Immature cardiac cells or cardiomyoblasts have no contractile filaments yet but divide actively. Their cytoplasm is full of free ribosomes. The energy required for the cytosol protein production, is provided by functioning of mitochondria with a relatively simple design.

Mitochondria are amazing organelles. Their seemingly simple structure may disguise a rather complicated functioning while this machinery ensures an effective ATP synthesis.

Unlike other organelles, mitochondria are constituted by two membranes: external and internal ones. They are separated by a narrow (10-20 nm) intermembranous space. The internal mitochondrial membrane forms some long and flat ridges or cristae #1, #2, #3, and they increase the membrane surface significantly. Their number and thus the internal membrane area correlate with energetic needs of cells.

The inner mitochondrial space is called a matrix #1, #2, #3. It is limited by the internal membrane. The matrix is a very viscous and concentrated liquid. There are key substrates for the ATP synthesis.

The mitochondrial native DNA encodes about 13% of mitochondrial proteins only. The rest of proteins are produced in the cellular cytoplasm with a help of the cellular genome. They come into the matrix through special channels called translocases, which are found near the contact sites #1, #2 where the external and internal membranes almost fuse together.