Biomolecules are the fundamental organic molecules of life, built primarily from elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. They serve as structural components, participate in biological reactions, and store genetic information, underpinning all biological processes from energy production to cellular communication.
Carbohydrates
Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen atoms. These molecules serve as the primary source of immediate energy for cells and also contribute to structural support. They are broadly categorized into monosaccharides, disaccharides, and polysaccharides based on their structural complexity.
Monosaccharides
Monosaccharides are the simplest carbohydrates. These single sugar units contain three to seven carbon atoms, including glucose, fructose, and galactose. Glucose, a six-carbon sugar, is the main fuel source for cellular respiration in most organisms. Fructose, found in fruits, and galactose, a component of milk sugar, are common monosaccharides that can be converted into glucose for energy.
Disaccharides
Disaccharides are formed when two monosaccharides are joined. Sucrose, commonly known as table sugar, is a disaccharide composed of one glucose and one fructose. Lactose, the sugar found in milk, consists of glucose and galactose, while maltose is made from two glucose units. These sugars provide readily available energy.
Polysaccharides
Polysaccharides are complex carbohydrates formed from many monosaccharide units linked together in long chains. Starch and glycogen are examples of polysaccharides that serve as energy storage molecules. Starch is the primary energy storage in plants, found abundantly in potatoes and grains, while glycogen is the main storage form of glucose in animals, stored primarily in the liver and muscles. Cellulose, another polysaccharide, provides structural support in plants, forming the rigid cell walls that give plants their strength and shape.
Lipids
Lipids are a diverse group of organic compounds characterized by their hydrophobic nature. This insolubility stems from their predominantly nonpolar hydrocarbon structures. Unlike carbohydrates and proteins, lipids do not share a common monomeric unit but are instead grouped by this shared physical property. They perform a variety of functions, including long-term energy storage, structural components of cell membranes, and signaling molecules.
Fats and Oils (Triglycerides)
Fats and oils (triglycerides) are composed of a glycerol molecule bonded to three fatty acid chains. These molecules are the most efficient form of long-term energy storage in living organisms, providing more than twice the energy per gram compared to carbohydrates. Fats, which are solid at room temperature, contain saturated fatty acids with no double bonds in their carbon chains. Oils, which are liquid at room temperature, contain unsaturated fatty acids with one or more double bonds, leading to kinks in their structure.
Phospholipids
Phospholipids are lipids that are the primary components of cell membranes. Each molecule consists of a glycerol backbone attached to two fatty acid chains and a phosphate group. The phosphate group is hydrophilic (water-loving), while the fatty acid tails are hydrophobic (water-fearing), creating a molecule with both water-attracting and water-repelling ends. This amphipathic nature allows phospholipids to spontaneously form a double-layered structure, or bilayer, in watery environments.
Steroids
Steroids are lipids characterized by a distinctive four-ring carbon structure. These molecules include cholesterol, various hormones, and vitamin D. Cholesterol is a component of animal cell membranes, contributing to their fluidity and stability. It also serves as a precursor for the synthesis of other steroids, such as steroid hormones like testosterone and estrogen. These hormones act as signaling molecules, regulating a wide range of physiological processes, including metabolism, inflammation, and reproduction, by binding to specific receptors within target cells.
Proteins
Proteins are large, complex biomolecules that perform a vast array of functions within living organisms. They are polymers constructed from smaller monomer units called amino acids, linked together by peptide bonds. The specific sequence of amino acids in a protein determines its unique three-dimensional shape, and this precise shape is directly responsible for the protein’s particular function.
Protein Structure
The complexity of protein function arises from their structural organization, described at four distinct levels.
Primary Structure
This refers to the linear sequence of amino acids in the polypeptide chain, dictated by genetic information. This sequence is fundamental, as any change can alter the protein’s subsequent folding and function.
Secondary Structure
This involves the localized folding of the polypeptide chain into specific, repeating patterns, primarily alpha-helices and beta-pleated sheets. These structures are stabilized by hydrogen bonds between the backbone atoms of the amino acids.
