A carbohydrate is a macro-nutrient. It is called “macro” because it is needed in large proportions in the diet. Carbohydrates have the basic formula Cn(H2O)n, hence they are considered hydrated carbons. They fall in four major categories.
Let’s take a closer look at each of these.
These are the smallest and simplest units of carbohydrates. They are sweet and hence are used in foods to improve sweetness. They have the ability to chemically react with metal salts such as copper in Benedicts solution, to produce a color change. Hence they are said to be reducing sugars. Reducing sugars are important in food production as they react with amino acids to produce dark colors and flavors in a reaction called Maillard browning.
Monosaccharides may be classified as:
- Triose – 3 carbons
- Tetrose – 4 carbons
- Pentose – 3 carbons
- Hexose – 3 carbons
Common monosaccharides include:
- Ribose and deoxyribose
Monosaccharides are further categorized as:
- Aldose: Carbohydrates with an aldehyde group
- Ketose: Carbohydrates with a ketone group
L and D Sugars
Monosaccharides may be in the L or D form. The D form is most common. L a D sugars are enantiomers (mirror images) of each other. If the OH group on the bottom chiral center is on the right, the sugar is a D sugar. If the OH on the bottom chiral center points to the left, the sugar is an L sugar.
Fischer and Haworth Projections
Monosaccharides may be drawn in either the Fischer (straight line) or Haworth projection (cyclic). In the case of glucose, carbon number 1 reacts with carbon number 5 to form a 6-carbon ring called a pyranose ring.
In the case of fructose, carbon number 2 reacts with carbon number 5 to produce a five membered ring called a furanose ring.
The Anomeric Carbon
The carbon derived from the carbonyl carbon (the aldehyde or ketone group) is called the anomeric carbon. It is a very important site as it is the site at which the ring opening occurs, becoming the functional carbonyl group. The substituent of the anomeric carbon may either be in the alpha (α) or beta (β) position.
Drawing the Structure of Monosaccharides
In an exam you may be given the Fischer projection and asked to draw the Haworth projection. Here are rules that you will need to follow, using hexose and pentose sugars as examples.
- Number the carbons in the Fischer projection
- Draw either a 6 or 5-membered ring
- Add the position of the oxygen
- Number the ring clockwise starting next to the oxygen
- Right-Down Rule – If the substituent (H or OH) is to the right in the Fischer projection, it will be drawn down in the Haworth projection. (Leave out assigning direction for the anomeric carbon and the highest numbered carbon until the next steps).
- DHUP Rule (pronounce as dup) – For D sugars the highest numbered carbon is drawn up. For L sugars the highest numbered carbon is drawn down.
- The OHDD-α Rule (pronounce as odd) – For D sugars, the OH group at the anomeric carbon is drawn down if it is α.
- The OHUL-α Rule (pronounce as owl) – For L sugars, the OH group at the anomeric carbon is drawn up if it is α.
Practice: Follow the above steps to draw the Haworth projection for each of the following sugars with the OH constituent on the anomeric carbon in the alpha position.
Disaccharides are carbohydrates consisting of two monosaccharide units. The bond that connects them is called a glyosidic bond. Common disaccharides include:
Lactose and maltose are reducing sugars since they both have free anomeric carbons. However, sucrose is not a reducing sugar. Both of its anomeric carbons are tied up to form an α (1-2) bond.
Oligosaccharides are carbohydrates having 3 to 10 monosaccharide units. They are classified as:
The most common oligosaccharides are raffinose and stachyose, found in pea and beans. They are a source of soluble dietary fiber.
Some oligosaccharides are linked to other biomolecules such as proteins and lipids. These are referred to as glycoproteins and glycolipids respectively. Glycoproteins and glycolipids play an important role in cell recognition and cell binding.
Polysaccharides are carbohydrates that consist of more than 10 monosaccharide units bonded together in a branched or linear form. They may consist of only one type of monosaccharides (homoglycans) or more than one (heteroglycan). They are tasteless, and generally act as dietary fiber. Types of polysaccharides include:
Starch is the main form of energy storage in plants. It is a homoglycan made up of glucose units. It has a crystalline structure comprising of two sub-structures with different functional properties. These are amylose, a linear chained molecule of glucose connected at the α (1-4) glycosidic link, and amylopectin consisting of α (1-6) branches in addition to chains connected at the α (1-4) bond.
Glycogen is the energy storage molecule in animals. It has a similar structure to starch except that does not have subunits (amylose and amylopectin), and is more highly branched.
Cellulose is an unbranched homoglycan found in plant cell walls. Their glucose units are connected by β(1-4) glycosidic bonds. These bonds cannot be broken down by digestive enzymes in the human digestive system. Therefore though abundant in plant foods, cellulose is not a source of energy for humans.
Hemicellulose are heteroglycans. They are less complex than cellulose, and may be either linear or branched. Hemicellulose bind with pectin and cellulose in plant cell walls to provide structure. They are a source of soluble dietary fiber. In food they are used for water-binding, thickening and gelling. Based on their main backbone they may classified as:
- D-Xylans: Consists of β(1-4) linked D-xylose
- D-Mannans: Consists of β(1-4) linked D-mannose
- D-Galactans: Consists of β(1-3) linked D-Galactose
- D-Xyloglycans: Consists of D-Xylopyranose residues attached to cellulose
Pectin is a soluble dietary fiber found in the cell walls of plants. It consist of linear chains of α (1-4) galacturonic acid and “hairy” variable branched regions. The linear regions may be methylated or acetylated to various degrees. Highly methylated pectin is called high-methoxy pectin while those with low methylation are referred to as low-methoxy pectin.
Pectin is used to provide the gel structure of jams. When dissolved at neutral pH it has a negative charge. This causes repulsion of the pectin chain, preventing them from binding. However with the addition of an acid such as lime juice to the jam, pectin approaches zero charge, bringing the strands together, producing a gel.
Gums are extracted mostly from plants but may also be derived from bacteria e.g. xanthan gum. There numerous hydrophilic groups and long chains allows them to bind water and trap materials that they are mixed with. Hence they are excellent as thickenings and binders in food and pharmaceutical products. Common plant-based gums include agar, guar gum, locust bean gum, gum acacia, sodium alginate, and carrageenan.
Inulin is a naturally occurring polysaccharide in plants. The compound is used by plants as a means of energy reserve and are mostly found in roots and rhizomes. The FDA has approved inulin as a natural source of dietary fiber. Chemically, inulin is a heterogenous collection of fructose polymers linked by β(1-2) bonds, attached to a terminal glucosyl residue by an α(1,2) bond.
Chitin is found in the cell walls of fungi and and the exoskeleton of insects, arthropods, crustaceans and the scales of fish. It provides a scaffold support structure and protects against dehydration. Chemically, chitin is made up of a long chain of N-acetylglucosamine (an amide derived from glucose) linked through β(1,4)-glycosidic bonds.