Overview of carbohydrates, including structure and properties of monosaccharides, disaccharides, and polysaccharides.
What’s in a spud? Besides water, which makes up most of the potato’s weight, there’s a little fat, a little protein…and a whole lot of carbohydrate (about 37 grams in a medium potato).
Some of that carbohydrate is in the form of sugars. These provide the potato, and the person eating the potato, with a ready fuel source. A bit more of the potato's carbohydrate is in the form of fiber, including cellulose polymers that give structure to the potato’s cell walls. Most of the carbohydrate, though, is in the form of starch, long chains of linked glucose molecules that are a storage form of fuel. When you eat French fries, potato chips, or a baked potato with all the fixings, enzymes in your digestive tract get to work on the long glucose chains, breaking them down into smaller sugars that your cells can use.
Carbohydrates are biological molecules made of carbon, hydrogen, and oxygen in a ratio of roughly one carbon atom () to one water molecule (). This composition gives carbohydrates their name: they are made up of carbon (carbo-) plus water (-hydrate). Carbohydrate chains come in different lengths, and biologically important carbohydrates belong to three categories: monosaccharides, disaccharides, and polysaccharides. In this article, we’ll learn more about each type of carbohydrates, as well as the essential energetic and structural roles they play in humans and other organisms.
Monosaccharides (mono- = “one”; sacchar- = “sugar”) are simple sugars, the most common of which is glucose. Monosaccharides have a formula of , and they typically contain three to seven carbon atoms.
Most of the oxygen atoms in monosaccharides are found in hydroxyl () groups, but one of them is part of a carbonyl () group. The position of the carbonyl () group can be used to categorize the sugars:
- If the sugar has an aldehyde group, meaning that the carbonyl C is the last one in the chain, it is known as an aldose.
- If the carbonyl C is internal to the chain, so that there are other carbons on both sides of it, it forms a ketone group and the sugar is called a ketose.
Sugars are also named according to their number of carbons: some of the most common types are trioses (three carbons), pentoses (five carbons), and hexoses (six carbons).
Structures of monosaccharides. By carbonyl position: glyceraldehyde (aldose), dihydroxyacetone (ketose). By number of carbons: glyceraldehyde (triose), ribose (pentose), and glucose (hexose).
Structure of aldehyde: carbonyl bonded to a H on one side and to an R group (carbon-containing group) on the other. Structure of ketone: carbonyl bonded to R and R' groups (carbon-containing groups) on both sides.
Glucose and its isomers
One important monosaccharide is glucose, a six-carbon sugar with the formula . Other common monosaccharides include galactose (which forms part of lactose, the sugar found in milk) and fructose (found in fruit).
Glucose, galactose, and fructose have the same chemical formula (), but they differ in the organization of their atoms, making them isomers of one another. Fructose is a structural isomer of glucose and galactose, meaning that its atoms are actually bonded together in a different order.
Glucose and galactose are stereoisomers (have atoms bonded together in the same order, but differently arranged in space). They differ in their stereochemistry at carbon 4. Fructose is a structural isomer of glucose and galactose (has the same atoms, but bonded together in a different order).
Glucose and galactose are stereoisomers of each other: their atoms are bonded together in the same order, but they have a different 3D organization of atoms around one of their asymmetric carbons. You can see this in the diagram as a switch in the orientation of the hydroxyl () group, marked in red. This small difference is enough for enzymes to tell glucose and galactose apart, picking just one of the sugars to take part in chemical reactions.
Ring forms of sugars
You may have noticed that the sugars we’ve looked at so far are linear molecules (straight chains). That may seem odd because sugars are often drawn as rings. As it turns out both are correct: many five- and six-carbon sugars can exist either as a linear chain or in one or more ring-shaped forms.
These forms exist in equilibrium with each other, but equilibrium strongly favors the ring forms (particularly in aqueous, or water-based, solution). For instance, in solution, glucose’s main configuration is a six-membered ring. Over 99% of glucose is typically found in this form.
Even when glucose is in a six-membered ring, it can occur in two different forms with different properties. During ring formation, the from the carbonyl, which is converted to a hydroxyl group, will be trapped either “above” the ring (on the same side as the group) or “below” the ring (on the opposite side from this group). When the hydroxyl is down, glucose is said to be in its alpha (α) form, and when it’s up, glucose is said to be in its beta (β) form.
Linear and ring forms of glucose. The linear form can convert into either the alpha or the beta ring form, with the two forms differing in the position of the hydroxyl group derived from the carbonyl of the linear form. If the hydroxyl is up (on the same side as the CHOH group), then the molecule is beta glucose, while if it is down (on the opposite side), then the molecule is alpha glucose.
Also pictured ring forms of ribose and fructose. Unlike the six-membered glucose rings, these rings are five-membered.
Disaccharides (di- = “two”) form when two monosaccharides join together via a dehydration reaction, also known as a condensation reaction or dehydration synthesis. In this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another, releasing a molecule of water and forming a covalent bond known as a glycosidic linkage.
