Origins of Cell Compartmentalization Basics
The foundations of the Endosymbiotic Theory and the Origins of Cell Compartmentalization are covered in this portion of the AP Biology curriculum (section 2.11). We’ll begin with a brief introduction to evolution and some key terminology for understanding endosymbionts. Then we’ll look at why compartmentalization is favourable evolutionarily.
Following this brief introduction, we’ll delve right into the hypothesis of symbiogenesis (an endosymbiotic theory) and examine a substantial body of data that supports it. For example, we’ll look at contemporary-day prokaryotes that seem to be developing a nucleus, how chloroplasts and mitochondria preserve circular DNA, and some of the biological macromolecules shared by organelles and modern bacterial cells!
- Multiple lines of evidence support evolution, which is defined as a change in the genetic composition of a population through time.
- Describe the similarities and/or differences between prokaryotic and eukaryotic cell compartmentalization.
- Explain how the roles of endosymbiotic organelles differ from those of their free-living ancestral counterparts.
- Endosymbiosis allows membrane-bound organelles to emerge from free-living prokaryotic cells.
- Interior membrane-bound organelles are rare in prokaryotes, although they do contain internal areas with specific structures and functions.
- Internal membranes of eukaryotic cells divide the cell into specialised areas.
- Endosymbiosis developed membrane-bound organelles from previously free-living prokaryotic cells.
Origins of Cell Compartmentalization Overview
Theory of evolution
When the theory of evolution was initially suggested, it was met with widespread opposition. Not only did many disputes that humans were linked to the Great Apes, but even Charles Darwin struggled to link the complex lives of humans to the earliest single-celled creatures! However, science has progressed significantly since Darwin’s voyage around the globe aboard the H.M.S Beagle.
We can now readily connect the dots between humans and single-celled creatures using modern microscopy, genetic research, and decades of meticulous observation. This overview starts at the beginning of the narrative, when cells begin to become segmented and perform intricate roles. The Endosymbiotic Theory is what this is called, and ideas from it will undoubtedly appear on the AP exam! So stay with us as we go through all you need to know about cell compartmentalization’s origins!
We’ll go through the facts in section 2.11 of the AP Biology Curriculum on the beginnings of cell compartmentalization in this overview. We’ll start with some evolution principles and nomenclature, then go into the advantages of compartments and the complexity they produce rapidly. We’ll look at the Endosymbiotic Theory and see how organelles arose from free-living creatures once we go over these basics. Finally, we’ll go through all the data that backs up the idea.
If you’ve been following along with our AP Biology overviews, we’ll now shift our focus from cell shapes and functions to hypotheses about how cells develop.
Let’s go over some evolutionary terminology and fundamentals before diving into the complexity of the Endosymbiotic Theory. Evolution has been taking place on Earth since the very first cells formed roughly 2.3 billion years ago! Because of limited resources and a changing environment, evolution occurs as a result of natural selection, which permits certain varieties of a species to reproduce while others are destroyed.
To put it another way, only creatures with appropriate adaptations are permitted to live and reproduce in the following generation, resulting in a population of organisms slowly changing over time.
Many people assume that species at the bottom of the food chain are less developed than organisms at the top of the food chain. This is a myth. While certain kinds of life are more complicated than others, all living species have evolved for about the same period of time.
The endosymbiotic hypothesis looks at the very beginnings of this journey, when the earliest cells began to alter owing to natural selection forces.
You must first comprehend the notion of symbiosis in order to completely comprehend the endosymbiotic hypothesis. Symbiosis is a term that describes a partnership between two organisms. There are three types of symbiosis. Both species benefit from a mutualistic connection. One organism benefits while the other remains unaffected in a commensal relationship. The parasite benefits while the other creature is damaged in a parasitic relationship.
Consider this… Cells can only originate from other cells, as far as we know. The genetic molecule DNA is responsible for cell growth and reproduction, and it is constantly inventing new methods to reproduce itself. Cells may have stayed the same way they were when they originally emerged between 1.6 and 2.3 billion years ago if resources were unrestricted.
