NOTE3 LU3 Carbohydrates

Carbohydrates

Prepared by: Dr. Mulham Alfatama
Delivered by: Dr. Fathurrahman Lananan
Learning Unit 3 ASH10303 Principles Biochemistry in Animal Sciences ASQ10103 Principles Biochemistry

Learning Objectives

Understand carbohydrates as an important biomolecule.
Learn the classifications of carbohydrates.
Understand the basic structures and properties of carbohydrates.
Learn the functions of carbohydrates.
Know the deficiency and excess of carbohydrates in the human body.

Introduction

Carbohydrates are the most abundant biomolecules.

They serve as:

The major source of energy for the plant kingdom and humankind ($4 ext{ kcal/g}$).
Supportive structural components in plants (cellulose).
Essential components in the mechanism of genetic control of growth and development in living cells (ribose and deoxyribose).
Serve as stored forms of energy as glycogen in liver & muscles.
Exert a sparing action on proteins, helping to conserve proteins.
Considered to be a carbon skeleton for the synthesis of some non-essential amino acids & fats.
Dietary fiber (cellulose).
Important components of brain cells as neuraminic acids, cerebrosides (glycolipids), and gangliosides.
Important components of nucleic acids as pentose sugars - ribose & deoxyribose.
Function as a physiological anticoagulant (heparin).

Structure of Carbohydrates

Carbohydrates are also referred to in a structural context:

Electron micrograph of the erythrocyte surface shows a thick carbohydrate coat, called the glycocalyx, consisting of closely packed oligosaccharides attached to cell-surface proteins and lipids.

Definitions

Carbohydrates are polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis (hydrates of carbon).
Some carbohydrates also contain nitrogen, phosphorus, or sulfur.
They are referred to as saccharides from the Latin word "saccharum," which means "sugar."
The simpler members of the carbohydrates family often give a sweet taste. They have the suffix -ose.
Empirical formula: $ (CH2O)n $

Classifications of Carbohydrates

Monosaccharides
- Glucose
- Fructose
- Galactose
- Triose ($C3H6O_3$)
- Tetrose ($C4H8O_4$)
- Pentose ($C5H{10}O_5$)
- Hexose ($C6H{12}O_6$)
- Heptose ($C7H{14}O_7$)
- Octose ($C8H{16}O_8$)
Disaccharides
- Sucrose (table sugar)
- Lactose (milk sugar)
- Maltose (malt sugar)
Polysaccharides
- Starch (corn starch)
- Glycogen (liver)
- Cellulose (cotton fiber)
- Hemi-cellulose (dietary fiber)

Properties of Monosaccharides

They are colorless crystalline solids that are freely soluble in water but insoluble in nonpolar solvents (like alcohol and ether).
The backbone is an unbranched carbon chain where all the carbon atoms are linked by single bonds.
They consist of a single polyhydroxy aldehyde or ketone unit.
One of the carbon atoms is double bonded to oxygen to form a carbonyl group; each of the other carbon atoms has a hydroxyl group.
Depending upon the position of the carbonyl group, they are categorized as aldose (if the carbonyl is at the end) or ketose (if the carbonyl is within the carbon chain).

Examples of Monosaccharides

Triose: 3 carbons example includes D-Glyceraldehyde.
Tetrose: 4 carbons example includes D-Erythrose and D-Threose.
Pentose: 5 carbons example includes D-Ribose (an aldopentose), and 2-Deoxy-D-ribose.
Hexose: 6 carbons example includes D-Glucose (an aldohexose) and D-Fructose (a ketohexose).

Cyclic Forms of Monosaccharides

Common monosaccharides (pentoses and hexoses) can occur in cyclic forms (closed chain).

The five-membered sugar ring system is known as furanose and the six-membered sugar ring system is known as pyranose.
The stereochemistry of cyclic forms of sugar is depicted by Haworth projections, which standardize the way of depicting the positions of hydroxyl groups in space.

Optical Activity

The D and L forms of handed pairs of stereoisomers are identical in their physical and chemical properties, except that they rotate plane-polarized light in opposite directions.
Optical isomers are handed stereoisomers that rotate plane-polarized light in opposite directions, for example, D-glyceraldehyde and L-glyceraldehyde which rotate plane-polarized light to the right and left respectively.

Reducing Properties

Simple monosaccharides are reducing agents; they can be oxidized by mild oxidizing agents like ferric ($Fe^{3+}$) or cupric ($Cu^{2+}$) ions.
This property is the basis for Fehling’s reaction, a qualitative test for the presence of reducing sugars, such as glucose.

Tests for Reducing Sugars

Benedict's Test:

Reactions yield various precipitates based on the concentration of reducing sugar
Fehling’s Solution:
- Requires fresh preparation.
- Deteriorates quickly.

Forms of Carbohydrates

Fischer Projections allow representing three-dimensional organic molecular structure in two dimensions.

In Fischer projections, the functional group is always at the top, while the carbon chain is written vertically with substituent groups positioned to the left and right.

