Final answer:
The correct statement is that before reaching the transport maximum, the reabsorption rate of amino acids is the same as the filtration rate. There is a transport maximum for amino acids, and since they are not secreted, the excretion rate will rise only after the Tm is exceeded.
Step-by-step explanation:
In the kidneys, the amino acids that are freely filtered are also 100% reabsorbed and not secreted. This means that under normal conditions, before reaching the transport maximum (Tm), the rate of reabsorption is equivalent to the filtration rate. Therefore, the excretion of amino acids would essentially be zero since excretion can be calculated by the formula: Excretion = Filtration - Reabsorption + Secretion. Given that reabsorption is complete, and no secretion occurs, excretion of amino acids under these conditions would typically not occur.
A transport maximum does exist for amino acids, which is the maximal rate at which the kidney can reabsorb these molecules. When the plasma concentration of an amino acid exceeds this limit, reabsorption plateaus and any excess is excreted in the urine. Since no secretion mechanism is involved for amino acids, there is no renal threshold for their secretion. Also, as there is no secretion, stating that the excretion rate is the same as the filtration rate after reaching the transport maximum is not correct, since any amino acid filtered beyond the Tm would be excreted.