Glucose

This is the first in a multitude of comprehensive articles related to the function and worth of the nutritional giants known as the macronutrients. This will be an introductory article to what the glucose molecule is, what it does, where it can be found, among many other aspects related to this important molecule. In this article you will learn everything you might want to know, without too much detail bogging you down, on glucose and its role in the carbohydrate family.

What is Glucose?

The term “glucose” is interchangeably replaced with the term “sugar” and/or “dextrose”, and while this is not exactly inaccurate, for the purpose of this article, we will be addressing the molecule glucose by its respected scientific name. We do this, because glucose is just one of the three possible sugars and generalizing it as sugar can lead to confusion. Glucose is, like anything, a molecule. However, it is a special molecule, because it does a wide array of things for human life (discussed in the next section).

Monosaccharide

Glucose is a monosaccharide, one of three (fructose and galactose being the other two), actually; hence the confusion calling glucose a “sugar” as there are, technically, two other simple “sugars” [6]. We will not discuss the other two monosaccharides, but the reason all three of these molecules are called monosaccharides is because they are the simplest form of carbohydrates [6]. Mono, meaning “one”, and cannot be further broken down, but can be chained together with more units of itself or units of the other two monosaccharides to create more complex saccharides (di- and poly- saccharides) [6]. It is actually these more complex saccharides that are found in nutritional content (fruits, vegetables, bread, etc.), not the individual monosaccharides themselves, but the body must break these di- and poly-saccharides down into their monosaccharide constituents to be absorbed [6][12].

Types of Glucose

There are two major types of glucose, D-glucose and L-glucose. Both have the same molecular make up as both contain 6 carbons, 12 hydrogens, and 6 oxygens, yet only one of them is found in abundance and used by the body for a massive number of metabolic processes – D-glucose [1]. Why, though?

Well, apart from L-Glucose being made in a lab and not found in nutrition useful to humans, the L-glucose molecule is slightly structurally different from D-glucose as the molecule, although the same atoms used (C6H12O6), is structured in a mirrored sequence to D-glucose and cannot be digested by the body [2][3].

So, our focus is on D-glucose, because the entire body’s metabolic processes are designed to fit well with the way this particular glucose molecule is structured [4][5]. It is called D-glucose for its molecular structure and the common name “dextrose” that it is also referred [1].

L-glucose is the mirror image of D-glucose, and as such does not fit in the enzymatic reactions fit for D-glucose. Both glucose molecules are shown here in their "stretched" versions, which is not their common formation.

D-glucose in its usual formation. This specific structure allows it to attach and interact with various enzymes in the body.

What function does D-glucose serve?

Glucose plays countless roles in the body, but most notably, it keeps you alive. Without glucose, we die. Luckily, we do not even have to consume glucose to have glucose in our system (this will be explained later in this article). However, glucose does involve itself in many processes related to metabolism. As mentioned before, glucose’s structure and molecular make up is such that it fits well with our entire physiology.

First, we should understand how glucose affects metabolism. Our cells go through a series of metabolic systems, but all of them are dependent on glucose existence one way or another. Because the glucose molecule is structured the way it is, once it is accepted into the cells of the body for energy, it is then phosphorylated (a phosphate group is added to it) for several reasons.

1. Phosphorylated glucose cannot dissociate back out of the cell as its structure has changed [5].
2. Phosphorylated glucose can go through the storage metabolic pathway (glycogenesis) [8].
3. Phosphorylated glucose can go through the immediate energy production pathway (glycolysis and pentose phosphate pathway)[7].

The key to remember here is that phosphate groups are closely linked to energy states (Adenosine Tri Phosphate – ATP), and a molecule that can attach a phosphate group to it and then carry that phosphate group through various sequences of metabolism is incredibly valuable – ergo, glucose is incredibly valuable. Without it, we have no molecules to hold onto phosphate groups and continue the sequences necessary to create more ATP (energy) for daily function. Glucose offers 30 ATP per molecule, and in practical terms, offers 4 calories like most other carbohydrates [14][15].

Without enough glucose in our system, the body does not have enough phosphate transporters to continue metabolism and our organs shut down until we die.

 

Reason #1: Phosphate group attaches to glucose upon entering the cell, so the "doorway" that it came through no longer fits for glucose to leave the cell again. Discussed further in Glycogenesis.

Reason #2: Once glucose is trapped inside the cell due to the added phosphate group, it can be stored as glycogen using the metabolic pathway of glycogenesis. Discussed, in detail, in Glycogenesis.

Reason #3: Once the phosphate group has been added, if glucose is not stored via glycogenesis, then it is used for energy via glycolysis

Body regulation of glucose?

Too little or too much glucose in the body leads to eventual health issues and premature death. However, the body is quite adept at handling low and excess amounts of glucose.

First, we need to understand that glucose is measured in the blood. The amount of glucose in the blood tells us how well the body is regulating glucose between the cells of the body and how much it needs to dump, or take out, into and out of, the circulatory system to attenuate a high level of energy demand or low level of energy demand. If the body has excess glucose, it will store it and if it has such high excess levels that it can no longer store it, it dumps it into the blood system. High blood glucose level is called hyperglycemia and if this persists it develops into diabetes.

Too little glucose in your blood system is called hypoglycemia and if the body is either inept at adding more into the blood system from storage due to a series of possible issues (illness, disease, not eating enough, etc.) or the body has been starving to a point it has no more glucose to release (literally not eating until near death), then our metabolism shuts down, our organs shut down, and we die (yippee!).

So, how do we prevent these issues?

