02/12 02 Chemistry of Life

Atoms, Elements, and the Chemical Basis of Life

Living organisms can be understood as chemical systems. Their membranes, enzymes, genetic material, and energy-transfer pathways depend on how atoms are arranged and how molecules interact.

Matter consists of substances that have mass and occupy space. Each element is made of one kind of atom. An atom contains positively charged protons and uncharged neutrons in its nucleus, with negatively charged electrons surrounding the nucleus. The number of protons is the atomic number and identifies the element.

Electrons in the outermost energy level are valence electrons. They largely determine how an atom bonds. Atoms may share, gain, or lose electrons to reach more stable arrangements. Atoms of the same element can differ in neutron number; these forms are isotopes. Charged atoms or molecules are ions: positively charged ions are cations, and negatively charged ions are anions.

The elements most prominent in biological molecules include carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Carbon is especially versatile because it can form four covalent bonds, creating the frameworks of many complex molecules.

Takeaway: Biological chemistry begins with atomic structure, especially the behavior of valence electrons and the bonding capacity of carbon.

Chemical Bonds and Molecular Interactions

Chemical bonds and intermolecular attractions determine how biological molecules form and behave.

A covalent bond forms when atoms share electrons. In a nonpolar covalent bond, electrons are shared relatively equally. In a polar covalent bond, electrons are shared unequally because one atom attracts them more strongly. This attraction is called electronegativity. In water, oxygen attracts shared electrons more strongly than hydrogen, giving oxygen a partial negative charge and hydrogen a partial positive charge.

An ionic bond or ionic interaction results from attraction between oppositely charged ions. In water, polar molecules surround ions and reduce the attraction holding them in a crystal. A hydrogen bond is a weaker attraction involving a partially positive hydrogen and an electronegative atom, usually oxygen or nitrogen. Many hydrogen bonds acting together can stabilize DNA, proteins, carbohydrates, and water networks.

Temporary charge differences can produce van der Waals interactions. Nonpolar groups in water can also cluster through the hydrophobic effect. This effect is not a conventional chemical bond; it reflects how water behaves around nonpolar substances.

Molecular shape is essential. A molecule’s bond polarities may cancel if its three-dimensional arrangement is symmetrical, while a different arrangement of the same types of bonds may produce an overall polar molecule.

Takeaway: Biological structure reflects the combined effects of covalent bonds, charge-based attractions, hydrogen bonds, van der Waals interactions, and the hydrophobic effect.

Chemical Reactions and Enzymes

Chemical reactions rearrange atoms to form new substances. Reactants become products through bond breaking, bond formation, electron transfer, or hydrogen-ion transfer.

In dehydration synthesis, two smaller molecules join by a covalent bond while water is removed. Hydrolysis reverses this pattern: water is added to break a covalent bond. In simplified form, dehydration synthesis can be represented as $\text{monomer}+\text{monomer}\rightarrow\text{larger molecule}+\mathrm{H_2O}$, while hydrolysis can be represented as $\text{larger molecule}+\mathrm{H_2O}\rightarrow\text{smaller molecules}$.

Redox reactions involve electron transfer. Oxidation is the loss of electrons, and reduction is the gain of electrons; the two processes occur together. Electron carriers such as $\mathrm{NAD^+}$ and $\mathrm{FAD}$ can temporarily accept high-energy electrons and hydrogen atoms during metabolism.

Reactions require an initial energy input called activation energy. A catalyst lowers the activation energy without being consumed. Enzymes are biological catalysts that bind particular reactants, called substrates, and orient them to favor chemical change. Temperature, pH, substrate concentration, inhibitors, and activators can all affect enzyme activity.

Takeaway: Chemical reactions in cells are controlled by energy barriers, electron transfer, water movement, and catalysts such as enzymes.

Water: Polarity, Solubility, and Temperature Regulation

Water’s polarity and its ability to form hydrogen bonds explain why it is the principal solvent of life. Water molecules attract one another through cohesion, while adhesion describes attraction between water and other substances. Together, cohesion and adhesion support capillary action, including the movement of water through narrow plant vessels.

Water has a relatively high specific heat, so it can absorb considerable heat with a modest temperature increase. Its high heat of vaporization means that evaporation removes substantial heat when hydrogen bonds are disrupted, as during sweating or plant transpiration.

In ice, hydrogen bonds create an open structure. Ice is therefore less dense than liquid water and floats, allowing floating ice to insulate the water below.

A solvent dissolves a solute to form a solution. Water dissolves many ionic and polar substances because its partial charges surround and stabilize them. For sodium chloride, the process can be written as $\mathrm{NaCl(s)\rightarrow Na^+(aq)+Cl^-(aq)}$. Water molecules form a hydration shell around the separated ions.

Hydrophilic substances interact readily with water, whereas hydrophobic substances do not. An amphipathic molecule has both regions. Phospholipids are amphipathic: their phosphate-containing heads interact with water, while their hydrocarbon tails avoid it. This arrangement drives the formation of membrane bilayers.

Takeaway: Water’s polarity, hydrogen bonding, thermal properties, and interactions with solutes make it essential for cellular organization and chemical reactions.

Acids, Bases, pH, and Buffers

Water can undergo self-ionization according to $\mathrm{H_2O\rightleftharpoons H^+ + OH^-}$. In aqueous solution, hydrogen ions are generally associated with water as hydronium, $\mathrm{H_3O^+}$, although $\mathrm{H^+}$ is commonly used as a convenient representation.

