Living organisms are complex systems, and at their core are large, intricate molecules known as macromolecules. These compounds are the fundamental building blocks that enable life to exist and function. They perform many roles, from storing genetic information and providing energy to forming cellular structures and carrying out chemical reactions. Understanding these molecules is foundational to comprehending how all living things operate.
General Characteristics of Macromolecules
Many large biological molecules are formed by linking smaller, repeating units. These building blocks are called monomers, and when many monomers join, they create a long chain known as a polymer. This assembly process is consistent across different types of macromolecules.
The formation of these polymers occurs through a chemical reaction called dehydration synthesis, also known as a condensation reaction. During this process, a covalent bond forms between two monomers, and a molecule of water is removed. This allows monomers to connect, extending the polymer chain.
Conversely, polymers can be broken down into their monomer units through a process called hydrolysis. Hydrolysis involves the addition of a water molecule, which breaks the covalent bond linking monomers. This reaction is how organisms digest complex molecules from food, breaking them down into smaller units for absorption and utilization.
Carbohydrates
Carbohydrates are a diverse group of macromolecules composed of carbon, hydrogen, and oxygen atoms. Their basic units are simple sugars, or monosaccharides, such as glucose, which is a six-carbon sugar, and fructose, a five-carbon sugar. Two monosaccharides can link to form a disaccharide, like sucrose (table sugar) or lactose (milk sugar).
Long chains of monosaccharides form polysaccharides. Starch, a common polysaccharide in plants, stores energy and is found in foods like potatoes and grains. Glycogen, a similar branched polysaccharide, is the primary energy storage molecule in animals, stored mainly in the liver and muscle cells.
Cellulose, another plant polysaccharide, provides structural support to plant cell walls. While humans cannot digest cellulose, it functions as dietary fiber. Carbohydrates serve as the primary energy source for cellular activities, fueling muscle contraction and brain function.
Lipids
Lipids are a varied group of biological molecules that are largely hydrophobic, meaning they do not mix well with water. This group includes fats, oils, phospholipids, and steroids. Triglycerides, a common type of fat, are formed from a glycerol molecule attached to three fatty acid chains.
These molecules are efficient for long-term energy storage, holding more than twice the energy per gram compared to carbohydrates. Adipose tissue not only stores energy but also provides insulation against cold temperatures and protective cushioning for internal organs.
Phospholipids are a type of lipid that forms the fundamental structure of cell membranes. Their design, with a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails, allows them to form a double layer that separates the cell’s interior from its external environment. Steroids, such as cholesterol and various hormones like testosterone and estrogen, are also lipids characterized by a four-ring carbon structure.
Proteins
Proteins are versatile macromolecules, performing many functions within living organisms. Their fundamental building blocks are amino acids, of which there are 20 common types. These amino acids link together via peptide bonds, forming long chains called polypeptides.
The sequence of amino acids in a polypeptide chain is its primary structure. This linear sequence dictates its folding into complex three-dimensional shapes. The secondary structure includes local folding patterns, such as alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds between backbone atoms.
Further folding of these secondary structures into a unique three-dimensional shape forms the tertiary structure, often stabilized by interactions between amino acid side chains. Some proteins, composed of multiple polypeptide chains, also possess a quaternary structure, the arrangement of individual subunits. The precise three-dimensional shape of a protein determines its specific function.
Proteins serve as enzymes, biological catalysts that speed up nearly all biochemical reactions in cells, such as digestive enzymes breaking down food molecules. They provide structural support, with examples like collagen forming connective tissues and keratin making up hair and nails. Proteins also transport substances, as hemoglobin carries oxygen in the blood, and they function in defense, with antibodies recognizing and neutralizing foreign invaders.
Nucleic Acids
Nucleic acids are macromolecules that store and transmit genetic information, playing a central role in heredity and protein synthesis. The monomers of nucleic acids are nucleotides, each composed of three parts: a five-carbon sugar, a phosphate group, and a nitrogen-containing base. These nucleotides link together to form long polymer chains.
The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA forms a double helix structure, resembling a twisted ladder, where two polynucleotide strands are wound around each other. DNA serves as the genetic blueprint, containing the instructions for building and maintaining an organism, passed down from one generation to the next.
RNA, a single-stranded molecule, expresses the genetic information stored in DNA. Messenger RNA (mRNA) carries genetic codes from DNA to ribosomes, transfer RNA (tRNA) brings amino acids to the ribosomes, and ribosomal RNA (rRNA) is a component of ribosomes, where proteins are assembled. Adenosine triphosphate (ATP) is a nucleotide that functions as the primary energy currency of the cell, powering various cellular processes.