Proteins often consist of hundreds or thousands of individual parts—the amino acids. These are connected like the links of a chain. However, protein molecules would not be able to fulfill their task as a long filament flapping back and forth. Each of them is therefore folded in its own, very specific way during its creation. For proteins that are released from the cell to the outside or transported to intracellular stores, this folding takes place at a specific location in the cell: the endoplasmic reticulum (ER). Here, the partial strands that end up lying next to each other in the course of this protein folding can also be linked together at defined points. This prevents the ball from unraveling and retains its functional shape.
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Source: Phys.org