Transport proteins do more than just ferry substances across the membrane they accumulate solutes against the concentration gradient. The necessity for carrier mediated transport is easy to understand. If diffusion were the only mechanism by which solutes entered a cell, cells would never achieve the intracellular concentrations necessary to carry out biochemical reactions; that is, their rate of uptake and intracellular concentration would never exceed the external concentration, which in nature is often quite low (Figure 3.9). Hence, cells must have mechanisms for accumulating solutes- most of which are vital nutrients- to levels higher than those in their habitats, and this is the job of transport proteins.