Atoms, chemical bonds, water, pH, carbon, and chemical reactions — the molecular foundation beneath every biological process this course will study
Living things are made of matter and run on chemistry. Every time a cell divides, a muscle contracts, a leaf captures sunlight, or a nerve sends a signal, chemical reactions are happening. Biology cannot be understood without some understanding of chemistry — not because biologists need to memorize the periodic table, but because the molecules inside living things obey chemical rules, and those rules explain how life works.
This chapter introduces the chemical ideas you need for the rest of this course. You will not need advanced chemistry. You will need enough chemistry to understand why water is so important to cells, why proteins can change shape, why some molecules carry energy, and why the acid-base balance in your blood has to stay within a very narrow range. These ideas will come back in nearly every chapter that follows.
Your body is about 60–70% water. Your DNA is a long chain of carbon-containing molecules held together partly by weak attractions called hydrogen bonds. The enzymes that speed up every chemical reaction in your cells are proteins that can stop working if the pH of your blood shifts by even a fraction of a unit. How does all of this chemistry connect to being alive? By the end of this chapter, you will have the foundation to answer that question — and to understand every biological system this course covers.
When you eat a meal, your digestive system breaks large molecules into smaller ones that your cells can absorb. When your cells use oxygen to release energy from sugar, they are running a carefully controlled set of chemical reactions. When DNA is copied before a cell divides, chemical bonds are being formed and broken with remarkable accuracy. When a hormone travels through the bloodstream and changes the behavior of a distant cell, it works by fitting into a protein receptor the way a key fits a lock — chemistry explaining biology.
Biology is the study of living systems, but living systems are made of chemicals, and their processes are chemical processes. A cell is not a bag of mystery. It is a highly organized collection of molecules doing specific jobs, following the same laws of chemistry that govern reactions in a laboratory or a kitchen.
· What atoms and molecules are, and how they combine
· Why some molecules dissolve in water and others do not
· How acids and bases behave, and why pH matters
· Why carbon is the backbone of every important molecule in living things
· How chemical reactions release, store, or transfer energy
The chemistry in this chapter is the foundation beneath cells (Chapters 6–7), enzymes (Chapter 8), photosynthesis and respiration (Chapters 9–10), DNA and genes (Chapters 12–13), digestion and blood chemistry (Chapters 25–26), and nutrient cycles (Chapter 28). It will not stay in Chapter 4 — it will come back continuously.
All matter — whether living or nonliving — is made of atoms. An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms are extraordinarily small: a single cell contains trillions of them. Every atom has a nucleus at its center, containing protons (positively charged) and neutrons (no charge). Surrounding the nucleus is a region where electrons (negatively charged) move. The number of protons defines what element an atom is — an atom with six protons is always carbon; an atom with eight protons is always oxygen.
| Element | Symbol | Approx. % of living matter (by mass) | Key roles in biology |
|---|---|---|---|
| Oxygen | O | ~65% | Water, cellular respiration, many biological compounds |
| Carbon | C | ~18% | Backbone of all biological molecules |
| Hydrogen | H | ~10% | Water, organic molecules, acids and bases |
| Nitrogen | N | ~3% | Proteins, DNA, RNA, chlorophyll |
Carbon, Hydrogen, Oxygen, and Nitrogen make up about 96% of living matter by mass. The acronym CHON helps you remember the four most abundant elements in life. Other elements — phosphorus, sulfur, calcium, potassium, iron — are also essential but present in smaller amounts.
When atoms of different elements bond together, they form a compound. Water (H₂O) is a compound of hydrogen and oxygen. Carbon dioxide (CO₂) is a compound of carbon and oxygen. The biological molecules that carry out life's functions — sugars, fats, proteins, DNA — are all compounds. A molecule is two or more atoms bonded together. The properties of a compound are different from the properties of its individual elements: hydrogen and oxygen are both colorless gases, but combined as water they form a liquid that is essential for life.
