Endosymbiotic Theory - Online Tutor, Practice Problems & Exam Prep

The endosymbiotic theory posits that mitochondria and chloroplasts originated from independent bacteria engulfed by eukaryotic cells. Mitochondria evolved from aerobic bacteria, providing ATP through oxidative metabolism, while chloroplasts arose from photosynthetic cyanobacteria, facilitating photosynthesis. Mitochondria feature a double membrane, cristae, and their own circular DNA, crucial for cellular respiration. Chloroplasts also possess a double membrane, with thylakoids stacked in grana, where light reactions occur, converting solar energy into glucose and oxygen. Understanding these organelles is essential for grasping energy production in cells.

Endosymbiotic Theory

In this video, we're going to do a quick recap on the endosymbiotic theory. So, recall that the endosymbiotic theory explains the development of complex eukaryotic organisms such as ourselves, and it suggests that mitochondria and chloroplast organelles were once independently living bacteria. It also suggests that about 1,500,000,000 years ago, there was an aerobic bacterium or a bacteria that uses oxygen in its metabolism that was engulfed by an anaerobic host cell or a host cell that does not use oxygen in its metabolism. This created cell was provided with aerobics by the host cell and the host cell was provided with aerobic metabolism by the bacterium. Over time, this aerobic bacterium would have lost many of its genes and abilities, including the ability to survive on its own, and it would have developed into today's mitochondria. In a similar fashion, a photosynthetic cyanobacteria would have also been engulfed by a host cell and over time, it would have evolved into today's chloroplasts.

Let's take a look at our example below. Notice on the far left, we have our host cell, which again is anaerobic and does not use oxygen. Notice that our host cell already has a nucleus, which means it's already a eukaryotic cell. The endosymbiotic theory does not suggest the creation of a eukaryotic cell; it suggests the development of a eukaryotic cell. This anaerobic host cell would have engulfed an aerobic bacterium, and this aerobic bacterium would have developed into today's mitochondria. In a similar fashion, the cyanobacterium would have been engulfed and developed into the chloroplast. Host cells that had both organelles would have developed into plant cells and into today's plants. Now, host cells that only engulfed the aerobic bacterium would have only had mitochondria. These host cells would have developed into today's animal cells and into animals, such as this Rafiki-looking monkey up here with some yellow teeth.

There's a lot of supporting evidence to support this theory, and most of it has to do with the fact that mitochondria and chloroplasts resemble prokaryotes. Mitochondria and chloroplasts both have small circular DNA, just like prokaryotes. They have 70S ribosomes, just like prokaryotes, and they replicate via binary fission, just like prokaryotes. Mitochondria and chloroplasts also have a double membrane, which resulted from being engulfed. In our next video, we're going to do a quick recap on the mitochondria. So, I'll see you guys in that video.

Mitochondria

So the mitochondria and the processes that occur inside of it are a major focus of biochemistry, and later in our course, we're going to talk a lot about the mitochondria and all of those processes. But in this video, we're just going to do a quick review. So, that being said, recall that the primary function of mitochondria is to produce ATP, or the high-energy molecule for a cell. And the bulk of this ATP is produced via oxidative energy metabolism or reactions that require the presence of oxygen. Now most textbooks tend to show the shape of a mitochondria as bean-shaped, similar to this one here. But what most people don't realize is that the mitochondrial shape varies greatly from cell to cell. Also, a separate fact to remember from our previous videos is that mitochondria have their own small circular DNA that is separate and independent from the nuclear DNA. Also, mitochondria have a double membrane. So they have an outer as well as an inner membrane. And the inner membrane is highly folded into structures called cristae that increase the surface area and lead to more energy production. Also, mitochondria are the location of major processes of cellular respiration, which include the citric acid cycle and the electron transport chain. So down below in our example is a chemical reaction that's probably burned into your guys' brains by now by how many times you've seen it in your previous bio courses, and that's the overall chemical reaction of cellular respiration. So on the reactant side, we have glucose as well as oxygen gas, and cellular respiration converts these reactants into the products of carbon dioxide, water, and energy in the form of ATP. And so, to review the typical structure of a mitochondria, recall that it has a double membrane. So it has an outer membrane that goes around the perimeter, and then it also has an inner membrane that's interior to that and is highly folded into structures called cristae. And so, between the inner and the outer membrane is a space, and the space is called the intermembrane space, and recall that this is very important to the electron transport chain processes. Now the most interior area of the mitochondria is referred to as the mitochondrial matrix. And this is the location of the citric acid cycle. Now, there's an enzyme that's known as the ATP synthase, and this is the enzyme that produces the bulk of the ATP that's associated with the mitochondria, and really, they're the main reason that mitochondria are associated with ATP. And so from our previous videos, we talked about how mitochondria have their own ribosomes, and these are 70S ribosomes, which are prokaryotic ribosomes. And they also have their own DNA, which is small circular DNA, just like the DNA of prokaryotes, and they have multiple copies of that DNA, which tends to be very common. And so, again, we're going to talk a lot more about mitochondria and cellular respiration later in our course, but for now, this is a good summary as we move forward. And so, in our next video, we're going to talk about the chloroplast. So I'll see you guys in that video.

