Living things are built from a remarkably small selection of the periodic table. Just six elements, carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, account for the overwhelming majority of biological mass, and a handful of ions and trace metals round out the toolkit. From those raw materials, chemistry assembles the molecules that store energy, carry information, form boundaries, and catalyze the thousands of reactions a cell needs to stay alive. Understanding how atoms become organisms means following a chain of increasingly complex chemical interactions, each level enabling new properties that the level below could not produce on its own.
A Small Corner of the Periodic Table
Of the roughly 90 naturally occurring elements, life relies heavily on fewer than a dozen. Carbon, hydrogen, nitrogen, and oxygen form the backbone of every major biological molecule, while phosphorus and sulfur play critical supporting roles in energy transfer, genetic material, and protein structure. Beyond these bulk elements, cells depend on ions like potassium, sodium, calcium, and magnesium to maintain electrical gradients, trigger muscle contraction, and stabilize molecular structures. A global survey of how elements contribute to life has classified them into five tiers, from those essential for all known organisms down to those with no known biological use, highlighting just how selective living systems are in the atoms they recruit.1PubMed Central. The elements of life: A biocentric tour of the periodic table
Trace metals deserve special mention. Iron, zinc, copper, manganese, and a few others show up in tiny amounts but punch far above their weight. Transition metal ions play key structural and functional roles throughout biochemistry, affecting the shapes of large molecules and the behavior of enzymes.2PubMed. Highlighting the roles of transition metals and speciation in chemical biology Iron sits at the center of hemoglobin, shuttling oxygen through your bloodstream. Zinc holds together the finger-like protein structures that grip DNA to switch genes on and off. Copper helps mitochondria finish the job of extracting energy from food. Remove any one of these trace elements and entire metabolic pathways grind to a halt, even though a human body contains only milligrams of each.
Why Water Is Not Just a Backdrop
Water makes up roughly 60 percent of the human body, and it would be easy to think of it as just the liquid everything else floats in. It is far more than that. Water’s distinctive properties arise from its orientation-dependent hydrogen bonding, which creates an open, cage-like structure unlike simpler liquids. That structure gives water unusually high cohesion, anomalous volume behavior (ice floats rather than sinking), and an exceptional ability to dissolve ions and polar molecules.3PubMed Central. How Water’s Properties Are Encoded in Its Molecular Structure and Energies
These properties are not trivia. The fact that ice floats keeps lakes and oceans liquid beneath a frozen surface, protecting aquatic life. Water’s high heat capacity buffers organisms against rapid temperature swings. Its ability to dissolve a vast range of molecules means that the inside of a cell is a crowded but well-mixed solution where reactants find each other quickly. Even the unusually fast diffusion of hydrogen ions in water turns out to be critical: it underpins the proton gradients that cells use to generate energy, a topic we will get to shortly. Life did not just happen to appear in water. Water’s molecular quirks made life chemically possible.
Membranes Build Themselves
Every cell is wrapped in a membrane, and so are most of the compartments inside it. These membranes are made primarily of phospholipids, molecules with a water-loving head and two water-avoiding tails. Drop enough of them into water and they spontaneously arrange into a two-layered sheet, tails facing inward and heads facing outward, because that arrangement minimizes the energetically unfavorable contact between the oily tails and the surrounding water.4PubMed Central. Simulation of lipid bilayer self-assembly using all-atom lipid force fields No enzyme builds a membrane from scratch. The physics of amphiphilic molecules in water does most of the work.
This self-assembly is one of the clearest examples of how chemistry generates biological structure without needing a blueprint. A membrane is not coded for in DNA the way a protein is. It emerges from the thermodynamic preferences of its components. Once formed, though, a membrane does far more than act as a passive barrier. It controls what enters and leaves the cell, hosts the protein machinery that senses the environment, and maintains the electrical and chemical gradients that power much of cellular metabolism.
