☰ Year plan · Science

Carbohydrates, fats and proteins

Week 9 · Lesson 3 Nutritional Chemistry: carbohydrates, fats and proteins, tests for them, their importance in nutrition and industry · 3 lessons
1

Carbohydrates: sugars, starch and fibre

Science Prep 2 Term 1, Unit 2: pp. 57–63
GoalClassify carbohydrates by size, name the main ones and say what each does in living things.
New words
carbohydrate · an organic compound of carbon, hydrogen and oxygen, such as sugar or starchglucose · a single sugar that cells use for energystarch · a long chain of glucose that plants use as a store
Explanation

Carbohydrates, fats and proteins are organic compounds made by living things. Carbohydrates contain only carbon, hydrogen and oxygen. We find them in bread, rice, potatoes, grains, fruit and vegetables. We sort them by size. Single sugars have one unit: glucose and fructose. Double sugars have two units: sucrose (glucose + fructose) and maltose (glucose + glucose). Small chains have 3 to 9 units, like raffinose in beans. Long chains have more than 9 units: starch and cellulose. Glucose is the sugar our cells use for energy, and the brain relies on it. Plants store their food as starch, and use cellulose in their cell walls; paper is mostly cellulose. Extra carbohydrate in our bodies is stored as glycogen in the liver and muscles, or as fat. Be careful: the idea that all carbohydrates taste sweet is wrong. Starch and cellulose are not sweet.

SizeUnitsExamples
single sugar1glucose, fructose
double sugar2sucrose, maltose
small chain3–9raffinose (in beans)
long chainmore than 9starch, cellulose
Worked examples
Sucrose, table sugar, is made of one glucose unit joined to one fructose unit. It is a double sugar, so it has two units. Maltose is two glucose units joined.
potatoricebread
Potatoes and rice are rich in starch, which is not sweet. When we digest starch, our bodies break the long chain into glucose units.
Why do plants store starch and not glucose? A long chain does not dissolve or move easily, so it makes a good store. A pile of loose glucose would dissolve and leak away. A seed full of starch can wait for months.
Class activity

Kitchen sort, with a grown-up. Choose eight foods (for example bread, rice, apple, potato, beans, honey, oats, lettuce). Sort them by main nutrient. Stir a spoon of sugar and a spoon of cornflour in two glasses of cold water and describe what you see in each glass. Do not use iodine or any test chemicals, and do not taste a test mixture.

Practice
1
Which three elements do carbohydrates contain?
2
Which sugar do cells use for energy?
3
Sort into single sugar, double sugar or long chain: glucose, sucrose, starch.
4
A student says all carbohydrates taste sweet. Give an example that shows he is wrong.