes. The green chloroplasts, which are a critical part of plant cells, evolved from an entirely different organism than the plant cell. The chloroplast is thought to have evolved from a cyanobacterial cell that managed to survive the cell's defenses.

SOOO… How did it happen? Endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a symbiotic relationship. The larger unicellular organism engulfed other, smaller, single-celled organisms. At that point, they were no longer independent colonial organisms but instead were one large cell. When the larger cell that had engulfed the smaller cells went to divide, copies of the smaller prokaryotes inside were made and passed down to the daughter cells.

SOOO… How did it happen? Eventually, the smaller prokaryotes that had been engulfed adapted and evolved into some of the organelles we know of today in eukaryotic cells like the mitochondria and chloroplasts. Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell. Other organelles eventually arose from these first organelles, including the nucleus where the DNA in a eukaryote is housed, the endoplasmic reticulum and the Golgi Apparatus. In the modern eukaryotic cell, these parts are known as membrane-bound organelles.

How did the eukaryotes become so complicated How did the eukaryotes become so complicated? And where did these battery-like organelles come from?

A A prokaryote ingested some aerobic bacteria. These bacteria were protected by the prokaryote and produced energy for it. B Over a long period of time, these bacteria became mitochondria, and could no long live on their own. C Some prokaryotes ingested some cyanobacteria which contained photosynthetic pigments. D Over time, the cyanobacteria became chloroplasts and could no longer live on their own.

Looking at it another way …

Endosymbiotic theory A scientist named Lynn Margulis put all of this information together and published it in 1967. Her paper was called On the Origin of Mitosing Cells. Eukaryotic cells undergo mitosis. Today scientists know her paper is very important, but it took many years before they accepted her theory.

Supporting evidence Similarities between mitochondria, chloroplasts, and prokaryotes … Circular DNA Ribosomes Binary fission

REVIEW You just learned about the Endosymbiotic theory. This theory explains how organelles inside eukaryotic cells are descended from ancient … Unicellular eukaryotic organisms. Unicellular prokaryotic organisms. Multicellular eukaryotic organisms. None of the above is correct.

REVIEW In the beginning, endosymbiosis was considered to be a controversial theory. What evidence was presented to support this theory? Mitochondria are similar in size and function to prokaryotic cells. Chloroplasts are similar in size and function to prokaryotic cells. The presence of ribosomes. All of the above are correct.