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Article Mitochondria: Read and annohighlight the background information.
I-Background: The microscopic world is full of one celled organisms. Bacteria are unique to microscopic world because they do not have a
nucleus. They are called prokaryotic cells because they do not have a nucleus. You and I and the other kingdoms of organisms are all
eukaryotes because we have a nucleus full of DNA in all of our cells. Bacteria are also the most ancient of all life forms and date back in
geologic time to the Precambrian. Protists are another kingdom of organisms that often referred to as the odds and ends kingdom as they have
so many different characteristics. Most are usually found in or near water but they move and take in or make food in a variety of ways.
Glycosis- The most pressing need of all cells in the body is for an immediate source of energy. Some cells such as brain cells have severely
limited storage capacities for either glucose or ATP, and for this reason, the blood must maintain a fairly constant supply of glucose. Glucose is
transported into cells as needed and once inside of the cells, the energy producing series of reactions commences.
Fermentation- The anaerobic (without oxygen) conversion of sugar to carbon dioxide and alcohol by yeast. Fermentation can also occur in
muscle cells with lactic acid production.
Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants
and some algae (Kingdom Protista). Plants need only light energy, CO2, and H2O to make sugar. The process of photosynthesis takes place in the
chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.
Extracellular-In cell biology, molecular biology and related fields, the word extracellular (or sometimes extracellular space) means "outside
the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid
II-Pre Reading-
1) What purpose do the mitochondria serve in the cell?______________________
2) Do the mitochondria have DNA? _______________________________
3) What is a symbiotic relationship? _________________________________________
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(see page 472)
III- Read the article on page 2 and go through my underlining of key information and make notes or questions in
the margins.
IV- Respond to analysis questions after you read article. You may need to reread article and background to do
your best on questions:
1. The authors felt it was important to conduct write this article, because: (State the relevant background information used to
justify their work.)
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2. The key questions the author is addressing are: (Identify the key questions in the mind of the author when they wrote the
article.) __________________________________ _____________________________________________________
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3. Will the researchers be able to quantify the evidence to prove their theory? If no, why not?
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4. If we take this line of reasoning seriously, the implications are: (What consequences are likely to follow if people take the
author’s reasoning seriously?)
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Evolutionary Origin of Mitochondria
Created by David R. Caprette (caprette@rice.edu), Rice University
Unlike any other organelle, except for chloroplasts, mitochondria appear to originate only from other
mitochondria. They contain their own DNA, which is circular as is true with bacteria, along with their own
transcriptional and translational machinery. Mitochondrial ribosomes and transfer RNA molecules are similar to
those of bacteria, as are components of their membrane. These and related observations led Dr. Lynn Margulis, in
the 1970s, to propose an extracellular origin for mitochondria.
Some species of present day protists contain living organisms within their cytoplasm. For example, Paramecium
bursaria are hosts for zoochlorellae, photosynthetic protists, that reside within the cytoplasm. The relationship
appears to be symbiotic. The endosymbiont (organism living in the protest) gains protection and possibly some
essential nutrients from the host cytoplasm. The host has a readily available food source when its usual food
source is depleted. If you have the opportunity to observe P. bursaria, note that the endosymbionts are not
incorporated into food vacuoles. They are residents within the cytoplasm itself, and either are descended from
organisms that survived endocytosis or have some mechanism for escaping food vacuoles once they are ingested.
Protists are eukaryotes, of course, meaning that their genetic material is organized into a compartment, the
nucleus, that is surrounded by membrane, and that they have membrane-delineated organelles. In the warm seas
of the ancient earth, the first living things would have been prokaryotes. The endosymbiotic hypothesis for the
origin of mitochondria (and chloroplasts) suggests that mitochondria are descended from specialized bacteria
(probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some
other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular
respiration in host cells that relied on glycosis and fermentation would have provided a considerable evolutionary
advantage. Similarly, host cells with symbiont bacteria capable of photosynthesis would also have an advantage. In
both cases, the number of environments in which the cells could survive would have been greatly expanded.
Mitochondria do not contain anywhere near the amount of DNA needed to code for all mitochondria-specific
proteins, however, a billion or so years of evolution could account for a progressive loss of independence. The
endosymbiotic hypothesis might be called a theory, but experimental evidence can't be provided to test it. Only
circumstantial evidence is available in support of the proposal, which is the most likely explanation for the origin of
mitochondria. The evidence needed to change the model from hypothesis to theory is likely forever lost in
antiquity.