Ever wonder why some cells are very simple and others are complex with countless organelles to remember? We will look at how simple cells evolved to become more complex cells – as well as evidence to support this.
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.
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.
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.
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.
Evidence for Endosymbiosis
You may be wondering, at this point in time, why it is that we think endosymbiosis explains the presence of certain organelles in eukaryotic cells. Let’s go back to the cell parts that we talked about earlier. Remember that mitochondria are responsible for cellular respiration and converting chemical energy into cellular energy – and that chloroplasts are responsible for photosynthesis.
The prokaryotic cell that was able to perform aerobic respiration is the ancestor of the mitochondria, and the cell that was able to perform photosynthesis is the ancestor of the chloroplast.
Evidence that leads us to believe this is vast, but we’ll look at a few important aspects.
First, both the mitochondria and chloroplast have enzymes and transport systems in their membranes that are very similar to those found in the membranes of current prokaryotic cells. Second, both organelles replicate in the same way that some prokaryotic cells do. Third, they both have their own DNA that is circular – just like the DNA found in bacteria, which are prokaryotic. Fourth, they are able to replicate this circular DNA independently of other cell parts.
Lastly, they exhibit similar antibiotic resistance as seen in some prokaryotes.Now, much of this may seem abstract, but it allows scientists to see a direct relationship between both organelles and prokaryotic cells, indicating that the mitochondria and plastids – including the chloroplast – came from prokaryotic origins.
Eukaryotic cells can be compared to mythical chimeras because of the origin scientists postulate for some of their organelles. Remember that the classic Chimera was an animal that was part goat, lion and serpent.
Scientists believe that the eukaryotic cell is made of several other types of cells, making it similar to this hodge-podge of organisms.Remember that prokaryotic cells are simple and do not contain membrane-bound organelles and that eukaryotic cells are more complex and do contain membrane-bound organelles, as well as a nucleus.The endosymbiosis theory explains how eukaryotic cells may have evolved from prokaryotic cells.
Symbiosis is a close relationship between two different organisms. We saw that, over time, one ancient cell became the host for other ancient cells that were capable of specific activities. This host cell engulfed – or ate – a prokaryotic cell capable of aerobic respiration. This is where the mitochondria originated. Later, a host cell engulfed a prokaryotic cell capable of photosynthesis.
This is where the chloroplast and other plastids originated.There is much evidence – including structures, genetic information and biological processes – that supports the relationship between these organelles and their prokaryotic ancestors.
At the end of this lesson, you will be able to explain the history of and the evidence that supports the endosymbiosis theory.