02.03 Early Cells Honors

Describe how endosymbiosis worked.

According to the endosymbiotic theory, a large prokaryotic cell ingested another prokaryotic cell such as a bacterium. The bacterium was not digested by the cell but stayed intact inside its new host. The ingested prokaryotic bacterium cell performed aerobic respiration, a way to extract energy from organic molecules.The energy shared by the ingested bacterium gave the host cell an advantage over the other primitive cells. The ingested bacterium gained an advantage as well. It lives in a nutrient-rich environment inside its host cell. This bacterium is believed to be the ancestor of the mitochondria found in modern-day eukaryotic cells.

DNA

As DNA sequencing projects decode more genomes, we are learning that eukaryotic cells have a lot in common with a group of prokaryotes called Archaea. These prokaryotes went completely undiscovered until 1977 because they live in such extreme environments as Antarctica, the Dead Sea, deep-sea vents, and hot springs. The relationship between the DNA of the eukaryotic "host" cells and these prokaryotes gives additional support to the endosymbiotic theory of eukaryote evolution.

DNA

Because of what we know from the endosymbiotic theory, scientists have been able to map out evolutionary relationships between various organisms. Mitochondrial DNA (mtDNA) has a unique pattern of inheritance. It is passed down directly from mother to child, and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA provides important clues about evolutionary history. Differences in mtDNA are examined to estimate how closely related one species is to another.

DNA

Biologists have studied the DNA in mitochondria to discover if and how they are related to prokaryotic bacteria. These studies have shown that mitochondria are most closely related to the bacteria called Rickettsia Prowazekii.

What did scientists believe was the early ancestors to our modern day eukaryotes?

Eukaryotic cells.

Describe Professor Kwang Jeon' experiment and what he observed. What is one more recent piece of evidence related to DNA that scientists have found?

In 1987, Professor Kwang Jeon of the University of Tennessee observed a number of amoebas (single-celled eukaryotes) that had ingested bacteria cells. The bacteria were killing off most of the amoebas, but a small number of the amoeba survived and returned to their normal modes. However, the invading bacteria were still present within surviving amoebas. Jeon continued to experiment with the amoebas and discovered that these surviving amoebas, and their offspring, could not survive without the bacteria. Jeon's discovery and experimentation proves it is possible for a cell to become dependent on an invading organism.

According to the endosymbiotic theory, which organelles were believed to be once simple prokaryotic cells?

Mitochondria and Chloroplast

Describe how endosymbiosis worked.

Most of the membrane-bound organelles inside eukaryotic cells seem to have originated from deep folds in the cell membrane. However, the mitochondria and chloroplasts have distinct properties that make them different from the other organelles. These two organelles have characteristics similar to those of single-celled prokaryotic cells.

What is an example of an endosymbiotic relationship that is very important to us as humans?

One example of endosymbiosis is bacteria that live in the digestive track of humans and other animals. These bacteria assist in digestion and other functions of the host's digestive track, while benefiting from the protection and nutrients provided by its surroundings.

Describe how endosymbiosis worked.

Over time, both the host cell and the bacteria cell it ingested developed a mutually satisfying or beneficial existence. Both entities lost their ability to function without the other. The endosymbiotic theory says that over time the internal cells evolved to become organelles, mitochondria, and chloroplasts within the eukaryotic cells.

2. They contain their own ribosomes.

Ribosomes are a small organelle that manufactures protein molecules. The ribosomes found inside chloroplasts and mitochondria are smaller than those found in the cytoplasm of the eukaryotic cell but similar in size to those found inside prokaryotic cells.

Describe how endosymbiosis worked.

Some of the oldest cells on Earth were single-cell organisms called bacteria, a type of prokaryotic cell. Fossil records show that prokaryotic bacteria cells covered the early Earth. Some of these bacteria adapted to make their own food using carbon dioxide from the atmosphere and energy from the sun. This process, called photosynthesis, produced oxygen that was released into the Earth's atmosphere. As the oxygen levels in the atmosphere increased, other organisms adapted and used that oxygen.

Endosymbiosis

Symbiosis in which one of the symbiotic organisms lives inside the other.

What is a symbiotic relationship?

Symbiosis is a close relationship between two species in which at least one species benefits. For the other species, the relationship may be positive, negative, or neutral. There are three basic types of symbiosis: mutualism, commensalism, and parasitism.

1. They contain their own DNA.

The DNA found in mitochondria and chloroplasts are in a circular formation. This is similar to how the DNA is arranged in prokaryotic cells but different than the linear DNA found in the chromosomes of a eukaryote's nucleus.

3. They are responsible for their own reproduction.

The duplication of other organelles in a eukaryotic cell is controlled by the cell's nucleus, but mitochondria and chloroplasts are responsible for their own reproduction. Mitochondria and chloroplasts reproduce by pinching in half, the same process used by bacteria. Every new mitochondrion must be produced from a parent mitochondrion in this way. If a cell's mitochondria are removed, it cannot build new ones.

What does the endosymbiotic theory hypothesize?

The endosymbiotic theory hypothesizes that the ancestors of modern-day eukaryotic cells were actually two or more prokaryotic cells in a symbiotic relationship. When one organism lives inside the other, it is called endosymbiosis. The relationship is symbiotic because the two organisms live together in a close relationship, and the prefix endo- indicates that one organism lives inside the other. One example of endosymbiosis is bacteria that live in the digestive track of humans and other animals. These bacteria assist in digestion and other functions of the host's digestive track, while benefiting from the protection and nutrients provided by its surroundings.

Describe how endosymbiosis worked.

The evidence supporting the endosymbiotic theory applies not just to mitochondria but also to the chloroplasts found in plant cells. Evidence suggests chloroplast organelles were once free-living bacteria that converted energy from sunlight in the process called photosynthesis.

Which two organelles found in eukaryotic cells have features in common with whole prokaryotic cells?

Two organelles found inside eukaryotic cells, mitochondria and chloroplasts, have several features in common with whole prokaryotic cells.

What does the prefix endo indicate?

When one organism lives inside the other, it is called endosymbiosis. The relationship is symbiotic because the two organisms live together in a close relationship, and the prefix endo- indicates that one organism lives inside the other.