What is Endosymbiosis Theory?

Brittany Stork, Danielle Weber, Christianlly Cena
  • Author
    Brittany Stork

    Brittany taught high school mathematics for two years. They have a B.S. in Biological Sciences and Secondary Mathematics Education from the University of Nebraska-Lincoln and a Ph.D. in Cellular and Molecular Biology from Baylor College of Medicine. They tutored student-athletes at University of Nebraska-Lincoln for 5 years in various math and science classes. Brittany has served as a TA for various undergraduate and graduate level biology classes. They also are a CLRA Level II certified tutor.

  • Instructor
    Danielle Weber

    Danielle teaches high school science and has an master's degree in science education.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master's degree in Physics and is currently pursuing his doctorate degree.

Learn what endosymbiosis theory is through the endosymbiosis definition and examples, and discover the evidence that supports the endosymbiotic theory. Updated: 02/24/2022

Cells: Overview and Types

All living things, from the mighty blue whale swimming in the ocean to a bacterium living in the soil, share one thing in common: the cell. Cells can be thought of as the basic building blocks of life and they make up all living organisms. Each cell contains all the necessary information, proteins, and structures necessary to keep it alive. Even though cells contain all of the equipment, they must still rely on their environment for access to nutrients.

In the human body alone, there are over 200 different kinds of specialized cells. That means there are millions of specialized cells across all life on Earth. Even with this vast number of cell types, all cells fall into one of two overarching categories: prokaryotic or eukaryotic.

Prokaryotic cells are characterized by their lack of membrane-bound organelles, including nuclei. Additionally, all prokaryotic cells have a cell wall. The cell wall protects the interior of the cell, adds more structure to the cell, and prevents the cell from bursting after ingesting water. Prokaryotic cells generally have some external feature that aids in movement, including a flagellum, cilia, or pseudopod. All prokaryotic organisms, which include bacteria and archaea, are single-celled. Finally, prokaryotes represent the oldest life form, with protobionts being their direct ancestor.


Prokaryotic cells have a cell wall but lack membrane-bound organelles and a defined nucleus.

Image of a general prokaryotic cell


Conversely, eukaryotic cells are characterized by the presence of membrane-bound organelles, including nuclei. Arguably, there is more diversity amongst eukaryotic cells. Some eukaryotic cells have cell walls, including plant cells, while others do not, like the animal cell. Similar to prokaryotic cells, most eukaryotic cells have an appendage that aids in locomotion, including higher-order plant cells. Eukaryotic organisms can be single- or multi-cellular. As eukaryotes evolved from prokaryotes, they are not as old as the prokaryotes.


Plant cells are eukaryotic cells that have a cell wall, membrane-bound organelles, including mitochondria and chloroplasts, and a defined nucleus.

Image of a general plant cell



Animals cells are eukaryotic cells that have membrane-bound organelles, including mitochondria, and a defined nucleus..

Image of a general animal cell.


There are many more characteristics that could be talked about regarding similarities and differences between prokaryotic and eukaryotic cells, but the ones that have been highlighted are the most pertinent here. Specifically, the difference in the presence of membrane-bound organelles will be important in understanding the remainder of the lesson.

Evolutionary Chimera

Several mythological creatures are combinations of animals. For example, the griffin is a combination of a lion and an eagle, while the jackalope is a combination of a jackrabbit and an antelope. The most notorious combination of animals is that of a lion, serpent and goat.

While this may seem like an odd combination, in Greek mythology, it was known as the Chimera. This compilation of animals had powers and traits of each of the animals involved. Now, we often use the term 'chimera' to represent an organism that is a combination of others. In our case, we will look at how the cells that make up our bodies are chimeras of older, simpler cells.

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What Is Endosymbiosis?

Many students who have taken a biology class have been asked to identify whether a relationship is mutualistic, commensalistic, or parasitic. These relationships are shared by a species that lives close to, on, or in another species. Endosymbiosis is used to describe the symbiosis where one species lives inside another species. The species that lives inside the other species is referred to as the endosymbiont, while the species containing the endosymbiont is referred to as the host. The definition of endosymbiosis perhaps paints this arrangement in a negative way, as is the case with parasites, but this is not always the case. Humans have a wide variety of bacteria living in the gut which help with digestion, maintenance of a healthy immune system, and other beneficial functions in exchange for access to nutrients. This example highlights a mutualistic endosymbiotic relationship. In this example, the gut bacteria are the endosymbionts and the humans are the hosts.

Recall from the beginning of the lesson that eukaryotic cells have membrane-bound organelles and prokaryotic cells do not. Popular theory suggests that two of these organelles, the mitochondria and the plastids, which include chloroplasts, evolved from once free-living prokaryotic cells. At some point in evolutionary history, aerobic and photosynthetic prokaryotes were endosymbionts in a proto-eukaryotic cell host. It is believed that this was a mutualistic relationship as the endosymbionts were offered more protection from the environment and the proto-eukaryotic cell had increased access to energy.

What is the Endosymbiotic Theory?

