Mitochondria – Definition, Structure, Function


Mitochondria Diagram
A mitochondrion (plural: mitochondria) is an organelle in plants, animals, and fungi that produces chemical energy for cells.

Mitochondria are the powerhouses of the cell. They are unique organelles present in almost all eukaryotic cells that are responsible for generating the cell’s supply of adenosine triphosphate (ATP), the energy currency of the cell.

Discovery and Word Origin

Albert von Kölliker discovered mitochondria in 1857 in insect voluntary muscles. Carl Benda coined the word “mitochondrion” in 1898.The term “mitochondrion” comes from the Greek words “mitos,” meaning thread, and “chondrion,” meaning granule. This term reflects their thread-like or granular appearance under the microscope. The plural of “mitochondrion” is “mitochondria.”

Functions of Mitochondria

While best-known for producing energy, mitochondria actually serve several important functions within the cell:

  1. ATP Production: Mitochondria are the site of oxidative phosphorylation and play a critical role in the production of ATP through the electron transport chain.
  2. Calcium Storage: They regulate calcium ion concentrations within the cell, influencing various cellular activities.
  3. Programmed Cell Death (Apoptosis): Mitochondria initiate and regulate apoptosis, a process of programmed cell death necessary for the removal of damaged cells.
  4. Heat Production: In certain cells, mitochondria generate heat, a process known as thermogenesis.
  5. Metabolic Functions: Mitochondria play a role in several metabolic pathways, including the citric acid cycle (Krebs cycle), which is crucial for energy production.
  6. Signaling: Mitochondria play roles in hormone, immune, and intercellular signaling.

Structure of a Mitochondrion

Mitochondria have a unique structure which is key to their function:

  • Outer Membrane: This smooth membrane encloses the entire organelle and is permeable to ions and small molecules. It is similar in composition to the cell’s plasma membrane. Integral membrane proteins called porins allow for transport between the mitochondrion and the cell’s cytosol.
  • Inner Membrane: The inner membrane is highly convoluted, forming folds known as cristae. This membrane contains proteins involved in the electron transport chain and ATP synthesis. The cristae greatly increase the surface area of the inner membrane, allowing for more ATP production. Small round bodies called oxysomes dot the surface of the cristae.
  • Intermembrane Space: The intermembrane space is the region between the inner and outer membranes. Its fluid composition resembles the cell’s cytosol in terms of ions and sugars. However, cytochrome c and certain other large proteins also occur here.
  • Matrix: The matrix is innermost compartment, which contains enzymes, mitochondrial DNA, and ribosomes. Essentially, it is the mitochondrial analog to the cell’s cytoplasm. Enzymes in the matrix play roles in the citric acid cycle and oxidation of fatty acids and pyruvate.

Mitochondrial Ribosomes

Mitochondrial ribosomes differ from cytoplasmic ribosomes within the cell. These differences are evident in several aspects:

  1. Size and Composition: Mitochondrial ribosomes are smaller than their cytoplasmic counterparts. In eukaryotic cells, cytoplasmic ribosomes are typically 80S (composed of a 40S small subunit and a 60S large subunit), whereas mitochondrial ribosomes are usually 55S to 70S, depending on the species. The ‘S’ here stands for Svedberg units, a measure of sedimentation rate during ultracentrifugation.
  2. Structural Features: The structure of mitochondrial ribosomes also differs from that of cytoplasmic ribosomes. There are differences in terms of their shape, the arrangement of their RNA and protein components, and in their molecular structure.
  3. RNA and Protein Content: Mitochondrial ribosomes have a different ratio of RNA to protein compared to cytoplasmic ribosomes. Mitochondrial ribosomes are richer in proteins and have less ribosomal RNA (rRNA).
  4. Genetic Origin: Mitochondrial DNA encodes the rRNA in mitochondrial ribosomes, whereas nuclear DNA encodes the rRNA in cytoplasmic ribosomes.
  5. Sensitivity to Antibiotics: Mitochondrial ribosomes are similar to bacterial ribosomes in terms of antibiotic sensitivity. Certain antibiotics that inhibit bacterial ribosomes also affect mitochondrial ribosomes.
  6. Function and Specificity: Although both types of ribosomes perform protein synthesis, mitochondrial ribosomes specialize in synthesizing proteins for use within the mitochondria. These proteins are often integral to mitochondrial functions, such as oxidative phosphorylation.

