Carbohydrate Basics

What’s A Carb?

Technically speaking a carbohydrate or a sugar is a carbon molecule with a 2:1 hydrogen to oxygen ratio. What does this mean though? Basically, a carbohydrate is a carbon-containing molecule with a bunch of OHs attached. Carbs can be pretty small like the three-carbon trioses or bigger like the six-carbon hexoses. On the MCAT we usually see the hexoses and pentoses since these are the biologically relevant ones.

Functional Groups

Linear and Cyclic


When talking about sugars in the context of biochemistry we refer to them as saccharides. So a single sugar molecule, such as glucose, is a monosaccharide. When multiple monosaccharides come together they form disaccharides, oligosaccharides, and polysaccharides depending on how many subunits are present.