Energy Storage in Biological Systems

Living organisms use two major types of energy storage. Energy-rich molecules such as glycogen and triglycerides store energy in the form of covalent chemical bonds. Cells synthesize such molecules and store them for later release of the energy. The second major form of biological energy storage is electrochemical and takes the form of gradients of charged ions across cell membranes. This learning project allows participants to explore some of the details of energy storage molecules and biological energy storage that involves ion gradients across cell membranes.

Energy storage molecules

What is an energy storage molecule? All molecules have energy "stored" in the chemical bonds that link their atoms. However, relatively few molecules found inside living organisms function as significant energy storage molecules. Most molecules of living organisms have other functions such as forming structures, holding and organizing genetic information, transforming existing molecules into other molecules, sending signals that coordinate the behavior of cells. Some molecules have important primary function related to the storage of chemical energy and such molecules can be thought of as "energy storage molecules".

Key aspects for all energy storage molecules are the details of how they are formed by chemical reactions, where they are stored, and how they are metabolized to release the stored energy. In some cases, the location in an organism where an energy storage molecule is formed, stored or where it is during release of the stored energy are not the same and transport of the molecule through an organism's body becomes an important issue.

Among the energy storage molecules, one of the key distinctions that can be made is the normal length of time between the formation of the molecule and its metabolism and release of the stored energy. Molecules such as ATP can be formed, diffuse a short distance within a cell and be metabolized to release the stored energy within a very short time period (on the order of seconds). Fat molecules can be stored in fat tissue and then used many months later, for example, during hibernation, or even years, in the case of seeds or eggs. Other energy storage molecules are specialized to hold energy for intermediate periods time, from minutes to days.

Examples of energy storage molecules

Glucose is a major energy storage molecule used to transport energy between different types of cells in the human body.

Starch

Fats

Fat itself has a high energy or calorfic value and can be directly burned in a fire. In the human body and presumably other animals, it serves a number of roles as there are different kinds of fats, but for the purpose of the discussion here, fats are frequently found associated with each of the organs in the body.

For example there is a deposit of fat on the heart and it was only relatively recently that it was realized that this acts as a temporary storage of buffer for energy. If one thinks of the blood system flowing around a heart, the glucose levels in it can fluctuate depending on a number of factors, such as whether one has recently digested a meal or engaged in strenous activity. As with most machines, steady inputs cause less strain on the system than large fluctuations. And so the role of the fat is to help smooth out these lean periods and allow the heart to continue operating without putting additional strain on it.

Adenosine triphosphate

Adenosine triphosphate or ATP is one of the key molecular energy carriers in the biological world.

Chemiosmosis

The term chemiosmosis refers to the inter-conversion of chemical energy (energy in the form of chemical bonds) and energy in the from of a transmembrane electrochemical gradient. The idea of "chemiosmotic coupling" arose largely from the work of Peter D. Mitchell and revolutionized the way biologists think about energy storage in biological systems.

ATP

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Transmembrane ion gradients