Transport proteins are responsible for moving materials such as nutrients and metabolic products through a cell's outer membrane, which seals and protects all living cells, to the cell's interior. These transported molecules include sugars, which can be used to fuel growth or to respond to chemical signals of activity or stress outside of the cell. Measuring the activity of transporter proteins in a living organism has been a challenge for scientists, because the methods are difficult, often require the use of radioactive tracers, and are difficult to use in intact tissues and organs.
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