Atoms bond to one another through forces of attraction between their electrons. The type of bond formed affects how easily it can be broken, how much energy it stores, and how molecules behave in water.
| Bond Type | How It Forms | Relative Strength | Biological Importance |
|---|---|---|---|
| Covalent (nonpolar) | Two atoms share electrons equally | Strong | Carbon-hydrogen bonds in organic molecules; stable backbones of proteins, DNA, fats |
| Covalent (polar) | Two atoms share electrons unequally; one end is slightly negative | Strong | O–H bonds in water; creates water's polarity and solvent properties |
| Ionic | One atom transfers electrons to another; opposite charges attract | Moderate (weaken in water) | Salt ions in body fluids; mineral nutrition; cell membrane charges |
| Hydrogen | Weak attraction between partially positive H and partially negative O or N | Weak individually; strong collectively | Holds DNA strands together; determines protein shape; gives water its properties |
"Breaking bonds releases energy." This is a common half-truth. Breaking bonds actually requires energy. Energy is released when new, stronger bonds form in the products. In an exergonic reaction, the bonds formed in the products are collectively stronger than the bonds broken in the reactants — so the net result is energy release. This concept will reappear in Chapter 8 (Enzymes) and Chapters 9–10 (Photosynthesis and Respiration).
Of all the molecules that matter for biology, water is the most important. Life as we know it originated in water, depends on water, and in most organisms consists mostly of water. A human body is roughly 60–70% water by mass. A bacterial cell may be 70–90% water. All of water's special properties trace back to one feature: its polarity.
A water molecule (H₂O) has two hydrogen atoms bonded to one oxygen atom. Oxygen pulls shared electrons strongly toward itself, making the oxygen end slightly negative (δ−) and the hydrogen ends slightly positive (δ+). The molecule is bent, not linear, so the partial charges are not symmetrically arranged. Water is a polar molecule, and this polarity explains everything below.
| Property | Cause | Biological Significance |
|---|---|---|
| Cohesion | Hydrogen bonds between water molecules | Water clings together; allows water to be pulled up tall plants in continuous columns; creates surface tension |
| Adhesion | Water is attracted to other polar surfaces | Water clings to walls of vessels in plants and blood vessels; assists upward movement of water against gravity |
| High specific heat | Hydrogen bonds resist temperature change | Body temperature is stabilized; blood resists rapid heating or cooling; coastal climates are moderated |
| Evaporative cooling | High-energy water molecules escape as vapor, leaving cooler liquid behind | Sweating cools the body; plants release water vapor through leaves to cool tissues |
| Excellent solvent | Polarity allows water to surround and dissolve polar and ionic substances | Nutrients, wastes, gases, and signals dissolve in body fluids and move through organisms |
| Ice floats | Frozen water forms a less-dense crystal lattice than liquid water | Ice forms on the surface of lakes and insulates liquid water below, allowing aquatic life to survive winter |
If ice sank, lakes would freeze solid from the bottom up each winter. Fish and other aquatic organisms would have no liquid water refuge. Because ice is less dense than liquid water — an unusual property traceable to water's hydrogen bonding — it floats on the surface and acts as an insulating layer, keeping the water beneath above freezing.
A solution is a uniform mixture of two or more substances. The substance that does the dissolving is the solvent; the substance that is dissolved is the solute. In biology, water is almost always the solvent. The concentration of a solution describes how much solute is dissolved in a given volume of solvent.
The fluids inside and outside your cells are solutions. Blood plasma, cytoplasm, the fluid in your digestive tract, and the liquid in your eyes are all aqueous (water-based) solutions containing dozens of dissolved substances — ions, gases, nutrients, proteins, hormones, and waste products. The concentration of these dissolved substances affects nearly every biological process.
| Mixture type | Particle size | Behavior | Biological example |
|---|---|---|---|
| Solution | Very small (ions, small molecules) | Uniform; particles do not settle | Salt dissolved in blood plasma; glucose dissolved in cytoplasm |
| Colloid | Intermediate | Particles dispersed but too large for true solution | Cytoplasm (gel-like interior of cells) |
| Suspension | Large (visible particles) | Particles may settle over time | Red blood cells suspended in plasma |
If the concentration of ions inside a cell changes, the cell's chemistry changes. If blood glucose rises too high (as in uncontrolled diabetes) or falls too low (hypoglycemia), serious health problems result. Homeostasis at the molecular level is largely about maintaining the right concentrations of dissolved substances in cells and body fluids. You will see this again in Chapters 7 (osmosis and transport), 24 (blood sugar regulation), and 26 (kidney function).
