Serine is an amino acid found in many foods.  Serine diffuses from the digestive tract into the cells lining the digestive tract through serine transporters.  A team of physiologists measured the serine diffusion rate from the digestive tract into the cells of several different mice.  Surprisingly, they found that the serine diffusion rate was greatest in the mouse that already had the highest serine concentration in its cells.

a) Identify as many ways as you can that could cause the greatest serine diffusion rate in the mouse that already had the highest concentration of serine in its cells.

b) Explain how the ways you identified could cause the greatest serine diffusion rate in the mouse that already had the highest concentration of serine in its cells.