Educerie · AP · Biology
AP Biology 2.10 Origins of Cell Compartmentalization
In one breath
- Prokaryotes typically lack membrane-bound organelles but have specialized internal regions: a nucleoid holding the DNA, ribosomes, plasmids, and in some, infolded membranes.
- Eukaryotes keep internal membranes that partition the cell.
- Mitochondria and chloroplasts evolved by endosymbiosis: an ancestral cell engulfed a free-living aerobic prokaryote, which became the mitochondrion; later, a photosynthetic prokaryote became the chloroplast.
- The evidence is that both have double membranes, circular DNA, bacterial-type ribosomes, division by splitting, bacterial size, and DNA sequences closest to particular bacteria.
- Argue it as claim, evidence, reasoning.
What you must be able to do
- Describe the endosymbiotic origin of mitochondria and chloroplasts, and support it with evidence"Provide two pieces of evidence that support the claim that chloroplasts evolved from free-living prokaryotes"
- Describe how prokaryotes organise their insides without membrane-bound organelles"Describe one way a prokaryotic cell has regions with specialized functions"
- Compare compartmentalization in prokaryotic and eukaryotic cellsA table of features: "Identify which belongs to prokaryotes, eukaryotes or both"
Once you have read the note, tick off each one you can do in Check your understanding.
In the exam. Section I multiple choice gives a table of features of mitochondria, chloroplasts, bacteria and the rest of the eukaryotic cell, or data on the effect of an antibiotic, and asks which observation supports endosymbiosis or how prokaryotes and eukaryotes differ. In Section II it feeds Conceptual analysis (support the claim that mitochondria were once free-living bacteria with evidence), Analyze model or visual representation (a diagram of endosymbiosis or of a prokaryotic cell), and Analyze data.
1Compartments in prokaryotes
A prokaryotic cell typically has no internal membrane-bound organelles: no nucleus, no ER, no Golgi, no mitochondria. That does not make its inside a featureless soup. It has internal regions with specialized structures and functions:
- The nucleoid: the region where the cell's DNA, usually one circular chromosome, is concentrated. It is not enclosed by a membrane.
- Ribosomes, scattered through the cytoplasm, making proteins.
- Small extra DNA loops called plasmids, in many bacteria.
- In some bacteria, the plasma membrane is folded inward, giving extra area for particular reactions. Photosynthetic bacteria carry their light-capturing pigments on such membranes.
Figure 1 shows a typical bacterium: a cell wall around a plasma membrane, cytoplasm with ribosomes, and a nucleoid with no membrane around it.
Eukaryotic cells, by contrast, maintain internal membranes that partition the cell into specialized regions: the nucleus, the endomembrane system, mitochondria and, in plants and algae, chloroplasts (2.9).
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Plasma membrane | Yes | Yes |
| Ribosomes | Yes | Yes |
| DNA location | Nucleoid region, no membrane | Nucleus, inside the nuclear envelope |
| Membrane-bound organelles | Typically none | Yes |
| Typical size | About 1–5 µm | About 10–100 µm |
2Endosymbiosis
Endosymbiosis is one organism living inside another to the benefit of both. The theory for mitochondria and chloroplasts runs like this.
- An ancestral cell, already with some internal membranes, took in a free-living aerobic prokaryote, one that used oxygen to release energy from food. It took it in by folding its membrane around it, as in endocytosis (2.5), so the prokaryote ended up wrapped in a second membrane.
- Instead of being digested, the prokaryote survived and kept dividing inside its host. The host got a supply of ATP; the guest got food and protection.
- Over many generations, the two became dependent on each other. Many of the guest's genes moved to the host's nucleus, and the guest could no longer live alone. It had become a mitochondrion.
- Later, in one line of these cells, the same thing happened with a photosynthetic prokaryote. It became the chloroplast. That line gave rise to algae and plants.
That order explains a pattern you can check: almost all eukaryotes have mitochondria, but only plants and algae have chloroplasts.
Mitochondria and chloroplasts are the descendants of free-living prokaryotes taken in by an ancestral cell: endosymbiosis.
3The evidence
A theory about events more than a billion years ago is tested by what it predicts we should find today. Each prediction comes true.
| Observation | Why it supports endosymbiosis |
|---|---|
| Mitochondria and chloroplasts have a double membrane | The inner membrane is the prokaryote's own; the outer one came from the host's membrane as it was engulfed |
| They contain their own DNA, as a circular molecule | Prokaryotes have circular DNA; the nucleus has linear chromosomes |
| They have their own ribosomes, smaller and more like bacterial ribosomes than like those in the eukaryotic cytosol | They make some of their own proteins, as a once-independent cell would |
| They divide by splitting in two, like binary fission, and a cell cannot build one from scratch | New ones come only from old ones, as with bacteria |
| They are about the size of a bacterium | Consistent with having once been one |
| Their DNA sequences are most similar to particular bacteria: mitochondria to a group of aerobic bacteria, chloroplasts to cyanobacteria | Close sequence similarity shows close relationship |
Some antibiotics work by blocking bacterial ribosomes, and some of these also interfere with mitochondrial ribosomes while leaving the cytosol's ribosomes untouched. That is what you would expect if mitochondrial ribosomes are bacterial in origin.
4Making the argument
This topic's skill is supporting a claim with evidence. Use claim, evidence, reasoning.
Claim: chloroplasts evolved from free-living photosynthetic prokaryotes. Evidence: chloroplasts contain their own circular DNA, and its sequence is most similar to the DNA of cyanobacteria. Reasoning: prokaryotes have circular DNA, and organisms with similar DNA sequences share a recent common ancestor; so the chloroplast's DNA is best explained by descent from a cyanobacterium-like prokaryote.
One piece of evidence, explained, earns more than three listed with no reasoning.
Practice questions
4 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.