However, since natural selection is driven by limited resources and competition, these cells evolved into more sophisticated forms of life that could collect resources and exploit varied niches in the environment. So, today’s variety of life is simply the result of billions of years of creatures attempting to optimise their reproductive success! Keep this in mind as we begin to explore the endosymbiotic idea!
Compartmentalization of organisms allows them to carry out catabolic and anabolic processes at the same time, as we saw in section 2.10. In other words, organisms may be far more effective in gathering nutrients, growing, and reproducing if they can divide off sections of their cells. Because all creatures compete for resources, those with the ability to compartmentalise their cells may reproduce more rapidly.
While eukaryotic cells have several membrane-bound organelles that divide their cells into distinct compartments, prokaryotic cells also contain compartmentalization mechanisms. Prokaryotic cells may compartmentalise distinct sections of their cytoplasm by utilising proteins to complete anabolic and catabolic events in separate places, in addition to creating a chamber around themselves in the periplasmic space. The endosymbiotic idea relies heavily on compartmentalization!
So, what is the endosymbiotic hypothesis, exactly? This idea, also known as symbiogenesis, proposes that the common ancestor of all life on Earth was a prokaryotic cell identical to the ones we observe today. This ancient prokaryote may have developed the nuclear envelope and endoplasmic reticulum membranes to safeguard its DNA and become more efficient via compartmentalization. This is just the beginning of the endosymbiotic adventure!
When this primordial eukaryotic cell met a much smaller, bacteria-like cell, the true symbiosis began. This early eukaryote may have accepted the bacteria into its cytoplasm via the mechanism of phagocytosis.
Normally, a lysosome would disintegrate this ingested bacterium, but a mutation may have enabled the bacterial cell to live and multiply inside the bigger, primitive eukaryote. This is most likely what gave rise to the mitochondria found in all eukaryotes today!
This scenario may have occurred with a photosynthetic bacteria, which would have developed into the chloroplasts and other plastids that algae and plants employ today!
While we’ll get to the complete list of evidence for the endosymbiotic idea in a moment, one of the most convincing pieces of evidence that these organelles were previously free-living creatures is that they both maintain circular DNA strands, much like modern-day bacterial cells! Though this genome is significantly smaller than a bacterial genome, it is thought that when endosymbionts coordinated their cell division with the host cell, the bacterial genome transferred some genes to the eukaryotic nucleus and lost others that were no longer necessary for life.
The endosymbiotic idea is backed up by a huge amount of data. This was not always the case, though. Konstantin Mereschkowski, a Russian scientist, initially proposed the hypothesis in 1901. Until Evolutionary Biologist Lynn Margulis produced a huge amount of microbiological data in the 1960s, the idea remained mainly undetected.
The parallels between different organelles and live bacteria are one of the most compelling pieces of evidence. While evolution alters some animals, it leaves others essentially unchanged from billions of years ago. The nucleus of eukaryotic creatures, for example, is similar to the basic membranes created by Planctomycetes bacteria.
These membranes may easily have developed into the nuclear membrane and the endoplasmic reticulum. Chloroplasts are similar to cyanobacteria, which are often found in water across the globe. Mitochondrial bacteria are remarkably similar to Rickettsial bacteria, which infect people and cause Spotted Fever and Typhus.
These organelles and bacteria are not only superficially linked, but DNA evidence and comparable biological macromolecules connect them much more. For example, the membranes of mitochondria and bacterial cells, for example, are substantially comparable in terms of integral membrane proteins and lipids. The chloroplasts of certain eukaryotic algae species have kept their peptidoglycan cell walls, similar to how some cyanobacteria do.
During cell division, the last piece of evidence supporting the endosymbiotic idea is present. Though eukaryotic cells divide via mitosis, the mitochondria and chloroplasts inside those cells divide by binary fission. Furthermore, if you remove the chloroplasts or mitochondria from a cell artificially, the cell is unable to recreate those organelles since they contain a major percentage of the cell’s own DNA!