Sugar (GLUCOSE)
In humans, the main substrate responsible for energy production is glucose, a carbohydrate (or sugar) that provides energy to different tissues and structures. Derived from carbo- (carbon) and hydrate (water), these substances are compounds such as sugars, starches, and cellulose that contain carbon, hydrogen, and oxygen. The ratio of hydrogen to oxygen atoms is 2 to 1, the same as that for water (H2O). In essence, all carbohydrates are some combination of carbon and water. Since sugars are carbohydrates, there are, again, two hydrogen atoms for every oxygen atom. Sucrose is C12H22O11. Glucose is C6H12O6.
Sugar consumption digests and enters the bloodstream. As blood sugar levels rise, the pancreas produces insulin, a hormone that prompts cells to absorb blood sugar for energy or storage. In muscle, fat, and liver cells, insulin triggers the translocation of glucose transporter proteins (GLUT4) to the cell membrane. These transporter proteins act as channels, facilitating the movement of glucose from the bloodstream into the cells. Once inside the cells, glucose can be:
- Used immediately for energy through cellular respiration processes.
- Converted to glycogen and stored in the liver and muscles for later use as energy.
- Converted to fatty acids in the liver and adipose tissue for storage as fat. If cells become resistant to insulin's effects (insulin resistance), they fail to absorb glucose efficiently from the bloodstream, leading to persistently high blood sugar levels. This can eventually contribute to the development of type 2 diabetes. 1
The metabolism of glucose, as well as other six carbon sugars (hexoses) begins with the catabolic pathway called glycolysis.
Glucose + Oxygen = Water + Carbon Dioxide + Energy
The oxidation or breakdown of glucose into usable products such as energy or ATP is referred to as cellular respiration. To undergo oxidation, glucose undergoes three major pathways — glycolysis, the Krebs cycle, and oxidative phosphorylation via the electron transport chain. Everyone needs these three things: a source of energy, a source of carbon, and a source of electrons.
"Thirty Thousand Foot" View Of The Six Carbohydrate Pathways 2 In this pathway, sugars are oxidized and broken down into pyruvate molecules; the product of glycolysis, pyruvate, can be completely oxidized to carbon dioxide. 3 Carbohydrates are aldehydes or ketones, with many hydroxyl groups attached, that can exist as straight chains or rings. Monosaccharides can be linked together to form polysaccharides in almost limitless ways.
The main types of sugars that we metabolize are monosaccharides, which are the simplest forms of sugars:
- Glucose (C6H12O6)
- Fructose (C6H12O6)
- Galactose (C6H12O6)
We also metabolize disaccharides, which are made up of two monosaccharides joined together:
- Sucrose (C12H22O11) - a disaccharide composed of glucose and fructose
- Lactose (C12H22O11) - a disaccharide composed of glucose and galactose
- Maltose (C12H22O11) - a disaccharide composed of two glucose molecules
Additionally, our bodies can metabolize other sugars like:
- Allulose (C6H12O6)
- D-tagatose (C6H12O6)
- Isomaltulose (C12H22O11)
These sugars are metabolized differently than traditional sugars like sucrose, and may have different effects on blood glucose levels and calorie intake. These more rare sugars are metabolized differently because they are absorbed, metabolized, and excreted at different rates compared to traditional sugars, resulting in reduced caloric values and lower impacts on blood glucose and insulin levels. 4
Polysaccharides
Polysaccharides are complex compounds consisting of long chains of monosaccharides linked together. The sugar units are galactose and 3,6-anhydrogalactose.
- Starch (C6H10O5)n - a polysaccharide
- Glycogen (C6H10O5)n - a polysaccharide
- Cellulose (C6H10O5)n - a polysaccharide
- Chitin (C8H13O5N)n - a polysaccharide
- Pectin (C6H10O5)n - a polysacchar
- Dextran (C6H10O5)n - a polysacchar
- Inulin (C6H10O5)n - a polysaccharide
- Xylan (C6H10O5)n - a polysaccharide Polysaccharides are broken down into monosaccharides through the process of hydrolysis. [^5] Polysaccharides are of linked sugar molecules.
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Simple sugars, starches and cellulose are organic compounds that have the approximate formula C (H O), which accounts for the name carbohydrate (or hydrate of carbon) that is usually applied to this group of compounds. They are not truly hydrates of carbon but are polyhydroxy (alcohol) compounds that contain an aldehyde or ketone functional group.
Simple sugars are called monosaccharides (one sugar), or disaccharides (2 sugars). Some monosaccharides are glucose, fructose, galactose, and xylose. Note that xylose is a pentose and fructose is a ketose.
When many sugar molecules are linked together into a polymer, the resulting compound is called a polysaccharide. Starches and celluloses are polysaccharides. Amylose is a linear chain polymer of glucose, whereas amylopectin (a plant starch) and glycogen (an animal starch) are branched polymers of glucose.
Foot Notes:
Footnotes
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Nutrition.org Sugar sweetened beverage consumption promotes dyslipidemia in adults (opens in a new tab) ↩
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New York University carbohydrates/introduction (opens in a new tab) ↩
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LibreText: Biology 6.1: Metabolism - Sugars (opens in a new tab) ↩
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US Food & Drug Sugars That Are Metabolized Differently Than Traditional Sugars (opens in a new tab) ↩