The theory concerning the origin of mitochondria and chloroplasts, along with other plastids, is named the endosymbiotic theory. The endosymbiotic theory states that mitochondria and chloroplasts were once free-living prokaryotic cells that formed an endosymbiotic relationship with a proto-eukaryotic or eukaryotic host cell. Some experts believe that the aerobic cell was the first endosymbiont because all eukaryotic cells have mitochondria, while only some have chloroplasts. How did this endosymbiotic relationship come about? To understand this, it is important to note that not all prokaryotic cells had aerobic or photosynthetic abilities. The ones that did likely reproduced more quickly and successfully. Instead of trying to compete with these advantaged cells, other cells developed endocytosis. So, instead of trying to beat them, they could eat them. The endosymbiotic theory diagram often depicts the host cell engulfing the soon-to-be endosymbionts.


The endosymbiotic theory diagram depicts aerobic and photosynthetic prokaryotes being engulfed by another cell and eventually evolving into mitochondria and chloroplasts.

The endosymbiotic theory diagram.


Cells

Eukaryotes are complex, can be multicellular, and can change shape.
Eukaryote

Before we can look at how current cells evolved from ancient cells, we first need to talk a little bit about cells themselves. Cells are the basic unit of structure and function for all living things. While these cells may vary greatly between and within organisms, they all have the same basic properties. There are two basic types of cells: prokaryotic and eukaryotic.

Prokaryotic cells are simple cells with no membrane-bound organelles. They evolved from protobionts and are the oldest form of life on Earth. They do not have structural components that allow them to change shape, and they are always single-celled.

Eukaryotic cells are complex cells with membrane-bound organelles and a nucleus. They evolved from prokaryotes and, therefore, haven't been around quite as long as prokaryotic cells. They have structural components that allow them to change shape, and they can either be unicellular or multicellular.

Eukaryotic cells are the cells that make up our bodies. They also have many cell parts. While we aren't going to need to know about all of these parts to understand the evolution of cells, there are a few essential cell parts to cover.

First, we know that they have a nucleus. The nucleus is basically the brain of the cell; it tells other cell parts what to do in addition to containing the genetic material of the cell. The mitochondria are often called the powerhouse of the cell because they are responsible for making cellular energy through aerobic respiration. In plant cells, there are structures known as plastids. The plastid that you are probably most familiar with is the chloroplast. This structure is where photosynthesis - the conversion of light energy into chemical energy - takes place.

Endosymbiosis

Now that we know about both prokaryotic and eukaryotic cells, let's look at the endosymbiosis theory. This theory suggests that mitochondria and plastids in eukaryotic cells were once independent prokaryotic cells. Basically, this means that once upon a time, there were three prokaryotic cells: one that was capable of aerobic respiration and converting energy, one that was capable of photosynthesis and one that was incapable of doing either of these processes. The lazy cell that was incapable of doing respiration and photosynthesis engulfed - or ate - the other cells.

Cells living inside a host cell are called endosymbionts.
Endosymbionts

Now, this may have been intentional or accidental. However, when the lazy cell engulfed the respiration cell, it was then able to make useful energy. When it engulfed the photosynthesis cell, it was then able to convert energy from the sun into stored chemical energy.

Let's go back to the term endosymbiosis to understand the role of each of these cells. You may remember that symbiosis is a close relationship between two different organisms. In this case, we are looking at the relationships between several ancient cells. The cell that ate the other cells is called the host, while the other two - the respiration and photosynthesis cells - are endosymbionts because they live within the host. It may sound like the only one benefiting in this situation is the cell that ate the other cells, but it could be that all cells are benefiting. While the lazy cell now has a source of energy and a way to convert that energy, the other cells have a safe place to live and grow.

Because all eukaryotic cells have mitochondria but not all have chloroplasts, it is thought that the host cell first engulfed the respiration cell. Quite a while later, one of these cells that contained the respiration cell engulfed the photosynthesis cell.

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Video Transcript

Evolutionary Chimera

Several mythological creatures are combinations of animals. For example, the griffin is a combination of a lion and an eagle, while the jackalope is a combination of a jackrabbit and an antelope. The most notorious combination of animals is that of a lion, serpent and goat.

While this may seem like an odd combination, in Greek mythology, it was known as the Chimera. This compilation of animals had powers and traits of each of the animals involved. Now, we often use the term 'chimera' to represent an organism that is a combination of others. In our case, we will look at how the cells that make up our bodies are chimeras of older, simpler cells.

Cells

Eukaryotes are complex, can be multicellular, and can change shape.
Eukaryote

Before we can look at how current cells evolved from ancient cells, we first need to talk a little bit about cells themselves. Cells are the basic unit of structure and function for all living things. While these cells may vary greatly between and within organisms, they all have the same basic properties. There are two basic types of cells: prokaryotic and eukaryotic.

Prokaryotic cells are simple cells with no membrane-bound organelles. They evolved from protobionts and are the oldest form of life on Earth. They do not have structural components that allow them to change shape, and they are always single-celled.