Energy Currency and the Machinery That Mints It
Cells run on a molecule called ATP, adenosine triphosphate. When a cell needs energy for almost anything, from contracting a muscle fiber to copying DNA, it breaks one of ATP’s phosphate bonds and captures the released energy. The concept of “energy-rich” molecules in biochemistry refers to those containing weak bonds that, when swapped for stronger bonds, release energy on the order of 20 or more kilojoules per mole. Nucleoside triphosphates like ATP, along with thioesters and molecular oxygen, are the principal energy-rich molecules in metabolism.5PubMed Central. Energy-Rich Molecules and Group Transfer Potentials in Energetic Coupling Reactions
Most of a cell’s ATP is produced by a rotary enzyme called ATP synthase, a molecular machine that literally spins. It sits in the inner membrane of mitochondria, where a flow of protons through its rotor drives the assembly of ATP from its components. The thermodynamic ratio is about four protons consumed per ATP molecule synthesized, though the structural ratio based on the enzyme’s architecture is closer to 4.7 protons per ATP, with the difference likely going toward internal processes like activating the enzyme.6PubMed. Thermodynamics of proton transport coupled ATP synthesis Proton-powered rotation of the enzyme’s c-ring proceeds through distinct intermediates, with a high energy barrier preventing backward rotation and ensuring the machine turns in only one direction, the direction that makes ATP rather than destroying it.7PubMed Central. The molecular mechanism of ATP synthase constrains the evolutionary landscape of chemiosmosis
What makes ATP synthase particularly fascinating is that it works in both directions. Run protons through it one way and it synthesizes ATP. Force it to hydrolyze ATP and it pumps protons the other way. This reversibility has implications for understanding how energy metabolism first evolved: early versions of the enzyme may have started as proton pumps before being co-opted for ATP production, or vice versa.7PubMed Central. The molecular mechanism of ATP synthase constrains the evolutionary landscape of chemiosmosis
Proteins Fold, Catalyze, and Get Customized
Proteins are the workhorses of the cell. A single human cell may contain thousands of distinct protein types, each folded into a specific three-dimensional shape that determines what it does. The energetics of this folding process, how much energy is gained or lost as a chain of amino acids collapses into its functional form, can now be computed with reasonable accuracy using molecular simulations, at least for smaller proteins.8Journal of Chemical Information and Modeling. Calculation of Protein Folding Thermodynamics Using Molecular Dynamics Simulations But the physical picture is straightforward: a protein’s amino acid sequence dictates the shape it naturally wants to adopt, and that shape dictates its function.
Enzymes, the proteins that catalyze biochemical reactions, work by lowering the energy barrier a reaction needs to overcome. The classic explanation, proposed by Linus Pauling, is that enzymes bind the transition state of a reaction more tightly than they bind the starting material, effectively stabilizing the hardest step of the reaction and making it happen faster.9PubMed Central. Electrostatic transition state stabilization rather than reactant destabilization provides the chemical basis for efficient chorismate mutase catalysis More recent work has refined this picture, showing that some enzymes instead destabilize the starting material, effectively pushing the reaction forward from behind rather than pulling it from the front. Both strategies achieve the same outcome, a lower energy barrier, but they do so by altering the charge densities of key atoms at different points in the catalytic cycle.10PubMed Central. Key difference between transition state stabilization and ground state destabilization: increasing atomic charge densities before or during enzyme–substrate binding
After a protein is made, the cell can modify it chemically, attaching or removing small chemical groups to alter its behavior. These post-translational modifications dynamically adjust a protein’s location, stability, activity, and ability to interact with partners.11PubMed Central. Functional analysis of protein post-translational modifications using genetic codon expansion This is how a cell fine-tunes its machinery in real time without needing to make entirely new proteins. The result is a vast expansion of functional diversity: the human genome codes for roughly 20,000 proteins, but post-translational modifications multiply the number of functionally distinct protein forms many times over.12PubMed. Protein diversification through post-translational modifications, alternative splicing, and gene duplication
Information Storage, Readout, and the Ribosome
DNA and RNA are the information molecules of biology. Both are built from chains of nucleotides, but they differ in a small chemical detail that has large consequences: RNA has a hydroxyl group on its sugar ring that DNA lacks. That single extra group affects how each molecule folds and behaves. In RNA hairpin structures, for example, the additional hydrogen bonds and stacking interactions created by that hydroxyl group lead to more misfolded intermediate states, slowing the folding process compared to equivalent DNA structures.13PubMed. Hydrogen-bonding modulation of RNA versus DNA hairpin folding pathways This might sound like a disadvantage, but RNA’s folding complexity is precisely what lets it adopt the elaborate shapes needed to function as a catalyst, not just a passive information carrier.
