Carbohydrate Notes

Carbohydrates

3.1 Introduction

Carbohydrates serve as the primary energy source for living cells.
They provide structural support in plants and insects.
Carbohydrates are hydrates of carbon, represented by the general formula $Cx(H2O)_y$ , where $x$ and $y$ are variable numbers.
Hydrogen and oxygen are in a $2:1$ ratio, similar to water.
Carbohydrates are classified into three main groups: monosaccharides, disaccharides, and polysaccharides, with names ending in '-ose'.

3.2 Monosaccharides

Monosaccharides represent the simplest carbohydrate form, composed of single sugar units (monomers).
Cells utilize them as a direct energy source.
Monosaccharides exist as either aldehydes (aldoses) or ketones (ketoses), both containing several hydroxyl (OH) groups.
Aldoses (terminal carbonyl group) oxidize more readily than ketoses (non-terminal carbonyl group).
Aldoses function as strong reducing agents.
Monosaccharides are categorized based on the number of carbon atoms: trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), and heptoses (7C).
Trioses, pentoses, and hexoses are the most prevalent.
Monosaccharides can exist in open chain or ring forms.
Open chain forms exhibit optical isomerism, existing as D and L isomers.
D-isomers are more common and rotate polarized light to the right (dextro = D), while L-isomers are rare and rotate light to the left (Levo = L).

3.2.1 Trioses

During glycolysis, glucose (6-carbon) is broken down into two 3-carbon (triose) sugars: glyceraldehyde and dihydroxyacetone.
Glyceraldehyde has a terminal carbonyl group, while dihydroxyacetone's carbonyl group is in the center.
Each triose sugar has D and L optical isomers. D-isomers rotate polarized light to the right (dextro), while L-isomers rotate light to the left (levo).
Figure 3.1a: Structures of D- and L-glyceraldehyde.
Figure 3.1b: Structures of D- and L-dihydroxyacetone.

3.2.2 Pentose Sugars

Pentoses are 5-carbon sugars with the formula $C5H{10}O_5$ .
Examples include ribose, deoxyribose, and ribulose.
Ribose and deoxyribose are vital nucleic acid components.
Ribulose facilitates carbon dioxide acceptance to produce glucose during photosynthesis.

3.2.2.1 Ribose and Deoxyribose

These 5C sugars constitute nucleic acids (RNA and DNA).
Ribose is a constituent of RNA, while deoxyribose is a constituent of DNA.
Ribose has one OH group and one H atom at carbon 2, while deoxyribose has two H atoms.
Figure 3.2: Structures of D and L isomers of ribose and deoxyribose (open chains).
Open chain pentose sugars exist as D and L isomers.
Open chain forms convert to ring forms when carbon 4 combines with carbon 1, forming a furanose ring.
The oxygen of carbon 4 links with carbon 1 to create a closed ring.
The ring form can exist as alpha (α) or beta (β) isomers.
The oxygen of the C=O bond on C1 combines with hydrogen of C4 to form an OH group, which is above the plane in β ribose and deoxyribose and below the plane in α ribose and deoxyribose.
α and β forms are stereoisomers due to the asymmetric (chiral) C1, which is attached to four different groups: OH, H, O, and C.
Figure 3.3: Formation of ribose furanose ring into α-D-ribose (OH group on C1 below the ring) or β-D-ribose (OH group on C1 above the ring).
Figure 3.4: Formation of deoxyribose furanose ring into α-D-deoxyribose (OH group on C1 below the ring) or β-D-deoxyribose (OH group on C1 above the ring).

3.2.3 Hexose Sugars

Hexoses are 6C sugars with the formula $C6H{12}O_6$ . Examples are glucose, fructose, and galactose.
They serve as a direct energy source and are oxidized during cellular respiration.
Glucose is the most prevalent respiratory monosaccharide.
Hexoses are used to synthesize disaccharides (2 monomers), oligosaccharides (2-10 monomers), and polysaccharides (more than 10 monomers).