Chloroplasts

In this video, we're going to brush up on the chloroplast. Recall that the primary function of the chloroplast is to perform photosynthesis. You have learned in your previous biology courses that photosynthesis produces chemical energy from solar energy. Let’s take a look at our example below. We've got the sun here that's providing solar energy, and we also have a plant that's absorbing that solar energy. Recall that plants have chloroplast organelles, and the chloroplast organelles perform photosynthesis for the plant.

Below, we have the overall chemical reaction for photosynthesis, where on the reactant side, we have carbon dioxide, water, as well as sunlight. Photosynthesis converts these three reactants into glucose as well as oxygen gas. This glucose here is what we're referring to as chemical energy.

To review the structure of the chloroplast, recall that it has a double membrane. It has an outer membrane on the perimeter and an inner membrane that's interior to that. The stroma here is referring to the fluid-filled space that fills the internal portion of the chloroplast. Notice that within this stroma, lie these green pancake-looking structures that are individually called thylakoids. Notice these thylakoids are stacked on top of each other. This particular stack has five thylakoids, and each stack of thylakoids is called a granum. There are a bunch of grana in the stroma.

Recall that photosynthesis consists of two major processes and multiple reactions, and these reactions take place in different parts of the chloroplast. Some take place within the thylakoid membranes and others take place within the stroma. We'll talk more details about photosynthesis later in our course, but for now, this is a good summary and I'll see you guys in the practice problem videos.

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

What is the primary purpose of cristae in the mitochondria?

• A. Provide a large surface area for chemical reactions
• B. Prevent the mitochondria from folding onto itself
• C. Protect the mitochondrial DNA
• D. No purpose has been identified yet

Here’s what students ask on this topic:

What is the endosymbiotic theory and what evidence supports it?

The endosymbiotic theory posits that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells. Over time, these bacteria formed a symbiotic relationship with their host cells, evolving into the organelles we see today. Evidence supporting this theory includes the fact that both mitochondria and chloroplasts have their own circular DNA, similar to bacterial DNA. They also have 70S ribosomes, which are characteristic of prokaryotes, and replicate via binary fission, a process used by bacteria. Additionally, both organelles have double membranes, consistent with the engulfing mechanism proposed by the theory.

How did mitochondria evolve according to the endosymbiotic theory?

According to the endosymbiotic theory, mitochondria evolved from an aerobic bacterium that was engulfed by an anaerobic host cell around 1.5 billion years ago. The aerobic bacterium provided the host cell with the ability to perform oxidative metabolism, producing ATP more efficiently. Over time, the engulfed bacterium lost many of its genes and abilities, becoming dependent on the host cell and evolving into the mitochondria. This symbiotic relationship allowed the host cell to thrive in oxygen-rich environments, leading to the development of complex eukaryotic cells.

What are the structural features of mitochondria that support the endosymbiotic theory?

Mitochondria have several structural features that support the endosymbiotic theory. They possess a double membrane, with the inner membrane being highly folded into structures called cristae, which increase the surface area for ATP production. Mitochondria also contain their own small circular DNA, separate from the nuclear DNA, and have 70S ribosomes, similar to those found in prokaryotes. Additionally, mitochondria replicate through binary fission, a process typical of bacteria. These features suggest that mitochondria originated from an ancestral aerobic bacterium that was engulfed by a eukaryotic cell.

How do chloroplasts support the endosymbiotic theory?

Chloroplasts support the endosymbiotic theory through their structural and functional similarities to cyanobacteria. They have a double membrane, with the inner membrane containing thylakoids stacked into grana, where light-dependent reactions of photosynthesis occur. Chloroplasts also possess their own circular DNA and 70S ribosomes, similar to those of prokaryotes. They replicate via binary fission, like bacteria. These features indicate that chloroplasts originated from photosynthetic cyanobacteria that were engulfed by a eukaryotic host cell, forming a symbiotic relationship and evolving into the chloroplasts found in plant cells today.

What is the primary function of mitochondria and how is it related to their structure?

The primary function of mitochondria is to produce ATP, the high-energy molecule used by cells. This function is closely related to their structure. Mitochondria have a double membrane, with the inner membrane folded into cristae, which increase the surface area for ATP production. The inner membrane houses the electron transport chain and ATP synthase, essential components of oxidative phosphorylation. The mitochondrial matrix contains enzymes for the citric acid cycle, which generates electron carriers used in the electron transport chain. These structural features enable efficient ATP production through cellular respiration.