Carbohydrates and Lipids: Structure, Function, and Biological Importance

Macromolecules in Biology

Definition and Importance

Macromolecules are large, complex molecules essential for life, formed by the joining of smaller subunits. They are the building blocks of cells and perform a wide range of biological functions.

• 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 of starch.

• Polymer: A long molecule consisting of many similar or identical monomers linked together. Example: cellulose is a polymer of glucose.

Four main classes of biological macromolecules:

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Formation and Breakdown of Polymers

Dehydration Synthesis and Hydrolysis

Polymers are formed and broken down by specific chemical reactions:

• Dehydration Synthesis (Condensation Reaction): A reaction in which two monomers are joined by removing a water molecule, forming a covalent bond.

• Hydrolysis: A reaction in which a polymer is broken down into monomers by the addition of water, breaking covalent bonds.

Example Equation (Dehydration Synthesis):

$ext{Monomer}_1 + ext{Monomer}_2 \rightarrow ext{Polymer} + H_2O$

Example Equation (Hydrolysis):

$ext{Polymer} + H_2O \rightarrow ext{Monomer}_1 + ext{Monomer}_2$

Application: Digestion involves hydrolysis reactions to break down food polymers into absorbable monomers.

Carbohydrates

Definition and Functions

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

• Functions:

  • Energy storage (e.g., starch in plants, glycogen in animals)

  • Structural support (e.g., cellulose in plant cell walls)

  • Cell recognition and signaling (e.g., oligosaccharides on cell surfaces)

Types of Carbohydrates

• Monosaccharides: Simple sugars with a single carbon backbone (e.g., glucose, fructose, galactose). They vary in carbon number (trioses, pentoses, hexoses) and arrangement of functional groups.

• Disaccharides: Composed of two monosaccharides joined by a glycosidic bond (e.g., sucrose = glucose + fructose).

• Oligosaccharides: Short chains of 3–10 monosaccharides. Often involved in cell recognition.

• Polysaccharides: Long chains (>10) of monosaccharides. Examples include starch, glycogen, and cellulose.

Monosaccharide Structure and Isomerism

• Monosaccharides can exist in linear or ring forms.

• Isomerism arises from different arrangements of -H and -OH groups and the position of the carbonyl group (aldose vs. ketose).

• Common monosaccharides: glucose (hexose), ribose (pentose).

Glycosidic Bonds

Monosaccharides are joined by glycosidic bonds, formed via dehydration synthesis.

• Glycosidic bond: Covalent bond linking two monosaccharides.

• Example: Formation of maltose from two glucose molecules.

Polysaccharides: Structure and Function

• Starch: Energy storage in plants; composed of amylose (unbranched) and amylopectin (branched).

• Glycogen: Energy storage in animals; highly branched structure, stored in liver and muscle.

• Cellulose: Structural polysaccharide in plant cell walls; composed of β-glucose monomers, forms straight, rigid fibers.

Table: Comparison of Major Polysaccharides

Name	Monomer	Function	Branching
Starch	α-glucose	Energy storage (plants)	Some (amylopectin)
Glycogen	α-glucose	Energy storage (animals)	Highly branched
Cellulose	β-glucose	Structural (plants)	Unbranched

Lipids

Definition and General Properties

Lipids are a diverse group of hydrophobic molecules, primarily composed of carbon and hydrogen atoms. They are not true polymers but are assembled from smaller molecules via dehydration reactions.

• Functions:

  • Long-term energy storage

  • Insulation and cushioning

  • Major component of cell membranes

  • Precursors for hormones and signaling molecules

Types of Lipids

• Triglycerides (Fats and Oils): Composed of glycerol and three fatty acids. Formed by dehydration reaction between hydroxyl group of glycerol and carboxyl group of fatty acid.

• Phospholipids: Contain a glycerol backbone, two fatty acids, and a phosphate group. Amphipathic (hydrophilic head, hydrophobic tails); form the lipid bilayer of cell membranes.

• Steroids: Characterized by four fused carbon rings. Examples: cholesterol (membrane component), steroid hormones (e.g., testosterone).

Saturated, Unsaturated, and Trans Fats

• Saturated fatty acids: No double bonds between carbon atoms; saturated with hydrogen. Straight chains allow tight packing, solid at room temperature (e.g., butter).

• Unsaturated fatty acids: One or more double bonds; causes kinks in the chain, preventing tight packing. Usually liquid at room temperature (e.g., olive oil).

• Trans fats: Unsaturated fats with trans double bonds, produced by partial hydrogenation. Trans configuration allows tighter packing, associated with increased risk of coronary heart disease.

Table: Comparison of Fatty Acid Types

Type	Bond Structure	Physical State	Health Impact
Saturated	No double bonds	Solid at room temp	Excess linked to heart disease
Unsaturated (cis)	One or more cis double bonds	Liquid at room temp	Generally healthier
Trans	One or more trans double bonds	Semi-solid	Increases heart disease risk

Phospholipids and Membranes

• Phospholipids are amphipathic, with hydrophilic (phosphate) heads and hydrophobic (fatty acid) tails.

• In water, they spontaneously form bilayers, the fundamental structure of cell membranes.

• This arrangement creates a selective barrier, with hydrophobic tails shielded from water and hydrophilic heads exposed.

Steroids

• Steroids have a core structure of four fused carbon rings with various functional groups attached.

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

• Steroid hormones: Include testosterone and estrogen, which regulate various physiological processes.

Key Terms and Concepts

• Macromolecule

• Monomer

• Polymer

• Dehydration synthesis

• Hydrolysis

• Carbohydrate

• Monosaccharide

• Disaccharide

• Oligosaccharide

• Polysaccharide

• Glycosidic bond

• Lipid

• Fatty acid

• Triglyceride

• Phospholipid

• Steroid

• Saturated/Unsaturated/Trans fat

Additional info: This guide expands on the provided notes with definitions, examples, and tables for clarity and completeness, as would be expected in a mini-textbook summary for General Biology students.