Membranes inside cells
Membranes are also crucially important inside cells for two reasons: first, to provide surfaces on which chemical reactions can proceed, and secondly to provide separate areas inside the cell, allowing chemical reactions to proceed which might otherwise interfere with each other. In bacteria, the inner surface of the plasma membrane defi nes the position of everything within the cell and provides attachment points for intracellular contents that need to be in specific positions. Using the analogy of the cell as a factory, the internal membranes provide the workbenches, floors, ceilings, and walls for all the different parts of cell production, with the nucleus centrally positioned as the offi ce in which the information is stored. In small cells, such as bacteria, which are usually rod shaped, the inside of the plasma membrane provides a large surface area in relation to the cellinterior, so that anything that needs a fixed position can be ‘hung’ on the inside, and consequently bacteria and other prokaryotes generally have little or no internal membrane. As mentioned earlier, the internal volume of eukaryote cells is a thousand times that of a bacterium, so that the eukaryotic cell requires a vast internal membrane system around a hundred times the area of the plasma membrane itself. This internal segregation of biochemical activities is crucial as there are hundreds of chemical reactions going on that can seriously interfere with each other. Prokaryotes, with no internal membranes (and eukaryotes to some extent), get round this problem by aggregating groups of specific enzymes into multiprotein complexes, which work as free entities inside the cell. In addition, eukaryotes confi ne different metabolic processes within membrane-bounded compartments. The major internal membrane system in eukaryotic cells is the endoplasmic reticulum (usually shortened to ER), which forms a network throughout the entire cell. This part of the cell is called the cytoplasm (everything inside the cell membrane excluding the nucleus; Figure 5a , b) and everything in it is surrounded by the cytosol, a complex mixture of substances dissolved in water, like a very crowded ‘molecular soup’.