Carbohydrates

What you’ll learn on this page

  • What a carbohydrate is
  • Why carbohydrates are important
  • The structure of carbohydrates
  • The differences between monosaccharides, disaccharides and polysaccharides
  • Examples of important carbohydrates

 

Carbohydrates are one of the four main categories of molecules found in living things – the other three being proteins, lipids and nucleic acids. They are vital for life on Earth and perform a range of functions such as providing energy, structural support and cellular communication.

A carbohydrate is either a sugar or a polymer of sugars. A polymer is two or more simple sugars joined together. Carbohydrates are carbon based molecules with hydrogen and oxygen bonded to a chain of carbon atoms.

A simple sugar is known as a monosaccharide. Monosaccharides can bond together to form disaccharides and polysaccharides. These are the three different types of carbohydrates and all three are important for different purposes in the natural world.

Why are carbohydrates important?

All life on Earth requires carbohydrates. They have found their way into the lives of not only animals and plants but also fungi, bacteria, archaea and protists.

Carbohydrates in fruitThe most important role of carbohydrates is as a source of energy. The chemical energy in sugars is the main source of energy for most living things.

Plants use the sun’s energy and CO₂ to create carbohydrates. These carbohydrates form the foundations of almost all ecosystems on Earth.

Using carbohydrates for energy prevents proteins being used for energy. This is important because it allows proteins to be used for other purposes such as metabolism and muscle contraction.

Some of the more complex carbohydrates provide structural support and protection. Plant and fungal cells have cell walls made from carbohydrates. These cell walls provide protection and support for the cell and the whole organism.

Carbohydrates are also involved in cell-cell recognition. Cells have carbohydrates on the external surface of their cell membranes that act as receptors. The receptors may interact with the carbohydrates on the membranes of other cells and help cells to identify each other.

Structure of carbohydrates

The chemical structure and composition of a carbohydrate is relatively simple compared to proteins and lipids. Most carbohydrates are composed entirely of carbon, hydrogen and oxygen atoms. A carbohydrate has three or more carbon atoms, at least two oxygen atoms and multiple hydrogen atoms. Some carbohydrates also contain nitrogen atoms, such as chitin which is found in insect shells.

Carbon atoms have the ability to bond to four other atoms. In carbohydrates, carbon atoms form a linear chain by bonding to two other carbon atoms. The chain ends when a carbon uses three of their bonds with oxygen and hydrogen rather than bonding to two carbons.

The oxygen atoms of a carbohydrate can be bonded to carbon with double or single bonds. If an oxygen forms a double bond to a carbon atom (C=O) along the carbon chain, this is known as the carbonyl group.

Oxygen can be bonded to the carbon chain in a hydroxyl group (an oxygen atom bonded to a hydrogen atom -OH) with a single bond to a carbon atom from the carbon chain. A carbohydrate can contain more than one hydroxyl group.

Hydrogen atoms take up most of the remaining carbon bonds. Generally there is around twice as many hydrogen atoms in a carbohydrate as there are oxygen atoms.

In reality, carbohydrates don’t always form linear chains but are often arranged into rings. This occurs because the double bond between the carbon and oxygen of the carbonyl group is reduced to a single bond and the oxygen instead bonds to another carbon atom along the chain. This creates a ring containing multiple carbon atoms and a single oxygen atom.

Monosaccharides – simple sugars

Monosaccharides are the most basic carbohydrates and are commonly known as simple sugars. They include well-known sugars such as glucose and fructose. A monosaccharide includes all the necessary components of a carbohydrate i.e. the carbon chain, carbonyl group and hydroxyl group.

Monosaccharides are the building blocks for larger carbohydrates and are also used in cells to produce proteins and lipids. Sugars that aren’t used for their energy are often stored as lipids or more complex carbohydrates.

It is the monosaccharides that are mostly used by cells to get energy. Glucose is arguably the most important monosaccharide because it is used in respiration to provide energy for cells. The energy stored in the bonds of a glucose molecule is converted by a series of reactions into energy that is usable by cells.

Disaccharides

A disaccharide is a carbohydrate made from two monosaccharides joined together. They are still considered sugars but are no longer simple sugars.

Monosaccharides bond together in what is known as a dehydration reaction because a water molecule is removed as the two sugars bond together. The reaction occurs between two hydroxyl groups (-OH) of the two monosaccharides.

The hydroxyl group is completely removed from one monosaccharide and from the second monosaccharide, a hydrogen atom is removed from a hydroxyl group. The removed hydroxyl group and hydrogen produces a water molecule i.e. OH + H → H₂O

From the second monosaccharide the oxygen from the hydroxyl group still remains. This oxygen bonds with the carbon atom where the hydroxyl group was removed from on the first monosaccharide. The bond links the two monosaccharides together creating a disaccharide.

The best known disaccharide is sucrose, which we use in our homes as sugar because of its sweetness. Sucrose is made by bonding together one fructose and one glucose molecule.

glucose + fructose = sucrose

Another well-known disaccharide is lactose, the sugar found in dairy products. Lactose is made from one molecule of glucose and one molecule of galactose.

It is not uncommon for humans to have difficulties breaking down lactose into glucose and galactose after eating dairy products. This is the cause of the health condition known as lactose intolerance which can cause diarrhea, bloating, gas and throwing up.

The names of monosaccharide and disaccharide carbohydrates finish with the suffix -ose. For example fructose, glucose, galactose, sucrose and lactose.

Polysaccharides

A polysaccharide is three or more monosaccharides joined together. In the exact same way that a disaccharide is formed, polysaccharides form through multiple dehydration reactions between carbohydrates.

A single monosaccharide in a polysaccharide is referred to as a monomer. A polysaccharide, which is made from many monomers, can be called a polymer. Some polymers are more than 1000 monomers (or monosaccharides) long.

monomer = monosaccharide; polymer = polysaccharide

monomer + monomer + monomer = polymer

Polysaccharides have a range of biological functions. A key function they fill is as a temporary storage of energy. Plants store energy in the form of the polysaccharide known as ‘starch’. Many crops, such as corn, rice and potatoes, are important because of their high starch content. Humans and other animals store energy in our muscles and liver using a polysaccharide known as ‘glycogen’.

A second important role of polysaccharides is providing structural support. Plants have two very important structural polysaccharides: cellulose and lignin.

Cellulose is the key compound that makes up the cell walls of plant cells. Cell walls are responsible for protecting and maintaining the shape of plant cells. Lignin is the structural compound that makes wood and has enabled plants to evolve into giant trees.

Animals and fungi also use polysaccharides as structural materials. Chitin is a polysaccharide found in the exoskeletons of insects, spiders and crustaceans as well as in the cell walls of fungi.


Summary

  • Carbohydrates are one of the four main molecules of life
  • A carbohydrate is a molecule made from carbon, hydrogen and oxygen atoms and include a carbonyl group (C=O) and a hydroxyl group (-OH)
  • Carbohydrates are a main source of energy for most organisms and are also important as structural compounds and cell-cell recognition
  • The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides
  • Monosaccharides are simple sugars e.g. glucose and fructose
  • Disaccharides are formed by linking two monosaccharides together
  • Polysaccharides contain three or more monosaccharides and are also known as polymers
  • Polysaccharides are important for storing energy and for providing support and protection for cells and whole organisms

Last edited: 23 April 2016

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