The cell is the basic biological unit, as the elementary particles and the elements are the basic units of chemistry. In spite of the enormous variation in the structure and function of different organisms, the fundamental unit, the cell, is with some variations basically the same. Why is the cellular structure the same? First of all, early in evolution the cell demonstrated its functionality. But the use of structural units of small size has also ensured effective transportation by diffusion. Most cells have a diameter between 1 and 20 ^m (Table 2.4). Cells have, therefore, a relatively high openness (see Table 2.3), that is necessary for the biochemistry of organisms to work. The hierarchical structure, which was presented in Box 2.2 and Figure 2.7 and will be further discussed in Chapters 3 and 7, is a precondition for the needed openness for each level in the hierarchy.
Let us, however, demonstrate the importance of openness by focusing on the cell. The problem is for the cells to have an openness that would match the need for diffusive transportation for the matter needed for the biochemical syntheses that take place in the cells, first of all for the synthesis of proteins.
Protein synthesis takes place in about ten steps from primary gene expression in DNA inside the nucleus to final production of the mature protein at its final destination outside the nucleus but within the plasma membrane. First there is transcription in which the DNA region encoding the gene is transcribed into a complementary messenger RNA (mRNA). Next, in eucaryotes, initial pre-mRNA is spliced and processed to mature mRNA. This is exported across the nuclear envelope into the cytosol. There, codons in ribosomes progressively translate the genetic code into a mature cytosolic protein. This is followed by several steps of sorting and modification involving cytoplasmic ultrastructures such as the endoplasmic reticulum and Golgi apparatus. All the genes of an organism make up its genome. Of these,
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