Question

One substance has no membrane transporters to move it but can diffuse freely through open leak channels if there is a concentration gradient. Initially, this substance's concentrations in the filtrate and extracellular fluid are equal. Later, however, the active transport of Na+ and other solutes creates a gradient by removing water from the lumen of the tubule where it is located. What substance is this?

Answer 1

Urea

Explanation:

Urea is a small nitrogenous compound which is the main end product of protein catabolism in mammals

• Urea is a nitrogen-containing substance normally cleared from the blood by the kidney into the urine
• It is made predominantly in the liver from ammonia and bicarbonate and is one of the main components of urine
• The rate of synthesis varies from 300 to 600 mmol/day depending on the protein intake
• All of this urea eventually finds its way into the urine
• Because urea makes up a large part of the obligatory solute excretion, its osmotic pressure requires significant volumes of water to carry the urea
• Urea passively crosses biological membranes, but its permeability is low because of its low solubility in the lipid bilayer
• Some cells speed up this process through urea transporters, which move urea by facilitated diffusion
• Urea is passively reabsorbed in the proximal tubule, but its route of transport is not clear
• Urea transporters have not yet been identified for the proximal tubule