Mitochondrial ribosomes are more similar to those found in present-day bacteria than to the ribosomes in the eukaryotic cytoplasm.

Where Are Mitochondria Within a Cell?

Mitochondria are in the cytoplasm of eukaryotic cells, but their specific location and distribution within the cell varies depending on the cell type and its energy requirements. Here are some key points about the location of mitochondria in cells:

  1. General Cytoplasmic Distribution: In most cells, mitochondria disperse more or less equally throughout the cytoplasm. This distribution allows for efficient supply of ATP to various parts of the cell where energy is needed.
  2. Near High Energy Demand Sites: In cells with high energy demands, such as muscle cells or neurons, mitochondria occur in greater concentrations near sites where energy consumption is highest. For example, in muscle cells, mitochondria are abundant near the contractile apparatus to rapidly supply energy for muscle contraction.
  3. Associated with Other Organelles: Mitochondria sometimes occur near other organelles, such as the endoplasmic reticulum (ER), with which they share close functional relationships. This proximity facilitates inter-organelle communication and metabolic coupling.
  4. Cell-Specific Distribution: In certain specialized cells, mitochondria concentrate in specific regions. For instance, in sperm cells, mitochondria wrap around the flagellum to provide the energy needed for motility.
  5. Dynamic Movement and Morphology: Mitochondria are not static; they move throughout the cell and change their shape and size. This dynamic behavior responds to the cell’s energy needs, signaling pathways, and the life cycle of the mitochondria themselves.
  6. Adaptation to Cellular Conditions: The number and location of mitochondria within a cell changes in response to metabolic conditions. For example, in response to increased energy demand or during cellular stress conditions, cells increase the number of mitochondria through a process called mitochondrial biogenesis.

Do All Cells Contains the Same Number of Mitochondria?

The number of mitochondria in cells is not uniform across all cell types and it changes within a cell over time. Several factors influence the number and form of mitochondria:

  1. Cell Type and Energy Demand: The number of mitochondria in a cell correlates to the cell’s metabolic activity and energy requirements. Cells with high energy demands, such as muscle cells, heart cells, and neurons, have a higher number of mitochondria compared to cells with lower energy requirements.
  2. Mitochondrial Biogenesis: Cells increase their number of mitochondria through a process called mitochondrial biogenesis. Factors that stimulation the process include increased energy demands, physical exercise, exposure to cold, and certain cellular stressors.
  3. Cell Cycle and Growth Conditions: Cells increase their number of mitochondria during cell growth and division.
  4. Mitochondrial Fusion and Fission: Mitochondria change their shape and size through processes known as fusion (joining together) and fission (splitting apart). These processes alter the number of distinct mitochondria within a cell. Fusion creates elongated, interconnected mitochondria, while fission results in smaller, individual mitochondria.
  5. Physiological and Pathological Conditions: Hormonal changes and pathological conditions influence mitochondrial number.
  6. Cellular Stress and Damage: Cells remove damaged mitochondria through a process called mitophagy, a type of autophagy specific to mitochondria. This also alters the number of mitochondria within a cell.
  7. Developmental Stage and Tissue Type: The number of mitochondria varies during different developmental stages and among different tissue types. For example, rapidly dividing embryonic cells have a different mitochondrial content compared to mature, differentiated cells.

Disorders Associated with Mitochondria

Mitochondrial disorders are often the result of failures in mitochondrial functions. These can include:

  • Mitochondrial DNA Mutations: Mitochondrial mutations cause diseases like Leber’s hereditary optic neuropathy and mitochondrial myopathy.
  • Reactive Oxygen Species (ROS) Production: Excess ROS damages cellular components, contributing to aging and diseases like Parkinson’s and Alzheimer’s.
  • Metabolic Disorders: Mitochondrial dysfunction can result in metabolic syndromes due to impaired energy production.

Theories of Mitochondrial Origins

The most accepted theory regarding the origin of mitochondria is the endosymbiotic theory. This theory suggests that mitochondria originated from free-living prokaryotes that entered into a symbiotic relationship with early eukaryotic cells. Over time, these prokaryotes evolved into the modern mitochondria, losing some of their autonomy but becoming integral to the host cell’s metabolism.