Tertiary Structure
This describes the overall three-dimensional shape of a single polypeptide chain, resulting from interactions between the side chains (R-groups) of the amino acids. These interactions include hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions, causing the polypeptide to fold into a compact, globular structure. This unique 3D conformation defines a protein’s functional state.
Quaternary Structure
This exists in proteins composed of two or more polypeptide chains, or subunits, that associate together to form a larger, functional complex. Hemoglobin, for example, consists of four separate polypeptide chains that work together to transport oxygen. The precise arrangement of these subunits is important for the protein’s overall activity.
Protein Functions
The diverse structures of proteins enable them to perform an extensive range of biological functions.
Enzymes
These specialized proteins act as biological catalysts, speeding up biochemical reactions without being consumed. For instance, amylase breaks down starch into simpler sugars during digestion.
Structural Proteins
These provide physical support and shape to cells and tissues. Collagen, a fibrous protein, is the most abundant protein in mammals, forming connective tissues like tendons, ligaments, and skin. Keratin is the main component of hair, nails, and the outer layer of skin.
Transport Proteins
These are responsible for moving substances throughout the body or across cell membranes. Hemoglobin in red blood cells transports oxygen from the lungs to the body’s tissues. Channel proteins embedded in cell membranes facilitate the passage of specific ions or molecules.
Antibodies and Receptors
Antibodies, produced by the immune system, recognize and neutralize foreign invaders like bacteria and viruses. Receptor proteins embedded in cell membranes receive signals from outside the cell, initiating specific cellular responses. Proteins are integral to virtually every cellular process.
Nucleic Acids
Nucleic acids are macromolecules that carry and transmit genetic information, serving as the blueprints for building and operating living organisms. These polymers are constructed from monomer units called nucleotides. Each nucleotide is composed of three distinct parts: a five-carbon sugar, a phosphate group, and a nitrogen-containing base. The specific arrangement of these nucleotides encodes the vast amount of information necessary for life.
Deoxyribonucleic Acid (DNA)
Deoxyribonucleic acid, or DNA, is the primary molecule responsible for storing and transmitting genetic instructions from one generation to the next. Its structure is a double helix, resembling a twisted ladder, where two strands of nucleotides are coiled around each other. The backbone of each strand is formed by alternating sugar and phosphate groups, while the nitrogenous bases extend inward, pairing specifically across the two strands. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C), forming the “rungs” of the ladder. This precise base pairing ensures accurate replication of genetic information.
Ribonucleic Acid (RNA)
Ribonucleic acid, or RNA, plays diverse roles in expressing the genetic information stored in DNA. Unlike DNA, RNA is a single-stranded molecule, and its sugar component is ribose instead of deoxyribose. In RNA, the base uracil (U) replaces thymine (T), so adenine (A) pairs with uracil (U) during RNA-DNA interactions. There are several types of RNA, each with specific functions. Messenger RNA (mRNA) carries genetic instructions from DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are synthesized. Ribosomal RNA (rRNA) is a structural component of ribosomes, and transfer RNA (tRNA) delivers specific amino acids to the ribosome during protein assembly.
Synthesis and Breakdown of Biomolecules
The formation and degradation of large polymeric biomolecules—carbohydrates, proteins, and nucleic acids—occur through universal chemical reactions. These processes, known as dehydration synthesis and hydrolysis, allow cells to efficiently build complex structures from smaller units and break them down when energy or building blocks are needed. This continuous cycle of synthesis and breakdown is fundamental to maintaining cellular life.
Dehydration Synthesis
Dehydration synthesis, also known as a condensation reaction, joins monomers to form polymers. During this reaction, a water molecule is removed as a covalent bond forms between two monomers. For example, when two amino acids link to form a dipeptide, a water molecule is formed, creating a peptide bond. This process requires energy and builds larger molecules like starch, proteins, and DNA from their monomeric units.
Hydrolysis
Conversely, hydrolysis breaks down polymers into their individual monomer units. This reaction is the reverse of dehydration synthesis, involving the addition of a water molecule across the covalent bond linking two monomers. The water molecule splits, with a hydrogen atom attaching to one monomer and a hydroxyl group to the other, breaking the bond. Digestion is an example of hydrolysis, where enzymes break down large food molecules into smaller units for absorption.