Carbohydrates and Lipids: Structure, Function, and Biological Importance

Macromolecules in Biology

Definition and Classes of Macromolecules

Macromolecules are large, complex molecules essential for life, formed by joining smaller molecules. They constitute the majority of the cellular structure and function.

• Macromolecule: A large molecule composed of thousands of atoms, typically formed by polymerization of smaller subunits.

• Monomer: A simple molecule that can join with other similar molecules to form a polymer. Example: Glucose is a monomer for starch.

• Polymer: A long chain molecule made by linking many monomers together. Example: Cellulose is a polymer of glucose.

• Four main classes of biological macromolecules:

  1. Carbohydrates

  2. Lipids

  3. Proteins

  4. Nucleic acids

Formation and Breakdown of Polymers

Dehydration Synthesis and Hydrolysis

Polymers are formed and broken down by specific chemical reactions involving water.

• Dehydration Synthesis: A reaction that joins two monomers by removing a water molecule, forming a covalent bond. This process is essential for building proteins, carbohydrates, and nucleic acids.

• Hydrolysis: A reaction that breaks the covalent bond between monomers by adding water, splitting polymers into monomers. This is crucial for digestion and cellular metabolism.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

Carbohydrates

Structure and Types

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a hydrogen:oxygen ratio of 2:1. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars with a single carbon backbone. Examples: Glucose (hexose, 6 carbons), Ribose (pentose, 5 carbons).

• Disaccharides: Two monosaccharides joined by a glycosidic bond. Example: Sucrose (glucose + fructose).

• Oligosaccharides: Short chains (3-10) of monosaccharides. Example: Raffinose (trisaccharide).

• Polysaccharides: Long chains (>10) of monosaccharides. Examples: Starch (energy storage in plants), Glycogen (energy storage in animals), Cellulose (structural support in plants).

Monosaccharide Structure and Isomerism

Monosaccharides vary in carbon number and arrangement of functional groups, leading to structural diversity.

• Isomers: Molecules with the same chemical formula but different structures. Example: Glucose and Fructose are both C6H12O6 but differ in structure.

• Ring and Linear Forms: Monosaccharides can exist in linear or ring forms, with the ring form being predominant in aqueous solutions.

Glycosidic Linkages

Monosaccharides are joined by glycosidic bonds formed via dehydration synthesis.

• Glycosidic bond: Covalent bond between two sugar molecules.

• Types of glycosidic bonds: Alpha and beta linkages, which affect the digestibility and structure of polysaccharides.

Functions of Carbohydrates

• Energy Storage: Starch (plants) and glycogen (animals) store glucose for energy.

• Structural Support: Cellulose provides rigidity to plant cell walls.

• Cell Recognition: Oligosaccharides on cell surfaces are involved in cell-cell recognition.

Lipids

General Properties and Types

Lipids are hydrophobic molecules, primarily composed of long hydrocarbon chains or rings. They are not true polymers but share similarities in formation via dehydration reactions.

• Types of lipids:

  • Triglycerides (fats and oils)

  • Phospholipids (cell membranes)

  • Steroids (cholesterol, hormones)

• Functions: Long-term energy storage, insulation, cushioning, and membrane structure.

Triglycerides

Triglycerides are formed by the reaction of glycerol and three fatty acids.

• Formation: Dehydration reaction between the hydroxyl group of glycerol and the carboxyl group of fatty acids.

• Equation:

$\text{Glycerol} + 3\ \text{Fatty Acids} \xrightarrow{\text{Dehydration}} \text{Triglyceride} + 3\ H_2O$

Saturated, Unsaturated, and Trans Fats

The structure of fatty acids determines their physical and chemical properties.

• Saturated Fatty Acids: No double bonds between carbon atoms; saturated with hydrogen. Pack closely, solid at room temperature.

• Unsaturated Fatty Acids: One or more double bonds; causes kinks, preventing tight packing. Liquid at room temperature.

• Trans Fats: Unsaturated fats with trans double bonds, produced by hydrogenation. Associated with increased risk of coronary heart disease.

Comparison of Fatty Acid Types

Type	Bond Structure	Physical State	Health Impact
Saturated	No double bonds	Solid at room temperature	Can raise LDL cholesterol
Unsaturated	One or more cis double bonds	Liquid at room temperature	Generally considered healthier
Trans	Trans double bonds (from hydrogenation)	Solid at room temperature	Increases risk of heart disease

Phospholipids

Phospholipids are major components of cell membranes, consisting of two fatty acids, a glycerol, and a phosphate group.

• Amphipathic: Contain both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Bilayer Formation: In water, phospholipids self-assemble into bilayers, forming the basic structure of cell membranes.

Steroids

Steroids are lipids with a structure of four fused rings and various functional groups.

• Cholesterol: Essential for membrane fluidity and precursor for steroid hormones.

• Steroid Hormones: Examples include testosterone and estrogen, which regulate various physiological processes.

Summary Table: Biological Macromolecules

Class	Monomer	Polymer	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid, steroid	Energy storage, membranes, hormones
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport
Nucleic acids	Nucleotide	DNA, RNA	Genetic information

Example: Starch is a polysaccharide made of glucose monomers, serving as energy storage in plants. Phospholipids form the bilayer of cell membranes, providing structural integrity and selective permeability.

Additional info: Expanded explanations and tables were added for clarity and completeness, including health impacts of fats and summary of macromolecule classes.