Metabolic Energy: Types, Sources, Process of Transformation of Chemical Energy into Metabolic Energy

The metabolic energy it is the energy obtained by all living beings 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.

Foods are formed by 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 allows 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. On the other hand, the plants convert the light energy of the Sun into chemical energy to synthesize other molecules; They do this during the process of photosynthesis.

Types of metabolic reactions

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

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

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 endergonals.

Metabolic energy sources

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 starchy vegetables. When there is little glucose in the blood, it can also be obtained from the glycogen molecules stored in the liver.

During the prolonged fast, or in the processes that require an additional expenditure of energy, it is required to obtain this energy from the fatty acids that are mobilized from the 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 in 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

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

Scientists can measure the amount of energy stored in food using a device called a calorimetric pump. 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 work like calorimeters. Instead of burning the energy in a large reaction, the 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. Food molecules act as electron donors.

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

At the same time, the electron acceptor molecules capture part 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 of a complex organic molecule are completely oxidized (at the end of the reaction chain) they are released in the form of carbon dioxide.

The cells do not use the energy of the 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, which can be used throughout the cell to boost metabolism and build new cellular components.

Reserve power

When energy is abundant, eukaryotic cells create larger, energy-rich molecules to store this excess energy. The resulting sugars and fats are kept in deposits within the cells, some of which are large enough to be visible in the electron micrographs.

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

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

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

Even so, each storage mechanism is important because the cells need energy deposits in both the short and long term. Fats are stored in droplets in the cytoplasm of cells. Humans usually store enough fat to supply their cells with energy for several weeks.

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