Carbohydrates: Life’s Sweet Molecules – Study Guide

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Carbohydrates: Life’s Sweet Molecules

6.1 Basics of Carbohydrates

Carbohydrates are essential biomolecules commonly referred to as sugars. They serve as a primary energy source and play structural and functional roles in living organisms.

Energy Source: Carbohydrates provide energy for cellular processes.
Complex Carbohydrates: Includes starches and insoluble fibers found in plants.
Structural Roles: Simple sugars are components of genetic material such as DNA and RNA.
Cell Surface Markers: Certain carbohydrates act as markers for cell recognition.
Health Implications: Carbohydrates are involved in diseases like diabetes and lactose intolerance.
Example: Glucose is a key energy source; lactose intolerance results from inability to digest lactose, a disaccharide.

6.1 Classes of Carbohydrates

Carbohydrates are classified based on the number of sugar units they contain.

Monosaccharides: The simplest carbohydrates; example: glucose (C6H12O6).
General Formula: where .
Disaccharides: Two monosaccharides joined; example: sucrose (C12H22O11), hydrolyzes to glucose and fructose.
Oligosaccharides: Three to nine monosaccharide units; ABO blood groups are oligosaccharides.
Polysaccharides: Ten or more monosaccharides; example: starch.

6.2 Alcohols in Monosaccharides

Monosaccharides contain alcohol functional groups, which are classified by the number of alkyl groups attached to the carbon bonded to the hydroxyl group.

Primary (1°) Alcohol: One alkyl group attached.
Secondary (2°) Alcohol: Two alkyl groups attached.
Tertiary (3°) Alcohol: Three alkyl groups attached.
Monosaccharides: Contain both primary and secondary alcohols.

6.2 Aldehydes and Ketones in Monosaccharides

Monosaccharides may contain aldehyde or ketone functional groups, which influence their classification and reactivity.

Aldehyde: Carbonyl group with a hydrogen atom and an alkyl/aromatic group.
Ketone: Carbonyl group with two alkyl/aromatic groups.
Biological Relevance: Both groups are found in many important biomolecules.

6.2 Aldoses and Ketoses

Monosaccharides are further classified based on their functional groups and carbon number.

Aldose: Contains an aldehyde group.
Ketose: Contains a ketone group.
Classification by Carbon Number:
- Triose: 3 carbons
- Tetrose: 4 carbons
- Pentose: 5 carbons
- Hexose: 6 carbons

6.3 Chiral Centers in Monosaccharides

Chirality is a key feature in monosaccharides, affecting their stereochemistry and biological activity.

Chiral Center: A carbon atom with four different groups attached.
Enantiomers: Compounds with one chiral center exist as two mirror-image forms.

6.3 Stereoisomers of Monosaccharides

The number of stereoisomers increases with the number of chiral centers in a molecule.

Formula: where is the number of chiral centers.
Example: Glucose has 4 chiral centers, so stereoisomers; only one is used as the main energy source in humans.

6.3 Fischer Projections

Fischer projections are a method for representing the 3D structure of sugars in 2D.

Horizontal Lines: Represent bonds coming out of the plane (wedges).
Vertical Lines: Represent bonds going behind the plane (dashes).
D and L Designation: Based on the position of the –OH group on the chiral carbon farthest from the carbonyl group.
D-sugars: –OH on the right; most natural carbohydrates are D-sugars.
L-sugars: –OH on the left; L is the enantiomer of D.

6.3 Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other.

Definition: Stereoisomers that are not enantiomers.
Example: Glucose and galactose are diastereomers.

6.3 Important Monosaccharides

Several monosaccharides are biologically significant.

Glucose: Most abundant; found in fruits, vegetables, corn syrup; main energy source; broken down via glycolysis.
Galactose: Component of lactose; present in milk; epimer of glucose.
Mannose: Found in cranberries; difficult to absorb.
Fructose: Found in fruits, vegetables, honey; forms sucrose with glucose; sweetest monosaccharide.

6.3 Ribose and Deoxyribose

Pentose sugars are components of nucleic acids.

Ribose: Found in RNA, vitamin riboflavin, and other molecules.
2-Deoxyribose: Found in DNA; differs from ribose by lacking an oxygen atom at carbon 2.
Example: Ribose and deoxyribose are essential for genetic material structure.

