Biomolecules are organic compounds fundamental to life’s processes. These molecules are built around a framework of carbon atoms, often incorporating hydrogen, oxygen, nitrogen, phosphorus, and sulfur. They serve as the structural material and the functional machinery within cells. The cell’s activities and structure rely entirely on the precise interactions of four primary classes of these molecules.
Carbohydrates: Structure and Energy Storage
Carbohydrates are molecules composed of carbon, hydrogen, and oxygen atoms, often in a 1:2:1 ratio. The simplest form is a monosaccharide, or simple sugar, such as glucose, which serves as the primary energy source for most cells.
These simple sugars link together to form larger structures known as polysaccharides, or complex carbohydrates. Starch and glycogen are examples of storage polysaccharides. Starch stores glucose in plants, while glycogen performs the same function in animals, primarily in the liver and muscle cells.
Carbohydrates also play a structural role, particularly in plants. Cellulose, a polysaccharide found in plant cell walls, provides rigid support. Unlike starch, the chemical linkages in cellulose make it largely indigestible for many organisms.
Lipids: Membranes and Insulation
Lipids are a diverse group of compounds defined by their hydrophobic, or water-fearing, nature. The major categories include fats and oils (triglycerides), phospholipids, and steroids.
Triglycerides, composed of a glycerol molecule and three fatty acid chains, function as the body’s long-term energy reserve. Lipids contain a greater amount of energy per gram than carbohydrates. They also provide insulation and cushioning for organs.
Phospholipids are structural lipids essential components of the cell membrane. These molecules have a dual nature, possessing a hydrophilic head and two hydrophobic tails. In an aqueous environment, phospholipids spontaneously arrange themselves into a double-layered sheet called a bilayer, which forms the boundary of every cell.
Other lipids, such as steroids, have a distinct four-ring structure. Cholesterol is a common steroid component of animal cell membranes that helps maintain fluidity. Steroids also serve as signaling molecules, including hormones like testosterone and estrogen.
Proteins: Catalysis and Structural Support
Proteins are polymers constructed from smaller units called amino acids, which are linked together by peptide bonds. They perform the majority of the work within cells. A protein’s function is connected to its complex three-dimensional shape.
The amino acid chain must fold precisely into a unique structure for the protein to become active. This folding process is directed by the sequence of amino acids. Errors in this folding can result in a non-functional protein.
Enzymes are proteins that act as biological catalysts, accelerating chemical reactions without being consumed. They are responsible for the chemical transformations that occur in a cell, allowing life’s processes to happen quickly.
Proteins also provide structural support, forming the fibrous components of tissues and cells. For example, collagen provides strength to skin and tendons, while keratin is the main component of hair and nails. Other proteins function in movement, such as actin and myosin, and in transport, like hemoglobin carrying oxygen.
Nucleic Acids: Genetic Information Storage
Nucleic acids are the information-carrying molecules of the cell, responsible for storing, transmitting, and expressing genetic instructions. These molecules are polymers made up of repeating units called nucleotides. Each nucleotide consists of a phosphate group, a five-carbon sugar, and a nitrogenous base.
There are two primary types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material found in all cellular life, structured as a double helix formed by two strands of nucleotides. The sequence of bases along the DNA strand constitutes the genetic code.
RNA is primarily involved in taking the genetic information encoded in DNA and using it to build proteins. Different forms of RNA act as intermediaries, carrying the instructions from the DNA in the nucleus to the protein-synthesizing machinery in the cell’s cytoplasm.