However, there are other theories that explain the origin of mitochondria:

  1. Hydrogen Hypothesis: This is a variation of the endosymbiotic theory. It suggests a symbiotic relationship between an anaerobic or facultatively anaerobic eukaryote and a hydrogen-producing bacterium (the future mitochondrion). The theory emphasizes the role of hydrogen and energy transfer in the establishment of the symbiotic relationship.
  2. Serial Endosymbiosis Theory (SET): Developed by Lynn Margulis, this theory is a broader hypothesis that includes the endosymbiotic origin of mitochondria. SET suggests a sequential acquisition of symbiotic partners, where an ancestral host cell absorbed bacterial cells that eventually evolved into organelles like mitochondria and chloroplasts.
  3. Autogenous Hypothesis: This hypothesis posits that organelles like mitochondria originated from the invagination of the plasma membrane within a prokaryotic cell, which then differentiated into a distinct organelle. This theory doesn’t explain the presence of separate mitochondrial DNA or the double-membrane structure of mitochondria.
  4. Endosymbiotic Gene Transfer: While not an alternative theory for the origin, this concept complements the endosymbiotic theory. It suggests that over time, genes from the endosymbiotic bacteria (the ancestors of mitochondria) transferred to the host cell’s nucleus, which is why mitochondria and the cells they reside in are so interdependent.
  5. Symbiogenetic Theory: This is a broader theory that suggests that new organisms evolve through the long-term symbiosis and eventual merging of two separate organisms. This theory encompasses the origin of mitochondria as a specific case of a more general evolutionary process.

Frequently Asked Questions (FAQs)

1. What are mitochondria?

  • Mitochondria are organelles found in the cells of most eukaryotic organisms. They are the powerhouses of the cell because they generate most of the cell’s supply of adenosine triphosphate (ATP), which is used as a source of chemical energy.

2. Why are mitochondria important?

  • Mitochondria are crucial for energy production in cells. They also play significant roles in various other cellular processes, including the regulation of the cell cycle and cell growth, signaling, cellular differentiation, and even cell death (apoptosis).

3. Do all cells contain mitochondria?

  • They do no occur in prokaryotic cells. Mitochondria occur in the cells of nearly all eukaryotic organisms, including animals, plants, fungi, and most algae. However, they are absent in a few cell types, such as mature red blood cells in humans. Some eukaryotic species completely lack mitochondria, while they have transformed into other structures in other species.

4. How do mitochondria produce energy?

  • Mitochondria produce energy through a process called oxidative phosphorylation. This process involves the electron transport chain and the enzyme ATP synthase, which convert oxygen and nutrients into ATP.

5. Do mitochondria have their own DNA?

  • Yes, mitochondria contain their own DNA, known as mitochondrial DNA (mtDNA). This DNA is distinct from the DNA found in the cell’s nucleus and is inherited maternally in most organisms.

6. What is the endosymbiotic theory regarding the origin of mitochondria?

  • The endosymbiotic theory suggests that mitochondria originated from free-living prokaryotes that entered into a symbiotic relationship with early eukaryotic cells. Over time, these prokaryotes evolved into modern mitochondria.

7. Can problems with mitochondria lead to diseases?

  • Yes, dysfunctions in mitochondria leads to a range of diseases. These affect various systems in the body, with symptoms such as muscle weakness, neurological problems, and organ dysfunction.

8. How are mitochondria inherited?

  • Mitochondrial DNA gets passed down from mothers to their children.. The mitochondria present in a fertilized egg come from the mother’s egg cell.

9. Can mitochondrial function be improved?

  • Lifestyle factors such as exercise, diet, and certain supplements might improve mitochondrial function, but research in this area is ongoing.

10. Are mitochondria involved in aging?

  • Some research suggests that changes in mitochondrial function are linked to the aging process. Mitochondrial dysfunction shares a connection to age-related decline in various biological systems.

References

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  • McBride, H.M.; Neuspiel, M.; Wasiak, S. (2006). “Mitochondria: more than just a powerhouse”. Current Biology. 16 (14): R551–R560. doi:10.1016/j.cub.2006.06.054
  • Seo, A.Y.; Joseph, A.M.; et al. (2010). “New insights into the role of mitochondria in aging: mitochondrial dynamics and more”. Journal of Cell Science. 123 (Pt 15): 2533–2542. doi:10.1242/jcs.070490
  • Siekevitz, P. (1957). “Powerhouse of the cell”. Scientific American. 197 (1): 131–140. doi:10.1038/scientificamerican0757-131