Water molecules occasionally break apart spontaneously, releasing a hydrogen ion (H⁺) and a hydroxide ion (OH⁻). The concentration of hydrogen ions in a solution determines whether it is an acid or a base.
An acid is a substance that releases H⁺ when dissolved in water, increasing hydrogen ion concentration. A base is a substance that accepts H⁺ or releases OH⁻, decreasing hydrogen ion concentration. The pH scale measures H⁺ concentration on a scale from 0 to 14. The scale is logarithmic — each step represents a tenfold change in H⁺ concentration.
· pH below 7 = acidic (higher H⁺ concentration)
· pH 7 = neutral (pure water)
· pH above 7 = basic / alkaline (lower H⁺ concentration)
· Each step of 1 pH unit = a 10-fold change in H⁺ concentration. A solution at pH 4 has 100 times more H⁺ than one at pH 6.
| Substance | Approximate pH | Acidic / Neutral / Basic |
|---|---|---|
| Stomach acid | 1.5 – 3.5 | Strongly acidic |
| Lemon juice | 2 | Strongly acidic |
| Black coffee | 5 | Weakly acidic |
| Urine | 4.5 – 8 | Varies |
| Pure water | 7 | Neutral |
| Blood | 7.35 – 7.45 | Slightly basic |
| Pancreatic fluid | 8 – 8.3 | Weakly basic |
| Seawater | 8.1 | Weakly basic |
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Buffers work by absorbing excess H⁺ when pH drops and releasing H⁺ when pH rises. The bicarbonate buffer system in human blood uses carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻) to maintain blood pH within the narrow range of 7.35–7.45. A drop below 7.35 is called acidosis; a rise above 7.45 is called alkalosis. Both are medical emergencies.
Enzymes are proteins, and their shapes depend on weak interactions (hydrogen bonds, ionic attractions) that are sensitive to H⁺ concentration. If pH shifts outside the enzyme's normal range, its shape changes and it stops working. Stomach enzymes work best at pH 1.5–2; intestinal enzymes work best near pH 7–8. Chapter 8 will cover this in depth.
Of the approximately 118 known elements, carbon has a special role in biology. Almost every important molecule in living things — sugars, fats, proteins, DNA — is built on a backbone of carbon atoms. Chemistry that uses carbon as a framework is called organic chemistry.
Why carbon? Because a carbon atom has four electrons available for bonding, meaning it can form four covalent bonds simultaneously. Carbon atoms can bond to other carbon atoms (forming long chains and rings), to hydrogen, oxygen, nitrogen, phosphorus, and sulfur. This versatility allows carbon to form an enormous variety of stable molecular structures. Life has taken advantage of this diversity over billions of years of evolution.
| Carbon Skeleton Shape | Example | Biological significance |
|---|---|---|
| Straight chain | Fatty acid chains in fats | Long nonpolar chains are hydrophobic — key to membrane structure |
| Branched chain | Some amino acid side chains | Branching creates three-dimensional shapes that determine protein function |
| Ring | Glucose (6-carbon ring), DNA bases | Rigid, compact structure; important for sugar chemistry and genetic information |
Attached to carbon skeletons are clusters of atoms called functional groups. Functional groups are specific arrangements of atoms that give molecules particular chemical properties — making them acidic, basic, reactive, hydrophilic, or hydrophobic. The chemical behavior of a large biological molecule depends significantly on which functional groups it carries.
| Functional Group | Composition | Property | Found in |
|---|---|---|---|
| Hydroxyl | −OH | Polar, hydrophilic | Alcohols, sugars |
| Carboxyl | −COOH | Acts as an acid; releases H⁺ | Amino acids, fatty acids |
| Amino | −NH₂ | Acts as a base; accepts H⁺ | Amino acids, DNA bases |
| Phosphate | −PO₄ | Charged, hydrophilic; stores energy in bonds | DNA, RNA, ATP |
| Carbonyl | C=O | Polar; reactive | Sugars, some lipids |
You do not need to memorize this functional group table. What matters is the concept: the chemical behavior of a large biological molecule depends on what functional groups are attached to its carbon skeleton. The same carbon backbone with different functional groups is an entirely different molecule with entirely different properties.
