By what process are most molecules secreted from the blood into the tubule? a. osmosis b. diffusion c. active transport d. facilitated diffusion

To determine the correct answer, let's first understand the four processes briefly: a. Osmosis: Osmosis is the passive movement of water molecules across a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. b. Diffusion: Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached. c. Active transport: Active transport is the movement of molecules against their concentration gradient (from lower to higher concentration) using cellular energy (ATP). d. Facilitated diffusion: Facilitated diffusion is the process in which molecules are passively transported across the cell membrane with the assistance of transport proteins.

In the context of the given exercise, we need to consider how most molecules are secreted from the blood into the tubule. Secretion in the kidneys is the process of moving substances (such as ions, drugs, and waste metabolites) from the blood into the renal tubules to be excreted in the urine. This process often involves moving molecules against their concentration gradient, which suggests that passive processes like osmosis and simple diffusion might not be the primary mechanism for the secretion of most molecules.

Since we have determined that osmosis and simple diffusion are not efficient for the secretion of most molecules, we are left with two options: active transport and facilitated diffusion. Considering that the secretion process often involves the movement of molecules against their concentration gradients, we must consider which of the remaining processes can accomplish this. Active transport uses cellular energy to move substances against their concentration gradients, making it the most efficient and effective mechanism for secreting molecules from the blood into the tubule. Therefore, the correct answer is: c. Active transport