3.2.3.1 Glucose

Glucose (grape sugar) is a simple monosaccharide found in plants.
It is an essential product of photosynthesis and fuel for cellular respiration.
Glucose is water-soluble due to hydrogen bonding with water molecules.
The open chain form of glucose is an aldehyde (aldose), existing as the common D isomer and the rare L isomer.
Figure 3.5: Structures of D-glucose and L-glucose.
Carbon 1 (C1) of the open chain combines with the OH group of C5 to form a six-membered pyranose ring, where the oxygen of the C=O bond links C1 and C5.
Figure 3.6: Formation of the glucose pyranose ring.
The ring structure makes carbon 1 asymmetric or chiral.
When the OH group on carbon 1 is below the ring, it results in the α-isomer of glucose; when above, it results in the β-isomer.
The existence of α- and β-glucose leads to the formation of various dimers and polymers like starch and glycogen (from α-glucose) and cellulose (from β-glucose).
Glucose is a strong reducing agent due to the aldehyde group at carbon 1 in the open chain (aldose).
The D-isomer of glucose is common, while the synthetic L-isomer is used as a low-calorie sweetener and laxative.
Living organisms cannot use L-glucose as an energy source, as they lack the necessary enzyme.

3.2.3.2 Galactose

Galactose is a water-soluble monosaccharide (aldose sugar).
It is a structural isomer of glucose.
The OH group on C4 of glucose is below the plane, while on C4 of galactose, it's above the plane.
It forms a pyranose ring that differs from glucose at C4.
Galactose combines with glucose to form lactose (milk sugar).
Galactose is a powerful reducing sugar due to its aldehyde group in the open chain.
Figure 3.7: Formation of the galactose pyranose ring.

3.2.3.3 Fructose

Fructose (fruit sugar) is a water-soluble monosaccharide in honey, fruits, flowers, berries, and root crops.
The open chain of fructose is a ketone (ketose) where carbon 2 combines with the OH group of carbon 5 to form a furanose ring.
Like glucose, fructose is used as an energy source in cellular respiration.
Fructose is a weak reducing sugar because its open chain structure can isomerize into an aldose (glucose).
Carbon 2 of fructose is asymmetric, leading to α- and β- isomers, similar to glucose. Fructose is a structural isomer of glucose.
Figure 3.8: Formation of the fructose furanose ring.

3.2.3.3 Summary of Hexose Isomers

Isomers are molecules with the same molecular formula but different spatial arrangements of atoms.
Examples of monosaccharide isomers include:
- Optical isomers: molecules rotating polarized light to the right (D) or left (L), e.g., D-glucose and L-glucose.
- Stereoisomers: molecules with the same molecular formula and structural formula but differing in spatial arrangement, e.g., α-D-glucose and β-D-glucose, glucose and galactose.
- Structural isomers: molecules with the same molecular formula but different structural formulas, e.g., glucose (aldose) and fructose (ketose).

3.3 Disaccharides

Disaccharides are composed of two sugar units (dimers). They are crystalline, sweet, and water-soluble.
They are formed by condensation (dehydration) reactions between two monosaccharides through a glycosidic bond.
They serve as temporary energy stores, broken down by hydrolysis into monomers for energy production through cellular respiration.
Common disaccharides include sucrose, lactose, and maltose.

3.3.1 Sucrose

Sucrose (cane sugar) consists of glucose and fructose linked via a 1,2-glycosidic bond between C1 of α-glucose and C2 of fructose.
Its molecular formula is $C{12}H{22}O_{11}$ .
It is a temporary energy store that is broken down into glucose and fructose for cellular respiration.
It is abundant in plants, transported through the phloem tissue.
Figure 3.9: Formation and breakdown of sucrose.
Sucrose is a non-reducing sugar because it is linked through carbon 1 of glucose and carbon 2 of fructose, which contain the reducing aldehyde and keto groups in the open chain structures.
It is used in industry as a sweetener, for baking, confectionery, and as a food preservative.

3.3.2 Lactose

Lactose (milk sugar) consists of galactose and glucose linked through a β-(1, 4) glycosidic bond between C1 of β-galactose and C4 of either α-glucose or β-glucose.
It has a formula of $C{12}H{22}O_{11}$ and is water-soluble.
Figure 3.10: Formation and breakdown of lactose.
Lactose is a reducing sugar because carbon 1 of glucose is free to form an open chain, exposing the reducing aldehyde.
It can be broken down into galactose and glucose for energy generation in tissue respiration.
It is extracted from sweet whey and added to tablets as a filler and to some beers (stouts) to reduce bitterness.

3.3.3 Maltose

Maltose (malt sugar) is water-soluble and composed of two glucose molecules linked through an α 1→4 glycosidic linkage.
It is a reducing sugar because carbon 1 on one glucose monomer is free and can form an open chain, exposing the reducing aldehyde.
Figure 3.11: Formation and breakdown of maltose.
Maltose is produced by the hydrolysis (digestion) of starch by amylase during animal digestion and in germinating seeds.
Maltose is then broken down into glucose units by maltase.
It is an important intermediate in producing fermented beverages (e.g., beers) from cereals like barley, millet, maize, and rice.