The most dramatic demonstration of RNA’s catalytic ability is the ribosome, the cellular machine that reads genetic instructions and assembles proteins. Despite containing many protein components, the ribosome’s catalytic core is made of RNA, making it a “ribozyme,” an RNA enzyme. Crystallographic studies have shown that peptide bond formation, the key chemical step in building a protein chain, is facilitated by the ribosomal RNA’s architectural frame rather than by any protein component.14PubMed. Ribosome crystallography: catalysis and evolution of peptide-bond formation, nascent chain elongation and its co-translational folding Even stripped of most of its proteins, the large ribosomal subunit can still catalyze peptide bond formation, and laboratory-evolved ribozymes as small as 196 nucleotides can do the same job.15PubMed. Peptide bond formation by in vitro selected ribozymes
Beyond the genetic code itself, cells regulate which genes get read and when through chemical modifications to the proteins that DNA wraps around. These histone modifications influence not only which genes are actively transcribed but also processes like DNA repair and replication, giving cells another layer of chemical control over their own information.16PubMed. Lateral Thinking: How Histone Modifications Regulate Gene Expression
Signaling Through Shape
Cells need to sense and respond to their environment, and they do this largely through receptors, proteins embedded in the membrane that detect signals on the outside and trigger responses on the inside. The unifying principle across the major receptor families, from ion channels to G protein-coupled receptors, is allostery: a molecule binding at one site on the receptor changes the protein’s shape at a distant site, flipping a molecular switch.17PubMed. Allosteric Modulation as a Unifying Mechanism for Receptor Function and Regulation
These shape changes are often subtle. Unlike the large mechanical movements of motor proteins, signal transduction in receptors is mediated by small structural shifts that are hard to detect experimentally.18PubMed Central. Coarse-grained modeling of allosteric regulation in protein receptors Recent work on G protein-coupled receptors has identified at least five distinct conformational states along the activation pathway, regulated by specific molecular interactions that act as gatekeepers, controlling how far the receptor opens and which downstream signaling partners it engages.19PubMed Central. Intermediate-state-trapped mutants pinpoint G protein-coupled receptor conformational allostery The practical upshot is that a single receptor can produce graded, nuanced responses rather than a simple on-or-off signal, and drugs can be designed to nudge a receptor into one specific conformational state rather than fully activating or fully blocking it.
The Cost of Using Oxygen
Oxygen is essential for the energy-extraction process that produces most of a cell’s ATP, but it comes with a price. Normal cellular metabolism constantly generates reactive oxygen species, highly reactive molecules that can damage proteins, DNA, lipids, and sugars.20PubMed Central. Oxidative stress and antioxidant defense In controlled amounts, reactive oxygen species actually serve as signaling molecules, helping cells communicate and adapt. But when production outstrips the cell’s ability to neutralize them, the resulting oxidative stress is linked to aging, cardiovascular disease, neurodegeneration, and cancer.21PubMed Central. ROS Generation and Antioxidant Defense Systems in Normal and Malignant Cells
Cells defend themselves with both enzymatic and non-enzymatic antioxidant systems. Enzymes like superoxide dismutase and catalase neutralize specific reactive species, while small molecules like glutathione and vitamins C and E act as chemical sponges, soaking up free radicals before they cause harm.20PubMed Central. Oxidative stress and antioxidant defense The balance between production and defense is a central theme in cellular chemistry, not something that went wrong, but an inherent trade-off of living in an oxygen-rich world.
