Mission 1: So the problem is???
Mission Objectives. You should be able to...
1. Describe exceptions to the cell theory.
2. Explain a significant missing component to cell theory.
Mission Objectives. You should be able to...
1. Describe exceptions to the cell theory.
2. Explain a significant missing component to cell theory.
UnderstandingsCells can only be formed from the division (or mitosis) of existing cells.
The first cells must have arisen from non-living material. The origin of eukaryotic cells can be explained by endosymbiotic theory. |
Applications & SkillsEvidence from Pasteur's experiments that spontaneous generation of cells no longer happens on Earth.
|
Exceptions to the Cell Theory. Back in August, we discussed cell theory. The three tenets are: (1) all organisms are composed of cells, (2) cells are the smallest units of life, and (3) cells come from pre-existing cells. What we didn't discuss were the exceptions to and problems with this current theory. Recall that a theory is a substantiated explanation of natural phenomena that incorporates tested hypotheses and laws. This makes a theory a valuable endgame in science because it represents understandings that have developed from the scientific process. Cell theory is an example of this. It has been altered since it was first proposed in the 1800s. As technology improves, so will research, and more advanced research will lead to continued refinement of what we already know.
Back to the original point: One of the flaws with cell theory is that it does not explain where the first cells came from. There is no evidence today to suggest that new cells come from non-living material. However, the first cells must have. Early research by Louis Pasteur and Francesco Redi disproves spontaneous generation, as well as future research by other scientists. This leads to exceptions to cell theory:
Endosymbiotic theory. This is an explanation of how cells progressed from simple prokaryotes to complex eukaryotes. Key points of the theory include:
The eukaryote helped the bacteria by providing shelter and the bacteria helped back (eventually) by specializing in supplying the euk with energy in the form of ATP. There is evidence to support this theory because mitochondria
This theory is also supported by the behavior of chloroplasts in plant cells. Page 41 provides examples of two organisms, the Hatena protist and the Elysia chlorotica that go through endosymbiosis. Be sure to read about the results of their symbiotic relationships.
DNA is the final bit of evidence for endosymbiosis. It has a code with 64 different codons that have the same meaning in nearly all organisms. There are only slight variations (or mutations) in the codons that are explained by the changing conditions of Earth over time. Mitochondrion DNA more closely resembles bacterial DNA than euk DNA. It is assumed that the more DNA two organisms have in common, the more closely related they are to one another.
According to the cell theory, cells are the smallest units of life and come from pre-existing cells. But where did the first cells originate? This short video postulates on some possible origins. You should be able to name three of them.
Back to the original point: One of the flaws with cell theory is that it does not explain where the first cells came from. There is no evidence today to suggest that new cells come from non-living material. However, the first cells must have. Early research by Louis Pasteur and Francesco Redi disproves spontaneous generation, as well as future research by other scientists. This leads to exceptions to cell theory:
- There are multi-nucleated cells of striated muscle cells, fungal hyphae and several types of giant algae
- There are very large cells with continuous cytoplasm that are not compartmentalized into separate smaller cells
- Existence of viruses, which are not classified as "living"
- The problem of explaining the origin of cells without spontaneous generation
Endosymbiotic theory. This is an explanation of how cells progressed from simple prokaryotes to complex eukaryotes. Key points of the theory include:
- A bacterial cell moved in with a eukaryotic cell about two billion years ago
- The eukaryotic cell was the one who invited the bacterial cell to move in
- A symbiotic relationship developed between the two cells
- The bacterial cell then got a makeover and turned into a mighty mitochondrion.
The eukaryote helped the bacteria by providing shelter and the bacteria helped back (eventually) by specializing in supplying the euk with energy in the form of ATP. There is evidence to support this theory because mitochondria
- are the same size of most bacterial cells
- divide by fission, like bacterial cells
- divide independent of the host cell
- have their own ribosomes so they can produce their own proteins
- have their own DNA, which is closer to prokaryote DNA than eukaryote DNA
- have two membranes on their exterior, which is consistent with a engulfing process
This theory is also supported by the behavior of chloroplasts in plant cells. Page 41 provides examples of two organisms, the Hatena protist and the Elysia chlorotica that go through endosymbiosis. Be sure to read about the results of their symbiotic relationships.
DNA is the final bit of evidence for endosymbiosis. It has a code with 64 different codons that have the same meaning in nearly all organisms. There are only slight variations (or mutations) in the codons that are explained by the changing conditions of Earth over time. Mitochondrion DNA more closely resembles bacterial DNA than euk DNA. It is assumed that the more DNA two organisms have in common, the more closely related they are to one another.
According to the cell theory, cells are the smallest units of life and come from pre-existing cells. But where did the first cells originate? This short video postulates on some possible origins. You should be able to name three of them.
Though most cells follow standard behavior protocols, there's always some examples that proves the rule. Continued research must be done to see where these examples fall within cellular biology.
Exceptions are:
Mission 2: Endosymbiosis...Or That Time When A Bacteria Boo'ed Up With a Eukaryote.
Mission Objective. You should be able to...
1. Describe and explain endosymbiosis and name two organisms that demonstrate this theory.
Exceptions are:
- There are multi-nucleated cells of striated muscle cells, fungal hyphae and several types of giant algae
- There are very large cells with continuous cytoplasm that are not compartmentalized into separate smaller cells
- Existence of viruses, which are not classified as "living"
- The problem of explaining the origin of cells without spontaneous generation.
Mission 2: Endosymbiosis...Or That Time When A Bacteria Boo'ed Up With a Eukaryote.
Mission Objective. You should be able to...
1. Describe and explain endosymbiosis and name two organisms that demonstrate this theory.
Who is Lynn Margulis and why is she significant? What is an obligate symbiont? Give two examples.
Homework: Review the material for Section 1.5. Use this review PowerPoint.
Homework: Review the material for Section 1.5. Use this review PowerPoint.