Interconversion of Forms

The α and β forms of monosaccharides are readily interconvertible in aqueous solution.
The change in specific rotation during this interconversion is known as mutarotation.
For example, a freshly prepared solution of α-D-glucose shows an initial rotation of +112° to +52°.

Disaccharides

When two monosaccharides join together, they form a disaccharide.
Example: Lactose

Formation of Glycosidic Bond

A glycosidic bond is formed when the hydroxyl group of one sugar reacts with the anomeric carbon of the other sugar, resulting in the elimination of one water molecule.

Structures of Disaccharides

Maltose: a-D-glucopyranosyl-(1-4)-D-glucopyranose.
Lactose: β-D-galactopyranosyl-(1-4)-β-D-glucopyranose.
Sucrose: β-D-fructofuranosyl-(α-D-glucopyranoside).
Trehalose: a-D-glucopyranosyl-a-D-glucopyranoside.

Polysaccharides

Polysaccharides are carbohydrates containing many monosaccharides units connected by glycosidic bonds.

Types of Polysaccharides

Storage Polysaccharides
- Starch (plants) includes Amylose (unbranched) and Amylopectin (branched).
- Glycogen (animal storage).
Structural Polysaccharides
- Cellulose (plant cell walls).
- Chitin (arthropod exoskeleton).
- Hyaluronic acids (connective tissues).

Starch Structure

Amylose consists of glucose units (60-300) linked by α-1,4-glycosidic bonds.
Amylopectin is highly branched, linked by both α-1,4-glycosidic links and α-1,6-glycosidic links.

Cellulose Structure

Cellulose is a linear polymer of D-glucose units (300-15,000), linked by β-1,4-linkage.
It forms the material of plant cell walls, providing structural support.

Glycogen Structure

Glycogen is a branched polymer similar to amylopectin, with nonreducing ends. Its structure allows for rapid mobilization of glucose when needed for energy.

Conclusion

Carbohydrates play a vital role as energy sources, structural components, and in various biological processes. Their diverse structures and functions make them essential biomolecules in both plants and animals.

Here are the formulas and equations found in the note:

FORMULA

The major source of energy for the plant kingdom and humankind ( $4 ext{ kcal/g}$ ).
Empirical formula: $(CH2O)n$
Triose $(C3H6O_3)$
Tetrose $(C4H8O_4)$
Pentose $(C5H{10}O_5)$
Hexose $(C6H{12}O_6)$
Heptose $(C7H{14}O_7)$
Octose $(C8H{16}O_8)$
Oxidized by mild oxidizing agents like ferric ( $Fe^{3+}$ ) or cupric ( $Cu^{2+}$ ) ions.

EXPLANATION

Energy Value

Energy per gram from carbohydrates: $4 ext{ kcal/g}$

Explanation: This represents the major source of energy provided by carbohydrates for organisms in both the plant kingdom and humankind.

General Carbohydrate Formula

Empirical formula of Carbohydrates: $(CH2O)n$

Explanation: This formula indicates that carbohydrates are composed of carbon, hydrogen, and oxygen, with hydrogen and oxygen typically in the same ratio as water ( $2:1$ ).

Monosaccharide Formulas (by carbon count)

Triose: $(C3H6O_3)$

Explanation: A monosaccharide containing three carbon atoms.

Tetrose: $(C4H8O_4)$

Explanation: A monosaccharide containing four carbon atoms.

Pentose: $(C5H{10}O_5)$

Explanation: A monosaccharide containing five carbon atoms.

Hexose: $(C6H{12}O_6)$

Explanation: A monosaccharide containing six carbon atoms, such as glucose and fructose.

Heptose: $(C7H{14}O_7)$

Explanation: A monosaccharide containing seven carbon atoms.

Octose: $(C8H{16}O_8)$

Explanation: A monosaccharide containing eight carbon atoms.

Ions involved in Reducing Properties

Ferric ion: $Fe^{3+}$
Cupric ion: $Cu^{2+}$

Explanation: These are the mild oxidizing agents (like in Fehling's reaction) that can oxidize simple monosaccharides, demonstrating their reducing properties.

Carbohydrates : The most abundant biomolecules.
Carbohydrates : Polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis (hydrates of carbon).
Saccharides : A term for carbohydrates, from the Latin word "saccharum," meaning "sugar."
Simpler carbohydrates : Members of the carbohydrate family that often give a sweet taste and have the suffix -ose.
Aldose : A monosaccharide where the carbonyl group is at the end of the carbon chain.
Ketose : A monosaccharide where the carbonyl group is within the carbon chain.
Furanose : A five-membered sugar ring system.
Pyranose : A six-membered sugar ring system.
Haworth projections : A method to depict the stereochemistry of cyclic forms of sugar, standardizing the way of depicting the positions of hydroxyl groups in space.
Optical isomers : Handed stereoisomers that rotate plane-polarized light in opposite directions.
Mutarotation : The change in specific rotation during the interconversion of
delta
and
beta
forms of monosaccharides in aqueous solution.
Glycosidic bond : A bond formed when the hydroxyl group of one sugar reacts with the anomeric carbon of another sugar, resulting in the elimination of