If you look at the word carbohydrate you can probably work out what elements it must be made from. The letters ate at the end of a compound indicate that there is oxygen built into the molecule. Examples would be copper sulfate (copper, sulfur & oxygen) or calcium carbonate (calcium, carbon & oxygen). Carbohydrate does contain the element oxygen.
The letters hydr in the name of a compound shows that hydrogen is present. Sometimes people mistake this for water as in anhydrous copper sulfate. In that case the hydr is not in the chemical name but in the description showing that the water associated with copper sulfate crystals has been driven off. Carbohydrate, like all organic molecules, contains hydrogen.
And the carbo bit refers to carbon. When you are pondering what gas might be given off by copper carbonate when it is heated, the presence of the letters carbo are strongly pointing you towards carbon dioxide. The more that you listen to the words as you read them off the page, the easier this subject becomes. If you are making a wild guess, then something has gone wrong. And so, of course, carbohydrates contain the element carbon.
There are three major carbohydrates in your diet and they are sugar, starch and cellulose. They are all made from carbon, hydrogen and oxygen but their molecular structure makes them behave in quite different ways.
Sugar
The most important sugar is glucose. It is a relatively simple molecule, it is highly soluble and is one of the two raw materials for aerobic respiration from which almost all organisms on Earth get their energy. Glucose is a monosaccharide – the simplest kind of sugar. Another monosaccharide is fructose, sometimes called fruit sugar. The two molecules are very similar. They both have the chemical formula C6H12O6 but the atoms are in different arrangements. Two molecules with the same chemical formula but different shapes are called isomers.
There is a chemical test to identify glucose. Dissolve a little of the food that you suspect contains glucose in some water, then add a blue solution called Benedict’s reagent and warm the mixture gently for ten minutes. If it turns red then there is glucose present.
If two monosaccharides link up you get a disaccharide. When glucose bonds to fructose, sucrose is produced. This is the refined white sugar that you put in your tea or on your cornflakes and which generally makes things taste sweet and delicious. There are also things called trisaccharides which are very interesting and I hope to write about them on here at some point in the future.
Starch
A more complex carbohydrate is starch. Starch is a polysaccharide because it is made from long chains of 500 or so glucose molecules linked together. It is insoluble and used as a storage material by plants. There is lots to be found in wheat, rice and potatoes. It is easily broken down in the digestive system by an enzyme called amylase (another name for starch is amylum). There is amylase in your saliva so starch digestion starts in the mouth. Starch can be identified by adding iodine which causes a blue/black colour to appear.
Because it is easily broken down into glucose in your body it is a good source of “slow release” energy.
Cellulose
Another polysaccharide is the even more complex cellulose. I think I have said on this blog that cellulose is the most abundant organic molecule on the planet because it forms the cell wall around every single plant cell. It is indigestible and forms much of the component of your diet known as dietary fibre or roughage. Adults should eat between 25 and 30 grams of fibre each day. It helps to keep the digestive system healthy by giving the intestinal muscles something to work against, keeping everything moving through your digestive system. Good sources of it include fruit and green vegetables and whole grains, brown bread, cereals etc.
Questions…
- What reagent would you use to test for the presence of glucose and what would a positive result be?
- What does it mean when cellulose is described as an organic molecule?
- If a molecule of glucose were completely burned in oxygen, what two products would be made?
- In the video the person doing the commentary suggests three examples of a reducing sugar. Glucose was one, can you name either of the others?
- The element iodine is a non-metal, solid at r.t.p. Suggest one physical characteristic most solid non-metals share.
- When heated iodine turns straight into a purple gas. What is this change of state called?
- There are only two elements that are liquids at r.t.p. Can you name either of them?
- The relative atomic mass (RAM) of hydrogen is 1, of carbon is 12 and oxygen is 16. What is the relative molecular mass (RMM) of (a) glucose and (b) sucrose?
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