Macromolecules: Carbohydrates & Lipids – Structure, Function, and Biological Roles

Macromolecules: An Overview

Definition and Classification

Macromolecules are large, complex molecules essential for life, built from smaller units called monomers. The four main classes are carbohydrates, proteins, nucleic acids, and lipids. Most are polymers, except lipids, which are structurally unique and often not considered true polymers.

• Carbohydrates: Monomer = monosaccharide

• Proteins: Monomer = amino acid

• Nucleic acids: Monomer = nucleotide

• Lipids: Not true polymers; main types include triglycerides, phospholipids, and steroids

Carbohydrates

Monomers: Monosaccharides

Monosaccharides are simple sugars that cannot be broken down further. They are categorized by the number of carbon atoms and typically have the formula CxH2xOx. Common examples include glucose, fructose, galactose, and ribose.

• Triose: 3 carbons (e.g., glyceraldehyde)

• Pentose: 5 carbons (e.g., ribose)

• Hexose: 6 carbons (e.g., glucose, galactose)

Functional Groups in Monosaccharides

Monosaccharides contain several functional groups:

• Hydroxyl (-OH): Found at almost every carbon

• Carbonyl (C=O): Either as an aldehyde (at the end) or ketone (within the chain)

Isomerism in Monosaccharides

Monosaccharides can exist as structural isomers (same formula, different arrangement) or stereoisomers (different spatial arrangement). For example, glucose and galactose are both hexoses but differ in the orientation of one hydroxyl group.

Ring Formation

Most pentoses and hexoses form ring structures in aqueous solutions, which is the predominant form in biological systems.

Disaccharides and Glycosidic Bonds

Disaccharides are formed by linking two monosaccharides via a glycosidic bond through a dehydration reaction (removal of water). Common disaccharides include maltose (glucose + glucose), lactose (galactose + glucose), and sucrose (glucose + fructose).

• Dehydration Reaction: $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Dimer} + H_2O$

Oligosaccharides and Polysaccharides

Oligosaccharides (3–10 sugars) and polysaccharides (10+ sugars) are formed by repeated glycosidic linkages. Polysaccharides include starch, glycogen, and cellulose.

• Starch: Storage polysaccharide in plants; composed of amylose (unbranched) and amylopectin (branched)

• Glycogen: Storage polysaccharide in animals; highly branched

• Cellulose: Structural polysaccharide in plants; unbranched, linear fibers

Biological Roles of Carbohydrates

Fuel Source

Monosaccharides, especially glucose, are the primary energy source for cells. Glucose is transported in the blood and taken up by cells via specific transporters.

Energy Storage

Excess monosaccharides are stored as polysaccharides. Plants store energy as starch (amylose and amylopectin), while animals store it as glycogen in muscles and liver.

Structural Support

Carbohydrates provide structural support in organisms. Cellulose forms plant cell walls, while chitin is found in fungal cell walls and arthropod exoskeletons.

Comparison of Starch and Cellulose

Starch and cellulose are both polymers of glucose but differ in their glycosidic linkages:

• Starch: α-1,4 linkages (digestible by humans)

• Cellulose: β-1,4 linkages (not digestible by humans)

Lipids

General Properties

Lipids are primarily hydrocarbons, making them nonpolar and hydrophobic. They are structurally diverse and not true polymers, but are considered macromolecules due to their size and biological importance.

• Three main types: Triglycerides, phospholipids, steroids

• Interactions: Van der Waals and hydrophobic interactions

Triglycerides

Triglycerides consist of three fatty acid chains attached to a glycerol head via ester bonds formed by dehydration reactions. Fatty acids can be saturated (all single bonds), monounsaturated (one double bond), or polyunsaturated (multiple double bonds).

• Saturated fatty acids: Tightly packed, solid at room temperature

• Unsaturated fatty acids: Kinked, less tightly packed, liquid at room temperature

• Trans fats: Chemically altered unsaturated fats, unhealthy

Biological Roles of Triglycerides

Triglycerides serve as energy storage, insulation, and cushioning in organisms. They are stored in the liver or subcutaneously.

Phospholipids

Phospholipids are major components of cell membranes. They are amphipathic, with a hydrophilic head (glycerol + phosphate group) and hydrophobic tails (fatty acids). This structure allows them to form bilayers in aqueous environments.

• Structure: Glycerol backbone, two fatty acid tails, one phosphate group

• Function: Structural component of plasma membranes

Steroids

Steroids are lipids composed of four fused hydrocarbon rings. They are hydrophobic and not built from fatty acids or glycerol. Examples include cholesterol (membrane fluidity), sex hormones (progesterone, estrogen, testosterone), and lipid-soluble vitamins (A, D, E, K).

• Structure: Four carbon rings

• Function: Membrane fluidity, hormones, vitamins

Key Vocabulary

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions

• Amylose: Unbranched starch

• Amylopectin: Branched starch

• Bilayer: Double layer of phospholipids in membranes

• Cellulose: Structural polysaccharide in plants

• Cholesterol: Steroid important for membrane fluidity

• Dehydration: Reaction removing water to form bonds

• Disaccharide: Two monosaccharides linked

• Ester linkage: Bond between fatty acid and glycerol

• Fatty acid: Hydrocarbon chain with carboxyl group

• Glycogen: Storage polysaccharide in animals

• Glycosidic linkage: Bond between monosaccharides

• Hydrophilic: Water-loving

• Hydrophobic: Water-fearing

• Monomer: Single unit of a polymer

• Monounsaturated fatty acid: One double bond

• Oligosaccharide: Few sugars

• Phospholipid: Lipid with phosphate group

• Polymer: Many monomers linked

• Polysaccharide: Many sugars

• Polyunsaturated fatty acid: Multiple double bonds

• Saturated fatty acid: All single bonds

• Starch: Storage polysaccharide in plants

• Steroid: Lipid with four rings

• Trans fatty acid: Chemically altered unsaturated fat

• Triglyceride: Three fatty acids + glycerol

• Unsaturated fatty acid: At least one double bond

Summary Table: Carbohydrate Polysaccharides

Polysaccharide	Monomer	Structure	Function
Cellulose	Glucose (β)	Unbranched, linear	Plant cell wall structure
Amylose	Glucose (α)	Unbranched, coiled	Plant energy storage
Amylopectin	Glucose (α)	Branched	Plant energy storage
Glycogen	Glucose (α)	Highly branched	Animal energy storage

Quiz Yourself

• What is the monomer of a carbohydrate?

• How do you identify a monosaccharide?

• What type of bond holds the monomers of carbohydrates together?

• What are the functions of carbohydrates?

• What is the monomer of a lipid?

• How do you identify a fatty acid?

• What is meant by saturated/unsaturated/trans fatty acid?

• What type of bond holds fatty acids to the glycerol head?

• What are the three types of lipids?

• What is the function of triglycerides?

• What is the structure of triglycerides?

• What is the function of phospholipids?

• What is the structure of phospholipids?

• What is the structure of steroids?

• What is the function of steroids?

Additional info:

• Dehydration reactions are catalyzed by enzymes, speeding up polymer formation.

• Hydrolysis reactions break polymers into monomers by adding water.

• Cellulose is the most abundant macromolecule on Earth.

• Chitin contains amino groups and is used for structural support in fungi and arthropods.