Glucose is one of the simplest sugars or monosaccharides that serves as the building block of all carbohydrates.
It’s also the primary source of energy for every known biological organism. Glucose is typically derived from carbohydrate- and sugar-rich food products such as wheat, rice and cakes.
Continue reading to learn more about glucose, how it’s made, and its function in the body.
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Glucose: a primary source of energy
Glucose comprises six carbon atoms and has the chemical formula C6H12O6. It’s the main energy source for every known biological organism, including plants. Cells release chemical energy through aerobic and anaerobic cellular respiration. Glucose is needed for both of these processes.
When glucose enters the body, it’s converted into energy via a series of complex chemical reactions. In the absence of glucose, fats and proteins are secondary sources of energy. In the first step of aerobic respiration, glucose is broken down in a catabolic process known as glycolysis. When this happens, energy is released in the form of ATP.
During glycolysis, glucose and oxygen react and form the following products per molecule of glucose:
- 2 molecules of ATP
- 2 molecules of NADH
- 2 molecules of pyruvate
As we explain below, the NADH and pyruvate molecules then enter the mitochondria to continue the next two steps of aerobic respiration – the Krebs cycle and the electron transport chain.
What is glucose?
Glucose is one of the simplest types of sugar and the building block (monomer) of more complex carbohydrates. It’s classed as a monosaccharide because it only contains one sugar unit.
A glucose molecule consists of six carbon atoms and an aldehyde group, making it an aldohexose. Other examples of aldohexoses include allose, altrose, galactose, gulose, idose, mannose, and talose.
Glucose is naturally present in sugar-rich natural sweeteners such as honey, as well as many processed treats like pastries and cakes. It’s also found in carbohydrate-derived foods like wheat, rice, and potatoes.
When dried and in powdered form, glucose molecules exist as linear or straight-chain molecules. However, they form cyclic molecules when dissolved in water. See the illustration below for an example:
The chemical formula of glucose
The chemical formula for glucose is C6H12O6 but it has several isomers. Both galactose and fructose have the same chemical formula as glucose, although they’re structurally different.
Where does glucose come from?
Glucose is naturally present in many plant products as a result of photosynthesis. It’s typically found in fruits, root crops, flowers (particularly in the nectar), and leaves. Glucose also comes from the breakdown of complex sugars such as sucrose during digestion.
The function of glucose in the body
Glucose plays a vital role in the body; our brain, skeletal muscles, and other organs simply wouldn’t be able to function without it.
After we’ve eaten, glucose is absorbed by the body via the small intestines and then carried around our body through the circulatory system. The blood carries nutrients (including glucose), oxygen, and waste products, to and from the cells to sustain them.
Virtually all of our organs and organ systems rely on glucose as their main source of energy. This is particularly true for the brain, which requires about 120g of glucose, equivalent to 420 kcal of energy, per day.
In humans, the metabolism of glucose is regulated by the hormone insulin. Without enough insulin, the cells can’t absorb and utilise glucose for energy and they will begin to malfunction.
How is glucose made?
Plants make glucose via photosynthesis. During this process, they capture sunlight in their leaves and then use the energy to convert carbon dioxide and water into glucose. Plants use glucose to perform other cellular activities and make substances such as starch and cellulose
Photosynthesis involves both light-dependent reactions and light-independent reactions. It’s in the light-independent reactions that glucose is produced. This cyclical series of reactions is known as the Calvin-Benson cycle. It’s fuelled by the ATP molecules produced during the light-dependent reactions.
The process of glycolysis
Glycolysis is the first step in the breakdown of glucose into energy. During glycolysis, glucose is converted into pyruvic acid. This occurs in the cytoplasm, where ATP is released for later use by the cell to supply energy for various metabolic processes and syntheses.
The overall reaction of glycolysis can be written as a balanced chemical equation:
C6H12O6 + 2 NAD+ + 2 ADP + 2 P —–>
2 pyruvic acid + 2 ATP + 2 NADH + 2 H+
Glycolysis is followed by the other two main steps in respiration – the Krebs cycle and the electron transport chain – which occur in the mitochondria.
The Krebs cycle
The Krebs cycle produces two molecules of carbon and one molecule of ATP or GTP per molecule of glucose. It also produces three molecules of NADH and one molecule of FADH2, both of which are necessary for the third phase of aerobic respiration (the electron transport chain).
The Krebs cycle is sometimes called the citric acid cycle because it involves citrate.
Electron transport chain
Also known as oxidative phosphorylation, the electron transport chain is the third phase of aerobic cellular respiration. It occurs in the inner part of the mitochondria.
During this step, electrons flow and create electrical potential between the inner mitochondrial membrane and the exterior of the mitochondria. This process produces NAD+, FAD and protons (H+), which facilitate the formation of water and ATP molecules.
In cellular metabolic processes, energy is released when ATP is converted into ADP as high-energy molecular bonds are broken. In a simplified chemical equation, cellular respiration can be summarised as the oxidation or burning of glucose to produce energy.
One molecule of glucose can produce up to 36 ATP molecules through aerobic cellular respiration. Water and carbon dioxide are waste products of this process but ATP is the most important byproduct because it releases energy.
How does glucose enter the cell?
Glucose enters the cell with the aid of glucose transporters. There are two main types of transporters – sodium-glucose linked transporters (SGLTs) and facilitated diffusion glucose transporters (GLUT). They can further be classified into various subtypes. These transporters differ in terms of their specific substrate, distribution, and mechanism of regulation.
Insulin helps multicellular organisms such as humans to absorb and utilise glucose. It binds with receptors on the surface of the cells to attract GLUT4 molecules, which then transport glucose inside the cells.
Conclusion
Glucose is a simple sugar and the primary source of energy for every known biological organism. It’s made by plants during photosynthesis. In humans, glucose is broken down into energy through a series of complex biochemical phases including glycolysis, the Krebs cycle and oxidative phosphorylation (the electron transport chain). This process releases energy in the form of ATP.