Eukaryotic cells are complex cells with membrane-bound organelles and a nucleus. They evolved from prokaryotes and, therefore, haven't been around quite as long as prokaryotic cells. They have structural components that allow them to change shape, and they can either be unicellular or multicellular.

Eukaryotic cells are the cells that make up our bodies. They also have many cell parts. While we aren't going to need to know about all of these parts to understand the evolution of cells, there are a few essential cell parts to cover.

First, we know that they have a nucleus. The nucleus is basically the brain of the cell; it tells other cell parts what to do in addition to containing the genetic material of the cell. The mitochondria are often called the powerhouse of the cell because they are responsible for making cellular energy through aerobic respiration. In plant cells, there are structures known as plastids. The plastid that you are probably most familiar with is the chloroplast. This structure is where photosynthesis - the conversion of light energy into chemical energy - takes place.

Endosymbiosis

Now that we know about both prokaryotic and eukaryotic cells, let's look at the endosymbiosis theory. This theory suggests that mitochondria and plastids in eukaryotic cells were once independent prokaryotic cells. Basically, this means that once upon a time, there were three prokaryotic cells: one that was capable of aerobic respiration and converting energy, one that was capable of photosynthesis and one that was incapable of doing either of these processes. The lazy cell that was incapable of doing respiration and photosynthesis engulfed - or ate - the other cells.

Cells living inside a host cell are called endosymbionts.
Endosymbionts

Now, this may have been intentional or accidental. However, when the lazy cell engulfed the respiration cell, it was then able to make useful energy. When it engulfed the photosynthesis cell, it was then able to convert energy from the sun into stored chemical energy.

Let's go back to the term endosymbiosis to understand the role of each of these cells. You may remember that symbiosis is a close relationship between two different organisms. In this case, we are looking at the relationships between several ancient cells. The cell that ate the other cells is called the host, while the other two - the respiration and photosynthesis cells - are endosymbionts because they live within the host. It may sound like the only one benefiting in this situation is the cell that ate the other cells, but it could be that all cells are benefiting. While the lazy cell now has a source of energy and a way to convert that energy, the other cells have a safe place to live and grow.

Because all eukaryotic cells have mitochondria but not all have chloroplasts, it is thought that the host cell first engulfed the respiration cell. Quite a while later, one of these cells that contained the respiration cell engulfed the photosynthesis cell.

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  • Activities
  • FAQs

Endosymbiosis Theory True or False Activity

In this activity, you will check your knowledge regarding the evolution of cells, as presented from the lesson.

Directions

Determine whether the following statements are true or false. To do this, print or copy this page on a blank paper and underline or circle the answer.

1. Prokaryotic cells are complex cells in which the genetic material is organized into membrane-bound nuclei.

True | False

2. The endosymbiotic theory states that some of the organelles in prokaryotic cells were once from eukaryotic organisms.

True | False

3. The mitochondria contain genetic material separate from that of the host and are responsible for the conversion of food to usable energy.

True | False

4. Both the mitochondria and chloroplast have enzymes and transport systems that are very similar to those found in the membranes of current prokaryotic cells.

True | False

5. The tissue is the basic structural, functional, and biological unit of all known organisms.

True | False

6. A photosynthetic cell is capable of converting radiant energy into stored chemical energy.

True | False

7. It was believed that the host cell first engulfed the respiration cell and then the photosynthetic cell.

True | False

8. Eukaryotes refer to organisms that live within the body or cells of another organism.

True | False

9. Prokaryotes refer to organisms whose cells are characterized by the absence of a nucleus or any other membrane-bound organelles.

True | False

10. Endosymbiosis is a phenomenon whereby a single-celled organism resides within another cell as part of a parasitic relationship.

True | False


Answer Key

1. False, because the correct statement is, Eukaryotic cells are complex cells in which the genetic material is organized into membrane-bound nuclei.

2. False, because the correct statement is, The endosymbiotic theory states that some of the organelles in eukaryotic cells were once from prokaryotic organisms.

3. True

4. True

5. False, because the correct statement is, A cell is the basic structural, functional, and biological unit of all known organisms.

6. True

7. True

8. False, because the correct statement is, Endosymbionts refer to organisms that live within the body or cells of another organism.

9. True

10. False, because the correct statement is, Endosymbiosis is a phenomenon whereby a single-celled organism resides within another cell as part of a mutually-beneficial relationship.

What is the endosymbiotic theory and why is it important?

The endosymbiotic theory states that many years ago, respiratory and photosynthetic prokaryotic cells were engulfed by eukaryotic cells. The endosymbiotic theory is important because it represents an event that allowed eukaryotic cells to make their own energy.

What is an example of endosymbiotic theory?

An example of the endosymbiotic theory is when a eukaryotic cell engulfed a respiratory prokaryotic cell and instead of digesting it, the eukaryotic cell kept it and incorporated it into its life cycle. These respiratory prokaryotic cells evolved into the present-day mitochondria in eukaryotic cells.

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