Cells, Mitochondria, and Cell Membranes

One-cell organisms such as bacteria have a diameter of 1-2 am. Cells in multicellular organisms are usually ten times larger. The inside of the cell, the protoplasm, contains enzymes, RNA, molecules of the degrading process (metabolism), mitochondria, and cell nucleus. Enzymes are proteins in union with active molecules called coenzymes. 

Most cells have a nucleus containing DNA with reproductive information (the genome). The cell is separated from the outer world by a plasma membrane. The latter consists of a double layer of phospholipids, arranged perpendicular to the membrane, with the hydrophobic end turned inward. The hydrophilic end is turned to the polar water solutions inside and outside the cell. The hydrophobic parts of each layer attract each other, probably due to great structural flexibility, increasing the entropy at room temperature. 

The free energy of easily mobile electrons is used to pump protons from the matrix across the inner mitochondrial membrane into the intermembrane space. There, an acidity gradient is built up, giving free energy for the protons to transfer back through the inner membrane and form ATP from ADP with the help of the protein enzyme ATP synthase. Apparently, a mechanical rotation first takes place in 

Since conserved strnctures with a very long lifetime have a great advantage in the evolution, a biological system is used over and over again. The result is cyclic reactions where reactants are fed in along a pathway and products are given away. There are several examples in Natnre, for example, the citric acid cycle and the Calvin cycle. 

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