3.4 Polysaccharides

Polysaccharides are non-sweet macromolecules, insoluble or slightly soluble in water.
They are formed by condensation (dehydration) reactions between many monosaccharides (10 or more).
Polysaccharides function as long-term food and energy stores (e.g., starch and glycogen) or as structural materials (e.g., cellulose and chitin).
Starch and glycogen are suitable energy storage molecules because:
- They are insoluble in water.
- They do not affect the osmotic potential of the cell.
- They are highly branched and fold into compact shapes within limited space.
- They are easily converted to disaccharides and monosaccharides by hydrolysis (digestion).
Cellulose and chitin are suitable as structural materials because:
- They are insoluble in water.
- They are made of long threads (fibers).
- They have very high tensile strength.

3.4.1 Starch

Starch is a polymer of α-glucose monomers that join to form maltose, which then forms starch.
It is insoluble in cold water due to its long molecule with many glycosidic bonds and a tightly packed structure.
Starch has two components:
- Amylose: A straight chain structure of thousands of α-glucose molecules linked through α-1,4 glycosidic bonds, coiling into a helix.
  - Amylose in water gives a blue-black color with iodine due to the iodide complex slipping into the α-helix coil.
  - It constitutes 20-30% of starch.
- Amylopectin: Similar to amylose but with additional branches formed by α-1, 6 glycosidic bonds and shorter chains.
  - Amylopectin in water gives a red-violet color with iodine due to little interaction between the iodide complex and amylopectin.
  - It constitutes 70-80% of starch.
Starch, a major energy store in plants, is the most common carbohydrate in human staple foods like rice, wheat, maize, cassava, and potatoes.
Starch is easily broken down into maltose (disaccharide) and glucose (monosaccharide) by hydrolysis (digestion).
In industry, starch is used as a thickening agent, stabilizer, or prebiotic.
Figure 3.12: Synthesis and breakdown of amylose (straight chain starch).
Figure 3.13: Synthesis and breakdown of amylopectin (branched starch).

3.4.2 Glycogen

Glycogen is the animal equivalent of starch (amylopectin) and is made from α-glucose.
It is water-insoluble due to numerous α(1-4) and α(1-6) bonds and its compact structure.
It exists as granules in the cytoplasm of many cells but is stored mainly in the liver and muscle cells of vertebrates.
Glycogen is hydrolyzed into glucose for use in respiration when needed.
Although similar to amylopectin, its branches are shorter and more frequent.
The glucose chains are organized in branches from a protein molecule at the center.
Figure 3.14: Structure of glycogen.

3.4.3 Cellulose

Cellulose is a water-insoluble polymer of β-glucose with -CH2OH groups alternating above and below the plane.
The hydroxyl (OH) groups projecting outwards form hydrogen bonds with neighboring chains.
Neighboring cellulose molecules lie close together in layers forming rigid microfibrils, which are arranged in larger macrofibrils.
These bundles are arranged in layers and have tremendous tensile strength.
Cellulose layers are fully permeable to water and solutes, important for plant cell function.
Cellulose is the most abundant organic compound on earth and the major structural material of plant cell walls.
Figure 3.15: Structures of cellulose, a fibril, and a microfibril.
Cattle, horses, ruminants, and termites have bacteria in their digestive systems that digest cellulose into soluble sugars used by both the bacteria and their hosts (symbiosis).
Cellulose is important in human diets as dietary fiber. In industry, cellulose is used to produce paper and cotton thread. Research aims to convert cellulose into biofuels.

3.4.4 Chitin

Chitin is an unbranched polymer of N-Acetyl-D-glucosamine found in cell walls of fungi and exoskeletons of insects, crabs, and shrimps.
It is similar to cellulose, but the hydroxyl group of carbon 2 of each glucose unit is replaced with an acetamido (NH(C=O)CH3) group.
Figure 3.16: Structure of chitin.

3.5 Metabolism of Carbohydrates

Table 3.1: Carbohydrate breakdown.
- Starch is broken down by amylase into maltose.
- Maltose is broken down by maltase into glucose.
- Sucrose is broken down by sucrase/invertase into glucose and fructose.
- Lactose is broken down by lactase into galactose and glucose.
- Glycogen is broken down by glycogen phosphorylase into glucose.
- Glucose, fructose, and galactose are broken down by various glycolytic enzymes into $CO2 + H2O + Energy$ .