Mitochondria

Overview

• Found in high concentrations in metabolically active cells
• Cell biology
  • double membrane structure
    • outer membrane
      • high permeability due to high concentration of porins
    • inner membrane
      • impermeable to ions and molecules
      • folded into cristae to ↑ surface area
      • contains enzymes for electron transport chain (ETC)
  • inner matrix
    • contains circular DNA (mtDNA)
      • maternal inheritance as mitochondria in the zygote come from the egg
        • non-Mendelian inheritance
      • synthesized in continous polycistronic manner including
        • 13 coding genes
        • 2 ribosomal RNA modules
        • 22 transfer RNA segments
      • examples of mtDNA disorders
        • Leber’s optic neuropathy 
          • degeneration of retinal ganglion cells
          • progressive loss of central vision
          • eventual blindness
        • Pearson marrow-pancreas syndrome
      • some protein synthesis occurs here
      • most proteins are synthesized in cytoplasm and translocated to mitochondria
• Function
  • oxidizes substrates
    • NADH/FADH₂ → NAH⁺/FAD 
  • generates ATP
    • converts NADH/FADH₂ into ATP via ETC
• stores Mg²⁺ and Ca²⁺

Mitochondrial inheritance pedigree

• Affected males
  • transmit to none of their children 
• Affected females
• transmit to all of their children  

Introduction

Mitochondria are organelles found in most eukaryotic cells that are responsible for producing energy in the form of ATP (adenosine triphosphate) through a process called cellular respiration. They are often described as the “powerhouses” of the cell.

Mitochondria are enclosed by two membranes, the outer membrane and the inner membrane, which creates two compartments within the organelle – the intermembrane space and the mitochondrial matrix. The inner membrane is folded into structures called cristae, which increase the surface area available for energy production.

Structure

Mitochondria are double-membraned organelles with a unique structure that allows them to carry out their functions. The outer membrane of the mitochondria is smooth and porous, allowing small molecules to pass through. The inner membrane is highly folded and forms structures called cristae, which increase the surface area available for energy production.

The space between the inner and outer membranes is called the intermembrane space, while the space inside the inner membrane is called the mitochondrial matrix. The matrix contains the mitochondrial DNA (mtDNA), ribosomes, enzymes involved in the citric acid cycle, and other proteins needed for function.

The inner membrane of the mitochondria is selectively permeable, meaning that it only allows certain molecules to pass through. This membrane contains a number of proteins that are essential for the process of oxidative phosphorylation, which is the main mechanism by which mitochondria generate ATP.

Types

1. Classical mitochondria: These are the most common type of mitochondria, found in most eukaryotic cells. They are responsible for energy production through oxidative phosphorylation.
2. Brown fat mitochondria: These mitochondria are found in brown adipose tissue and are specialized for heat production, rather than ATP production. They have a high concentration of uncoupling protein 1 (UCP1), which allows them to generate heat by uncoupling the electron transport chain from ATP synthesis.
3. Sperm mitochondria: Mitochondria are also found in sperm cells and are inherited only from the mother. They are specialized for ATP production, which is required for the motility of the sperm.
4. Plant mitochondria: Plant mitochondria are similar in structure to classical mitochondria but have some unique features. For example, they contain alternative oxidase, which allows them to generate heat in response to stress.
5. Mitochondria in muscle cells: Muscle cells contain a large number of mitochondria, which are specialized for ATP production to support the high energy demands of muscle contraction.
6. Mitochondria in neurons: Neurons have a high energy demand due to their extensive branching and signaling processes.

Studies

Here are some examples of key findings from mitochondrial research:

1. Endosymbiotic theory: The idea that mitochondria originated from a free-living bacterial ancestor and were subsequently incorporated into eukaryotic cells through endosymbiosis was first proposed in the 1960s. This theory has been supported by various lines of evidence, including the presence of mtDNA, the similarity between mitochondrial and bacterial ribosomes, and the fact that mitochondria can replicate independently of the cell.
2. Diseases: Dysfunction in mitochondria has been implicated in a number of human diseases, including mitochondrial disorders, cancer, and neurodegenerative diseases like Alzheimer’s and Parkinson’s.
3. Oxidative phosphorylation: The process by which mitochondria generate ATP through oxidative phosphorylation was first described in the 1960s. This process involves the transfer of electrons through a series of proteins in the inner mitochondrial membrane, which generates a proton gradient that is used to drive ATP synthesis.
4. Dynamics: Mitochondria are dynamic organelles that undergo constant fusion and fission to maintain their shape and function. This process is regulated by a complex network of proteins, and dysfunction in mitochondrial dynamics has been linked to a range of diseases.
5. Signaling: Are involved in a range of cellular signaling pathways, including calcium signaling, apoptosis, and autophagy. These processes are regulated by proteins that are localized to the mitochondria and can modulate it function.

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