Adsorption vs. Desorption

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Adsorption and desorption are opposite mechanisms that both play an important role in chemical reactions. We'll take a look at enzyme mechanics and chromatography to see how they can work together.

Connecting and Disconnecting

Have you ever tried using tape to attach something to the wall? If you are trying to attach something heavy, you may find that the tape does not stick to the wall well enough and simply falls off. Other times you may actually find tape that is strong enough to remain attached to the wall, but when you try to take it off, the tape brings a section of the wall with it! The ideal tape would stay attached when you want it attached, but easily peel away when you're done. Many companies have tried to find this sweet spot and have marketed products claiming they keep holding up when needed but come off with no damage to the wall.

In chemical reactions we also need to perfect this ability for compounds to attach strongly when the attachment is needed, and detach easily when we need it to detach. When something attaches this is called adsorption and when it detaches this is called desorption. Adsorption should not be confused with absorption, which is the full incorporation into the structure of another compound, not just attachment to the surface.

Enzymes

Enzymes are compounds which help speed up a chemical reaction. They often work by binding to one of the reacting compounds. This can help reorient a compound or bring it closer to other reacting compounds, allowing it to react more easily. Once the reaction has occurred, the enzyme needs to release the new compound; otherwise it isn't an effective enzyme because we don't actually have access to the new compound (and the enzyme can't be re-used).

The attachment of the reactant to the enzyme is a type of adsorption. If the adsorption is not strong then the reactant won't readily attach to the enzyme and the enzyme won't be effective. The release of the new product is desorption, which as discussed earlier needs to happen easily or else the enzyme is not effective.

The heme in hemoglobin can adsorb or desorb oxygen depending on its state
Heme

A great example of this process is the activity of hemoglobin. Hemoglobin's job is to transfer oxygen from the lungs to the muscles. When hemoglobin is in the lungs it needs to be able to have a strong adsorption to oxygen -- otherwise it would leave the lungs carrying no oxygen. But, once hemoglobin is in the muscles it needs to be able to release the oxygen, otherwise the muscles won't get the needed oxygen. Hemoglobin accomplishes this by having two states, the T (tense) and R (relaxed) states. The lungs have a slightly higher pH than the muscles, which puts hemoglobin into the R-state, increasing the adsorption. The lower pH in the muscles then changes hemoglobin to the T-state, which increases the desorption.

Chromatography

Chromatography is a method of separating materials into individual components that is extremely helpful in identifying compounds. Modern chromatography is done with machines, such as HPLC (high performance liquid chromatography), GC (gas chromatography), or IC (ion chromatography) machines. No matter the method used the principles are the same: the machine sends molecules through a thin panel or column that holds onto some molecules for longer than others, then eventually releases all the molecules. Take a moment to think about the use of adsorption and desorption in this scenario, then take a look at the real-world example below.

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