Carbohydrates Lincoln High School Version 1 12 1

Carbohydrates Lincoln High School Version 1. 12 1

Carbohydrates • Contain Carbon, Hydrogen and Oxygen • Can be characterized as – Monosaccharides – Disaccharides – Polysaccharides • Includes sugars, starches, cellulose, 2

Carbohydrates • Carbohydrates are produced in green plants in the presence of chlorophyll and sunlight in a process known as photosynthesis. • They serve as food sources for living organisms and provide the structural support for plants. • Many carbohydrates are large polymers composed of repeating units of simple sugars. 3

Types of Carbohydrates have the following basic composition: w Monosaccharides - simple sugars with multiple -OH groups. Based on number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. w Disaccharides - Two monosaccharides linked by a covalent bond. w Oligosaccharides - a few monosaccharides linked by covalent bonds w Polysaccharides - polymers consisting of chains of multiple monosaccharide or disaccharide units. 4

Carbohydrates Monosaccharides • Single (simple) sugars • Contain C, H, and O in a 1: 2: 1 ratio • Quick energy sources Examples: Glucose C 6 H 12 O 6 Fructose C 6 H 12 O 6 Galactose C 6 H 12 O 6 Glucose glucose Fructose 5

Monosaccharides • Empirical formula is CH 2 O • Both open chain and ring structures are possible • Mulitple structural isomers are possible • Multiple chiral carbon atoms lead to optical isomers • Monosaccharides generally have between 3 and 6 carbon atoms • The most common monosaccharides are: – Five carbons C 5 H 10 O 5 - called pentoses – Six carbons C 6 H 12 O 6 - called hexoses • Monosaccharide straight chains have at least one carbonyl group C=O. • If the carbonyl group is at the end it is an aldose sugar. If it is within the chain it is a ketose sugar 6

Monosaccharides Aldoses (e. g. , glucose) have an aldehyde group at one end. Ketoses (e. g. , fructose) have a ketone group, usually at C 2. 7

Optical Isomers: D and L Forms D or dextrorotatory & L or levorotatory are designations for optical isomers that are based on the configuration about the single asymmetric C in glyceraldehyde. The lower representations are Fischer Projections. 8

Sugar Nomenclature For sugars with more than one chiral center, D and L refer to the asymmetric C farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. 9

Steroisomers D & L sugars are mirror images of one another. They have the same name, e. g. , D-glucose & L-glucose. Other stereoisomers have unique names, e. g. , glucose, mannose, galactose, etc. The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The 6 -C aldoses have 4 asymmetric centers. Thus there are 16 possible stereoisomers (8 D-sugars and 8 L-sugars). 10

Ring Structures Pentoses and hexoses can form ring structures as the ketone or aldehyde reacts with a distal OH. Glucose forms an intra-molecular hemiacetal, as the C 1 aldehyde & C 5 OH react, to form a 6 -member ring known as a pyranose ring, These representations of the cyclic sugars are called Haworth projections. 11

Fructose Ring Structures Fructose may form either w a 6 -member pyranose ring, by reaction of the C 2 keto group with the OH on C 6, or w a 5 -member furanose ring, by reaction of the C 2 keto group with the OH on C 5. 12

Monosaccharides Some examples of pyranose ring structures for hexose sugars. The ring is not actually planar but exists in boat and chair conformers 13

Sugar Derivatives An Amino sugar is a sugar in which an amino group substitutes for a hydroxyl. An example is glucosamine. The amino group may be converted to an amide, as in N-acetylglucosamine. 14

Anomers of Glucose Cyclization of glucose produces a new asymmetric center at C 1. The 2 stereoisomers are called anomers, a & b. Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C 1: w a (OH below the ring) w b (OH above the ring). 15

Glycosidic Bonds The anomeric hydroxyl groups of two sugars can join together, splitting out water to form a glycosidic bond. Two glucose molecules combine to form a disaccharide known as maltose. 16

Disaccharides • Double sugars • Good source of energy • Break down into simple sugars Sucrose (glucose + fructose) Other disaccharides include: Lactose (glucose + galactose) -- Sucrose, common table sugar, has a glycosidic bond linking the anomeric hydroxyls of glucose & fructose. -- Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(1 2). The full name of sucrose is a-D-glucopyranosyl-(1 2)-b. D-fructopyranose. ) -- Lactose, milk sugar, is composed of galactose & glucose, H H with b(1 4) linkage from the anomeric OH of galactose. Its full name is b-D-galactopyranosyl-(1 4)-a-Dglucopyranose 17

