Cells constantly create new proteins in order to maintain their normal function, this is called protein turnover.
Why is that? Do the old molecules wear out as time passes, so that they need a replacement?
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Biology is an intricate orchestration of chemical reactions and their products. Generally, this fete is accomplished by enzymatic facilitation of certain reactions that would otherwise occur too slowly.
However, "unwanted" reactions occur spontaneously all the time, too. One important mechanism for these reactions is the presence of free radicals causing oxidative stress: reactive molecules are present in cells as a consequence of the energy necessary for metabolism, and these can react with proteins and other cellular components and change their structure. Modified proteins may fold incorrectly and lose their function (or gain harmful function). There is very little that can be done to prevent these reactions from occurring except to clean up afterwards.
Half-lives vary by protein, but for most proteins are measured in the scale of hours (Chen et al, 2016) to a couple days (Boisvert et al, 2012). By constantly degrading and replacing proteins, cells ensure that their proteins are functional and fresh. You might consider this as analogous to performing regular maintenance on a machine like an automobile, using replacement parts as the originals become worn.
Boisvert, F. M., Ahmad, Y., Gierliński, M., Charrière, F., Lamont, D., Scott, M., ... & Lamond, A. I. (2012). A quantitative spatial proteomics analysis of proteome turnover in human cells. Molecular & Cellular Proteomics, 11(3).
Chen, W., Smeekens, J. M., & Wu, R. (2016). Systematic study of the dynamics and half-lives of newly synthesized proteins in human cells. Chemical science, 7(2), 1393-1400.