Carbohydrates: Structure, Isomerism, and Biological Relevance

Study Guide - Smart Notes

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Chapter 13: Carbohydrates

Introduction to Carbohydrates

Carbohydrates are a major source of energy for the body and play essential roles in biological systems. They are produced by photosynthesis in plants and are found in foods such as bread, rice, and potatoes.

Definition: Organic compounds consisting of carbon, hydrogen, and oxygen, typically with the formula Cn(H2O)n.
Functions: Provide energy, serve as structural components, and participate in cell recognition.
Examples: Glucose, fructose, sucrose, starch, cellulose.

Core Chemistry Skills for Carbohydrates

Identifying functional groups (alcohols, aldehydes, ketones)
Naming alcohols and ketones
Writing equations for dehydration and oxidation of alcohols

Types of Carbohydrates

Classification

Monosaccharides: The simplest carbohydrates, consisting of a single sugar unit (e.g., glucose, fructose).
Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond (e.g., sucrose, lactose).
Polysaccharides: Long chains of monosaccharide units (e.g., starch, cellulose, glycogen).

Monosaccharides are classified by the number of carbon atoms and the type of carbonyl group present:

Aldose: Contains an aldehyde group.
Ketose: Contains a ketone group (always on carbon 2 in simple sugars).
Triose, tetrose, pentose, hexose: Indicate 3, 4, 5, or 6 carbons, respectively.

Example: Glucose is an aldohexose; fructose is a ketohexose.

Structural Representations

Fischer Projections

A two-dimensional representation of three-dimensional molecules.
The most highly oxidized carbon is placed at the top.
Vertical lines represent bonds going away from the viewer; horizontal lines represent bonds coming toward the viewer.

Isomerism in Carbohydrates

Structural Isomers

Molecules with the same molecular formula but different bonding arrangements.
Example: C3H8O can be either 1-propanol or 2-propanol.

Stereoisomers

Same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientation of atoms.
Cannot be superimposed if they are chiral.

Chirality and Chiral Carbon Atoms

Chiral molecule: A molecule that is not superimposable on its mirror image.
Chiral carbon: A carbon atom bonded to four different groups.
Achiral molecule: A molecule that is superimposable on its mirror image.

Example: Hands are chiral objects; a fork is achiral.

Enantiomers and Diastereomers

Enantiomers: Stereoisomers that are non-superimposable mirror images.
Diastereomers: Stereoisomers that are not mirror images.
Racemic mixture: A 50:50 mixture of two enantiomers.
Epimers: Diastereomers that differ at only one chiral center.

D and L Notations

Assigned based on the position of the –OH group on the chiral carbon farthest from the carbonyl group in a Fischer projection.
D-isomer: –OH group on the right.
L-isomer: –OH group on the left.

Biological Relevance of Enantiomers

Many biological molecules are chiral, and only one enantiomer is biologically active.
Enzymes and receptors are stereospecific; the wrong enantiomer may be inactive or harmful.
Example: L-dopa is effective in treating Parkinson’s disease, while D-dopa is not.

Summary Table: Types of Carbohydrates

Type	Number of Units	Examples	Key Features
Monosaccharide	1	Glucose, Fructose	Simple sugars, basic building blocks
Disaccharide	2	Sucrose, Lactose	Two monosaccharides joined by glycosidic bond
Polysaccharide	Many	Starch, Cellulose, Glycogen	Long chains, energy storage or structural

Key Equations and Concepts

General formula for carbohydrates:

Photosynthesis equation:

Chirality: A carbon atom is chiral if it is attached to four different groups.

Study Tips

Practice drawing Fischer projections and identifying D/L isomers.
Be able to distinguish between structural isomers, enantiomers, and diastereomers.
Understand the biological importance of chirality in drug action and metabolism.