4. Carbohydrates

Overview of Monosaccharides

Carbohydrates are produced in photosynthetic organs from the inorganic molecules, water and carbon dioxide (yes, H20 and C02 are inorganic!). In heterotrophs, carbohydrates are acquired from photosynthetic organisms, or they’re produced directly from other organic molecules.

Carbohydrates exist as monosaccharides, single sugar units used as fuel molecules. They also exist as complex polymers called polysaccharides, or complex carbohydrates.

Monosaccharides can have an unbranched linear structure containing three to six carbons, which consist of a carbonyl group and several alcohol functional groups.

In general, when a carbonyl is at the end of a molecule, it becomes an aldehyde. In a monosaccharide, this structure is called an aldose with a suffix of “-ose.”

When the carbonyl is anywhere else, it’s called a ketone group. In a monosaccharide, this is a ketose. In all-natural sugar, the second carbon, C-2, carries the ketone group. The suffix for ketoses is “-ulose.”

from harvardX MCB63X lecture slide, copyright harvardX and edX.org

Triose and Tetrose are monosaccharides with three and four carbons, respectively, which act as metabolic intermediates. They never occur in complex carbohydrates.

Pentose, with five carbons, is found in complex carbohydrates in plants. They’re also found in all organisms as part of nucleotides. For example, ATP contains a molecule of ribose (a pentose).

Hexoses, with six carbons, are important fuel molecules, most notably glucose. In plants, glucose is polymerized in the form of starch. In animals, it’s glycogen.

Asymmetric carbons

Some sugars are stereoisomers (mirror images) of one another called enantiomers.

Take glyceraldehyde, which has one asymmetric carbon (its chiral center) and two enantiomers. While both share many of the same properties, they differ in how they interact with plane-polarized light. When a light source passes through a polarizing filter and into a solution containing the sugar, the plane of the light will rotate differently depending on the enantiomer.

If the rotation is clockwise, the enantiomer is said to be dextrorotatory, designated by the + sign. If counterclockwise, it’s said to be levorotatory and is designated by the – sign.

Fisher established a similar nomenclature called the L and D series. Per this definition, if the hydroxyl group of the penultimate carbon (the bottom-most asymmetric carbon) is on the right, it’s called a D-series sugar. If it’s on the left, it’s an L-series sugar.

from harvardX MCB63X lecture slide, copyright harvardX and edX.org

Strangely, the L/D assignments are not always the same as the +/- assignments. Fisher got lucky with this choice. It wasn’t based on the optical properties. Annoying!

The diversity of monosaccharides

While there are an insane number of possible combinations (2^N), only a small number exist in nature. In cells, there are only a few types of hexoses, for example, all of which are in the D-series.

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