6.3 Diabetes

Diabetes is a metabolic disorder involving carbohydrate metabolism.

Prevalence: Affects nearly 10% of the U.S. population.
Type 1: Requires supplemental insulin.
Type 2: Accounts for 95% of cases; cells do not respond properly to insulin.

6.4 Hemiacetal Formation and Ring Structure

Monosaccharides often form ring structures through hemiacetal formation.

Hemiacetal Formation: The carbonyl carbon reacts with a hydroxyl group, forming a ring.
Anomeric Carbon: The carbon involved in ring formation; gives rise to α and β anomers.
D-glucose: Exists mainly in ring form; α and β anomers differ in the position of the –OH group on the anomeric carbon.

6.4 Redox Reactions and Reducing Sugars

Monosaccharides can undergo oxidation and reduction reactions.

Oxidation: Produces sugar acids.
Reduction: Produces sugar alcohols.
Reducing Sugars: Sugars that can be oxidized; all monosaccharides are reducing sugars.

6.4 Benedict’s Test

Benedict’s test is used to detect reducing sugars.

Principle: Aldehyde groups are oxidized, reducing Cu2+ to Cu+, forming a brick-red precipitate.
Application: Used to test for glucose in urine.
Ketoses: Can rearrange to aldoses and also act as reducing sugars.

6.5 Glycosidic Bonds

Disaccharides and polysaccharides are formed by glycosidic bonds between monosaccharides.

Formation: Condensation reaction at the anomeric carbon.
Specification: Bonds are named by the carbons involved and the α or β configuration.
Example: Maltose has an α(1 → 4) glycosidic bond.

6.5 Disaccharides

Disaccharide	Monosaccharide Units	Glycosidic Bond	Reducing Sugar?
Maltose	Glucose + Glucose	α(1 → 4)	Yes
Lactose	Glucose + Galactose	β(1 → 4)	Yes
Sucrose	Glucose + Fructose	α,β(1 → 2)	No

Maltose: Formed from starch breakdown; reducing sugar.
Lactose: Found in milk; reducing sugar.
Sucrose: Table sugar; not a reducing sugar.

6.6 Storage Polysaccharides: Starch

Starch is the main storage polysaccharide in plants, composed of amylose and amylopectin.

Amylose: Linear chain of D-glucose units, α(1 → 4) bonds.
Amylopectin: Branched chain, α(1 → 4) bonds with α(1 → 6) branches every ~25 units.
Proportion: Amylose (20%), Amylopectin (80%).

6.6 Storage Polysaccharides: Glycogen

Glycogen is the storage polysaccharide in animals, structurally similar to amylopectin but more highly branched.

Location: Liver and muscles.
Structure: α(1 → 4) bonds with α(1 → 6) branches every ~12 units.
Function: Hydrolyzed to glucose to maintain blood sugar levels.
Branching: Allows rapid mobilization of glucose.

6.6 Structural Polysaccharides: Cellulose

Cellulose is a structural polysaccharide in plants, composed of β(1 → 4) bonded glucose units.

Structure: Straight chains align to form rigid fibers.
Digestibility: Humans cannot digest cellulose but require it for digestive health.

6.6 Structural Polysaccharides: Chitin

Chitin is a structural polysaccharide found in the exoskeletons of insects and crustaceans, and in fungal cell walls.

Composition: Modified β-D-glucose (N-acetylglucosamine) with β(1 → 4) bonds.
Properties: Structurally strong and water repellent.

6.7 Carbohydrates and Blood: ABO Blood Groups

Carbohydrates are key components of blood group markers on red blood cells.

ABO Markers: Oligosaccharides with three or four monosaccharide units.
Immune Recognition: The immune system recognizes its own carbohydrate set and attacks foreign types.
Blood Donation: O-type blood is universally accepted because its trisaccharide is present in all blood types.

6.7 Heparin

Heparin is a polysaccharide that prevents blood clotting.

Structure: Highly ionic polysaccharide with repeating disaccharide units (glycosaminoglycans).
Function: Anticoagulant; sulfate groups contribute to high charge density.