For instance, the diagram below shows glucose and fructose monomers combining via a dehydration reaction to form sucrose, a disaccharide we know as table sugar. (The reaction also releases a water molecule, not pictured.)
Formation of a 1-2 glycosidic linkage between glucose and fructose via dehydration synthesis.
In some cases, it’s important to know which carbons on the two sugar rings are connected by a glycosidic bond. Each carbon atom in a monosaccharide is given a number, starting with the terminal carbon closest to the carbonyl group (when the sugar is in its linear form). This numbering is shown for glucose and fructose, above. In a sucrose molecule, the carbon of glucose is connected to the carbon of fructose, so this bond is called a glycosidic linkage.
Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide consisting of glucose and galactose and is found naturally in milk. Many people can't digest lactose as adults, resulting in lactose intolerance (which you or your friends may be all too familiar with). Maltose, or malt sugar, is a disaccharide made up of two glucose molecules. The most common disaccharide is sucrose (table sugar), which is made of glucose and fructose.
Common disaccharides: maltose, lactose, and sucrose
A long chain of monosaccharides linked by glycosidic bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched and may contain different types of monosaccharides. The molecular weight of a polysaccharide can be quite high, reaching daltons or more if enough monomers are joined. Starch, glycogen, cellulose, and chitin are some major examples of polysaccharides important in living organisms.
Starch is the stored form of sugars in plants and is made up of a mixture of two polysaccharides, amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose using light energy gathered in photosynthesis, and the excess glucose, beyond the plant’s immediate energy needs, is stored as starch in different plant parts, including roots and seeds. The starch in the seeds provides food for the embryo as it germinates and can also serve as a food source for humans and animals, who will break it down into glucose monomers using digestive enzymes.
In starch, the glucose monomers are in the α form (with the hydroxyl group of carbon sticking down below the ring), and they are connected primarily by glycosidic linkages (i.e., linkages in which carbon atoms and of the two monomers form a glycosidic bond).
- Amylose consists entirely of unbranched chains of glucose monomers connected by linkages.
- Amylopectin is a branched polysaccharide. Although most of its monomers are connected by linkages, additional linkages occur periodically and result in branch points.
Because of the way the subunits are joined, the glucose chains in amylose and amylopectin typically have a helical structure, as shown in the diagram below.
Top: amylose has a linear structure and is made of glucose monomers connected by 1-4 glycosidic linkages. Bottom: amylopectin has a branching structure. It is mostly made of glucose molecules connected by 1-4 glycosidic linkages, but has glucose molecules connected by 1-6 linkages at the branch points.
That’s great for plants, but what about us? Glycogen is the storage form of glucose in humans and other vertebrates. Like starch, glycogen is a polymer of glucose monomers, and it is even more highly branched than amylopectin.
Glycogen is usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down via hydrolysis to release glucose monomers that cells can absorb and use.
Although energy storage is one important role for polysaccharides, they are also crucial for another purpose: providing structure. Cellulose, for example, is a major component of plant cell walls, which are rigid structures that enclose the cells (and help make lettuce and other veggies crunchy). Wood and paper are mostly made of cellulose, and cellulose itself is made up of unbranched chains of glucose monomers linked by glycosidic bonds.
Cellulose fibers and molecular structure of cellulose. Cellulose is made of glucose monomers in the beta form, and this results in a chain where every other monomer is flipped upside down relative to its neighbors.
Unlike amylose, cellulose is made of glucose monomers in their β form, and this gives it very different properties. As shown in the figure above, every other glucose monomer in the chain is flipped over in relation to its neighbors, and this results in long, straight, non-helical chains of cellulose. These chains cluster together to form parallel bundles that are held together by hydrogen bonds between hydroxyl groups. This gives cellulose its rigidity and high tensile strength, which are important to plant cells.
The β glycosidic linkages in cellulose can't be broken by human digestive enzymes, so humans are not able to digest cellulose. (That’s not to say that cellulose isn’t found in our diets, it just passes through us as undigested, insoluble fiber.) However, some herbivores, such as cows, koalas, buffalos, and horses, have specialized microbes that help them process cellulose. These microbes live in the digestive tract and break cellulose down into glucose monomers that can be used by the animal. Wood-chewing termites also break down cellulose with the help of microorganisms that live in their guts.
Image of a bee. The bee's exoskeleton (hard outer shell) contains chitin, which is made out of modified glucose units that have a nitrogenous functional group attached to them.
Cellulose is specific to plants, but polysaccharides also play an important structural role in non-plant species. For instance, arthropods (such as insects and crustaceans) have a hard external skeleton, called the exoskeleton, which protects their softer internal body parts. This exoskeleton is made of the macromolecule chitin, which resembles cellulose but is made out of modified glucose units that bear a nitrogen-containing functional group. Chitin is also a major component of the cell walls of fungi, which are neither animals nor plants but form a kingdom of their own.
