Macromolecules, water, pH, and bonds — the chemical foundation of biology
Carbohydrates are made of carbon, hydrogen, and oxygen in a roughly 1:2:1 ratio (CH₂O)ₙ. Their primary job is energy — they're the body's preferred quick fuel source. They also provide structural support (cellulose in plants, chitin in fungi/insects) and play roles in cell communication.
Lipids are hydrophobic (water-fearing) molecules. They're not true polymers — they don't have a single repeating monomer unit. Instead they're diverse molecules that share the property of being insoluble in water. They store more energy per gram than carbohydrates, making them excellent long-term energy storage.
Proteins are the most diverse and functionally important macromolecules. Built from 20 different amino acids, they can fold into an almost infinite variety of 3D shapes — and shape determines function. Enzymes, antibodies, structural proteins, transport proteins, and hormones are all proteins.
Nucleic acids store, transmit, and express genetic information. DNA is the permanent information archive; RNA carries instructions from DNA to ribosomes for protein synthesis. Every living cell uses nucleic acids — they are the molecular basis of heredity and the central dogma of biology: DNA → RNA → Protein.
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Water is a polar molecule — oxygen pulls electrons toward itself more strongly than hydrogen, creating partial negative (δ−) and partial positive (δ+) charges. This makes water molecules attracted to each other (and to other polar/charged molecules), forming hydrogen bonds between the δ+ hydrogen of one water molecule and the δ− oxygen of another.
Cohesion is water sticking to water. Adhesion is water sticking to other polar surfaces. Together these properties allow water to move against gravity up plant stems (transpiration), create surface tension strong enough for insects to walk on, and form the droplets that roll off leaves.
Water resists temperature change better than almost any other substance — it takes a lot of energy to heat it up or cool it down. This is because breaking hydrogen bonds requires energy. This high specific heat stabilizes Earth's climate, keeps ocean temperatures stable, and allows organisms to maintain homeostasis.
Almost every substance is denser as a solid than as a liquid. Water is a major exception. When water freezes, hydrogen bonds lock into a crystalline lattice that holds molecules further apart than in liquid water. Ice is less dense than liquid water — so it floats. This has enormous biological consequences.
Water dissolves more substances than any other liquid — earning the title "universal solvent." Ionic compounds and polar molecules dissolve in water; nonpolar molecules (lipids) do not. This selective solvency is fundamental to how cells work — nutrients, waste products, and signals all travel dissolved in water.
All 5 water properties are exam-ready topics
An acid is a substance that releases hydrogen ions (H⁺) — also called protons — into solution. The more H⁺, the more acidic (lower pH). Strong acids (like HCl) dissociate completely. Weak acids (like acetic acid in vinegar) dissociate partially. pH is a logarithmic scale — pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.
A base (or alkali) accepts hydrogen ions from solution, reducing H⁺ concentration and raising pH. Bases feel slippery (soap), taste bitter, and can be just as dangerous as strong acids. Blood is slightly basic (pH 7.4). Most cellular processes work optimally at near-neutral to slightly basic pH.
Buffers are weak acid/base pairs that resist pH changes by absorbing excess H⁺ or OH⁻. They are essential for life — your blood pH must stay between 7.35 and 7.45. Even a shift to 7.0 or 7.8 can be fatal. Cells use buffers to maintain the precise pH their enzymes need to function.
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Covalent bonds form when two atoms share electrons. They are the strongest chemical bonds — requiring the most energy to break. All organic macromolecules (proteins, carbohydrates, lipids, nucleic acids) are held together primarily by covalent bonds. Can be nonpolar (equal sharing) or polar (unequal sharing).
Ionic bonds form when one atom transfers electrons to another, creating oppositely charged ions (cations + and anions −) that attract each other. Strong in dry conditions but weaker in water (water molecules surround and separate ions). Table salt (NaCl) is the classic example — it dissolves easily in water.
Hydrogen bonds are not true bonds — they're attractions between a δ+ hydrogen (attached to N or O) and a δ− atom (N or O) on another molecule. Individually weak (~1/20th the strength of a covalent bond), but when thousands form simultaneously they become collectively very important. They're responsible for water's unique properties and for DNA's double helix.
Van der Waals forces are fleeting, weak attractions between all molecules caused by temporary fluctuations in electron distribution. When electrons briefly cluster on one side of an atom, they create a temporary dipole that induces an opposite dipole in neighboring atoms. Incredibly weak individually — but when many nonpolar molecules pack closely together, these forces add up.
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