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Part of the TeachMe Series
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Original Author(s): Charlotte Smith
Last updated: 1st December 2020
Revisions: 18

Original Author(s): Charlotte Smith
Last updated: 1st December 2020
Revisions: 18

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Cell membranes are selectively permeable. This means that they allow the movement of some molecules freely across them, but do not allow the free passage of others. In broad terms, there are three ways in which molecules move across membranes. These processes are diffusionosmosis and active transport.

In this article we will describe the process of diffusion, discuss the different types of diffusion, and consider the clinical relevance of this process.

Mechanism of Diffusion

Diffusion is the movement of a molecule down a concentration gradient, from an area of its high concentration to an area of its low concentration. This process is passive, i.e. it requires no input of additional energy; the concentration gradient alone is enough to drive the process.

Types of Diffusion

Diffusion across the cell membrane is either simple or facilitated.

Simple diffusion Facilitated diffusion
Mechanism Molecules move directly across the membrane without the aid of a carrier protein Movement occurs passively down a concentration gradient, however the molecules require the help of carrier proteins to allow them to cross the lipid bilayer
Examples of molecules using this process
  • Hydrophobic molecules such as O2 and CO2
  • Small uncharged polar molecules such as urea
  • Charged molecules (ions)
  • Large uncharged polar molecules such as glucose


An example of a membrane transport protein involved in facilitated diffusion is the GLUT-2 protein, which is the primary protein involved in the transfer of glucose from the liver into the blood.

Figure 1 – Diagram of simple diffusion across the plasma membrane

Rate of Diffusion

Fick’s laws describe the factors affecting the rate of diffusion. Simplified, this states that ‘the rate of diffusion is proportional to the concentration gradient, the length of the diffusion pathway and the surface area available for diffusion‘. This can be written as follows:

Rate of diffusion ∝ (surface area x concentration gradient)/(length of diffusion pathway)

N.B. Fick’s laws of diffusion are in truth more complex, but beyond the scope of this article.

The effect of these factors is summarised in Table 1:

Factor Increased rate of diffusion Reduced rate of diffusion
Concentration gradient Large gradient Small gradient
Length of pathway Short pathway Long pathway
Surface area Greater surface area Reduced surface area

Clinical Relevance

Drug Targets

The channel proteins that allow facilitated diffusion can be exploited as pharmacological targets, particularly in the kidney. Some examples include:

  • SGLT-2 inhibitors: Reduce the reabsorption of glucose in the proximal tubule, and are used in the management of type 2 diabetes.
  • Loop diuretics: Reduce water reabsorption in the Loop of Henle by blocking the sodium/potassium/chloride co-transporter (NKCC2). They are used in conditions such as congestive cardiac failure.
  • Thiazide diuretics: Reduce water reabsorption in the distal convoluted tubule by blocking the sodium/chloride symporter (NCCT). These are primarily used in management of hypertension.

Clinical Relevance

Cystic Fibrosis

The Cystic Fibrosis Transmembrane Conduction Regulator (CFTR) protein is a ligand-gated chloride channel found in the cell membranes of epithelial cells of many organs such as the lungs, pancreas and reproductive tracts.

In healthy people, it allows chloride ions to flow freely out of cells. These chloride ions are followed passively by sodium ions to maintain electrochemical balance, and then water follows via osmosis. This keeps secretions in these organs thin and watery.

Cystic fibrosis results from various mutations that lead to either absent or dysfunctional CFTR proteins. This means that chloride ions cannot flow out of cells, and thus neither does sodium or water. This results in mucus which is thick and sticky, which is especially problematic in the lungs, pancreas and reproductive tract.

However, in the sweat glands the CFTR protein’s role is slightly different. Rather than allowing chloride ions to flow out of the cells, in sweat glands the CFTR protein is involved in the reabsorption of chloride, and subsequently sodium, from the sweat. In cystic fibrosis, this cannot occur, thus sodium and chloride remain in the sweat, giving rise to the classically “salty” sweat seen in CF.

Figure 2 – Diagram of a functioning (1) and faulty (2) chloride channel in cystic fibrosis