Passage 9: Mitochondria
Mitochondria are the primary source of energy production and are implicated in a
wide range of biological processes in most eukaryotic cells. Skeletal muscle heavily
relies on mitochondria for energy supplements. In addition to being a powerhouse,
mitochondria evoke many functions in skeletal muscle, including regulating
calcium and reactive oxygen species levels. A healthy mitochondria population is
necessary for the preservation of skeletal muscle homeostasis, while mitochondrial
dysregulation is linked to numerous myopathies.
Mitochondria generates energy in the form of adenosine triphosphate (ATP) from
energy-enriched molecules such as pyruvate, fatty acids, and amino acids via
oxidative phosphorylation. Electrons generated from oxidations of energy-enriched
molecules are transferred via nicotinamide adenine dinucleotide hydrogen (NADH)
to complex I (NADH ubiquinone oxidoreductase) or flavin adenine dinucleotide
(FADH2) to complex II (succinate dehydrogenase), then transported to coenzyme
Q. Coenzyme Q then delivers electrons generated from complex I or II via complex
III (cytochrome bc1 complex) to cytochrome c and then to complex IV (cytochrome
c oxidase), where oxygen is reduced to water. Finally, coupling with electron
generation, the protons (H+) are pumped to the intermembrane space from
complex I, III, and IV for ATP production in complex V (ATP synthase).
In addition to ATP generation, mitochondria have other functions, including
the production of reactive oxygen species (ROS) and the regulation of cellular
calcium homeostasis. Mitochondrial ROS is the side product of the incomplete
mitochondrial oxidative phosphorylation process from the electron leakage
predominately in complexes I and III. Excess ROS damages cells by oxidation of
nucleic acids, proteins, and lipids. Yet, the growing evidence reveals that ROS acts
as a secondary messenger that participates in a wide range of cell signaling to
stimulate cell proliferation, differentiation, death, etc.
Mitochondria in skeletal muscle form a dynamic network, named mitochondrial
reticulum, to minimize metabolite distribution and maximize energy utilization
efficiency. The mitochondrial reticulum is constantly reshaped by fusion and fission
events, allowing mitochondria to exchange their content, including mitochondrial
DNA (mtDNA). This is shown in Figure 1.
The control of skeletal muscle is voluntary by motor neurons to generate force
and locomotion. The coordination of differences in nerve impulse transmission,
membrane excitability, excitation–contraction coupling calcium flux between
sarcoplasmic reticulum and cytosol, and ATP hydrolysis rate of myosin ATPase
generates a variety of movements in our daily life. Most, if not all, of the cellular
actions controlling movement are highly dependent on mitochondrial activities. It
was not surprising that the common feature of mitochondrial diseases is muscle
dysfunction.
Which of the following is true according to the passage and Figure 1 about
muscle cell mitochondria?
I. They reproduce sexually
II. They reproduce asexually
III. They share nuclear DNA
A) only II
B) only III
C) I and III
D) II and III
Correct answer: A. Like with any Roman numeral question, it is first
important to decide which statement to analyze first, depending on how frequently it
appears in the answer choices. In this case, statement III appears the most, so it should
be analyzed first. Since the passage states that the mitochondria share mitochondrial
DNA when they reproduce, this is likel, not nuclear DNA since this DNA is specific to the
mitochondria. Therefore, even though they share DNA, the DNA would not be nuclear,
making the statement false. Thus, answer choices B, C, and D are eliminated, and we
are left with answer choice A. Regardless, let’s analyze each statement in depth. Since
Figure 1 shows how the mitochondrial DNA undergoes fission and fusion, we know that
this process is likely asexual. Fission occurs when a cell splits from its own organelles and
parts without sexual intervention, making it asexual reproduction. When fusion occurs,
the cell’s materials combine and produce one cell. In this case, according to Figure 1, the
cell reproduces asexually. This means that statement II is true and statement I is false.
It is important to notice if two statements of answer choices conflict with each other
because analyzing one will automatically help either rule in or out the other statement. In
this case, since statements I and II oppose each other and statement II is true, statement
I is false.
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