METABOLIC ENERGY: TYPES, SOURCES AND TRANSFORMATION - BIOLOGY - 2024

The metabolic energy is the energy that all living beings obtained from the chemical energy contained in food (or nutrients). This energy is basically the same for all cells; however, the way to obtain it is very diverse.

Food is made up of a series of biomolecules of various types, which have chemical energy stored in their bonds. In this way, organisms can take advantage of the energy stored in food and then use this energy in other metabolic processes.

All living organisms need energy to grow and reproduce, maintain their structures, and respond to the environment. Metabolism encompasses the chemical processes that sustain life and that allow organisms to transform chemical energy into useful energy for cells.

In animals, metabolism breaks down carbohydrates, lipids, proteins, and nucleic acids to provide chemical energy. Plants, for their part, convert light energy from the Sun into chemical energy to synthesize other molecules; they do this during the photosynthesis process.

Types of metabolic reactions

Metabolism comprises several types of reactions that can be grouped into two broad categories: the degradation reactions of organic molecules and the synthesis reactions of other biomolecules.

Metabolic degradation reactions constitute cellular catabolism (or catabolic reactions). These involve the oxidation of energy-rich molecules, such as glucose and other sugars (carbohydrates). Since these reactions release energy, they are called exergonic.

In contrast, synthesis reactions make up cellular anabolism (or anabolic reactions). These carry out processes of reduction of molecules to form others rich in stored energy, such as glycogen. Because these reactions consume energy, they are called endergonic.

Sources of metabolic energy

The main sources of metabolic energy are glucose molecules and fatty acids. These constitute a group of biomolecules that can be rapidly oxidized for energy.

Glucose molecules come mostly from carbohydrates ingested in the diet, such as rice, bread, pasta, among other derivatives of vegetables rich in starch. When there is little glucose in the blood, it can also be obtained from glycogen molecules stored in the liver.

During prolonged fasting, or in processes that require additional energy expenditure, it is necessary to obtain this energy from fatty acids that are mobilized from adipose tissue.

These fatty acids undergo a series of metabolic reactions that activate them, and allow their transport to the interior of the mitochondria where they will be oxidized. This process is called β-oxidation of fatty acids and provides up to 80% additional energy under these conditions.

Proteins and fats are the last reserve to synthesize new glucose molecules, particularly in cases of extreme fasting. This reaction is of the anabolic type and is known as gluconeogenesis.

Process of transformation of chemical energy into metabolic energy

Complex food molecules such as sugars, fats and proteins are rich sources of energy for cells, because much of the energy used to make these molecules is literally stored within the chemical bonds that hold them together.

Scientists can measure the amount of energy stored in food using a device called a bomb calorimeter. With this technique, the food is placed inside the calorimeter and heated until it burns. The excess heat released by the reaction is directly proportional to the amount of energy contained in the food.

The reality is that cells do not function as calorimeters. Instead of burning energy in one big reaction, cells release the energy stored in their food molecules slowly through a series of oxidation reactions.

Oxidation

Oxidation describes a type of chemical reaction in which electrons are transferred from one molecule to another, changing the composition and energy content of the donor and acceptor molecules. Molecules in food act as electron donors.

During each oxidation reaction involved in the decomposition of food, the product of the reaction has a lower energy content than the donor molecule that preceded it on the path.

At the same time, the electron acceptor molecules capture some of the energy that is lost from the food molecule during each oxidation reaction and store it for later use.

Eventually, when the carbon atoms in a complex organic molecule are completely oxidized (at the end of the reaction chain) they are released as carbon dioxide.

Cells do not use the energy from oxidation reactions as soon as it is released. What happens is that they convert it into small, energy-rich molecules, such as ATP and NADH, that can be used throughout the cell to boost metabolism and build new cellular components.

Standby power

When energy is abundant, eukaryotic cells create larger, energy-rich molecules to store this excess energy.

The resulting sugars and fats are held in deposits within cells, some of which are large enough to be visible on electron micrographs.

Animal cells can also synthesize branched polymers of glucose (glycogen), which in turn aggregate into particles that can be observed by electron microscopy. A cell can rapidly mobilize these particles whenever it needs fast energy.

However, under normal circumstances humans store enough glycogen to provide a day's worth of energy. Plant cells do not produce glycogen, but instead make different glucose polymers known as starches, which are stored in granules.

In addition, both plant and animal cells save energy by diverting glucose in the fat synthesis pathways. One gram of fat contains almost six times the energy of the same amount of glycogen, but the energy from fat is less available than that from glycogen.

Still, each storage mechanism is important because cells need both short-term and long-term energy stores.

Fats are stored in droplets in the cytoplasm of cells. Humans generally store enough fat to power their cells for several weeks.

References

1. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K. & Walter, P. (2014). Molecular Biology of the Cell (6th ed.). Garland Science.
2. Berg, J., Tymoczko, J., Gatto, G. & Strayer, L. (2015). Biochemistry (8th ed.). WH Freeman and Company
3. Campbell, N. & Reece, J. (2005). Biology (2nd ed.) Pearson Education.
4. Lodish, H., Berk, A., Kaiser, C., Krieger, M., Bretscher, A., Ploegh, H., Amon, A. & Martin, K. (2016). Molecular Cell Biology (8th ed.). WH Freeman and Company.
5. Purves, W., Sadava, D., Orians, G. & Heller, H. (2004). Life: the science of biology (7th ed.). Sinauer Associates and WH Freeman.
6. Solomon, E., Berg, L. & Martin, D. (2004). Biology (7th ed.) Cengage Learning.
7. Voet, D., Voet, J. & Pratt, C. (2016). Fundamentals of Biochemistry: Life at the Molecular Level (5th ed.). Wiley.