Endosymbiotic Theory - Online Tutor, Practice Problems & Exam Prep

The endosymbiotic theory posits that mitochondria and chloroplasts in eukaryotic cells originated from independent aerobic and photosynthetic bacteria, respectively. This symbiotic relationship began over 1.5 billion years ago when an anaerobic host cell engulfed these bacteria. Over time, these bacteria lost many genes, evolving into organelles. Supporting evidence includes similarities in DNA structure, ribosomes, and replication methods between these organelles and prokaryotes, as well as their double-membrane structure, indicating their engulfment. This theory highlights the evolutionary significance of these organelles in complex life forms.

Endosymbiotic Theory

In this video, we're going to introduce the endosymbiotic theory. The endosymbiotic theory is a theory that suggests that today's mitochondria and chloroplast organelles that we find inside of eukaryotic cells were once independently living bacteria. The endosymbiotic theory suggests that a really long time ago, about 1,500,000,000 years ago, there was an aerobic bacterium or a bacteria that uses oxygen in its metabolism. This aerobic bacterium was engulfed by an anaerobic host cell, or a host cell that does not use oxygen in its metabolism. When this aerobic bacterium was engulfed by the anaerobic host cell, it created a symbiotic relationship, or a beneficial relationship between the two organisms, since each of them had abilities that the other did not have.

Over a really long period of time, again about 1,500,000,000 years, the aerobic bacterium that was engulfed by the host cell lost many of its genes and abilities, and it developed and evolved into today's mitochondria. The theory of the endosymbiotic theory suggests that the mitochondria, it was once an independently living aerobic bacteria. Similarly, the endosymbiotic theory suggests that a photosynthetic cyanobacterium, or a bacterium that's capable of performing photosynthesis was also engulfed by a host cell, and, over a long period of time, it evolved into today's chloroplast.

Let's take a look at our image down below to get a better understanding of this endosymbiotic theory. Notice over here on the far left, we're showing you in blue a host cell, and this host cell is an anaerobic host cell, which means that it does not use oxygen in its metabolism. Here, we have an aerobic bacterium or a bacteria that does use oxygen in its metabolism. This aerobic bacterium is living separately and independently from the host cell. Eventually, this aerobic bacterium was engulfed by the host cell. Over a long period of time, it eventually developed into today's mitochondria that we find inside of animal cells. This is what allowed for complex living organisms to arrive, such as this animal and this Rafiki-looking monkey with a nice smile.

Similarly, a long time ago, there was a cyanobacterium, a photosynthetic cyanobacterium that was capable of performing photosynthesis, and this cyanobacterium was also engulfed. When it was engulfed over time, it developed into today's chloroplast. The host cell that had both mitochondria and chloroplast ended up developing into today's plant cell. You could see that plant cells today, they have both chloroplasts and mitochondria, and that developed into today's plant. This is once again a theory, and theories have lots of supporting evidence.

Some of the supporting evidence that supports the endosymbiotic theory includes the vast amount of similarities that there are between mitochondria and chloroplasts and prokaryotes, like the aerobic bacterium and the photosynthetic cyanobacterium. The similarities between mitochondria, chloroplasts, and prokaryotes include that they both have or do, they both have small circular DNA. Mitochondria, chloroplasts, and prokaryotes all have small 70S ribosomes, and they all replicate via a process called binary fission. This is a lot of similarities that mitochondria and chloroplasts have with prokaryotes, suggesting that supporting evidence to suggest that maybe mitochondria and chloroplasts were once independently living prokaryotes or bacteria.

In addition to these similarities, mitochondria and chloroplasts, they both have 2 membranes. They both have an outer membrane and they both have an inner membrane as well. Having an outer membrane and an inner membrane is consistent with this idea of being engulfed or engulfment, where a host cell engulfs them, and that makes them acquire a second membrane. All of this here, these similarities and the fact that both mitochondria and Chloroplasts have 2 membranes, is all consistent supporting evidence of the endosymbiotic theory. This concludes our introduction to the endosymbiotic theory and how it suggests that today's mitochondria and chloroplasts were once independently living bacteria.

