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The mitochondria are present in almost all eukaryotic cells, whereas chloroplasts appear only in photosynthetic plant cells. Both are included within the energetic organelles of cells.

If it weren't for the mitochondria, heterotrophic cells could only obtain their ATP from anaerobic glycolysis, but with them they can completely degrade glucose in the presence of oxygen to convert it into carbon dioxide and water, obtaining much more energy.

The number of mitochondria depends on the energy needs of the cell, being located in areas where a significant consumption of ATP is required.

The chondriome is the set of all the mitochondria in a cell.

Structure of the mitochondria

The mitochondria are cylindrical and are bounded by a double membrane, the outer mitochondrial membrane, which separates the hialoplasma, and the inner mitochondrial membrane, having a folds inward increasing its surface, the mitochondrial cristae. These membranes leave two compartments: the intermembrane space, between both membranes, and the mitochondrial matrix, the space that surrounds the inner mitochondrial membrane.

Chemical composition of mitochondria

The mitochondrial matrix

Limited by the inner mitochondrial membrane, in addition to 50% water, the content of the matrix includes:

  • DNA molecules (mitochondrial-DNA), double-stranded and circular (such as bacteria and chloroplasts), which contain information to synthesize mitochondrial proteins. This DNA is different from nuclear DNA.
  • Mitochondrial RNA molecules forming the mitorribosomes, different from the rest of the ribosomes of the cell.
  • Ribosomes (the mitorribosomes), which can be both free and attached to the inner mitochondrial membrane. They are similar to bacterial ribosomes (70 S), smaller than cytoplasmic ribosomes.
  • Calcium and phosphate ions, ADP,  ATP, coenzyme-A, etc.
  • Enzymes necessary for the replicationtranscription and translation of mitochondrial DNA to synthesize mitochondrial proteins.
  • Enzymes necessary to develop the Krebs cycle and the β-oxidation of fatty acids.

The inner mitochondrial membrane

The inner mitochondrial membrane has a large surface area because it has folds called mitochondrial ridges, where enzymes of the respiratory chain and mitochondrial ATPase are located.

It is impermeable to ions and other substances, which have to pass through it through transporter proteins.

It has more proteins (80%) than the other cell membranes and it has no cholesterol (as in bacteria) among its lipids (20%). Among the proteins, the following stand out:

  • The proteins that make up the respiratory chain that carry electrons to molecular oxygen.
  • An enzyme complex, mitochondrial ATP synthase, that catalyzes the synthesis of ATP (oxidative phosphorylation).
  • The carrier proteins, allowing the passage of ions and molecules between the mitochondrial matrix and cytosol.

The outer mitochondrial membrane

It is the membrane that surrounds the mitochondria, in contact with the cytosol. It has the same structure as the rest of cell membranes: a lipid double layer and associated proteins.

The outer mitochondrial membrane resembles other cell membranes, specifically those of the endoplasmic reticulum. Among its proteins we distinguish:

  • Proteins that form large water channels or porins and make it very permeable, unlike the inner mitochondrial membrane. These channels are not selective and allow large molecules to pass through.
  • Enzymes, such as those that activate fatty acids to be oxidized in the mitochondrial matrix.

The intermembrane space

Located between the outer and inner membranes, with a composition similar to that of the cytosol due to the permeability of the outer mitochondrial membrane, although it contains enzymes that allow it to transfer the energy from ATP obtained in the mitochondria to other nucleotides, for example, AMP.

Fundamental ideas about mitochondria

Mitochondria are energetic cellular organelles with an elongated shape, formed by a double membrane (outer mitochondrial membrane and inner mitochondrial membrane), an intermembrane space and a mitochondrial matrix.

The origin of mitochondria and chloroplasts is in some aerobic bacteria and other photosynthetic bacteria that were ingested by some prokaryotes and had a symbiotic relationship and evolved together. Therefore, mitochondria and chloroplasts are organelles similar to prokaryotic cells.

Thus, mitochondria and chloroplasts were formed, organelles very similar to prokaryotic cells, which gave rise to eukaryotic cells.

  • Mitochondrial matrix: Contains:
    • double-stranded and circular DNA.
    • Mitochondrial RNA molecules.
    • Mitoribosomes. Ribosomes similar to bacterial ribosomes (70 S), smaller than cytoplasmic ribosomes .
    • Calcium and phosphate ions, ADP, ATP, coenzyme-A, etc.
    • Enzymes necessary for the replication, transcription, and translation of mitochondrial DNA to synthesize mitochondrial proteins.
    • Enzymes necessary for the metabolic pathways that occur in the mitochondrial matrix:
  • Inner mitochondrial membrane
    • Mitochondrial cristae: folds where the enzymes of the respiratory chain and the mitochondrial ATPase (ATP synthesis) are.
    • It has carrier proteins for ions and other substances to pass through.
    • It has no cholesterol.
  • Outer mitochondrial membrane
    • in contact with the cytosol.
    • Proteins with large aqueous channels or porins that make it a very permeable membrane.
  • intermembrane space
    • Between the outer and inner membranes.
    • Composition similar to that of the cytosol by the permeability of the outer mitochondrial membrane.


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