Organization Without Membranes
Not every compartment inside a cell has a membrane around it. Over the past decade, researchers have recognized that many cellular structures form through liquid-liquid phase separation, a process similar to oil droplets forming in salad dressing. Proteins and RNA with certain properties can spontaneously condense into droplets that concentrate specific molecules, creating membraneless organelles that support activities like gene regulation and chromatin organization.22PubMed Central. Liquid-Liquid Phase Separation: Mechanisms, Roles, and Implications in Cellular Function and Disease
This phenomenon is not limited to the complex cells of animals and plants. Bacteria, which generally lack internal membrane-bound compartments, also form liquid-phase condensates. Protein assemblies in bacteria behave as dynamic liquid droplets consistent with phase separation under thermal equilibrium, and these structures increase bacterial fitness by helping cells tolerate environmental stress.23PubMed Central. Membraneless organelles formed by liquid-liquid phase separation increase bacterial fitness Phase separation provides a way to organize chemistry in space and time without building a permanent structure, and it likely predates the evolution of complex internal membranes.
From Chemistry to Biology on the Early Earth
How did all of this get started? The leading framework is the RNA world hypothesis: before DNA and proteins existed, RNA molecules served as both the genetic material and the catalysts of early life. There is strong evidence that such an RNA-dominated stage preceded the DNA-and-protein-based biology we see today.24PubMed Central. The origins of the RNA world One of the central challenges of this hypothesis is explaining how RNA’s building blocks formed in the first place. Laboratory experiments simulating alkaline hydrothermal conditions on the early Earth have shown that cyanide, a plausible prebiotic feedstock, can produce nucleobase-like molecules under the right temperature and pressure, suggesting that hot, alkaline environments near volcanic vents may have been nurseries for life’s first information molecules.25PubMed Central. Prebiotic synthesis of noncanonical nucleobases under plausible alkaline hydrothermal conditions
But having the right molecules is only part of the puzzle. For chemistry to become biology, those molecules needed to be enclosed in some kind of compartment, a protocell. Simple fatty acid vesicles can self-assemble in water, much like the phospholipid membranes of modern cells, and encapsulation of RNA within such vesicles promotes folding, substrate binding, and catalytic activity while protecting fragile molecules from degradation.26Current Biology. Origins of life: Encapsulating Darwinian evolution In laboratory models, enzymes trapped inside fatty acid vesicles can even modify the vesicle membrane itself, making it more robust and better at retaining its contents.27Nature Communications. Collaboration between primitive cell membranes and soluble catalysts This kind of feedback loop, where what is inside the compartment improves the compartment, may have been one of the first steps toward natural selection acting on chemical systems rather than just individual molecules.
A theoretical model for how self-replication might have worked proposes an “intramolecular replicase,” an RNA molecule containing two catalytic domains that each copy the other, allowing the whole molecule to replicate itself.28PubMed. Intramolecular RNA replicase: possibly the first self-replicating molecule in the RNA world The path from such molecular self-replicators to something recognizable as a cell likely passed through a protocell stage involving self-assembled membranes, non-enzymatic polymerization of amino acids and nucleotides, and encapsulation of those polymers into populations of compartments that could compete and evolve.29PubMed Central. Perspective: Protocells and the Path to Minimal Life
Expanding the Chemical Alphabet
All known natural life uses the same set of 20 standard amino acids to build proteins. Over the past two decades, researchers have figured out how to break that constraint. Using a technique called genetic code expansion, scientists can engineer cells to incorporate noncanonical amino acids, building blocks not found in nature, into specific positions in a protein. Over 40 such unnatural amino acids have been site-specifically incorporated into proteins in bacteria, yeast, and mammalian cells using an engineered transfer RNA and its matching enzyme that the cell’s own machinery does not interfere with.30PubMed Central. Expanding the genetic code for biological studies
The applications are broad. In one line of work, researchers introduced noncanonical amino acids into proteins of the human pathogen Salmonella, enabling new approaches to labeling proteins, studying how they interact, and investigating post-translational modifications inside a live pathogen.31PubMed Central. Expanding the genetic code of Salmonella with non-canonical amino acids More broadly, genetic code expansion gives synthetic biologists a toolkit to enhance and diversify biological designs beyond the constraints that natural evolution imposed, pushing proteins into chemical territory that four billion years of evolution never explored.32Chemical Reviews. Cellular Site-Specific Incorporation of Noncanonical Amino Acids in Synthetic Biology If the chemistry of life as we know it reflects a particular set of solutions to the problem of building and maintaining organisms, genetic code expansion is beginning to show that the solution space is much larger than nature sampled.