Beginner Topic 980 of 1102 Biochemistry, Environment & Industry

Starch

A plant energy store made of amylose and amylopectin.

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In-depth explanation of Starch

Starch is the main energy store of plants and consists of two polymers of alpha-glucose. Amylose is an essentially unbranched chain joined by alpha-1,4 links that coils into a helix, while amylopectin is branched by alpha-1,6 links at roughly every 25 residues. Amylose makes up about 20 to 30 percent of typical starch, and its helix traps iodine to give the characteristic blue-black colour of the iodine test. Enzymes such as salivary and pancreatic amylase hydrolyse the alpha-1,4 bonds to give maltose and glucose, while debranching enzymes are needed at the branch points. Starch granules in seeds, tubers and roots are a major source of human food energy.

Going deeper: Starch

Key ideas: A mix of amylose (about 20 percent, unbranched alpha-1,4 chains, helical) and amylopectin (about 80 percent, alpha-1,4 chains with alpha-1,6 branches every 24 to 30 units). Iodine fits inside the amylose helix to give a deep blue-black complex. Amylase begins hydrolysis in saliva, giving maltose, then maltase gives glucose.

Common mistakes and connections: Students think the iodine test detects glucose or any sugar; it detects the helical starch only and the color vanishes on heating as the complex breaks. Starch is not the same as glycogen, which is more branched. Links to cellulose, glycogen and digestion.

Visualization of Starch

Typical starch is about 20 to 30 percent helical amylose and 70 to 80 percent branched amylopectin. The amylose helix traps iodine to give the blue-black test. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Potatoes, rice and wheat supply large amounts of starch, which the mouth and gut digest into glucose.

Frequently asked questions about Starch

What is Starch?

A plant energy store made of amylose and amylopectin.

Is Starch a beginner, intermediate or advanced topic?

Starch is a beginner topic in Biochemistry, Environment & Industry. It needs no earlier chemistry lessons and is a good starting point.

What should I learn before Starch?

Study these earlier topics in Biochemistry, Environment & Industry first: Monosaccharides, Disaccharides, Polysaccharides.

What comes after Starch?

The next topics in Biochemistry, Environment & Industry are Cellulose and Glycogen.

Can I practise Starch with quizzes?

Yes. The interactive lesson includes a self-check question on Starch, and the Biochemistry, Environment & Industry quiz draws on every topic in this area.

Where Starch fits in Biochemistry, Environment & Industry

Starch is topic 5 of 127 in Biochemistry, Environment & Industry.