Types of Carbohydrates Section 17.1

Four Types of Carbohydrates Monosaccharides  Contain a single sugar unit  Examples: glucose and fructose Disaccharides  Contain two monosaccharides units joined through bridging oxygen atoms AKA glycosidic bond  Examples: sucrose and lactose

Four Types, continued Oligosaccharides  Three to ten monosaccharide units joined by glycosidic bonds Polysaccharides  Long, often highly branched, chains of monosaccharides  Examples: starch, glycogen, and cellulose

Monosaccharides Composed of carbon, hydrogen and oxygen Most follow the general formula (CH 2 O) n All names end in –ose Two types  Ketose – monosaccharide that contains a ketone carbonyl group  Aldose – monosaccharide that contains an aldehyde carbonyl group

Ketose OR Aldose?

Ketose OR Aldose

Another system – number of carbons Another system of nomenclature tells us the number of carbon atoms in the main skeleton. 3 carbon triose 4 carbon tetrose 5 carbon pentose 6 carbon hexose Etc.

Triose, tetrose, pentose, etc?

Answers TriosePentoseHexose

Combining the two systems Aldose Triose Aldotriose D-Glyceraldehyde Aldose Hexose Aldohexose D-Glucose Ketose Hexose Ketohexose D-Fructose

Chiral Objects Chiral compounds have the same number of atoms arranged differently in space. A chiral carbon atom has four different groups attached.

Mirror Images The three-dimensional structure of a chiral compound has a mirror image. Your hands are chiral. Try to superimpose your thumbs, palms, back of hands, and little fingers. Is it possible? Why or why not?

Chiral or NOT? Determine if there is a chiral carbon in each compound. A B

Solution A Yes, 4 different B No, the groups are attached 2 H atoms to the second C atom are identical

D and L Notation D,L tells to which of the two chiral isomers we are referring. If the –OH group on the next to the bottom carbon atom points to the right, the isomer is a D-isomer; if it points left, the isomer is L. The D form is usually the isomer found in nature.

D notation

Glucose

Homework Complete problems 17.3 and 17.4 on page 472; 17.5 and 17.6 on page 474-5; 17.24, 17.25, and on page 493

Importance of Glucose Most important sugar in the human body. Glucose is broken down in glycolysis and other pathways to release energy for body functions The concentration of glucose in the blood is carefully controlled by insulin and glucagon. Normal blood glucose levels are mg/100mL

Importance of Glucose Insulin stimulates the uptake of the excess glucose by most of the cells in the body. 1-2 hrs after eating the glucose levels return to normal. If glucose concentrations drop too low, the individual feels lightheaded and shaky. When this happens, glucagon stimulates the liver to release glucose in the blood.

Diabetes / Hypoglycemia and Glucose Type I diabetes or diabetes mellitus – caused by the inability to produce the hormone insulin; If untreated, end up with high blood sugar Type II diabetes or adult-onset diabetes – caused by insulin resistance (body fails to properly use insulin) combined with a relative insulin deficiency; If untreated, end up with high blood sugar Hypoglycemia – caused by the over excretion of insulin in response to a rise in blood sugar; If untreated, end up with low blood sugar

Structure of glucose In reality, the open-chain form of glucose is present in very small concentrations in cells. In most cases, the cyclic intramolecular hemiacetal is formed. The hydroxyl group on C-5 reacts with the aldehyde group to form a hemiacetal. Two isomers are formed because a new chiral carbon is created.

D-glucose α-D-glucose β-D-glucose

Haworth Structure for D-Glucose Write –OH groups on the right (C2, C4) down Write –OH groups on the left (C3) up The new –OH on C1 has two possibilites: down for  form, up for  form

α-D-glucose O

β-D-glucose O

You Try This One! Write the cyclic form of  -D-galactose

Solution  -D-galactose O

Benedict’s reagent Benedict’s reagent – a basic buffer solution that contains Cu 2+ ions It readily oxidizes the aldehyde group of aldoses to form a brick red Cu 2 O precipitate. It will also oxidize ketoses because of the –OH group on the carbon next to the carbonyl group.

Reducing Sugars Reducing sugar – a sugar that can be oxidized by Benedict’s reagent All monosaccharides and the common disaccharides (except sucrose) are reducing sugars.

Use of Benedict’s reagent Benedict’s reagent was commonly used to qualitatively monitor excess glucose in the urine by diabetics to insure proper dosage of insulin. These have been replaced by blood glucose tests that are more accurate.