Disaccharides • Compare the structures of these three common disaccharides H H • Sucrose is an a (1 -2) link between D-Glucose and DFructose • Lactose is an (1 -4) link between D glucose and Dgalactose 18

Polysaccharides • 3 or more sugars linked together • Complex sugars • Important for energy storage Examples: Starch- (plants) found in leaves, tubers… Glycogen- (animals) found in the liver and muscles Cellulose- (plants) make up cell walls Starch 19

Polysaccharides - Starches Reducing end Amylose • Plants store glucose as amylose or amylopectin. Both are glucose polymers collectively called starch. w Amylose is a glucose polymer with a (1 4) linkages. w The end of the polysaccharide with an anomeric C 1 that is not involved in a glycosidic bond is called the reducing end. • Glucose storage in polymer form minimizes osmotic effects. 20

Amylopectin w Amylopectin is a glucose polymer with mainly a(1 4) linkages, but it also has branches formed by a (1 6) linkages. Branches are generally longer than those shown in the diagram above. • The branches produce a compact structure & provide multiple chain ends at which enzyme activity can occur. 21

Glycogen • Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin found in plants • Glycogen has more a (1 6) branches than amylopectin • The ability to rapidly mobilize glucose is more essential to animals than to plants. • The highly branched structure permits rapid glucose release from glycogen stores, e. g. , in muscle during exercise. 22

Starch and Cellulose Amylose Cellulose The essential difference between amylose starch and cellulose is in the glycosidic link between the saccharide units. Amylose has a(1 -4) links. Cellulose has b(1 -4) links. 23

Cellulose • Cellulose is the major building component of plant cell walls • Long chain of glucose molecules would be expected to be a great source of energy, but humans lack the necessary enzyme to digest cellulose • The Endosymbiotic Protist in cow guts DOES have the enzyme 24

Dietary Fiber • Dietary fiber is mainly plant material that is not hydrolyzed by enzymes secreted by the human digestive tract but may be digested by microflora in the gut. • Examples of dietary fiber include cellulose, hemicellulose, lignin and pectin. • Dietary fiber may be helpful in the prevention of conditions such as diverticulosis, irritable bowel syndrome, constipation, obesity, Crohn’s disease, hemorrhoids and diabetes mellitus.

Carbohydrate Functions: Energy Sources • During metabolism animals break down carbohydrates to carbon dioxide and water vapor. • Monosaccharides and dissaccharides break down quickly and provide quick energy sources. • Starches take longer to metabolize but the end products are the same. • Human beings cannot break down cellulose, since we lack the appropriate enzyme to breakdown the b 1 -4 linkage 26

Carbohydrate Functions: Storage • The main storage polysaccharides are starches and glycogen. While plants use starch as their storage polysaccharides, animals use glycogen. • When the body has a high glucose concentration, the pancreas releases insulin, which converts glucose into glycogen and stores it in the liver. • When the glucose concentration is low, the hormone glucagon converts glycogen back into glucose. • Glycogen is the primary energy reserve in human beings. Metabolism of glucose provides the energy necessary for our bodies to function and carry out daily activities. • When it is broken down into glucose and oxidized, ultimately to CO 2 and H 2 O, through cellular respiration, large amounts of energy are released. 27

• • • Carbohydrate Functions: Structure Cellulose is a major component of plant cell walls. It is a polymer of -D-glucose and forms a very strong fiber, which is excellent building material in plants. Cows and other ruminants have enzymes that break down cellulose. In humans it is primarily bulk or roughage. Chitin is a structural polysaccharide found in the exoskeletons of some insects. Chitin is a leather like structural substance that eventually hardens when it is shed. Chitin is often used in medicine for sutures because it is both strong and flexible, but it also decomposes over time. 28

Carbohydrate Functions: Precursor Molecules • • Carbohydrates are precursors for the synthesis of certain biomolecules. Carbohydrates (ribose) form part of the skeletons of nucleic acids, DNA and RNA. The carbon skeletons of carbohydrates serve as raw material for the synthesis of other small organic molecules, such as amino acids and fatty acids. Disaccharides provide building material for structures that protect the cell or whole organism. 29

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