Module 8-Circulation, Immunity, and Excretion

Lesson 8-Urine Formation in the Nephron

 

A Review of Diffusion, Osmosis and Active Transport

 

Molecules and ions enter and leave cells through membranes in the body by two basic processes: passive transport and active transport. It is important to review these processes in order to understand nephron function.

  1. Passive transport is movement that does not rely on energy expenditure by the cell. One example is diffusion. This is movement of molecules or ions from an area of high concentration to one of lower concentration; that is, it is movement along the concentration gradient. The energy for this movement comes from the motion of the particles themselves and not from the ATP supplies in the cell.

    Osmosis is a special kind of diffusion, involving water and a membrane. Water (solvent) molecules diffuse across a differentially permeable membrane from a hypotonic solution (lower solute concentration) to a hypertonic solution (higher solute concentration). Osmotic pressure can be thought of as the tendency of a solution to gain water by osmosis. Thus, the hypertonic solution has a higher osmotic pressure than the hypotonic solution. As solute concentration is decreased, osmotic pressure is decreased.


    Inquiry into Biology (Whitby, ON: McGraw-Hill Ryerson, 2007), BLM 9.1.6. Reproduced by permission.

    Another example of passive transport is movement caused by electrostatic attractions. For example, negatively charged ions follow the movement of positive ions as they are pumped across a membrane, because of the attraction between unlike charges. The movement of the negative ions is passive because they merely "tag along" as the positive ions are actively transported.

  2. Active Transport involves movement that is powered by the ATP supplies in the cell. Molecules are pumped across a cell membrane from an area of low concentration to one of higher concentration. This process is energy-absorbing because the movement of particles is against the concentration gradient.

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