Chapter 13: Carbohydrates and Chiral Molecules – Structured Study Notes

Chapter 13: Carbohydrates

13.1 Carbohydrates

Carbohydrates are essential biomolecules that serve as a major source of energy in the human diet. They are composed of carbon, hydrogen, and oxygen, and are commonly referred to as saccharides or "sugars." Understanding their classification and structure is fundamental in biochemistry and health sciences.

• Major Source of Energy: Carbohydrates provide energy for cellular processes and physical activity.

• Composition: Made from the elements carbon (C), hydrogen (H), and oxygen (O).

• Photosynthesis: Plants synthesize carbohydrates from CO2, H2O, and sunlight energy.

• Oxidation in Cells: Carbohydrates are oxidized to produce CO2, H2O, and energy.

Photosynthesis Equation: $6CO_2 + 6H_2O + \text{energy} \rightarrow C_6H_{12}O_6 + 6O_2$ Respiration Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ Clinical Application: Monitoring blood glucose is crucial for diabetes management. Pre-meal glucose should be ≤110 mg/dL; an increase >50 mg/dL post-meal suggests excessive carbohydrate intake.

Types of Carbohydrates

Carbohydrates are classified based on the number of saccharide units:

• Monosaccharides: Simplest carbohydrates; single sugar units.

• Disaccharides: Composed of two monosaccharide units.

• Polysaccharides: Contain many monosaccharide units.

Hydrolysis Reactions

Disaccharides and polysaccharides can be hydrolyzed to yield monosaccharides:

• Disaccharide + H2O (acid/enzyme) → two monosaccharide molecules

• Polysaccharide + many H2O (acid/enzyme) → many monosaccharide molecules

Monosaccharides

Monosaccharides are the building blocks of carbohydrates, consisting of three to eight carbon atoms with one carbon in a carbonyl group. They are classified by the type of carbonyl group and the number of carbon atoms.

• Aldoses: Monosaccharides with an aldehyde group.

• Ketoses: Monosaccharides with a ketone group.

• Hydroxyl Groups: Present on all carbons except the carbonyl carbon.

Classification by Carbon Number

Type	Number of Carbon Atoms
Triose	3
Tetrose	4
Pentose	5
Hexose	6

• Aldopentose: Five-carbon saccharide with an aldehyde group.

• Ketohexose: Six-carbon saccharide with a ketone group.

Examples of Monosaccharides

Name	Type	Structure
Glyceraldehyde	Aldotriose	3C, aldehyde
Threose	Aldotetrose	4C, aldehyde
Ribose	Aldopentose	5C, aldehyde
Fructose	Ketohexose	6C, ketone

Fructose

• IUPAC Name: 1,3,4,5,6-Pentahydroxy-2-hexanone

• Structure: Contains a ketone group at C2 and hydroxyl groups at C1, C3, C4, C5, and C6.

Study Check: Identifying Monosaccharides

• Aldohexose: Six-carbon monosaccharide with an aldehyde group.

• Ketopentose: Five-carbon monosaccharide with a ketone group.

13.2 Chiral Molecules

Chirality and Chiral Molecules

Chirality is a property where an object or molecule cannot be superimposed on its mirror image. This concept is crucial in organic chemistry and biochemistry, especially for understanding the behavior of biomolecules.

• Chiral Objects: Hands, gloves, and shoes are examples of chiral objects.

• Achiral Objects: Baseball bats and glasses are examples of achiral objects.

Structural Isomers vs. Stereoisomers

Type	Description	Example
Structural Isomers	Same molecular formula, different bonding arrangement	Ethanol vs. Dimethyl ether
Stereoisomers	Same molecular formula and bonding sequence, different spatial arrangement	Enantiomers

Chiral Carbon Atoms

A carbon atom is chiral if it is bonded to four different groups. Molecules with at least one chiral carbon atom are chiral and have nonsuperimposable mirror images.

• Enantiomers: Stereoisomers that are nonsuperimposable mirror images of each other.

• Achiral Carbon: If the mirror image can be superimposed, the carbon is achiral.

Drawing Fischer Projections

Fischer projections are two-dimensional representations of three-dimensional molecules, commonly used for sugars and amino acids.

• Most Oxidized Carbon: Placed at the top.

• Vertical Lines: Represent bonds going back.

• Horizontal Lines: Represent bonds coming forward.

D and L Notations

D and L isomers are assigned based on the position of the —OH group on the chiral carbon farthest from the carbonyl carbon:

• L-Isomer: —OH group on the left.

• D-Isomer: —OH group on the right.

Study Check: Fischer Projections

• D-Isomer: —OH is on the right.

• L-Isomer: —OH is on the left.

Chemistry Link to Health: Enantiomers in Biological Systems

Biological Activity of Enantiomers

In biological systems, only one enantiomer of a compound is typically active. This is due to the chirality of enzymes and cell surface receptors, which interact specifically with one enantiomer.

• Chiral Receptors: Fit only the correct enantiomer; the mirror image does not fit properly.

• Drug Example: L-dopa is effective in treating Parkinson's disease, while D-dopa is not.

Drug Development

• Historically, drugs were produced as mixtures of enantiomers.

• Modern research focuses on producing only the biologically active enantiomer using chiral technology.

Summary Table: Enantiomers in Health

Compound	Active Enantiomer	Inactive Enantiomer	Application
Dopa	L-dopa	D-dopa	Treatment of Parkinson's disease

Additional info: Chirality is a foundational concept in organic chemistry, impacting drug design, metabolism, and molecular recognition in biological systems.