A chemical reaction is a process in which chemical bonds are broken, formed, or rearranged, converting reactants into products. Every biological process — from digesting a meal to copying DNA — is a chemical reaction or a linked sequence of them.
Exergonic reactions — release energy. Products have less stored energy than reactants. Cellular respiration is exergonic: glucose + oxygen → CO₂ + water + energy released.
Endergonic reactions — require energy input. Products have more stored energy than reactants. Building proteins, DNA, and other large molecules is endergonic — it costs energy.
In cells, exergonic and endergonic reactions are often coupled: energy released by breaking down food powers the synthesis of ATP, which then powers endergonic reactions elsewhere.
Even reactions that ultimately release energy require an initial input to get started. This initial energy is called activation energy. Think of pushing a boulder off a hilltop: you must push it up a small rise before it rolls down. In cells, molecules called enzymes lower activation energy, making reactions fast enough to sustain life. Without enzymes, most biological reactions would occur far too slowly. Enzymes are covered fully in Chapter 8.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energyMetabolism is the total of all chemical reactions in a living organism. Metabolic reactions are organized into pathways — sequences in which the product of one reaction becomes the reactant for the next. Anabolic pathways (anabolism) build complex molecules from simple ones and require energy input. Catabolic pathways (catabolism) break complex molecules into simpler ones and release energy. Living things maintain life by constantly running and regulating both types.
| Pathway type | Direction | Energy | Example |
|---|---|---|---|
| Anabolic | Small → large molecules | Requires energy (endergonic) | Building proteins from amino acids; making DNA; photosynthesis building glucose |
| Catabolic | Large → small molecules | Releases energy (exergonic) | Cellular respiration breaking down glucose; digestion breaking down proteins |
The body maintains careful chemical balance. When this regulation fails, illness follows. Acid reflux occurs when stomach acid (pH ~2) escapes into the esophagus, which lacks the protective lining of the stomach. Kidney disease can disrupt regulation of ion concentrations and pH in the blood. Diabetes involves disrupted regulation of glucose concentration in the blood. Each of these is a chemical problem with biological consequences.
Acid rain — rainfall made acidic by sulfur dioxide and nitrogen oxide pollution — can lower the pH of lakes and soils, harming aquatic organisms that cannot survive outside a narrow pH range. Ocean acidification — caused by the ocean absorbing excess carbon dioxide from the atmosphere — threatens shellfish and coral reefs, whose calcium carbonate structures dissolve in more acidic water.
Living things are made of carbon compounds. When organisms die and decompose, or when fossil fuels (the remains of ancient organisms) are burned, carbon dioxide is released into the atmosphere. Carbon dioxide is a greenhouse gas — it traps heat. Understanding the carbon cycle, covered in Chapter 28, requires understanding how carbon moves between living organisms, the atmosphere, the oceans, and Earth's crust.
Correction: Breaking bonds requires energy. Energy is released when new, stronger bonds form in the products. In an exergonic reaction, the bonds formed in the products are collectively stronger than those broken in the reactants — so the net result is energy release. Confusing breaking with releasing is a very common error in biology and chemistry.
Correction: Water is not a passive container. Its polarity, hydrogen bonding, high specific heat, and solvent properties actively participate in biological chemistry. Many reactions in cells would not occur at all without water.
Correction: Both acids and bases can be harmful at extremes. Stomach acid (pH ~2) is highly acidic and essential for digestion. Bleach (pH ~13) is highly basic and corrosive. What matters is concentration, context, and what tissues are exposed — not whether something is acidic or basic in general.
Correction: Carbon is found throughout the nonliving world — in carbon dioxide, limestone, diamonds, and coal. What is special about carbon in living things is the remarkable variety of carbon-containing molecules that organisms build, not that carbon is exclusive to life.
Chapter 4 introduced the chemical building blocks and forces that underlie all of biology. Chapter 5 builds directly on this foundation by introducing the four major classes of biological molecules: carbohydrates, lipids, proteins, and nucleic acids. Each class has a characteristic structure built on a carbon skeleton, a set of functional groups that determine its properties, and a set of functions in living systems. Understanding how molecular structure relates to biological function is one of the most powerful ideas in all of biology — and Chapter 5 is where that idea begins to take shape in detail.