A disaccharide, also called a double sugar, is a molecule formed by two monosaccharides, or simple sugars. Three common disaccharides are sucrose, maltose, and lactose. They have 12 carbon atoms, and their chemical formula is C12H22O11. Other, less common disaccharides include lactulose, trehalose, and cellobiose. Disaccharides are formed through dehydration reactions in which a total of one water molecule is removed from the two monosaccharides.
Functions of Disaccharides
Disaccharides are carbohydrates found in many foods and are often added as sweeteners. Sucrose, for example, is table sugar, and it is the most common disaccharide that humans eat. It is also found in other foods like beetroot. When disaccharides like sucrose are digested, they are broken down into their simple sugars and used for energy. Lactose is found in breast milk and provides nutrition for infants. Maltose is a sweetener that is often found in chocolates and other candies.
Plants store energy in the form of disaccharides like sucrose and it is also used for transporting nutrients in the phloem. Since it is an energy storage source, many plants such as sugar cane are high in sucrose. Trehalose is used for transport in some algae and fungi. Plants also store energy in polysaccharides, which are many monosaccharides put together. Starch is the most common polysaccharide used for storage in plants, and it is broken down into maltose. Plants also use disaccharides to transport monosaccharides like glucose, fructose, and galactose between cells. Packaging monosaccharides into disaccharides makes the molecules less likely to break down during transport.
Formation and Breakdown of Disaccharides
When disaccharides are formed from monosaccharides, an -OH (hydroxyl) group is removed from one molecule and an H (hydrogen) is removed from the other. Glycosidic bonds are formed to join the molecules; these are covalent bonds between a carbohydrate molecule and another group (which does not necessarily need to be another carbohydrate). The H and -OH that were removed from the two monosaccharides join together to form a water molecule, H2O. For this reason, the process of forming a disaccharide from two monosaccharides is called a dehydration reaction or condensation reaction.
When disaccharides are broken down into their monosaccharide components via enzymes, a water molecule is added. This process is called hydrolysis. It should not be confused with the process of dissolution, which happens when sugar is dissolved in water, for example. The sugar molecules themselves do not change structure when they are dissolved. The solid sugar simply turns into liquid and becomes a solute, or a dissolved component of a solution.
Examples of Disaccharides
Sucrose, commonly known as table sugar in its refined form, is a disaccharide found in many plants. It is made up of the monosaccharides glucose and fructose. In the form of sugar, sucrose is a very important component of the human diet as a sweetener. Sugar was first extracted and purified from sugar cane in India as early as the 8th Century BCE. In fact, the word candy gets its name in part from the word khanda, which was a name for sugar crystals in Sanskrit. Today, around 175 metric tons of sugar are produced each year.
People with congenital sucrase-isomaltase deficiency (CSID) are sucrose intolerant and cannot digest it well because they are missing the enzyme sucrose-isomaltase. Some people with CSID have trouble digesting starches as well. A person who is sucrose intolerant must limit sucrose as much as possible, and they may have to take supplements or medications.
Maltose, also known as malt sugar, is formed from two glucose molecules. Malt is formed when grains soften and grow in water, and it is a component of beer, starchy foods like cereal, pasta, and potatoes, and many sweetened processed foods. In plants, maltose is formed when starch is broken down for food. It is used by germinating seeds in order to grow.
Lactose, or milk sugar, is made up of galactose and glucose. The milk of mammals is high in lactose and provides nutrients for infants. Most mammals can only digest lactose as infants, and lose this ability as they mature. In fact, humans that are able to digest dairy products in adulthood actually have a mutation that allows them to do so. This is why so many people are lactose intolerant; humans, like other mammals, did not have the ability to digest lactose past childhood until this mutation became prevalent in certain populations around 10,000 years ago. Today, the number of people who are lactose intolerant varies widely between populations, ranging from <10% in Northern Europe to 95% in parts of Africa and Asia. The traditional diets of different cultures reflect this in the amount of dairy consumed.
Trehalose is also made up of two glucose molecules like maltose, but the molecules are linked differently. It is found in certain plants, fungi, and animals like shrimp and insects. The blood sugar of many insects, such as bees, grasshoppers, and butterflies, is made up of trehalose. They use it as an efficient storage molecule that provides rapid energy for flight when broken down.
Lactulose is formed from fructose and galactose. It can be used to treat constipation and liver disease and may also be used to test for overgrowth of bacteria in the small intestine. Some countries use it in food, but this is not allowed in the United States because it is viewed as a pharmaceutical that could potentially harm diabetics.
Like maltose and trehalose, cellobiose is also made up of two glucose molecules, but they are linked in yet another different way. It is found made cellulose, the main component of plant cell walls, is hydrolyzed. Cellobiose is used in bacteriology, the study of bacteria, to perform chemical analyses.
Chitobiose consists of two linked glucosamine molecules. Structurally, it is very similar to cellobiose except that it has a N-acetylamino group where cellobiose has a hydroxyl group. It is found in some bacteria, and used in biochemistry research in order to study enzyme activity. Also, it is found in chitin, which makes up fungi cell walls, exoskeletons of insects, arthropods, and crustaceans, and is also found in fish and cephalopods such as octopuses and squid.
Related Biology Terms
- Carbohydrate – An organic molecule consisting of carbon, hydrogen and oxygen with a hydrogen to oxygen ratio of 2:1.
- Glucose – A monosaccharide used for energy; it is found in the blood of animals and created during photosynthesis by plants.
- Monosaccharide – Simple carbohydrates; two of them join to form a disaccharide.
- Starch – Long chains of glucose produced by plants for energy storage; it is common in human diets.
1. Which disaccharide is made up of two glucose molecules and is found as a sweetener in many foods?
2. Which phrase describes the disaccharide lactose?
A. Commonly known as table sugar
B. Formed when grains soften and germinate in water
C. Used in order to perform chemical analyses of bacteria
D. Found in milk, provides nutrients for mammalian infants
3. What two monosaccharides join to form sucrose?
A. Glucose and fructose
B. Glucose and galactose
C. Glucose and glucose
D. Fructose and galactose
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