General structure
The architecture of the organelle is based on the presence of two membranes with fundamentally different properties:
- Outer membrane is pierced by wide hydrophilic channels. It is highly permeable to most low-molecular-weight substances, but retains large compounds.
- Inner membrane forms numerous folds — cristae. Mushroom-shaped structures — oxysomes (so-called $F_1$ particles) — are located on them. Respiratory chain enzymes and complexes for ATP synthesis are tightly embedded in this membrane.
- Matrix is the internal space between the cristae, filled with a semi-fluid medium.
Tissue Morphology
The appearance and internal structure of organelles vary depending on the functional load on the tissue. Their shape ranges from an almost ideal sphere to highly elongated threads.
The number and shape of cristae directly reflect the needs of the cell:
- Skeletal muscles: due to their high energy demand, they contain a huge number of lamellar cristae.
- Liver cells (hepatocytes): there are significantly fewer folds of the inner membrane.
- Adrenal cortex: cristae acquire a specific tubular shape, which is why they look like a scattering of small bubbles or vesicles on a histological section.
Semiautonomy and mtDNA
Mitochondria have their own genetic system located directly in the matrix. It includes small cyclic mtDNA molecules (1–50 copies per organelle) and their own ribosomes, which are smaller than cytoplasmic analogues.
This system can synthesize only about 5% of the proteins required by the organelle. The rest is encoded by nuclear DNA, assembled in the cytoplasm, and then transported inside. Such a cyclic DNA structure and small ribosome size confirm the symbiogenesis theory — the origin of organelles from ancient symbiotic bacteria.
Features of mitochondrial genetics:
- High mutability: mtDNA mutates 10 times more often 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, the spermatozoon's organelles do not enter the egg, so only maternal mitochondria are passed on to the offspring.
- Independent replication: the doubling of organelles occurs autonomously and does not depend on the phases of the cell cycle (except for the moment of cell mitosis itself).
Functional role
Mitochondria are the main power plants, but their physiology is not limited solely to ATP synthesis.
Key processes:
- Krebs cycle (tricarboxylic acid cycle): dehydrogenation of breakdown products of amino acids, carbohydrates, and fats (acetyl-CoA) occurs. Cycle enzymes are dissolved in the matrix, except for succinate dehydrogenase (SDH), which is firmly embedded in the inner membrane.
- Oxidative phosphorylation: localized on the cristae. Electrons are transferred along the respiratory chain to oxygen, and the released energy is used by ATP synthase (in oxysomes) to create ATP molecules.
- Additional metabolism: urea synthesis, fatty acid breakdown, and the transformation of pyruvate into acetyl-CoA also take place in the matrix.
Briefly: answers to topical questions
How do mitochondrial ribosomes differ from cytoplasmic ribosomes?
Mitochondrial ribosomes differ from cytoplasmic ones in smaller size, different composition, and similarity to prokaryotic ribosomes; in the mitochondrial matrix, they are represented by 70S-type ribosomes.
| Sign | Mitochondrial ribosomes | Cytoplasmic ribosomes |
|---|---|---|
| Size and composition | Smaller in size, different composition | Larger in size compared to mitochondrial ones |
| Similarity | Similar to prokaryotic ones | Different from mitochondrial ones |
| Localization | In the mitochondrial matrix | Freely in the hyaloplasm or fixed on the outer surface of ER membranes |
By means of which protein complexes does the translocation of proteins from the cytoplasm into the mitochondria occur?
The translocation of proteins from the cytoplasm into the mitochondrion is carried out using specialized receptors and translocases.
- TOM complex — the translocase of the outer membrane, which recognizes synthesized proteins and passes them through the outer membrane.
- TIM23 complex — the translocase of the inner membrane 23, through which proteins are transferred into the inner membrane and matrix.
Which marker enzymes are used to identify the outer mitochondrial membrane?
To identify the outer mitochondrial membrane, the enzyme monoamine oxidase is used. This enzyme uses oxygen regardless of the activity of the respiratory chain and belongs to extramitochondrial oxidation systems.
Which specific molecules (RNA and proteins) are encoded by the 37 genes of mitochondrial DNA?
Mitochondrial DNA encodes 37 genes that are responsible for the synthesis of a strictly defined set of molecules.
- Respiratory chain proteins — 13 subunits (7 for complex I, 1 for complex III, 3 for complex IV, and 2 for ATP synthase).
- Transfer RNAs — 22 molecules.
- Ribosomal RNAs — 2 molecules.
To avoid confusing the localization, remember: "Cristae are wires (electron transport chain and ATP synthesis), and the matrix is a boiler (where Krebs cycle enzymes are dissolved and breakdown reactions take place)."
Class and exam questions
How do new mitochondria appear in the cell?
They form from maternal organelles through simple constriction (division). This process is completely autonomous and can occur at any moment of the cell life cycle except mitosis.
Why does mtDNA mutate much more frequently than nuclear DNA?
It is located in the aggressive environment of the matrix, where free radicals actively form. In addition, mtDNA is not protected by special histone proteins.
How to visualize mitochondria using light microscopy?
Classical histology uses Altman's method (special staining). Organelles take the shape of bright red beads or chains, as seen in preparations of Ascaris intestinal cells.
What else to study on this topic
- Stages and complexes of the electron transport chain on the cristae
- Structure and operation principle of ATP synthase in oxysomes
- Mechanisms of destruction of old organelles by autophagosomes
- Symbiogenesis theory and the evolution of cellular respiration
- Features of the enzyme succinate dehydrogenase (SDH)
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