C = 3 = triose
C = 4 = tetrose
C = 5 = pentose
C = 6 = hexose
Trioses: (e.g. glyceraldehydes, Dihydroxyacetone), intermediates in respiration and photosynthesis.
Tetroses: rare.
Pentoses: (e.g. ribose, ribulose), used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP.
Hexoses: (e.g. glucose, fructose), used as a source of energy in respiration and as building blocks for larger molecules.
All but one carbon atom have an alcohol (OH) group attached. The remaining carbon atom has an aldehyde or ketone group attached.
One important aspect of the structure of pentoses and hexoses is that the chain of carbon atoms is long enough to close up on itself and form a more stable ring structure. This can be illustrated using glucose as an example. When glucose forms a ring, carbon atom number 1 joins to the oxygen on carbon atom number 5. The ring therefore contains oxygen, and carbon atom number 6 is not part of the ring. By studying the diagram above we can see that the hydroxyl group, -OH, on carbon atom 1 may be above or below the plane of the ring. The form of glucose where it is below the ring is known as alpha glucose and the form where it is above as beta glucose. Two forms of the same chemical are known as isomers, and the extra variety provided by the existence of alpha and beta isomers has important Biological consequences, as we shall see in the structure of starch, glycogen and cellulose.
Glucose is so small that it can pass through the villi and capillaries into our bloodstream. The molecules subsequently release energy as a result of respiration. Simple glucose molecules are capable of so much more. They can combine with others to form bigger molecules.
Monoshacarides have two major functions. Firstly, they are commonly used as a source of energy in respiration. This is due to the large number of carbon-hydrogen bonds. These bonds can be broken to release a lot of energy which is transferred to help make ATP (adenosine triphosphate) from ATP (adenosine diphosphate) and phosphate. The most important monosaccharide in energy metabolism is glucose.
Secondly, they are important as building blocks for larger molecules. For example, glucose is used to make the polysaccharides starch, glycogen and cellulose. Ribose (a pentose) is used to make RNA and ATP. Deoxyribose (a pentose) is used to make DNA .
Each glucose unit is known as a monomer and is capable of linking others. Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond. The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of a 1,4-glycosidic bond. Depending on the monosaccharides used, this can be an α-1,4-glycosidic bond or a β-1,4-glycosidic bond. The reaction involves the formation of a molecule of water (H2O). This diagram shows two molecules of α glucose forming a disaccharide:
This shows two glucose molecules joining together to form the disaccharide maltose. Because this bond is between carbon 1 of one molecule and carbon 4 of the other molecule it is called a 1-4 glycosidic bond. This kind of reaction, where water is formed, is called a condensation reaction. A condensation reaction means that as two carbohydrate molecules bond together a water molecule is produced. The link formed between the two glucose molecules is known as a glycosidic bond. A glycosidic bond can also be broken down to release separate monomer units. This is the opposite of the reaction shown above. The reverse process, when bonds are broken by the addition of water (e.g. in digestion), is called a hydrolysis reaction. Instead of water being given off a water molecule in need to break each glycosidic bond. This is called hydrolysis because water is needed to split up the bigger molecule. Like most chemical reactions in cells, hydrolysis and condensation reactions are controlled by enzymes. Here is a diagram to represent the hydrolysis reaction:
There are three common disaccharides:
-
Maltose (or malt sugar) is glucose & glucose. It is formed on digestion of starch by amylase, because this enzyme breaks starch down into two-glucose units. Brewing beer starts with malt, which is a maltose solution made from germinated barley. Maltose is the structure shown above.
-
Sucrose (or cane sugar) is glucose & fructose. It is common in plants because it is less reactive than glucose, and it is their main transport sugar. It's the common table sugar that you put in tea.
-
Lactose (or milk sugar) is galactose & glucose. It is found only in mammalian milk, and is the main source of energy for infant mammals.
Sucrose is used in many plants for transporting food reserves, often from the leaves to other parts of the plant. Lactose is the sugar found in the milk of mammals and maltose is the first product of starch digestion and is further broken down to glucose before absorption in the human gut.
Biochemical tests
All monosaccharides and some disaccharides including maltose and lactose are reducing sugars. These can be tested for, by adding Benedict's reagent to the sugar and heating in a water bath. If a reducing sugar is present, the solution turns green, then yellow and finally produces a brick red precipitate. Non-reducing sugars can also be tested for using Benedict's reagent but first require addition of an acid and heating to hydrolyze (break apart) the sugar. The acid must then be neutralized using an alkali like sodium hydroxide before carrying out the test as described above.
