Chapter 2 of 10 - AP Biology Course
Chemistry of Life
All living things are built from the same chemical toolkit. This chapter covers the properties of water, the pH scale, carbon chemistry, the four classes of biological macromolecules, and how enzymes catalyze the reactions that sustain life.
Water: The Solvent of Life
Water makes up 70-95% of most living cells. Its unique properties arise from the polarity of the molecule and the resulting hydrogen bonds between neighboring molecules. Each water molecule can form up to four hydrogen bonds, creating a dynamic network that gives water extraordinary physical characteristics.
Cohesion and Adhesion
Cohesion is the attraction between water molecules themselves. It produces surface tension, which allows insects like water striders to walk on ponds. Adhesion is water's attraction to other polar surfaces. Together, cohesion and adhesion drive capillary action, helping water travel up the narrow xylem vessels of plants against gravity.
High Specific Heat
Water has a specific heat of 4.184 J/(g*K), meaning it absorbs or releases a large amount of heat with only a small temperature change. This property stabilizes body temperature in organisms and moderates coastal climates. Hydrogen bonds must absorb energy before water molecules move faster, which resists temperature swings.
Universal Solvent
Because water is polar, it dissolves ionic compounds and other polar molecules effectively. Hydrophilic solutes dissolve readily; hydrophobic molecules (like fats and oils) do not. This distinction is critical for membrane structure, protein folding, and transport in living systems.
A 3D representation of a water molecule (H2O). The bent geometry creates a dipole with partial negative charge on oxygen and partial positive charges on the hydrogens, enabling hydrogen bonding.
Quick Check
Which property of water allows insects to walk on the surface of a pond?
pH and Buffers
The pH scale measures hydrogen ion concentration on a logarithmic scale from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic (alkaline). Each unit change represents a tenfold difference in H+ concentration.
Living organisms maintain narrow pH ranges through buffers - substances that resist changes in pH by accepting or donating hydrogen ions. The bicarbonate buffer system in human blood keeps pH between 7.35 and 7.45. Even small deviations outside this range can denature proteins and disrupt enzyme function.
Fill in the Blank
A solution with a pH of 3 has________times more hydrogen ions than a solution with a pH of 5.
Carbon: The Backbone of Life
Carbon is central to biological chemistry because it can form four covalent bonds, creating complex three-dimensional structures. Carbon atoms bond to other carbons to form chains, rings, and branched structures. The diversity of organic molecules comes from variations in carbon skeleton length, branching, double-bond positions, and the identity of attached functional groups.
Key functional groups in biology include hydroxyl (-OH), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), sulfhydryl (-SH), carbonyl (C=O), and methyl (-CH3). Each group confers specific chemical properties - for example, carboxyl groups act as acids by donating H+, while amino groups act as bases by accepting H+.
The Four Macromolecules
Biological macromolecules are large, complex molecules built by linking smaller subunits (monomers) through dehydration synthesis (condensation reactions that release water). The reverse process, hydrolysis, breaks polymers apart by adding water. The four major classes are carbohydrates, lipids, proteins, and nucleic acids.
The four classes of biological macromolecules. Each is built from characteristic monomers joined by specific bond types.
Carbohydrates
Sugars and polysaccharides for energy and structure
Lipids
Fats, phospholipids, and steroids for membranes and energy
Proteins
Enzymes, structural, transport, and signaling molecules
Nucleic Acids
DNA and RNA for information storage and transfer
| Macromolecule | Monomer | Polymer Example | Bond Type | Key Functions |
|---|---|---|---|---|
| Carbohydrates | Monosaccharides | Starch, glycogen, cellulose | Glycosidic | Quick energy, structural support |
| Lipids | Fatty acids + glycerol | Triglycerides, phospholipids | Ester | Long-term energy, membranes, insulation |
| Proteins | Amino acids | Hemoglobin, collagen, enzymes | Peptide | Catalysis, structure, transport, defense |
| Nucleic Acids | Nucleotides | DNA, RNA | Phosphodiester | Genetic information, protein synthesis |
Quick Check
Which macromolecule is primarily responsible for long-term energy storage in animals?
Glucose
Glucose is a six-carbon monosaccharide and the most important fuel molecule for cellular respiration. It is the monomer of glycogen, starch, and cellulose, and its oxidation in glycolysis and the citric acid cycle yields ATP.
Formula
C6H12O6
Mol. Weight
180.16 g/mol
Enzymes and Activation Energy
Enzymes are biological catalysts - most are proteins - that speed up chemical reactions by lowering the activation energy (Ea) required to reach the transition state. Enzymes are not consumed in the reaction and do not change the overall free-energy change (delta G) of the reaction; they only accelerate the rate at which equilibrium is reached.
The induced-fit model describes how the enzyme active site changes shape slightly when the substrate binds, creating a tighter fit. Enzyme activity is influenced by temperature, pH, substrate concentration, and the presence of inhibitors (competitive or noncompetitive). Allosteric regulation allows molecules to bind at sites other than the active site, changing the enzyme's conformation and either activating or inhibiting it.
Fill in the Blank
Enzymes speed up reactions by lowering the________needed to reach the transition state, without altering the overall free-energy change.
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