Section One

Lesson 1: Carbohydrates and Introduction to Monosaccharides

Sections 19.1-19.2

Objectives:

To be able to define carbohydrate, to state the three main functions of carbohydrates, and to indicate several specific uses of carbohydrates by plants, animals and bacteria.
To know the general chemical formula for monosaccharides and be able to determine the molecular formula for a monosaccharide given the number of carbons in its structure.
To be able to define monosaccharide and be able to classify monosaccharides according to the number of carbons and the presence of an aldehyde or ketone.
To be able to identify all stereocenters and the position which defines the molecule as being the D- or L-form in the Fischer projection.

I. Introduction

A. Carbohydrates

All the compounds below are carbohydrates. Notice the functional groups they have in common.

simplesugars

All of these compounds are alcohols with many hydroxyl groups -- they are polyhydroxylated. They are also either aldehydes or ketones.

Carbohydrates are:

polyhydroxylated aldehydes or ketones or
compounds that hydrolyze to give polyhydroxylated aldehydes or ketones.

Thought of another way, carbohydrates are:

polyhydroxylated aldehydes or ketones and their derivatives.

So, the word "carbohydrate" includes polymers and other compounds synthesized from polyhydroxylated aldehydes and ketones. They can be synthesized in the lab or in living cells.

Simple carbohydrates or the entire carbohydrate family may also be called saccharides.

B. Importance

Most people know that the body uses carbohydrates for energy. For example, the simple carbohydrate glucose (dextrose) is oxidized by liver cells. In exchange, the cells produce adenosine triphosphate (ATP), the main energy-providing compound in the cell. However, carbohydrates are used in a number of ways by plants, animals and bacteria, not just for energy.

The three main functions of carbohydrates are:

energy storage or use
structural support
source of carbon for biosynthesis of other compounds.

As part of their function in plants, animals and bacteria, carbohydrates may undergo chemical reactions. Since they are both alcohols and either aldehydes or ketones, they can participate in the same types of chemical reactions as we have previously studied for these functional groups.

In living cells, glucose is oxidized to give carbon dioxide and energy:

glucose + O₂

CO₂ + energy

The energy produced is in the form of ATP and heat. This reaction is just like the combustion reactions you have already studied, which is why we say that carbohydrates are "burned" by the body.

During photosynthesis in plants, glucose can be produced from CO₂ and energy:

CO₂ + energy

glucose + O₂

Here, the energy consumed is in the form of sunlight. This reaction is the reverse of the combustion reaction.

Assignment

W19.1 In your book or elsewhere, find examples of the three functions of carbohydrates and post them on the bulletin board.

W19.2 What chemical reactions can carbohydrates undergo? To get you started, they could be dehydrated by acid-catalysis of an alcohol group. Post your answers on the bulletin board.

II. Introduction to Monosaccharides

A. Monosaccharide Structure

Monosaccharides have a single aldehyde or ketone group and typically three to eight carbons. They have the general molecular formula C_nH_2nO_n. The compounds in section I (above) are all monosaccharides. Notice that compound 1 has the molecular formula C₃H₆O₃. Here n = 3 and 2n = 6. The formulas for some other monosaccharides are given in section 19.2 in your book. Tables 19.1 and 19.2 show the structures for some commonly occurring monosaccharides. Monosaccharides are often called simple sugars, or just sugars.

Assignment:

W19.3 Determine the molecular formulas for the other carbohydrates in section I (above). Post your answers on the bulletin board.

B. Monosaccharide Nomenclature

The suffix used when naming monosaccharides is "-ose." Monosaccharides are then classified according to:

whether they have an aldehyde or ketone and
number of carbons.

An aldehyde is indicated by the prefix "ald-" and is called an aldose. A ketone is indicated by the prefix "ket-" and is called a ketose.

The number of carbons is also indicated by a prefix:

# of carbons	prefix
3	tri- (NOT prop-)
4	tetr- (NOT but-)
5	pent-
6	hex-
7	hept-
8	oct-

For example, a three carbon sugar is called a triose, a four carbon sugar is called a tetrose and so on. The entire group of three carbon sugars are called trioses, and similarly for monosaccharides with other numbers of carbons.

To indicate both the number of carbons and whether the compound is an aldose or ketose, the class name is formed as follows:

(keto-/aldo-) + (# of carbons) + -ose

The class name for a monosaccharide that contains an aldehyde and three carbons is formed in this way:

aldo- + tri + ose ------> aldotriose

Likewise, a five carbon sugar that contains a ketone is called a ketopentose.

The class names refer to a group (class) of monosaccharides. Specific sugars have unique names according to their structure. Tables 19.1 and 19.2 give the names for several monosaccharides. Here you can see the difference between class names and names of specific sugars. For example, notice that there are two aldotetroses (Table 19.1), D-Erythrose and D-Threose.

C. Fischer Projections

Monosaccharides, except for dihydroxyacetone phosphate, have one or more stereocenters. Chemists use structure drawings called Fischer projections to show the configuration about the stereocenters. Fischer projections can be drawn quicker than the wedge drawings we have used previously.

A Fischer projection of D-glucose (an aldohexose) is shown below (on left). The Fischer projection represents the same compound as the structure on the right that uses wedges to indicate whether bonds project up or down from the page.

glucosefischer

Important things to remember about Fischer projections are:

Carbon 1 is always drawn at the top. For an aldose, carbon 1 is the aldehyde's carbonyl carbon. For the ketoses we will study, carbon 2 is always the ketone's carbonyl carbon.
Each cross-over point represents a chiral carbon atom (stereocenter) sitting in the plane of the "paper." Don't forget that there is a carbon at each of these positions, even though there is no "C".
Horizontal lines attached to the chiral carbons represent bonds projecting towards you, sticking up above the page.
Vertical lines attached to the chiral carbons represent bonds projecting away from you, sticking down below the page.

D. D- and L- Monosaccharides

In Chapter 15, you read about the R,S system for designating the configuration (chirality) about a chiral carbon. To name a compound with the R,S system, each stereocenter must be given its own R,S designation. In carbohydrate chemistry, there are so many hydroxyl groups and stereocenters that using IUPAC nomenclature and the R,S system would result in very long names. For this reason, common names are used more often than IUPAC names for monosaccharides, and the D,L system is used to designate configuration.

Only the highest-numbered chiral carbon is given a D or L designation. We also call this carbon the last chiral carbon (or penultimate carbon). In a Fischer projection, this carbon is the chiral carbon at the bottom of the structure.

dlsugars1

What about designations for all the other stereocenters in a monosaccharide? Simply stating D-glucose as the common name specifies that we are talking about the six-carbon sugar that, in the Fischer projection, has the carbon 2 and 4 hydroxyls on the right and the carbon 3 hydroxyl on the left. Its enantiomer (L-glucose) has the opposite configuration at every chiral carbon, not just the last chiral carbon. Most naturally occurring monosaccharides are D-sugars.

dlsugars2

Assignment:

W19.4 Work through the D,L exercises on the CD.

W19.5 Which sugars in Table 19.1 are diastereomers? In Table 19.2? Are there any pairs of enantiomers in Table 19.1 or 19.2? Post answers on the bulletin board.