Polysaccharides are long chains of many monosaccharides joined together by glycosidic bonds and through a condensation reaction. Each successive monosaccharide is added by means of a glycosydic bond, as in disaccharides. The final molecule may be several thousand monosaccharideunits long, forming a macromolecule. The most important polysaccharides are starch, glycogen and cellulose, all of which are polymers of glucose. Polysaccharides are not sugars.
Since glucose is the main source of energy for cells, it is important for living organisms to store it in an appropriate form. If glucose itself accumulated in cells, it would dissolve and make the contents of the cell to concentrated, which would seriously affect its osmotic properties. It is also a reactive molecule and would interfere with normal cell chemistry. These problems are avoided by converting it, by condensation reactions, to a storage polysaccharide, which is a convenient, compact, inert and insoluble molecule. This is in the form of starch in plants and glycogen in animals. Glucose can be made available again quickly by an enzyme- controlled reaction.
Starch is the principal polysaccharide used by plants to store glucose for later use as . Plants often store starch in seeds or other specialized organs, for example, common sources of starch include rice, beans, wheat, corn, potatoes, etc. When humans eat starch, an enzyme that occurs in saliva and in the intestines called amylase breaks the bonds between the repeating glucose thus allowing the sugar to be absorbed into the bloodstream. Once absorbed into the bloodstream, the human body distributes glucose to the areas where it is needed for or stores it as its own special - glycogen. Glycogen, another of glucose, is the polysaccharide used by animals to store . Excess glucose is bonded together to form glycogen , which the animal stores in the liver and muscle tissue as an "instant" source of . Both starch and glycogen are of glucose, however starch is a long, straight chain of glucose , whereas glycogen is a branched chain of glucose . It is insoluble and forms starch granules inside many plant cells. Being insoluble means starch does not change the water potential of cells, so does not cause the cells to take up water by osmosis. It is not a pure substance, but is a mixture of amylose and amylopectin.
Amylose is simply poly-(1-4) glucose, so is a straight chain. In fact the chain is floppy, and it tends to coil up into a helix.
Amylopectin is poly(1-4) glucose with about 4% (1-6) branches. This gives it a more open molecular structure than amylose. Because it has more ends, it can be broken more quickly than amylose by amylase enzymes.
Both amylose and amylopectin are broken down by the enzyme amylase into maltose, though at different rates. Amylose is made by many condensations between α-glucose molecules. In this way a long, unbranching chain of several thousand 1,4 linked glucose molecules is built up. The chains are curved and coil up into helical structures like springs, making the final molecule more compact. Amylopectin is also made of many 1,4 linked α-glucose molecules, but the chains are shorter than in amylase, and branch out to the sides. The branches are formed by 1,6 linkages.
Mixtures of amylase and amylopectin molecules build up into relatively large starch grains which are commonly found in chloroplasts and in storage organs such as the potato tuber and the seeds of cereals and legumes. Starch grains are easily seen with a light microscope, especially if stained; rubbing a freshly cut potato tuber in a glass slide and staining with iodine- potassium iodide solution is a quick method of preparing a specimen for viewing. Starch is never found in animal cells. Instead, a substance with molecules very like those of amylopectin is used as the storage carbohydrate.
This is called glycogen. Glycogen is similar in structure to amylopectin. It is poly (1-4) glucose with 9% (1-6) branches. It is made by animals as their storage polysaccharide, and is found mainly in muscle and liver. Because it is so highly branched, it can be mobilized (broken down to glucose for energy) very quickly. Glycogen molecules clump together to form granules, which are visible in liver and muscle cells where they form an energy reserve.
Another important polysaccharide is cellulose. Cellulose is yet a third of the monosaccharide glucose. Cellulose differs from starch and glycogen because the glucose form a two-dimensional structure, with hydrogen bonds holding together nearby , thus giving the added stability. Cellulose, also known as plant fiber, cannot be digested by human beings therefore cellulose passes through the digestive tract without being absorbed into the body. Some animals, such as cows and termites, contain bacteria in their digestive tract that help them to digest cellulose. Cellulose is a relatively stiff material, and in plants cellulose is used as a structural to add support to the leaves, stem and other plant parts. Despite the fact that it cannot be used as an source in most animals, cellulose fiber is essential in the diet because it helps exercise the digestive track and keep it clean and healthy.
