A plant cell was put into a solution of substance A at a concentration of 200 mmol/L. The rate of uptake into the cell was measured as 5 mmol/minute. When the cell was placed in a solution of substance A at a concentration of 400 mmol/L, the rate of uptake was 10 mmol per minute. When the experiment was repeated with substance B, the rate of uptake was 10 mmol per minute at both concentrations of the substance. Which of the following provides the best explanation of the results?