How & Why Is Chemical Energy Stored In Food?

Lucy Bell-Young

by Lucy Bell-Young

2nd September 2020

Food is basically solar energy that’s stored in the form of complex biochemical substances. The process of releasing stored energy from food is almost as complex as the process of storing energy in food. This article will explain how this process works, as well as why it happens. 

A Brief Overview Of Chemical Energy

Chemical energy is simply energy that is stored in compounds or elements, specifically in the atomic bonds that connect atoms and molecules. In food, it’s a form of potential energy because it’s stored away. Then, through a process called chemical digestion, the bonds containing the stored energy are broken, releasing energy that our bodies then use. 

Chemical energy can also be converted from one form to another. For instance, when hydrogen gas combines with oxygen gas, thermal energy and light energy are released by the process of combustion. New covalent bonds are then formed between the atoms of hydrogen and oxygen to form water.

Combustion is a simple, yet violent type of chemical reaction that releases thermal, heat, and sound energy. Many chemical reactions, however, are slow and barely perceptible, like the process of rusting, which is when metal becomes oxidised. Just like combustion, rusting is a one-step process.

On the other hand, some chemical reactions are complex. These more complex processes are usually found in living organisms. The conversion of chemical energy from food, for instance, is a multi-step process that involves breaking down complex substances, like carbohydrates and proteins, into their basic constituents, forming various intermediate compounds along the way.

Graphic showing how the human body digests food

In food, chemical energy is stored away as potential energy

Is Food An Example Of Chemical Energy?

Food is an example of stored chemical energy that is converted into usable energy by our cells. We all know what food is, but more specifically, it’s any edible part of an animal, plant, algae, fungus, plankton, bacteria, or other organism, that is absorbed by another organism as a source of nutrients and energy:

  • Some organisms, like plants and algae, manufacture their food through photosynthesis
  • Some archaebacteria, like chemoautotrophs, process food from inorganic and organic chemicals 
  • Chemoautotrophs living in deep caves, for example, can derive food directly from rock minerals

Luckily for us, finding food is easier than that, but the process of how it is converted in our bodies is just as complex. Food is utilised to make energy-rich molecules, such as ATP and NADPH. These molecules are then used by our cells to synthesise other molecules and form various biochemical products, like enzymes and hormones. These complex metabolic processes synthesise proteins and other materials that eventually become parts of cells, tissues, and organs in all multicellular organisms (which includes us).

During this process, food gets broken down into basic molecules, like amino acids, lipids and glucose. These, among other organic molecular components, become the building blocks of the organism that ate the food. In humans, for example, amino acids from food are used to build proteins that become part of our muscles. 

How Chemical Energy Is Stored In Food

At the most fundamental level, chemical energy is stored in food as molecular bonds. These molecular bonds represent potential energy, which is either very stable, such as in fat molecules, or very active and transitory, such as in ATP molecules.

In living organisms, energy is also stored through the electron potentials across membranes, such as those in the thylakoid membranes during photosynthesis. 

There are several ways of storing energy in food, including photosynthesis and chemosynthesis. Here on earth, the ultimate source of energy that gets converted into food is the sun. Chemosynthetic bacteria living in undersea darkness might seem to be the exception to this, but even these vampiric microorganisms are still indirectly dependent on the sun’s energy. 

There are two major ways that living organisms store energy:

  • Energy-rich molecules: Glycogen, carbohydrates, triglycerides, and lipids are energy storage molecules. These molecules store energy in the form of covalent bonds. In fact, any biologically synthesised molecules, like proteins, serve as energy storage. However, some molecules are more easily converted into energy than others.
  • Electrochemical potential: This type of energy storage is more active and readily used by cells. Electrochemical potential takes the form of ionic gradients across cell membranes, such as those in thylakoid membranes and mitochondrial membranes.