Cellulose is only found in plants, where it is the main component of cell walls. It is poly (1-4) glucose, but with a different isomer of glucose. Cellulose contains beta-glucose, in which the hydroxyl group on carbon 1 sticks up. This means that in a chain alternate glucose molecules are inverted.
This apparently tiny difference makes a huge difference in structure and properties. While the 1-4 glucose polymer in starch coils up to form granules, the beta1-4 glucose polymer in cellulose forms straight chains. Hundreds of these chains are linked together by hydrogen bonds to form cellulose microfibrils. These microfibrils are very strong and rigid, and give strength to plant cells, and therefore to young plants.
The beta-glycosidic bond cannot be broken by amylase, but requires a specific cellulose enzyme. The only organisms that possess a cellulose enzyme are bacteria, so herbivorous animals, like cows and termites whose diet is mainly cellulose, have mutualistic bacteria in their guts so that they can digest cellulose. Cellulose is a form of in which some 1500 glucose rings chain together. It is the chief constituent of cell walls in living organisms. Wood is mostly cellulose, making cellulose the most abundant type of organic compound on the Earth. Cellulose molecules tend to be straight chains, and the fibers which result from collections of cellulose molecules have the strength to form the supporting structures of plants. Even though human digestion cannot break down cellulose for use as a food, animals such as cattle and termites rely on the energy content of cellulose. They have protozoa and bacteria with the necessary enzymes in their digestive systems. Cellulose in the human diet is needed for fiber.
Cellulose is the most abundant organic molecule on the planet due to its presence in plant cell walls and its slow rate of breakdown in nature. It has a structural role, being a mechanically strong molecule, unlike starch and glycogen. However the only difference between cellulose and the latter is that cellulose is a polymer of β- glucose not α- glucose. Remember that in the β isomer the –OH group on carbon atom 1 projects above the ring. In order to form a glycosydic bond with carbon atom 4, where the –OH group is below the ring, one glucose molecule must be upside down relative to the other, that is rotated 180°. Thus successive glucose units are linked at 180° to each other (as seen in diagram A8).
Adjacent chains of long, unbranched polymers of glucose joined by b-1,4-glycosidic bonds hydrogen bond with each other to form microfibrils. This results in a strong molecule because the hydrogen atoms of –OH groups are weakly attracted to oxygen atoms in the same cellulose molecule (the oxygen of the glucose ring) and also to oxygen atoms of –OH groups in neighboring molecules. These hydrogen bonds are individually weak, but so many can form, due to the large number of –OH groups, that collectively they develop enormous strength. Between 60 and 70 cellulose molecules become tightly cross linked to from bundles called microfibrils. Microfibrils are in turn held together in bundles called fibers by hydrogen bonding.
How useful are polysaccharides?
- Starch is stored in organisms as a future energy source, e.g. potato as a high starch content to supply energy for buds to grow at a later stage.
- Glycogen is stored in the liver, which releases glucose for energy in times of low blood sugar.
Both starch and glycogen are insoluble which enables them to remain inside cells.
- Cellulose has long chains and branches which help form a tough protective layer around plant cells, the cell wall.
- Pectins are used alongside cellulose in the cell wall. They are polysaccharides which are bound together by calcium pecate.
Together the cellulose and pectins give exceptional mechanical strength. The cell wall is also permeable to a wide range of substances.
Here are some other polysaccharides that I find interesting.
- Chitin (poly glucose amine), found in fungal cell walls and the exoskeletons of insects.
- Pectin (poly galactose uronate), found in plant cell walls.
- Agar (poly galactose sulphate), found in algae and used to make agar plates.
- Murein (a sugar-peptide polymer), found in bacterial cell walls.
- Lignin (a complex polymer), found in the walls of xylem cells, is the main component of wood.
Functions of carbohydrates
The following table classifies carbohydrates.
Both and are which are classified as polysaccharides since they are composed of chains of glucose molecules. While they are similar, starches can be used as energy sources by the human body while cellulose cannot. Enzymes are important in the metabolism of foods, and these enzymes are very specific. They are somewhat like keys which will fit the geometry of the starch bonds, but not those of the cellulose bonds.
The carbohydrates are the compounds which provide energy to living cells. They are compounds of carbon, hydrogen and oxygen with a ratio of two hydrogens for every oxygen atom. The carbohydrates we use as foods have their origin in the photosynthesis of plants. The name carbohydrate means "watered carbon" or carbon with attached water molecules. Many carbohydrates have empirical formuli which would imply about equal numbers of carbon and water molecules. For example, the formula C6H12O6 suggest six carbon atoms and six water molecules.