Organic Building Block Molecules: Carbohydrates and Lipids

Organic Building Block Molecules

Introduction

Organic molecules are the foundation of biological systems. The four major classes of organic building block molecules in living organisms are carbohydrates, nucleic acids, proteins, and lipids. These molecules are primarily composed of carbon, hydrogen, oxygen, and sometimes nitrogen.

• Carbohydrates

• Nucleic acids

• Proteins

• Lipids

Carbohydrates, nucleic acids, and proteins are classified as polymers, which are long chains of repeating molecular units called monomers.

Polymers and Monomers

Polymers are large molecules made by joining many smaller units called monomers. The process of joining monomers to form polymers is called polymerization.

• Polymer: Long chain of repeating molecules (poly = many, mer = part)

• Monomer: Single repeating molecule (mono = one, mer = part)

Examples of monomers:

• Monosaccharide (carbohydrate monomer)

• Nucleotide (nucleic acid monomer)

• Amino acid (protein monomer)

Polymer Assembly and Disassembly

Polymers are assembled and disassembled through chemical reactions:

• Dehydration Reaction: Monomers are joined together by removing a molecule of water (H2O).

• Hydrolysis: Polymers are broken down into monomers by adding water; hydro = water, lysis = to break.

For example, building a polymer is like connecting train cars (monomers) by removing water (the blockage), while breaking a polymer is like adding water to separate the train cars.

Carbohydrates

Introduction

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

• General formula: $C_n(H_2O)_n$

• Ratio in carbohydrates: 1C : 2H : 1O (or close to it)

Functions of Carbohydrates

• Structural

  • Cell wall components:

    • Fungal cell wall: chitin

    • Plant cell wall: cellulose

  • Tunicates: Only animals that can make cellulose

  • Arthropod exoskeletons: Made of chitin

• Energy Storage

  • Short-term storage:

    • Glucose in animals

    • Sucrose in plants

  • Long-term storage:

    • Glycogen in animals (liver and muscles)

    • Starch in plants (e.g., potato tubers)

Types of Carbohydrates

• Monosaccharides (single sugar units)

  • Triose (3 carbons): $C_3H_6O_3$

  • Pentose (5 carbons): $C_5H_{10}O_5$

  • Hexose (6 carbons): $C_6H_{12}O_6$

  • Monosaccharides are the monomers of carbohydrates.

• Disaccharides (two sugar units)

  • Formed by dehydration reaction between two monosaccharides (covalent bond)

  • Examples:

    • Glucose + fructose = sucrose (table sugar), $C_{12}H_{22}O_{11}$

    • Glucose + glucose = maltose (beer sugar)

    • Glucose + galactose = lactose (milk sugar)

• Polysaccharides (many sugar units)

  • Examples:

    • Starch: polymer of glucose (energy storage in plants)

    • Cellulose: polymer of glucose (structural component in plants)

  • Linkages: alpha and beta glycosidic bonds

• Modified Complex Carbohydrates

  • Chitin: polymer of N-acetylglucosamine, held together by β (beta) linkages

  • Formed by addition of other elements to carbohydrate polymers

Structural Isomers

Monosaccharides with the same formula can have different structures (isomers). In water, pentose and hexose sugars often form ring structures.

Lipids (Building Block Molecules)

Introduction

Lipids are organic molecules composed mainly of carbon, hydrogen, and a small amount of oxygen. They are hydrophobic due to their nonpolar covalent bonds and are not polymers.

• General formula example: $C_{16}H_{32}O_2$

• Ratio: 1C : 2H, very little O

• Hydrocarbons are hydrophobic (do not interact with water)

Functions of Lipids in Living Things

• Energy Storage

  • Lipids store about 2.5 times more energy per gram than carbohydrates.

  • Important in animals for mobility (less weight to carry).

  • Plants store energy as carbohydrates since they do not move.

• Structural

  • Major component of cell membranes (phospholipids).

  • All living things have a cell membrane.

• Waterproofing

  • Lipids prevent water from penetrating surfaces (e.g., bird feathers).

• Regulatory

  • Lipid hormones regulate biological processes (e.g., sex hormones, adrenocortical hormones).

• Protection

  • Lipids protect delicate organs (e.g., fat around kidneys and eyes).

• Insulation

  • Lipids insulate animals against cold (e.g., polar bear, seal, penguin).

• Defense

  • Lipids can act as toxins in some organisms (e.g., toads, fireflies, milkweed plants).

  • Monarch butterflies have evolved to neutralize plant toxins.

Comparison Table: Carbohydrates vs. Lipids

Property	Carbohydrates	Lipids
Monomer	Monosaccharide	Fatty acid, glycerol (not true polymers)
General Formula	$C_n(H_2O)_n$	$C_{16}H_{32}O_2$ (example)
Energy Storage	Short and long term (glycogen, starch)	Long term, more energy per gram
Structural Role	Cell walls (cellulose, chitin)	Cell membranes (phospholipids)
Water Interaction	Hydrophilic	Hydrophobic

Additional info: Academic context was added to clarify the structure and function of carbohydrates and lipids, and to provide definitions and examples for key terms.