Sugars are a type of carbohydrate, which means they contain carbon, oxygen, and hydrogen. They’re soluble in water and, of course, sweet-tasting. Various types of sugar are found in our daily diet, from the table sugar we use in our morning brew to the sugar in fruits and chocolate we like to snack on.
These sugars all have similar repeating molecules of simple sugar subunits. Glucose is the simplest sugar (called a monosaccharide) with one sugar unit, while more complex sugars (called disaccharides) have two or more glucose and monosaccharide subunits.
Monosaccharides have the general formula (CH2O)n, where “n” ranges from 3 to 7. The most common monosaccharides are:
While some of the common sugars have the same chemical formula, they differ in terms of their molecular structure. For instance, glucose and fructose have the same chemical formula but their molecular structures are different, as shown in the illustration below:
Glucose and fructose can be combined as subunits, forming sucrose. These subunits and other monosaccharides can form chains, creating more complex sugars and other types of carbohydrates.
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Are There Different Types of Sugar?
Technically, there are only two categories of sugars, namely the monosaccharides and the disaccharides. As we mentioned above, monosaccharides are simple sugars composed of a single sugar. Disaccharides are composed of two monosaccharide subunits.
Longer chains of monosaccharides, which contain at least three linked monosaccharides and are otherwise known as polysaccharides, are not considered sugars but as complex carbohydrates instead.
As the name implies, monosaccharides have one sugar unit. As we touched on earlier, their general formula is (CH2O)n. The “n” ranges from 3 to 7, but the most common range is between 5 and 6. Monosaccharides are the subunits of disaccharides and polysaccharides.
Monosaccharide molecules can be arranged in two types of structures: a linear structure, and a ring or cyclic structure. The monosaccharide molecular structure is linear when the sugar is in a dry powdered state. The molecular structure becomes cyclic when the sugar is dissolved in water. Oxygen is a member of the ring, along with carbon.
The three most common monosaccharides are fructose, glucose, and galactose. These monosaccharides are commonly found in the food we eat. They’re easily absorbed in the digestive system, particularly by the small intestines. Therefore, they’re also called dietary monosaccharides.
While these common monosaccharides have the same chemical formula of C6H12O6, they have different molecular structures. Because they have six carbon atoms, they’re classified as hexoses.
Here’s a little bit more information about three of the most common monosaccharides:
- Fructose: This is commonly found in fruits but can also be found in root crops (e.g. potatoes), sugar cane, and honey. Commercially available fructose is in the form of corn syrup.
- Glucose: This is the primary form of sugar transported in the bloodstream of many animals, including humans. Many complex carbohydrates are reduced into glucose through digestion and metabolic processes. It’s commonly found in fruits and other plant products.
- Galactose: This monosaccharide doesn’t naturally exist in a free state. It’s typically found as constituent of the disaccharide lactose, as well as on the surface of red blood cells as a component of antigens that identify the various blood groups
What is Glucose?
Glucose is a simple sugar commonly transported in the bloodstream of many animals. It’s typically the byproduct of digesting complex carbohydrates from plants, like the starch in potatoes, grains, and other carbohydrate-rich food.
Glucose is primarily a product of photosynthesis. Just like other sugars, its molecules can exist in either a linear (open chain) structure or in a cyclic (ring) structure as shown below:
Where Does Glucose Come From?
Glucose is a basic subunit of many disaccharides and complex carbohydrates (polysaccharides), like starch. Glucose is a product of photosynthesis, whereby the carbon dioxide from the air and the water from the ground are converted into sugar and oxygen. The simplified balanced equation for this is shown below:
6CO2 + 6H2O → C6H12O6 + 6O2
In reality, however, the biological process of photosynthesis is complicated. It’s composed of both light-dependent reactions and light-independent reactions.
The light-dependent reactions occur in the thylakoids inside the chloroplast, which breaks down water and releases oxygen. The hydrogen nucleus, or proton, creates an energy gradient across the chloroplast membrane. This energy is used by an enzyme called ATP synthase to create ATP molecules. Meanwhile, carbon dioxide undergoes the Calvin cycle.
Below is a simplified diagram that illustrates the process of photosynthesis:
How is Glucose Used in the Body?
Glucose is used by the human body as the main source of energy for various physiological functions, like muscle contraction and brain activities. In fact, the brain has the highest consumption of glucose at 120g per day, which is about 60 to 70% of total glucose consumption.
In order to derive energy from glucose, it must undergo a metabolic pathway known as glycolysis, otherwise known as cellular respiration. This occurs in the cytoplasm of a cell and converts glucose into pyruvate. During the process, every molecule of glucose produces the following:
- Two pyruvate molecules
- Two molecules of ATP (the total is four but two molecules are used up)
- Two molecules of NADPH
Below is a schematic diagram of the complex series of chemical reactions involved in glycolysis:
What is Fructose?
Fructose is one of the most common monosaccharides found in food, particularly in fruits. It’s classified as a hexose because of its six carbons. It has the same formula as glucose and other hexoses, but has different molecular structure.
Fructose is commercially available as corn syrup. It’s a major ingredient in many food products such as apple pie, sauces, and salad dressings.
What is Fructose Found in?
As the name suggests, fructose is naturally found in fruits. However, it also naturally occurs in root crops, honey, sugar cane, and some vegetables. Below are some examples of fructose sources with corresponding free fructose content (grams) per 100 grams:
- Apple: 5.9 g
- Banana: 4.9 g
- Dried fig: 22.9 g
- Grapes: 8.1 g
- Pear: 6.2 g
- Pineapple: 2.1 g
Is Fructose Bad For You?
Anything that’s excessive in our diet is usually not good. This is particularly true for sugars like fructose. Eating food or drinking beverages with a high fructose content daily can have long-term health consequences. With that in mind, fructose can be bad for you in excessive amounts, and can lead to the following health problems:
- Heart disease
What is Sucrose?
Sucrose is common table sugar. It’s a disaccharide composed of two monosaccharides, namely fructose and glucose. Its chemical formula is C12H22O11 and it naturally occurs in plants, particularly in sugarcanes, from which it’s refined.
What is the Function of Sucrose?
Sucrose is a source of energy for the body. Sucrose can provide 1,620 kJ (390 kcal) of energy per 100 grams of serving. It’s a primary ingredient in many food items, particularly in sweets, pastries, ice cream, and cakes. It’s also a primary ingredient in soft drinks and other beverages.
The global demand for sucrose is high, with the world producing almost 200 million tonnes of sucrose each year. In fact, the global market demand for this sugar is projected to reach US$ 63.7 billion (£46.34 billion) in 2024.
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