CARBOHYDRATES

BIOCHEM

Carbohydrates

Are polyhydroxyl aldehydes and ketones. It follow the general formula C_nH_2nO_n

Monosaccharide

Simple sugars that have 3 to 7 carbons in chain, with carbonyl group at either the terminal carbon or carbon adjacent to it

Aldoses

Monosaccharides with an aldehyde group at C1

Ketoses

Monosaccharides with a ketone group at C2

D-configuration

The naturally occurring sugar configuration

L-Glucose

Sugar configuration that is not utilized by the body

D-Glucose

Sugar configuration that is utilized by the body

37% alpha α, 63% β, trace amount of hydroxyl aldehyde (open-chain form)

Equilibrium mixture of Monosaccharide that exists

Trans

α-D-glucose CH₂OH and anomeric OH configuration

Cis

β-D-glucose CH₂OH and anomeric OH configuration

Epimeric Pair

Glucose-Mannose (C2)
Glucose-Galactose (C4)

Hemiacetal

Formed when the aldehyde group of a sugar reacts with one of its own hydroxyl (–OH) groups, causing the sugar to form a ring structure

Anomeric Carbon

The carbon atom that was part of the carbonyl group in the open-chain form of a sugar and becomes a new chiral center when the sugar forms a ring

Mutarotation

The spontaneous change in the optical rotation of a sugar as its α- and β-anomers interconvert at equilibrium through ring–chain tautomerism

α-glycosidic bond

formed when the anomeric hydroxyl (–OH) involved in glycosidic bond formation is below the plane of the ring in D-sugars

β-glycosidic bond

formed when the anomeric hydroxyl (–OH) involved in glycosidic bond formation is above the plane of the ring in D-sugars

The β-anomer of D-glucose is the preferred form because its anomeric hydroxyl (–OH) is in the equatorial position, which reduces steric hindrance and makes the ring more stable

Why is the β-anomer the preferred form of glucose?

Glycosidic Bond

A covalent linkage between two monosaccharides, formed through a dehydration reaction that eliminates a water molecules

Reducing sugar

Kind of sugar if the anomeric carbon is free (not involved in bonding)

Non-reducing sugar

Kind of sugar if both anomeric carbons are linked Simp

Simple Carbohydrates

Monosaccharides that cannot be broken down by hydrolysis

Complex Carbohydrates

Consist of two or more monomers that linked through glycosidic bond

Oligosaccharides

Carbohydrates that are composed of 2–10 monosaccharide units linked by glycosidic bonds.

Disaccharide

A carbohydrate composed of two monosaccharides joined by a glycosidic bond.

Trisaccharide

A carbohydrate composed of three monosaccharides linked by glycosidic bonds

Sucrose (Table Sugar)

A disaccharide composed of glucose and fructose linked by an α(1→2) glycosidic bond.

Role: It is the primary transport sugar in plants and a major source of dietary energy.

Lactose (Milk Sugar)

A disaccharide composed of glucose and galactose linked by a β(1→4) glycosidic bond.

Role: It is the main sugar found in milk and serves as an energy source for infants.

Maltose (Malt Sugar)

A disaccharide composed of two glucose molecules linked by an α(1→4) glycosidic bond.

Role: It is an intermediate product formed during the digestion of starch and glycogen.

Raffinose

A well-known trisaccharide composed of galactose, glucose, and fructose.

Sources: Sugarcane, beets, seeds

Melezitose

A trisaccharide composed of two glucose molecules and one fructose molecule.
Role: It is found in plant sap and honeydew and serves as an energy source for some insects.

Stachyose

A trisaccharide composed of two galactose and one glucose molecule.

Sources: Soybeans, jasmine, lentils

Polysaccharide

Consist of more than 10 monosaccharide units

Homopolysaccharide

Composed of only one type of monosaccharide.
Examples: Starch, Glycogen, Cellulose, and Chitin.

Heteropolysaccharide

composed of two or more different types of monosaccharides. Examples: Hyaluronic acid, Heparin, Pectin, Carrageenan, and Alginic acid.

α-Glycosidic linkages

Produce flexible, coiled polysaccharides that are easily broken down by enzymes, making them ideal for energy storage

β-Glycosidic linkages

Produce straight, rigid polysaccharide chains that form strong fibers, making them ideal for structural support.

Starch

A polysaccharide composed of α-D-glucose units. (20% Amylose, 80% amylopectin)

Role: It is the primary energy storage carbohydrate in plants.

Glycogen

A highly branched polysaccharide composed of α-D-glucose units.

Role: It is the primary energy storage carbohydrate in animals, mainly stored in the liver and muscles.

Cellulose

A polysaccharide composed of β-D-glucose units linked by β(1→4) glycosidic bonds.

Role: It is the main structural component of plant cell walls.

Chitin

A polysaccharide composed of N-acetylglucosamine (GlcNAc) units linked by β(1→4) glycosidic bonds.

Role: It provides structural support in fungal cell walls and the exoskeletons of insects and crustaceans.

Alginic acid

A heteropolysaccharide composed of β-D-mannuronic acid and α-L-guluronic acid.

Role: It provides structural support in the cell walls of brown algae and is widely used as a thickening, gelling, and stabilizing agent in food and pharmaceutical products.

Carrageenan

A heteropolysaccharide composed of sulfated galactose units.

Role: It is extracted from red algae and is commonly used as a thickener, stabilizer, and emulsifier in foods such as dairy products and desserts.

Pectin

A heteropolysaccharide composed mainly of galacturonic acid.

Role: It is a structural component of plant cell walls and is widely used as a gelling agent in jams and jellies.

Cellulose is indigestible because humans lack the enzyme cellulase, which is required to hydrolyze the β(1→4) glycosidic bonds between glucose molecules.

Why Cellulose indigestible to humans?

Amylose

the linear (unbranched) component of Starch, composed of α-D-glucose units linked by α(1→4) glycosidic bonds

Amylopectin

The branched component of Starch, composed of α-D-glucose units linked by α(1→4) glycosidic bonds with α(1→6) branch points.

Cellobiose

A reducing disaccharide composed of two glucose molecules linked by a β(1→4) glycosidic bond. It is the repeating unit of cellulose and cannot be digested by humans because we lack the enzyme cellulase.

Gentiobiose

A reducing disaccharide composed of two glucose molecules linked by a β(1→6) glycosidic bond. It occurs naturally in some plants and is produced during the breakdown of certain plant glycosides.

Isomaltose

A reducing disaccharide composed of two glucose molecules linked by an α(1→6) glycosidic bond. It is produced during the digestion of amylopectin and glycogen, where α(1→6) branch points are broken down.

Stachyose

A non-reducing tetrasaccharide composed of 2 galactose + 1 glucose + 1 fructose. It belongs to the raffinose family of oligosaccharides and is commonly found in beans, soybeans, and legumes. Humans cannot digest it because they lack α-galactosidase, so intestinal bacteria ferment it, producing gas.