General Architecture
The organelle's architecture is based on two membranes with fundamentally different properties:
- Outer membrane is penetrated by wide hydrophilic channels. It is highly permeable to most low-molecular-weight substances but retains large compounds.
- Inner membrane forms numerous folds called cristae. Mushroom-shaped structures—oxysomes (so-called $F_1$ particles)—are located on them. Respiratory chain enzymes and ATP synthesis complexes are densely embedded in this membrane.
- Matrix is the internal space enclosed by the inner membrane, filled with a semi-fluid medium.
Tissue Morphology
The appearance and internal structure of the organelles vary depending on the functional load of the tissue. Their shape ranges from a near-perfect sphere to highly elongated threads.
The number and shape of cristae directly reflect the cell's metabolic demands:
- Skeletal muscle: due to high energy demand, they contain an enormous number of lamellar cristae.
- Liver cells (hepatocytes): folds of the inner membrane are significantly fewer.
- Adrenal cortex: cristae acquire a specific tubular shape, making them look like a scatter of small vesicles on histological sections.
Semiautonomy and mtDNA
Mitochondria possess their own genetic system located directly in the matrix. It includes small circular mtDNA molecules (1–50 copies per organelle) and their own ribosomes, which are smaller than cytoplasmic ones.
This system can synthesize only about 5% of the proteins required by the organelle. The rest are encoded by nuclear DNA, assembled in the cytoplasm, and then transported inside. Such circular DNA structure and small ribosome size support the endosymbiotic theory—the origin of organelles from ancient symbiotic bacteria.
Features of mitochondrial genetics:
- High mutation rate: mtDNA mutates 10 times faster than nuclear DNA. This is due to the aggressive environment (free radicals from oxidation) and the lack of protective histone proteins.
- Maternal inheritance: upon fertilization, sperm organelles do not enter the oocyte, so offspring inherit only maternal mitochondria.
- Independent replication: organelle duplication occurs autonomously and independently of cell cycle phases (except during cell mitosis).
Functional Role
Mitochondria are the primary power plants, but their physiology extends far beyond ATP synthesis.
Key processes:
- Krebs cycle (tricarboxylic acid cycle): dehydrogenation of breakdown products from amino acids, carbohydrates, and fats (acetyl-CoA) occurs here. Cycle enzymes are dissolved in the matrix, except for succinate dehydrogenase (SDH), which is tightly embedded in the inner membrane.
- Oxidative phosphorylation: localized on the cristae. Electrons are transferred along the electron transport chain to oxygen, and the released energy is used by ATP synthase (in oxysomes) to generate ATP molecules.
- Additional metabolism: the matrix also hosts reactions of urea synthesis, fatty acid breakdown, and the transformation of pyruvate into acetyl-CoA.