The body is usually extremely good at regulating blood glucose levels if you are consuming enough food. Here are the scenarios and their physiological solutions:


1. Too much glucose in the circulatory (blood) system:

Body stores glucose in the muscle and liver cells (primarily) via a process called glycogenesis [8]. However, the muscles and liver only store a certain amount (as glycogen) and once that capacity is reached, glucose is stored as adipose tissue (body fat) via de novo lipogenesis [9]. Both of these occur to maintain a strict range of acceptable blood glucose as too much blood glucose can lead to types of “clogging” of the arteries as the blood is oversaturated with glucose molecules.

2. Too little glucose in the circulatory system:

When blood glucose levels begin to dip, the body is then able to release glucose back into the circulatory, blood system via the liver, but cannot release glucose from the muscles [10]. This occurs when glycogenolysis releases stored glucose from storage within the cell, and then the hepatic cells pump glucose into the blood system [10]. On other hand, eating releases glucose in the blood stream, as well. Another process, called gluconeogenesis, uses other substrates such as fat and amino acids to produce glucose [11].

Here is a simple diagram showing the different reactions to hypo- and hyper-glycemia. 

Note: Insulin is released, but glycogen stores are not full, body will store glucose as glycogen until full - then proceed with storage in fat cells. 

Digestion and Absorption

Just as with anything, glucose needs to be digested before being absorbed. Also, as touched upon earlier, glucose is a monosaccharide and rarely comes in monosaccharide form, but usually chained together into disaccharides and polysaccharides. We also know that only monosaccharides are absorbed by the body, so the body needs to get to the glucose molecule in isolation, but the glucose molecule does not come, usually, in isolation; this is where digestion steps in and takes over [12].

Digestion of glucose begins in the mouth. Aside from mastication (chewing) that occurs with almost all foods, glucose also has salivary enzymes that help begin the break down process from disaccharide and polysaccharide glucose chains to isolated units (monosaccharides) of glucose [12][13]. This article will not cover every step of the digestion and absorption process, but know that enzymatic digestion begins in the mouth, unique to other macronutrients.

As with any other energy dense nutrient, glucose is then swallowed and sent to the stomach. There, the majority of the breakdown process occurs as it is mixed up with hydrochloric acid [13]. Once this occurs, the body allows the mixture of digested nutrients, known as chyme, to travel into the intestinal tract. Once it enters the intestinal system into the duodenum (first part of the small intestine), pancreatic juice packed with further digesting enzymes finishes off the digestion process by breaking remaining chained nutrients into their monosaccharide parts for absorption through the intestinal epithelial cells (wall of the intestine)[12][13].

After absorption, the body then shuttles glucose into the blood stream and then, depending on what need or lack of need, glucose is either stored or used in metabolism.


SUMMARY

Now we should have a good understanding of glucose. Glucose is a single molecule used by the body to create energy for day to day function; it is found in many carbohydrate rich foods, but not isolated in single units, but rather, chained together into longer, more complex structures. It is heavily regulated and can be obtained a multitude of different ways (feeding, breaking down glycogen, converting other macronutrients into glucose, etc.). It offers 4 calories, and with an insufficient supply of glucose, we die.

Writer: Nicolas Verhoeven

                                                                                                          Citations

[1] D-Glucose | C6H12O6 - PubChem. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/D-glucose

[2] Spinoff 2004-A NATURAL WAY TO STAY SWEET. (n.d.). Retrieved from http://spinoff.nasa.gov/Spinoff2004/ch_4.html

[3] Sugars & Polysaccharides. (n.d.). Retrieved from https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm

[4] Glucose metabolism | definition of glucose metabolism by Medical dictionary. (n.d.). Retrieved from http://medical-dictionary.thefreedictionary.com/glucose+metabolism

[5] Brandt. (n.d.). Endocrine Regulation of Glucose Metabolism. Retrieved from https://www.rose-hulman.edu/~brandt/Chem330/EndocrineNotes/Chapter_5_Glucose.pdf

[6] Carbohydrates. (n.d.). Retrieved from http://www.austincc.edu/emeyerth/carbohyd.htm

[7] Zhao, Y., Wieman, H. L., Jacobs, S. R., & Rathmell, J. C. (2008). Chapter Twenty‐Two Mechanisms and Methods in Glucose Metabolism and Cell Death. Programmed Cell Death,General Principles forStudying Cell Death, Part A, 439-457. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605583/


[8] Glycogenesis. (n.d.). Retrieved from http://chemistry.elmhurst.edu/vchembook/604glycogenesis.html

[9] How are carbohydrates converted into fat deposits? - Nutrition FAQ | Examine.com. (n.d.). Retrieved from https://examine.com/faq/how-are-carbohydrates-converted-into-fat-deposits/

[10] The Liver & Blood Sugar :: Diabetes Education Online. (n.d.). Retrieved from http://dtc.ucsf.edu/types-of-diabetes/type1/understanding-type-1-diabetes/how-the-body-processes-sugar/the-liver-blood-sugar/

[11] Gluconeogenesis. (n.d.). Retrieved from https://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/gluconeo.htm

[12] Absorption of Monosaccharides. (n.d.). Retrieved from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_sugars.html

[13] Digestion and Absorption of Carbohydrates. (n.d.). Retrieved from http://2012books.lardbucket.org/books/an-introduction-to-nutrition/s08-02-digestion-and-absorption-of-ca.html

[14] Remarkable Calorie. (n.d.). Retrieved from https://www.unm.edu/~lkravitz/Article%20folder/remarkablecalorie.html

[15] ATP Production of One Glucose. (n.d.). Retrieved from https://www.unm.edu/~lkravitz/Exercise%20Phys/ATPproduction.html

 

 

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