We'll be able to get some practice applying the concepts that we've learned here in our next few videos. I'll see you guys in our next one.

Endosymbiotic theory is supported by the discovery of non-nuclear DNA in the ______ and ________ organelles.

• A. Golgi apparatus and lysosomes.
• B. Mitochondria and lysosomes.
• C. Chloroplast and Golgi apparatus.
• D. Chloroplast and mitochondria.

According to the endosymbiotic theory, which of the following is likely the ancestor of the mitochondria?

• A. Aerobic eukaryotes.
• B. Aerobic bacteria.
• C. Anaerobic bacteria.
• D. Cyanobacteria.
• E. Chloroplasts.

Here’s what students ask on this topic:

What is the endosymbiotic theory and how does it explain the origin of mitochondria and chloroplasts?

The endosymbiotic theory posits that mitochondria and chloroplasts in eukaryotic cells originated from independent aerobic and photosynthetic bacteria, respectively. Around 1.5 billion years ago, an anaerobic host cell engulfed these bacteria, forming a symbiotic relationship. Over time, the engulfed bacteria lost many of their genes and evolved into the organelles we see today. Mitochondria evolved from aerobic bacteria, while chloroplasts evolved from photosynthetic cyanobacteria. Supporting evidence includes similarities in DNA structure, ribosomes, and replication methods between these organelles and prokaryotes, as well as their double-membrane structure, indicating their engulfment.

What evidence supports the endosymbiotic theory?

Several lines of evidence support the endosymbiotic theory. First, mitochondria and chloroplasts have their own small circular DNA, similar to prokaryotic DNA. Second, they possess small 70S ribosomes, akin to those found in bacteria. Third, both organelles replicate via binary fission, a method used by prokaryotes. Additionally, mitochondria and chloroplasts have double membranes, consistent with the engulfment process. These similarities suggest that mitochondria and chloroplasts were once independently living bacteria that became integral parts of eukaryotic cells through a symbiotic relationship.

How did the engulfment of aerobic bacteria and cyanobacteria lead to the evolution of mitochondria and chloroplasts?

The engulfment of aerobic bacteria and cyanobacteria by an anaerobic host cell led to a symbiotic relationship where each organism benefited from the other's capabilities. The aerobic bacteria, which used oxygen for metabolism, provided the host cell with efficient energy production, eventually evolving into mitochondria. Similarly, the cyanobacteria, capable of photosynthesis, provided the host cell with the ability to convert sunlight into energy, evolving into chloroplasts. Over time, these engulfed bacteria lost many of their genes and functions, becoming specialized organelles within the host cell, contributing to the complexity of eukaryotic life.

Why do mitochondria and chloroplasts have double membranes?

Mitochondria and chloroplasts have double membranes as a result of the engulfment process described by the endosymbiotic theory. When an anaerobic host cell engulfed the aerobic bacteria and cyanobacteria, the engulfed bacteria retained their original membrane, and an additional membrane was formed from the host cell's membrane. This double-membrane structure is consistent with the idea of engulfment and serves as evidence supporting the endosymbiotic origin of these organelles. The inner membrane is derived from the original bacterial membrane, while the outer membrane comes from the host cell's engulfing vesicle.

What are the similarities between mitochondria, chloroplasts, and prokaryotes?

Mitochondria, chloroplasts, and prokaryotes share several key similarities. First, they all have small circular DNA, unlike the linear DNA found in the nuclei of eukaryotic cells. Second, they possess small 70S ribosomes, which are characteristic of prokaryotes. Third, they replicate through binary fission, a process used by bacteria. Additionally, both mitochondria and chloroplasts have double membranes, which is consistent with the engulfment process described by the endosymbiotic theory. These similarities provide strong evidence that mitochondria and chloroplasts originated from independent prokaryotic organisms.