Want to join the conversation?
- Why are galactose and glucose isomers, when the hydroxyl group that is "flipped" is linked to the chain by single bonds only? I thought only double+triple bonds forced atoms to stay where they are. According to the previous lesson about ethene/ethane, galactose and glucose should be the same thing because the flip happens around single bonds.(17 votes)
- Well, single bonds allow the actual atom/molecule to rotate. However, in this case, when the article is talking about "flipped" molecules, the molecules are not able to rotate, they are just upside-down in relation to the neighboring molecules.(7 votes)
- Confused about differences between beta-glycosidic and alpha glycosidic linkages. Are beta-glycosidic linkages inherently stronger due to some chemical property, or is it just that humans lack the enzymes to digest it? Whatever the answer, this is not to be confused with the reason cellulose is so strong (which is due to the hydrogen bonds acting between different polymers of glucose, forming thin fibrils), am I correct?(8 votes)
- Well from what I learned beta-glycosidic linkages are stronger due to the way bonds criss-cross between every alternate glucose molecule such as cellulose making it both physically and chemically stable.
These beta-glycosidic linkages can be broken down by enzymes stored by the bacteria stored in a cow's gut for example which why cow's can ingest complex carbohydrates like cellulose and as you suggested humans cannot digest such carbohydrates as we do not have the enzymes that can specifically break beta-glycosidic linkages.(12 votes)
- What is the advantage of polysaccharides, storage wise?(3 votes)
- To add to the excellent reply from Okapi, another reason why glucose is stored as glycogen is that if it were stored as free glucose, this would cause osmotic pressure to increase such that cell membranes would rupture. Even the elevated glucose level (hyperglycemia) seen in uncontrolled diabetes cause changes in osmotic pressure that are responsible for some of the symptoms, such as increased urination and excessive thirst.(8 votes)
- What is the difference between a monomer and a monosaccharide?(2 votes)
- A monomer is the smallest unit of a polymer.
A polysaccharide (carbohydrate) is a polymer. A monosaccharide is the monomer that makes up a polysaccharide.
In short- Monomer is a general term, while monosaccharide is a specific term for the monomer that makes up a carbohydrate.
A monosaccharide is a single sugar molecule. This includes glucose, fructose, and galactose.
Disaccharides are double sugars, such as sucrose (table sugar).
Polysaccharides are long chains, such as plant and animal starches.(8 votes)
- With the glucose and galactose isomers, can't glucose's 3rd carbon spin around if it doesn't have a double bond? Then it would be exactly like galactose. Does this happen or is there a reason why it can't?(4 votes)
- Because it requires breaking off the bonds.
If you want to turn H, and OH into one another, you will have to break bonds and reattach them. This does not happen spontaneously in water.
You cannot turn one into another through mere rotations of a bond. :)(2 votes)
- we have looked at the linear structures of these sugars my question is how these atoms of sugars are arranged in real life means in linear form or in ringed form?(4 votes)
- In a solution of water, around 3% will be in the linear form, the rest are ringed.(1 vote)
- if single bonds can rotate freely do the stereoisomers become each other interchangeably ?(2 votes)
- No, single bonds stereoisomers such as some enatiomers are not interchangeably because they are stuck in the three dimension world(3 votes)
- Looking at my notes from the "Properties of Carbon" module, I would say that glucose and galactose could be further subcategorised as enantiomers, as they cannot be perfectly aligned one on top of the other. Is this correct?(2 votes)
- No. They are not mirror images of each other (which is what an enantiomer is). Instead, they are diastereomers, since they have 2 or more stereogenic centers.(4 votes)
- in case of fructose which is a pentose sugar how can we distinguish between alpha and beta fructose?(3 votes)
- If CH2OH is on the same side of OH then beta otherwise alfa(1 vote)
- How can you identify a carbohydrate? Are all molecules with a carbonyl group (C=O) and hydroxyl group (OH) carbohydrates? Or is it so that ONLY carbohydrates have one carbon (C) atom to one H2O? If the latter is true, why is deoxyribose a carbohydrate with a formula C5H10O4?(1 vote)
- Unfortunately there isn't a universally accepted definition for what makes up a carbohydrate.
A very restrictive definition is as follows:
Carbohydrates consist of carbon, hydrogen, and oxygen — with the hydrogen and oxygen occurring in a 2:1 ratio. There also must be at least three carbons.
In other words, these are organic molecules that incorporate multiple water molecules and have at least three carbons.
However, derivatives of molecules that meet the above definition are still generally thought of as carbohydrates.
(In addition, some molecules that fit these parameters aren't generally thought of as carbohydrates.)
For example: deoxyribose is a sugar (carbohydrate) found in DNA that has had an oxygen removed.
Other modifications including addition of groups containing nitrogen and sulfur are also commonly found.
I recommend the wikipedia article on this subject:
Does that help?(3 votes)