In our planet’s ecosystem, all organisms are potential food for other organisms. Therefore, all organisms are technically chemical energy storage units. Hierarchy is apparent in any ecological system, and you’ll remember this interconnectivity between organisms as the food chain:

  • At the base of the food chain are the producers, i.e. the photosynthetic organisms and chemoautotrophs. They serve as the main support for all other organisms, including themselves. 
  • Producers are eaten by primary consumers like herbivorous ungulates, including cows, horses, rhinos, and giraffes. The energy stored in the producers is then passed onto the herbivore.
  • The primary consumers are then eaten by the secondary consumers, carnivores and omnivores. Carnivorous predators and omnivores, like humans, are at the top of the food chain.

Then it’s the turn of the decomposers, like bacteria and fungi. These consume other organisms once they have died. The organic remains of other organisms are also absorbed by producers as nutrients from the ground. Thus, the cycle is complete.

Diagram of ATP molecule

Food is utilised to make energy-rich molecules, such as ATP, the key molecule used by cells in metabolic processes

How Do Cells Release Chemical Energy From Food?

Cells release chemical energy from food through the process of respiration, which can either be aerobic or anaerobic. This process occurs primarily in the mitochondria of cells.

Aerobic respiration requires oxygen, while anaerobic respiration does not. The release of energy is necessary to fuel the activities of the cells, such as biochemical synthesis, repairing the body, and reproduction. ATP, or adenosine triphosphate, is the key molecule that is utilised by the cells in various metabolic processes.

Virtually all activities of the cells require energy. These cellular activities are called metabolism, which can be divided into two categories:

  • Catabolism: During this process, molecular bonds are broken down to release energy from food
  • Anabolism: This is the process of synthesising biochemical compounds needed by the cells

The release of chemical energy from food is a catabolic process. But before cells can release energy, the food must first be digested, reduced to its basic constituents, absorbed by the cells, and stored.

During aerobic respiration, oxygen reacts with the basic constituents of food, like carbohydrates, fats, and proteins. These are consumed as reactants to create ATP molecules. For example, ATP is produced when glucose reacts with oxygen during aerobic cellular respiration.

An energy transfer then occurs, which is used to break the molecular bond of ADP in order to add a third phosphate group, forming ATP molecules. NADH and FADH2 are also involved in the phosphorylation process at the substrate level. 

The simplified chemical reaction is as follows:

C6H12O6 (s) + 6O2 (g) → 6 CO2 (g) + 6 H2O (l)

Anaerobic respiration, on the other hand, is the main metabolic path in many species of bacteria and archaebacteria. Since bacteria do not have specialised organelles, anaerobic respiration occurs in the cytoplasm. Instead of oxygen, anaerobic respiration uses sulfate, nitrate, sulfur, or fumarate as electron acceptors.

What Process Releases The Chemical Energy Stored In Food?

The chemical energy stored in food is released by cells through the process of respiration. This process has four steps, and mainly produces ATP as the energy-carrying molecule that can be used by cells in their metabolic activities. The four steps of the process are:

  1. Glycolysis: This uses 2 ATP molecules, but 4 ATP molecules are produced via substrate-level phosphorylation.
  2. Oxidative decarboxylation of pyruvate: 5 ATP molecules are produced through oxidative phosphorylation.
  3. Citric acid cycle: This is otherwise known as the Krebs cycle. A total of 20 ATP molecules are produced through the various stages of the process.
  4. Oxidative phosphorylation: A further 3 or 5 ATP molecules are produced with the aid of two NADH coenzymes through oxidative phosphorylation.

The total ATP yield of respiration is 30 or 32 per glucose molecule. Although the theoretical yield is 38 ATP molecules per glucose molecule, some are lost because of being spent in moving pyruvate from glycolysis, phosphate and ADP into the mitochondria.

Diagram showing how cellular respiration works

The chemical energy stored in food is released by cells through the process of respiration

Why Is Chemical Energy Stored In Food?

Chemical energy is stored in food because of the various molecular bonds in food and the electrochemical gradients that they create. Depending on the type of food, these bonds may either be easy or difficult to break. The constituents of food, such as carbohydrates, fibres, minerals, fats, and proteins, act as reactants.

They are broken down into basic molecules, like amino acids and glucose, which are either used as energy or reassembled and stored in other forms, like glycogen. The presence of chemical energy in food is crucial to providing our bodies with the energy they need to keep us moving and alive.


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