Lecture 10: Carbohydrates

What are Carbohydrates?

• Carbohydrates are a group of organic compounds made of oxygen, carbon and hydrogen. (CH2O)

• Due to their structure, highly polar and interact well with water

Carbohydrates Function

• Serve as energy sources, energy storage molecules, and structural components

• Found in sugars, starches, glycogen, cellulose and even in nucleic acids.

Carbohydrate Structure

• Made of oxygen, carbon and hydrogen

• Contain multiple -OH groups and often an aldehyde or ketone group

• Can contain ethers (ROR) and alcohols (OH) but NOT Chlorine (CL).

Classification of Carbohydrates

Carbohydrates are classified based on their size and complexity.

1. Monosaccharides

2. Disaccharides

3. Polysaccharides

Monosaccharides

• Simplest form, just single sugar units

• Ex: Glucose, Ribose*, Deoxyribose*

• Ex: Ribose* = Sugar (Aldopentose) found in RNA and ATP

• Ex: 2-Deoxyribose* = Sugar (Aldopentose) found in DNA

Monosaccharides Name depending on Functional Groups in it

1. Aldehyde in Monosaccharide = Aldose

2. Ketone in Monosaccharide = Ketose

Number of Carbons in Monosaccharides

3 = triose

4 = tetrose

5 = pentose

6 = hexose

7 = heptose

Disaccharides

• When two monosaccharides are linked together

• Linked together by = Glycosidic bond (acetal linkage) “The glue between them”

• Ex: Glucose + Fructose = Sucrose (A disaccharide)

Glycosidic bond: Acetal Linkage

The bond that links 2 monosaccharides together and forms a disaccharides.

Cellobiose/Cellulose

• It’s 2 glucose molecules stuck together, A disaccharide

• β-1,4 = a specific bond

• Humans can’t break this bond, meaning can’t digest it

Polysaccharides

• Long chains of many sugar units linked together (monosaccharides)

• Ex: Glycogen = the storage carbohydrate in humans

• Has α-1,4 and α-1,6 bonds

• Highly branched = more end points = quick glucose release, so body can release energy very quickly when needed

α-1,4 and α-1,6 Glycosidic Bonds

• Bonds in Glycogen polysaccharides

• α-1,4 bonds = straight chains

• α-1,6 bonds = branch points

• So glycogen is not straight and highly branched like a tree

Polymerization

The process of building a big molecule from many small repeating units using active sugar nucleotides

• Your body doesn’t just randomly build glycogen It uses “activated sugars” (like charged building blocks)

• ATP indirectly used to activate the building process

Glycogen vs Cellulose

• Glycosidic Linkage

- Glycogen = a-1,4 and a-1,6

- Cellulose = b-1,4

• Digestible

- Glycogen = Yes!

- Cellulose = No! Humans lack enzymes to hydrolyze B-1,4 bonds

Isomerism in Carbohydrates

Molecules that share the identical chemical formula, such as C₆H₁₂O₆, but feature distinct atomic arrangements.

1. Enantiomers

2. Epimers

3. Anomers

4. Conformers

Isomerism in Carbohydrates: Enantiomers

Exact mirror images: D- and L- forms of the same carbohydrate

• Same formula but react differently in enzyme reactions (D vs L)

• Ex: Aldohexose

Isomerism in Carbohydrates: Epimers

Molecules that share exact same molecular formula but differ in spatial arrangement of atoms at only one chiral center (ex: glucose and galactose)

Isomerism in Carbohydrates: Anomers

Special type of carbohydrate isomer that emerge when a linear sugar molecule undergoes cyclization into a ring structure

• a- and B- forms created when glucose cyclizes

Anomers: Mutarotation

Means that the two anomers of a sugar (α and β forms) can switch back and forth in solution.

• Can go form a form to chain to b form when in water

Isomerism in Carbohydrates: Conformers

Molecules have the same structure and connectivity, but differ by rotation around single bonds.

• Glucose can exist in chair and boat forms

• Results from rotation about C-C single bond

• Bulky groups more stable in equatorial position (opposite)

• Bulky groups not stable in axial position (same way): cause steric strain

2^n Rule

The number of possible stereoisomers depends on the number of chiral center a molecule has, where n is the number of chiral centers.

Ex: Aldotetroses has 2 chiral centers so 4 stereoisomers

Aldohexoses: Type of Enantiomers

A monosaccharide (simple sugar) that contains an aldehyde group at carbon-1.

• Has 16 stereoisomers so 8 D pairs and 8 L pairs

Function of Carbohydrates: 1. Energy Storage + Release

• Glucose = Primary energy molecule

• Glycogen = Stored glucose for later use

Maltose - Energy storage

2 glucose with a-1,4 linkage (not found in cellulose).

Function of Carbohydrates: 2. Structural Roles

Carbohydrates serve as structural roles such as cellulose in plant walls.

• Cellobiose = the repeating unit of Cellulose, linked by b-1,4 bonds.

Function of Carbohydrates: 3. Building blocks for Biomolecules

Ribose and Deoxyribose form the backbone of Nucleic acids like ATP, DNA, and RNA

• Fun fact: Ribose has one more hydroxyl group than Deoxyribose

Function of Carbohydrates: 4. Reducing Sugars

Some carbohydrates can be oxidized to form carboxylic acids

Function of Carbohydrates: 5. Cell Recognition

Carbohydrates can be involved in cell recognition,

Carbohydrates Key Points Review

1. Carbohydrates provide energy for life (glucose metabolism, glycogen storage)

2. Serve as structural materials (plant cell walls)

3. Involved in cell communication and recognition

4. Small structural differences (epimers,anomers,enantiomers) can completely change biological activity