What is Starch? – Definition, Function & Chemical Formula

In this lesson, you’ll learn what starch is composed of and some foods that contain it. In addition you will discover more about the chemical components of starch as well as its function inside and outside the body.
What is Starch?
Have you ever seen someone with so much energy you wish you could bottle it for yourself? Plants have their own way of ‘bottling’ energy. They do this by storing energy as starch.

But what exactly is starch? Starch is long chains of sugar molecules linked together like a chain. A single sugar molecule is a monosaccharide.

Monosaccharide
Many sugar molecules linked together is a polysaccharide. Starch, therefore, is a polysaccharide.

Polysaccharide
Specifically, starch is composed of the sugar glucose. Glucose is a sugar molecule made up of carbon (C), hydrogen (H), and oxygen (O) with a basic chemical formula of C6H12O6. Plants use glucose to produce energy, but they’re not always making glucose. Much like we store up energy reserves after eating, so do plants. Plants store extra glucose in the form of starch for use when they aren’t photosynthesizing (a plant’s equivalent to eating).

glucose molecule
Chemical Formula of Starch
Since starch is composed of glucose molecules, the basic formula of starch is very similar to that of glucose. However, in order to link together, glucose molecules have to lose some of their components.

Consider this: If you want to hold hands with someone, you can’t be holding anything else. Similarly, the glucose molecule has to empty its hand by letting go of H and O in order to hold hands with another glucose molecule. The H and O are given off as water.

Linked glucose shares oxygen.
Linked glucose shares oxygen
We can deduce that the basic chemical formula of starch is (C6H10O5)n, where n is the number of glucose molecules in the chain. For example if there are 100 glucose molecules in a starch molecule, the formula for that starch molecule would be (C6H10O5)100 or C600H1000O500.

Starch molecule chemical formula example
Starch molecule chemical formula example
Since starch is made solely of glucose molecules linked together, it is called a homosaccharide, a chain of sugars made up of one type of molecule.

Homosaccharide
Starch is a chain of glucose molecules, but the chain isn’t always straight. Sometimes, the sugar molecules branch off from the main chain and form their own, just like a tree has a main trunk and then branches. As such, starch actually has two forms: one form has no branches while the other form does.

The branchless form is amylose.

Amylose
Amylose can contain over 250 glucoses units per one molecule of amylose. Since it doesn’t have branches, amylose can form a 3-D helical structure, much like a slinky. Imagine a completely unwound slinky. It would be awkward to carry around and hard to store. The ‘slinky’ structure of amylose allows cells to store energy in a compact form, but also makes it easily accessible.

Slinky as an example of helix structure.
Slinky exemplifies helix wound and unwound structure.
Interestingly, since iodine can insert itself into the helix structure and stains blue, scientists often use iodine to test for the presence of starch.

Amylopectin is the branched form of starch and can contain over 1,000 glucose units. The main chain of hand-holding glucoses is still there, so where does the branching-off glucose attach? It attaches to a different carbon, rather like a third person holding on to your belt while you hold hands with two other people.

Amylopectin
Amylopectin structure
About every 20 to 25 glucose molecules, a branch point occurs where one molecule of glucose decides to go off in a different direction.

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