Chapter 5: An Introduction to Carbohydrates – Structure, Function, and Biological Roles

Study Guide - Smart Notes

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Practice Problems: Functional Groups and Macromolecule Structure

Functional Groups in Biological Molecules

Understanding the chemical groups that define biological macromolecules is essential for grasping their properties and functions.

Acid in Nucleic Acid: The 'acid' in nucleic acid comes from the phosphate group (–PO42−), which is acidic and imparts a negative charge to DNA and RNA.
Acid in Amino Acid: The 'acid' in amino acid comes from the carboxyl group (–COOH), which can donate a proton (H+), making the molecule acidic.

Identifying Macromolecules and Bonds

Dimer A: Structure shows two amino acids joined by a peptide bond (–CO–NH–), indicating a protein/peptide dimer.
Dimer B: Structure shows two monosaccharides joined by an oxygen bridge (glycosidic linkage), indicating a carbohydrate dimer (disaccharide).
Bond Types: Both peptide and glycosidic bonds are covalent bonds.
Bond Location: Peptide bond is between the carboxyl group of one amino acid and the amino group of another. Glycosidic bond is between the anomeric carbon of one sugar and a hydroxyl group of another.
Functional Groups:
- Peptide: Amino (–NH2), carboxyl (–COOH), side chains (R groups).
- Carbohydrate: Hydroxyl (–OH), carbonyl (C=O, as aldehyde or ketone).
Directionality:
- Peptide: N-terminus (amino end) to C-terminus (carboxyl end).
- Carbohydrate: Non-reducing end to reducing end (typically from C1 to C4 or C6).

Chapter 5: An Introduction to Carbohydrates

Overview of Carbohydrates

Carbohydrates are essential biomolecules that play critical roles in cell structure, cell identity, and energy storage. They are classified based on the number of sugar units they contain.

Monosaccharide: Single sugar unit (e.g., glucose, fructose).
Oligosaccharide: Short chains of monosaccharide units (few sugars).
Polysaccharide: Long chains of monosaccharide units (many sugars).

What are Carbohydrates?

The general molecular formula for a monosaccharide is . Carbohydrates typically have names ending in -ose (e.g., glucose, fructose, sucrose). Enzymes that act on carbohydrates often end in -ase (e.g., lactase, sucrase).

Simple carbohydrates: Monosaccharides and disaccharides.
Complex carbohydrates: Polysaccharides, which are polymers of monosaccharide monomers.

Monosaccharides

Structure and Function

Monosaccharides are the simplest carbohydrates and serve as the building blocks for more complex carbohydrates.

Examples: Glucose, fructose.
Functions:
- Provide chemical energy in cells (e.g., glucose is the main energy source in most organisms).
- Serve as building blocks for larger compounds.

Structural Variations in Monosaccharides

Monosaccharides vary in four primary ways:

Location of the carbonyl group:
- At the end: Aldose
- In the middle: Ketose
Spatial arrangement of –OH groups
Number of carbon atoms: Triose (3C), pentose (5C), hexose (6C)
Linear vs. ring structure: Sugars often form ring structures in aqueous solutions, which are more stable.

Isomers are molecules with the same molecular formula but different structures (e.g., glucose and galactose).

Disaccharides and Glycosidic Linkages

Formation and Examples

When two monosaccharides are joined by a condensation reaction, a disaccharide is formed. The bond formed is called a glycosidic linkage.

Examples:
- Lactose: Glucose + galactose
- Sucrose: Glucose + fructose
Glycosidic linkages can form between any two –OH groups on the monosaccharides.

Types of Glycosidic Linkages

α-1,4-glycosidic linkage: Common in starch and glycogen (energy storage polysaccharides).
β-1,4-glycosidic linkage: Common in cellulose (structural polysaccharide).
Enzyme specificity: Enzymes that hydrolyze α-linkages (e.g., amylase) cannot hydrolyze β-linkages (e.g., cellulase).

Polysaccharides

Structure and Function

Polysaccharides are long chains of monosaccharide units joined by glycosidic linkages. They serve various functions in living organisms.

Energy storage polysaccharides:
- Starch: Glucose polymer in plants (helical structure).
- Glycogen: Glucose polymer in animals (highly branched, stored in liver and muscle).
Structural polysaccharides:
- Cellulose: Polymer of β-glucose, provides structural support in plant cell walls, forms insoluble fiber.
- Chitin: Found in fungal cell walls and exoskeletons of arthropods.
- Peptidoglycan: Found in bacterial cell walls.

Biological Roles of Carbohydrates

Functions in Cells

Precursors to larger molecules: Carbohydrates are used to synthesize nucleotides, amino acids, and lipids (e.g., ribose in RNA, deoxyribose in DNA).
Structural materials: Cellulose and chitin form strong fibers or sheets, providing rigidity and protection.
Cell identity: Glycoproteins and glycolipids on cell surfaces are involved in cell recognition and signaling (e.g., blood types, immune response).
Energy storage: Carbohydrates store chemical energy, which is released during cellular respiration to produce ATP.

Carbohydrates and Energy

During photosynthesis, plants convert CO2 and H2O into carbohydrates using sunlight:

When energy is needed, glucose is broken down to produce ATP:

Carbohydrates have more C–H and C–C bonds (shared equally), which store more energy than C–O bonds.
Lipids have even more C–H bonds, making them higher in energy content per gram than carbohydrates.

Summary Table: Types of Carbohydrates

Type	Definition	Example	Function
Monosaccharide	Single sugar unit	Glucose, fructose	Energy source, building block
Disaccharide	Two monosaccharides joined by glycosidic bond	Sucrose, lactose	Transport, energy
Polysaccharide	Many monosaccharides joined in long chains	Starch, glycogen, cellulose	Energy storage, structure

Key Terms and Definitions

Monomer: A small molecule that can join with others to form a polymer.
Polymer: A large molecule made of repeating monomer units.
Glycosidic linkage: Covalent bond joining two monosaccharides.
Peptide bond: Covalent bond joining two amino acids.
Condensation reaction: Chemical reaction that joins two molecules with the loss of water.
Hydrolysis: Chemical reaction that breaks a bond by adding water.

Example: Directionality in Macromolecules

Proteins: Synthesized from N-terminus to C-terminus.
Carbohydrates: Synthesized from non-reducing end to reducing end.

Practice Problem Review

Functional groups: Phosphate (nucleic acid), carboxyl (amino acid).
Bond types: Peptide (protein), glycosidic (carbohydrate).
Directionality: N- to C-terminus (protein), non-reducing to reducing end (carbohydrate).