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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.

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Types of Carbohydrates

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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 (CH2O)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 ORAldose


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

Triose

Pentose

Hexose


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 17.26 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 100-120 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.


6

α-D-glucose

5

1

1

4

2

6

2

3

3

5

4

4

1

5

2

6

3

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 Cu2+ ions

  • It readily oxidizes the aldehyde group of aldoses to form a brick red Cu2O 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.


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