AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-10)
Unit 2 · Topic 2.10
2.10 Origins of Cell Compartmentalization
Mitochondria and chloroplasts most likely began as free-living prokaryotes that were engulfed by another cell and stayed, an idea called endosymbiotic theory. Strong evidence supports it, including their double membranes, their own circular DNA and their bacteria-like ribosomes. The topic also compares how prokaryotes and eukaryotes organize their insides.
Key terms
- endosymbiotic theory
- prokaryotic cell
- eukaryotic cell
- membrane-bound organelle
- circular DNA
Prokaryotes vs. eukaryotes
Prokaryotic cells (bacteria and archaea) usually have no membrane-bound organelles and no nucleus. That doesn't mean their insides are a random mix. They still have specialized regions: their DNA is gathered in an area called the nucleoid, they have ribosomes, and some have internal membranes. For example, photosynthetic cyanobacteria have folded internal membranes where photosynthesis happens.
Eukaryotic cells (plants, animals, fungi and protists) use internal membranes to split their insides into separate working spaces: the nucleus, ER, Golgi, mitochondria and so on (2.9).
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No; DNA in a nucleoid region | Yes, enclosed by a nuclear envelope |
| Membrane-bound organelles | Usually none | Many |
| Ribosomes | Yes | Yes |
| Plasma membrane | Yes | Yes |
| DNA | Usually one circular chromosome | Linear chromosomes (usually several) in the nucleus |
| Typical size | Usually smaller | Usually larger |
Endosymbiotic theory
'Endo' means inside and 'symbiosis' means living together. The theory says that, long ago, an ancestral host cell took in an aerobic prokaryote, one that could use oxygen to release energy from food. Instead of being digested, the prokaryote survived and kept living inside the host. The host gave it protection and nutrients, and it gave the host energy. Over many generations, it became the mitochondrion.
Later, a cell that already had mitochondria took in a photosynthetic prokaryote related to modern cyanobacteria. That one became the chloroplast. This order explains why nearly all eukaryotes have mitochondria (or are descended from ancestors that did), but only plants and algae have chloroplasts. The biologist Lynn Margulis gathered the evidence for this theory and argued for it in the 1960s and 1970s, and it's now widely accepted.
The evidence
- Double membranes: both organelles have two membranes, which fits the idea of a prokaryote being wrapped in host membrane when it was engulfed.
- Their own DNA: mitochondria and chloroplasts each have their own DNA, and it's circular, like a bacterial chromosome.
- Their own ribosomes: their ribosomes resemble bacterial ribosomes more than the ribosomes in the eukaryote's cytosol. Some antibiotics that target bacterial ribosomes can also affect mitochondrial ribosomes.
- Division: they reproduce by splitting in two, a lot like bacterial binary fission. A cell can't make a new mitochondrion from scratch, only by dividing existing ones.
- DNA comparisons: mitochondrial DNA sequences are most similar to a group of aerobic bacteria, and chloroplast DNA sequences are most similar to cyanobacteria.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Building an evidence-based argument
A scientist discovers a single-celled eukaryote containing a membrane-bound structure that photosynthesizes. Describe two pieces of evidence the scientist could look for to test whether this structure came from endosymbiosis, and explain what result would support the hypothesis.
Show the solutionHide the solution
- Step 1: Choose evidence that separates an engulfed prokaryote from a structure the host built itself.
- Step 2: Evidence 1: check whether the structure has its own DNA. Support = it has its own circular DNA, separate from the nucleus.
- Step 3: Evidence 2: compare that DNA's sequences to known organisms. Support = it's most similar to a group of free-living photosynthetic bacteria, such as cyanobacteria.
- Step 4: Other options: a double membrane, ribosomes that resemble bacterial ribosomes, or new copies forming only by division of existing ones.
Answer: For example: (1) its own circular DNA, and (2) DNA sequences most similar to free-living photosynthetic prokaryotes. A double membrane, bacteria-like ribosomes, or reproduction by fission would also support endosymbiosis.
- Example 2
Prokaryotes have no structure? (classic trap)
A student writes: 'Because prokaryotes don't have membrane-bound organelles, their insides have no organization and they can't make proteins.' Correct both parts of the statement.
Show the solutionHide the solution
- Step 1: Part 1: prokaryotes do have organized internal regions, such as the nucleoid where the DNA is concentrated, and some have internal membranes (like the photosynthetic membranes of cyanobacteria).
- Step 2: Part 2: protein synthesis happens on ribosomes, which are not membrane-bound and are found in all cells, including prokaryotes (2.1).
- Step 3: So lacking membrane-bound organelles doesn't mean lacking ribosomes or lacking organization.
Answer: Prokaryotes have specialized internal regions (like the nucleoid and, in some, internal membranes), and they make proteins on their own ribosomes.
Common mistakes
- Saying prokaryotes have no ribosomes. All cells have ribosomes.
- Listing 'mitochondria make energy' as evidence for endosymbiosis. Evidence means features shared with bacteria: double membrane, circular DNA, bacteria-like ribosomes, division by fission, DNA similarity.
- Saying chloroplasts came first, or that animal cells have chloroplasts. Mitochondria came first and are in nearly all eukaryotes; chloroplasts are only in plants and algae.
On the exam
- Expect 'support a claim with evidence' questions about endosymbiosis. Name a specific piece of evidence and explain how it points to a prokaryotic origin.
- Comparison questions ask for a similarity and a difference between prokaryotes and eukaryotes. Ribosomes, plasma membrane and DNA are similarities; a nucleus and membrane-bound organelles are differences.
Connected topics
Videos
Check yourself
4 questions on 2.10 Origins of Cell Compartmentalization. Pick an answer to see if you got it, and why.
Which of the following observations provides the strongest evidence that chloroplasts descended from free-living prokaryotes?
A newly discovered single-celled organism has ribosomes, a plasma membrane and DNA, but no nucleus and no membrane-bound organelles. How should it be classified?
| Source of rRNA compared | Similarity to mitochondrial rRNA (%) | Similarity to chloroplast rRNA (%) |
|---|---|---|
| α-proteobacterium (a free-living bacterium) | 79 | 61 |
| Cyanobacterium (a photosynthetic bacterium) | 62 | 84 |
| Archaeon | 51 | 50 |
| The same plant's nuclear genes | 48 | 47 |
Illustrative data: percent of matching positions when ribosomal RNA (rRNA) genes from a plant's mitochondria and chloroplasts were lined up with rRNA genes from other sources.
Which of the following claims is best supported by the data?
Which of the following results, if it had been found instead, would most weaken the claim that mitochondria descend from free-living bacteria?
0 of 4 answered