Carbohydrates

Carbohydrates Types of Carbohydrates Classification of Monosaccharides D and L Notations from Fischer Projections Structures of Some Important Monosaccharides

Carbohydrates Carbohydrates are: A major source of energy from our diet. Composed of the elements C, H and O. Also called saccharides, which means “sugars”. Produced by photosynthesis in plants.

Photosynthesis and Respiration Photosynthesis in plants requires CO2, H2O, and energy from the sun and produces carbohydrates, such as glucose [C6H12O6 = C(H2O)6]. In respiration, glucose is oxidized in living cells to produce CO2, H2O, and energy.

Types of Carbohydrates Monosaccharides are the simplest carbohydrates. Disaccharides consist of two monosaccharides. Polysaccharides contain many monosaccharides.

Classification of Monosaccharides 1) Functional group: Aldoses are monosaccharides with an aldehyde group and many hydroxyl (-OH) groups. Ketoses are monosaccharides with a ketone group and many hydroxyl (-OH) groups. Aldose Aldose Ketose

Monosaccharides 2) Number of carbon atoms: A triose has three carbons; a Tetrose has four carbons; a pentose has five carbons; and a hexose has six carbons. Triose Tetrose Hexose

Note Numerous Chiral Carbons 2 | 1—C—3 4 3—C—1 The two simplest sugars Note Numerous Chiral Carbons

Fischer Projections Is used to represent carbohydrates. Places the most oxidized group at the top. Uses horizontal lines for bonds that come forward. Uses vertical lines for bonds that go back.

D and L isomers By convention, the letter L is assigned to the structure with the —OH on the left. The letter D is assigned to the structure with —OH on the right.

D and L isomers The —OH on the chiral atom farthest from the carbonyl group is used to assign the D or L configuration. The D form is usually the isomer found in nature.

D-Glucose Found in grape, corn syrup, and honey. An aldohexose with the formula C6H12O6 Known as blood sugar in the body.

Blood Glucose Level In the body, glucose has a normal concentration of 80-110 mg/dL. In a glucose tolerance test, blood glucose is measured for several hours after ingesting glucose.

D-Fructose Is a ketohexose with formula of C6H12O6. Is the sweetest carbohydrate. Is found in fruit, juices and honey. Converts to glucose in the body.

D-Galactose Is not found free in nature. Is obtained from lactose, a disaccharide. Is important in cellular membranes.

Cyclic Structures Monosaccharides with 5-6 carbon atoms form cyclic structures The hydroxyl group on C-5 reacts with the aldehyde group or ketone group

Sugars Prefer To Be Cyclic

Haworth Structure for D-Isomers The cyclic structure of a D-isomer has the final CH2OH group located above the ring.

Haworth Structure for D-Glucose The new –OH on C1 has two possibilities: down for  anomer, up for  anomer

Cyclic Haworth Structures Stable cyclic when the C=O group and the -OH are part of the same molecule. The cyclic structure of a D-isomer has the last CH2OH group located above the ring.

Haworth Structure for D-Glucose   -D-Glucose -D-Glucose

Glucose Anomers

Mutarotation A small amount of open chain is in equilibrium with the cyclic forms. The most stable form of glucose is β-D-glucose. -D-glucose D-glucose (open) β-D-glucose (36%) (trace) (64%)

-D-Glucose In nature, open chain forms are rare. All 5 forms coexist with the pyranose forms occurring 99% of the time, Furanose about 0.5% each and a trace of open chain.  and  are “anomers” and differ in configuration at the new chiral center formed in the cycle