Carbohydrates

Carbohydrates are the human body’s main way of supplying its cells with energy. Cells can release this energy in a process known as cell respiration where they oxidise glucose (one type of carbohydrate). We will go into detail about the process of cell respiration later in this topic, but first we will discuss the chemical elements present in carbohydrates and their structure.

Key term

Cell respiration – the process which involves cells turning food into usable energy.

What are Carbohydrates?

Carbohydrates are large molecules that are made up from smaller basic units known as chemical elements. All carbohydrates contain the chemical elements of carbon, hydrogen and oxygen only. However, the quantity and the way that these chemical elements are arranged changes which type of carbohydrate it is – we will discuss the structure of these in detail. Carbohydrates are types of sugar that are organised into three groups: monosaccharides, disaccharides and polysaccharides. All sugars are sweet-tasting and are soluble in water. The ones that we are going to discuss are listed in the table below:

Monosaccharides

Monosaccharides include glucose, fructose and galactose. These are ‘single’ (or ‘simple’) sugars which means that they consist of just a single unit of sugar that cannot be broken down any further. Out of the three, glucose is the main ‘fuel’ for the body’s cells; whereas fructose participates in metabolism and galactose is the least common of the two and has a different function, which we will discuss shortly. Like most nutrients, monosaccharides are absorbed in the small intestine. However, unlike other nutrients, they don’t need to be broken down by intestinal enzymes before they are absorbed

Glucose molecules are made up of the chemical elements carbon, hydrogen and oxygen. The chemical formula for glucose is C₆H₁₂O₆.  Glucose is a natural sugar that is found in many sweet-tasting foods, for example fruits and vegetables. Glucose is absorbed by the body easily, faster and completely. Sometimes, the body will already have the energy it requires; when this happens, any glucose that is being ingested will be stored in liver and muscle cells for when the body next requires it. When stored, glucose is made up of numerous connected glucose molecules which are a type of reserve carbohydrate and polymer known as glycogen.

The chemical formula of fructose is the same as glucose: C₆H₁₂O₆. The difference between the two is that their chemical structure is arranged differently. Fructose is a different type of sugar that is found in honey and many types of fruit. It is sweeter and more soluble than glucose and is also referred to as the fruit sugar. Like glucose, fructose is an energy source for cells. It is also used to produce the reserve carbohydrate glycogen.

Like the other two monosaccharides we have looked at, galactose also has the same chemical formula of C₆H₁₂O₆. Again, the difference between this type of sugar is that its chemical structure is arranged differently. Similar to glucose, galactose is absorbed by the body easily, faster and completely. It is not as sweet as glucose and fructose. When galactose is ingested, the human body converts most of it into glucose

Disaccharides

Disaccharides are formed when two simple sugars (two monosaccharides) join together. This means that unlike monosaccharides, disaccharides are broken down during the digestion process by digestive enzymes. Two of the major disaccharides include lactose and sucrose.  We will discuss these individually.

Lactose is a sugar found in milk. It is a disaccharide sugar derived from the two monosaccharides glucose and galactose. The chemical formula of lactose is C₁₂H₂₂O₁₁. Another name for lactose is milk sugar.

Sucrose molecules are made up of two monosaccharides joined together: glucose and fructose. The chemical formula of sucrose is the same as lactose: C₁₂H₂₂O₁₁, however, the chemical elements in these two carbohydrates are arranged differently. Table sugar that people commonly use in their tea and coffee is sucrose. In regards to plants, sucrose is the main sugar which is transported through their stems. This is the reason as to why we can extract sucrose from the stems of a plant known as sugar cane.

Polysaccharides

Polysaccharides are larger carbohydrates which include a number of monosaccharides joined together; like disaccharides, this means that they are able to be broken down into simple sugars (monosaccharides). Polysaccharides are known as a key class of biological polymers. The polysaccharides that we will discuss are starch, glycogen and cellulose.

Starch is a carbohydrate that consists of many glucose sub-units that are joined together like a chain. The basic chemical formula of a molecule of starch is C₆H₁₀O₅. Of a person’s required intake of carbohydrates, they mainly receive this from ingesting starch. Starch is an insoluble molecule (meaning it does not dissolve in water) and is therefore found as a storage carbohydrate in many plants; some examples of plants containing starch include, oats, potatoes, rice etc. Starch is sometimes also referred to as a polymer of glucose and is only ever found in the tissues of plants.

We have mentioned glycogen briefly in this chapter. Glycogen is a similar carbohydrate to starch but glycogen is found in some animal cells and has a chemical formula of C₂₄H₄₂O₂₁. Glycogen is a type of reserve carbohydrate that is made up of a number or glucose molecules collected together in liver and muscle cells where it is stored until the body requires it; in other words, glycogen acts as a store of energy for liver and muscle cells.

You may remember discussing cellulose when we explored the structure of plant cells. Cellulose is a polymer of glucose that makes up the cell walls of plants. Cellulose is a carbohydrate that humans are not able to digest. This is because the human body’s gut does not contain the specific enzyme required to break down the particular molecules of this carbohydrate. However, although we cannot extract energy from this carbohydrate, it does have an essential and unique function in our diet as it forms dietary fibre/roughage. This helps to avoid constipation and it also helps to prevent some serious intestinal conditions (such as cases of inflammatory bowel disease and bowel cancer) because it enables the muscles in the gut to have something to push against whilst food is being moved down the intestine.

Starch and glycogen have to be broken down into simple sugars during digestion. This is achieved through digestive enzymes breaking them down so that they can be absorbed into the blood. We will discuss enzymes in a later chapter.

Although carbohydrates are an essential food substance, too much sugar in a person’s diet can cause negative effects. This includes serious