How is Glucose Used for Energy?

Lucy Bell-Young

by Lucy Bell-Young

3rd November 2021

Blog

Energy is derived from glucose when it’s converted into pyruvate through a series of complex chemical reactions known as aerobic cellular respiration. It’s a three-step process that includes glycolysis, Krebs cycle, and electron transport.

Glucose is one of the simplest monosaccharide (or single) sugars. This is because it’s a fundamental unit of other types of sugar (disaccharides) and complex carbohydrates (polysaccharides). It is typically derived from carbohydrate- and sugar-rich food products like potatoes, wheat, rice, and pastries.

What is Glucose?

Glucose is a six-carbon simple sugar classified as a monosaccharide. Typically, it’s the end-product of the digestion of complex carbohydrates (polysaccharides) and disaccharides like sucrose (table sugar). A molecule of sucrose is composed of two monosaccharide units: glucose and fructose, molecularly bonded together.

When dried and in powdered form, glucose molecules exist as linear or straight-chain molecules. When dissolved in water, they form cyclic molecules. See the illustration below for an example:

Glucose structure and properties

What Does Glucose Do?

In mammals, including humans, glucose is transported by the blood to different organs of the body. It’s then absorbed by the cells and metabolised into pyruvate, NADPH, and finally, ATP – these last two store energy and are electron donors. This process is called cellular respiration – here’s a look at how it works: 

Cellular respiration diagram

Glycolysis is the process whereby 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. 

Glycolysis is a catabolic process that breaks down glucose with the help of oxygen. The latter reacts with it to produce the following products per molecule of glucose:

  • 2 molecules of ATP
  • 2 molecules of NADH
  • 2 molecules of pyruvate

Pyruvate and NADH molecules produced from glycolysis enter the mitochondria to participate in the Krebs cycle and electron transport. Pyruvates participate in the Krebs cycle while NADH from the glycolysis participate in the electron transport.

The Krebs cycle is a ten-step cycle that  produces the following molecules per one molecule of pyruvate:

  • 2 molecules of carbon
  • 3 molecules of NADH
  • 1 molecule of FADH2
  • 1 molecule of ATP or GTP

The NADH molecules from the Krebs cycle also participate in electron transport. The electron transport chain 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 sustains the process like a battery. It involves four chemical complexes that serve as enzymes or catalysts. Meanwhile, protons are pumped out.

This process produces the following:

  • NAD+
  • FAD
  • Protons (H+)

In turn, these products 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 in order to produce energy.

Cellular respiration formula

Overall, 36 ATP molecules are produced by aerobic cellular respiration. Water and carbon dioxide are waste products of this process. ATP is the most important byproduct because it releases energy.

What is Glucose Used For in the Body?

The brain, skeletal muscles, and other organs would not function without glucose. Each organ, however, has a unique metabolic profile in terms of using this sugar and other biochemicals. For instance, the human brain uses about 60% of all the glucose in the blood.

The brain needs approximately 120 grams of glucose daily, or 420 kcal (1760 kJ) of energy. In fact, the brain generates a sufficient amount of electricity to power a small incandescent light bulb. Therefore, the energy this sugar supplies is crucial to keep the brain functioning. A person who has a very low level of blood sugar will feel dizzy and may lose consciousness.

Meanwhile, the skeletal muscles partly use glucose as a source of energy, along with fatty acids and ketone bodies. Skeletal muscles store energy in the form of glycogen, with the energy equivalent of 1200 kcal (5000 kJ). Glycogen is a polysaccharide, which is actually composed of multiple branches of glucose. It has the general chemical formula (C6H10O5)n. Approximately 75% of glycogen in the human body is stored in the skeletal muscles, while the rest is stored in the liver and soft tissues.

How Does Glucose Enter the Cell?

Glucose enters the cells through facilitated diffusion, which is regulated by specialised proteins called glucose transporters on the surface of the cell membrane. These transporters are the gatekeepers that prevent glucose saturation inside the cells while allowing the necessary amount for energy use.

Glucose molecules are too big to simply pass through the cell membranes through simple diffusion. Therefore, they need the assistance of transporters. Facilitated diffusion can either be through primary active transport or through secondary active transport.

Primary active transport utilises the energy from ATP to pump glucose into the cell even against the concentration gradient. Meanwhile, secondary active transport doesn’t utilise ATP. Instead, it uses sodium concentration gradients. 

Glucose transporters or GLUTs are classified into three classes:

  • Class I includes GLUTs 1 to 4. GLUTs 1 and 2 function as glucose sensors that detect it in the blood. In humans, they are mainly found in the pancreas, liver, and kidneys. GLUT3 is mainly present in the brain. It has a high affinity for glucose. GLUT4 is responsive to insulin. It has a large concentration in the heart, but is also found in the skeletal muscles and the brain.
  • Class II includes GLUTs 5, 7, 9, and 11. The first member of this class (GLUT5) is a fructose transporter that is primarily found in the small intestine and the kidneys. GLUT7 transports both glucose and fructose. It can be mainly found in the small and large intestines, the kidneys, prostate, and testes. GLUT9 is mainly found in the kidneys and liver. GLUT11 has three isoforms. They are present in the skeletal muscles, the heart, placenta, and the kidneys.
  • Class III includes GLUTs 6, 8, 10, 12, and 13. The brain and the spleen have GLUT6 transporters. GLUT8 transports glucose, but is inhibited by fructose and galactose. GLUT10 is located in almost all the cells of various tissues. GLUT12 is found in the skeletal muscle tissues, the kidneys, the adipose tissues, and small intestine. Finally, GLUT13, also known as HMIT, is abundant in the brain, especially in the brain stem, cerebellum, hypothalamus, and the hippocampus. 

What is the Chemical Formula For Glucose?

Glucose is a simple sugar and has the chemical formula C6H12O6, making it a hexose or a six-carbon sugar. There are other hexoses with the same formula but different molecular structures, such as fructose (fruit sugar).

Disclaimer

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