An acid increases hydrogen-ion concentration. A base decreases hydrogen-ion concentration by accepting hydrogen ions or releasing hydroxide ions. Strong acids and bases dissociate extensively, while weak acids and bases dissociate only partly. Strength is not the same as concentration: a dilute strong acid can contain fewer total acid molecules than a concentrated weak acid while still dissociating more completely.

The pH relationship is $\mathrm{pH=-\log_{10}[H^+]}$. At approximately $25\,^{\circ}\mathrm{C}$, pH 7 is neutral, values below 7 are acidic, and values above 7 are basic. Because pH is logarithmic, a solution at pH 3 has ten times the hydrogen-ion concentration of a solution at pH 4.

A buffer limits pH change by reversibly accepting or donating hydrogen ions. A typical weak-acid buffer system is $\mathrm{HA\rightleftharpoons H^+ + A^-}$. When hydrogen ions are added, $\mathrm{A^-}$ can bind them to form $\mathrm{HA}$; when hydrogen ions are removed, $\mathrm{HA}$ can dissociate to replace them. Buffers have a limited capacity and do not prevent pH change completely.

Small pH changes can alter amino-acid charges, hydrogen bonds, protein shape, and enzyme activity. Maintaining suitable pH conditions is therefore part of biological homeostasis.

Takeaway: pH links hydrogen-ion concentration to biological function, while buffers help keep cellular conditions within workable limits.

Biological Macromolecules and Their Functions

Biological macromolecules differ in structure, but each class supports essential cellular functions.

Carbohydrates include monosaccharides, disaccharides, and polysaccharides. Glucose and other monosaccharides can provide immediate energy. Starch stores energy in plants, glycogen stores energy in animals and fungi, cellulose supports plant cell walls, and chitin contributes to fungal cell walls and arthropod exoskeletons. Differences in the bonds and three-dimensional arrangements of sugar units produce different properties.

Lipids are largely hydrophobic and include fats, oils, phospholipids, steroids, and waxes. A triglyceride contains glycerol joined to three fatty acids. Saturated fatty acids have no carbon–carbon double bonds, whereas unsaturated fatty acids have one or more. Double bonds can bend fatty-acid chains and influence membrane fluidity. Steroids contain four interconnected carbon rings; cholesterol contributes to membrane organization and is a precursor for several hormones.

Proteins are polymers of amino acids. Amino acids are connected by peptide bonds, forming polypeptides. Protein structure is described as primary, secondary, tertiary, and quaternary structure. Shape determines whether a protein can function as an enzyme, receptor, transporter, channel, motor, antibody, structural fiber, or signal molecule. Denaturation changes shape and often impairs function.

Nucleic acids are made of nucleotides, each containing a five-carbon sugar, a phosphate group, and a nitrogen-containing base. DNA uses deoxyribose and the bases adenine, thymine, cytosine, and guanine. RNA uses ribose and uracil instead of thymine. DNA base pairing is stabilized by hydrogen bonds: adenine pairs with thymine, and cytosine pairs with guanine. ATP is a nucleotide with an important role in chemical energy transfer.

Takeaway: Carbohydrates, lipids, proteins, and nucleic acids have distinct structures that support energy management, membranes, catalysis, structure, signaling, and hereditary information.

Molecular Shape and Biological Function

Molecular function depends on both chemical composition and three-dimensional arrangement. Shape complementarity allows a substrate to fit an enzyme’s active site. Charge complementarity supports attraction between oppositely charged regions. Hydrogen-bond patterns guide DNA base pairing and help stabilize protein folding.

Hydrophobic clustering drives membrane formation and often places nonpolar groups inside globular proteins, away from water. Molecular flexibility can allow a protein or other molecule to change shape when it binds a partner. These principles explain why a small structural change can affect biological activity.

Many biological structures are stabilized by numerous weak interactions rather than by one overwhelming force. Their combined effect can be strong enough to maintain a folded protein or paired DNA strands, while remaining reversible enough for proteins to release ligands and DNA strands to separate during replication.

Protein structure illustrates the connection from sequence to function. The primary amino-acid sequence influences local secondary structures, which contribute to tertiary folding and, when present, quaternary assembly. A change in conditions can disrupt the interactions that maintain these levels of organization.

Takeaway: Biological molecules work because their shapes, charges, flexibility, and interaction patterns match the tasks they perform.

Applying Chemistry Through Investigation

Chemical principles can be tested by changing one condition at a time, measuring a response, and controlling other relevant variables.

Solubility

Compare water with a suitable nonpolar solvent using equal amounts of salt, sugar, and vegetable oil. Record whether each substance dissolves, forms a suspension, or separates into layers. Ionic and polar substances are generally more soluble in polar water, whereas nonpolar substances interact more favorably with nonpolar solvents.

Buffer capacity

Add equal increments of dilute acid or base to water and to a buffer. Measure pH after each addition. The solution type or amount added can be the independent variable, and measured pH can be the dependent variable. Keep volume, temperature, concentration, mixing time, and measurement method controlled. A buffer should show smaller pH changes until its capacity is approached or exceeded.

Enzyme activity and pH

Measure product formation or substrate disappearance at several pH values while keeping temperature, enzyme concentration, and substrate concentration constant. Activity often reaches an optimum pH and decreases outside that range because changes in charge and molecular interactions can alter the active site or cause denaturation.

Amphipathic organization

Observe phospholipid models, soap films, or another approved amphipathic system. Relate the arrangement of hydrophilic and hydrophobic regions to bilayers and micelles.

A strong investigation identifies a testable question, independent and dependent variables, controlled conditions, quantitative measurements, and sources of uncertainty.

Takeaway: Solubility, pH response, enzyme rate, and membrane organization provide observable evidence for molecular interactions.