Carbohydrates
Biological molecules such as carbohydrates and proteins are often polymers and are based on a small number of chemical elements.
Carbohydrate is a fancy way of saying "sugar." Scientists came up with the name because the compounds have many carbon atoms bonded to hydroxyl groups. Carbohydrates can be very small or very large molecules, but they are still sugars. What are they used for?
A carbohydrate is called an organic compound because it contains carbon. Sugars provide living things with energy and act as substances used for structure. Some examples of structures might be the shell of a crab or the stem of a plant.
The elements that make up a carbohydrate molecule are CARBON, OXYGEN and HYDROGEN. Generally speaking, a monosaccharide (mono-, sacchar-) has a formula that is some multiple of CH2O. For instance, glucose has a formula of C6H12O6. Glucose is the most common monosaccharide.
SUGARS IN STRUCTURES
An important structural polysaccharide is cellulose. Cellulose is found in plants. It is one of those carbohydrates used to support or protect an organism. Cellulose is in wood and the cell walls of plants. You know that shirt you're wearing? If it is cotton, that's cellulose, too!
Monosaccharides are the basic molecular units (monomers) of which carbohydrates are composed.
SACCHARIDES
Scientists also use the word saccharide to describe sugars. If there is only one sugar molecule, it is called a monosaccharide. If there are two, it is a disaccharide. If there are three, it is a trisaccharide. You get the idea.
SIMPLE SUGARS
What about the simplest of sugars? A sugar called glucose is the most important monosaccharide on Earth. Glucose is used in cellular respiration and created by photosynthesis. When you think of table sugar, like the kind in candy, it is actually a disaccharide. The sugar on your dinner table is made of glucose and another monosaccharide called fructose.
POLYSACCHARIDES
When several carbohydrates combine, it is called a POLYsaccharide ("poly" means many). Hundreds of sugars can be combined in a chain. These chains are also known as starches. You can find starches in foods such as pasta and potatoes. They are very good sources of energy for your body.
The linking of a-glucose by glycosidic bonds formed by condensation to form maltose and starch.
In carbohydrates, the sub-units to be joined are monosaccharides like glucose. Both of the groups which combine are -OH groups, (even though there are many single -H atoms on a glucose molecule). Joining two -OH groups with the removal of H2O results in a disaccharide containing an -O- bridge between the 2 monosaccharide units. Between glucose units, these bonds are usually between carbon 1 of one glucose molecule and carbon 4 or 6 on the other. Depending on the direction of the -OH group at carbon 1, it may be called an alpha or a beta linkage. The bond so formed is called a glycosidic bond or link. These links can be extended many times, resulting in the production of polysaccharides.
Sucrose is a disaccharide formed by condensation of glucose and fructose.
Sucrose consists of two monosaccharides, α-glucose and fructose, joined by a glycosidic bond between carbon atom 1 of the glucose unit and carbon atom 2 of the fructose unit. What is notable about sucrose is that unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, and not between the reducing end of one and the non-reducing end of the other. This linkage inhibits further bonding to other saccharide units. It is classified as a non-reducing sugar. Hydrolysis can be used to convert sucrose into glucose and fructose. This can be brought about by the action of the enzyme sucrase.
Lactose is a disaccharide formed by condensation of glucose and galactose.
Lactose is a disaccharide that consists of galactose and glucose.
Lactose intolerance.
Lactose intolerance is the inability to metabolize lactose, a sugar found in milk and other dairy products, because the required enzyme lactase is absent in the intestinal system or its availability is lowered.
Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream, so in the absence of lactase, lactose present in ingested dairy products remains uncleaved and passes intact into the large intestine. Bacteria in the large intestine switch over to lactose metabolism, and this results fermentation which produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane). This, in turn, may cause a range of abdominal symptoms, including stomach cramps, bloating, and flatulence. In addition, the presence of lactose and its fermentation products raises the osmotic pressure of the colon contents, so can cause diarrhoea.
Biochemical tests using Benedict's reagent for reducing sugars and non-reducing sugars.
All monosaccharides and most disaccharides (except sucrose) will reduce copper (II) sulphate, producing a precipitate of copper (I) oxide on heating, so they are called reducing sugars. To test for the presence of monosaccharides and reducing disaccharide sugars in food, the food sample is dissolved in water, and a small amount of Benedict's reagent is added. It is placed in a water bath (at about 60°C), for 5-10 minutes. If a reducing sugar is present the solution should progress in the colors of blue (with no glucose present), green, yellow, orange, red, and then brick red or brown (with high glucose present).
To test for the presence of a non-reducing sugar, first test the sample for reducing sugars, to see if there are any present before hydrolysis. Then, using a separate sample, boil the test solution with dilute hydrochloric acid for a few minutes to hydrolyse the glycosidic bond. Neutralise the solution by gently adding small amounts of solid sodium hydrogen carbonate (or sodium bicarbonate) until it stops fizzing, then add an equal quantity of Benedict's reagent. Shake, and heat for a few minutes at 60°C in a water bath. A precipitate indicates reducing sugar. The colour and density of the precipitate gives an indication of the amount of reducing sugar present, so this test is semi-quantitative.
Iodine / potassium iodide solution for starch.
To approximately 2 cm³ of test solution add two drops of iodine/potassium iodide solution. A blue-black colour indicates the presence of starch as a starch-polyiodide complex is formed. Starch is only slightly soluble in water, but the test works well in a suspension or as a solid.
Biological Molecules
Life - The Science of Biology - Chapter 2 and 3
Hypertextbook - Large Molecules
Biochemistry -Society - Essential Chemistry for Biochemistry
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