9.3 Biomolecules and the Chemistry of Life
Key Takeaways
- The four biomolecule families are carbohydrates (monomer: monosaccharide), lipids (glycerol plus fatty acids), proteins (monomer: amino acid), and nucleic acids (monomer: nucleotide).
- Polymers are assembled by dehydration synthesis, which removes a water molecule at each bond, and broken by hydrolysis, which adds a water molecule to split each bond — the chemistry behind digestion.
- Organic compounds are carbon-based and typically contain carbon-hydrogen bonds; water, carbon dioxide, and mineral salts are inorganic even though living systems depend on them completely.
- Enzymes are proteins that lower a reaction's activation energy without being consumed, and each enzyme's active site is specific to its substrate, so denaturing the protein by heat or extreme pH destroys function.
- Water's polarity gives it cohesion, adhesion, a high specific heat, expansion on freezing, and universal-solvent behavior — the properties that make cells, transpiration, and aquatic ecosystems possible.
Chemistry Is the Bridge Between Domain II and Domain III
Competency 011 ends with a descriptive statement asking teachers to "apply chemical principles" to biological structure and function. The exam uses that statement to build cross-domain items: a question about digestion is really about hydrolysis, and a question about why an enzyme stops working in a hot car is really about protein structure. Knowing the chemistry lets you answer both.
Organic Versus Inorganic
Organic compounds are built on carbon skeletons and almost always contain carbon-hydrogen bonds. Carbon is uniquely suited to this role because it forms four stable covalent bonds, allowing chains, branches, and rings of essentially unlimited length.
Inorganic compounds generally lack C-H bonds: water (H₂O), carbon dioxide (CO₂), table salt (NaCl), and mineral ions such as Ca²⁺, K⁺, and Fe²⁺. The common student error is equating "inorganic" with "unimportant to life." Water is inorganic and is the most abundant molecule in every cell; carbon dioxide is inorganic and is the carbon source for all photosynthesis.
The Four Biomolecule Families
| Family | Monomer | Elements | Primary functions | Examples |
|---|---|---|---|---|
| Carbohydrates | Monosaccharide (glucose, fructose) | C, H, O in roughly 1:2:1 | Quick energy; structural support | Glucose, sucrose, starch, glycogen, cellulose, chitin |
| Lipids | Glycerol + fatty acids (not a true polymer) | C, H, O with very little O | Long-term energy storage; membranes; insulation; hormones | Triglycerides, phospholipids, cholesterol, waxes |
| Proteins | Amino acid (20 kinds) | C, H, O, N, often S | Enzymes, structure, transport, defense, signaling | Hemoglobin, keratin, amylase, antibodies, insulin |
| Nucleic acids | Nucleotide | C, H, O, N, P | Store and transmit genetic information; energy currency | DNA, RNA, ATP |
Two element-based shortcuts are worth memorizing because items use them as identification clues: nitrogen signals a protein or nucleic acid, and phosphorus signals a nucleic acid (or a phospholipid or ATP). A molecule described as containing only C, H, and O in a 1:2:1 ratio is a carbohydrate.
Carbohydrates in More Detail
Monosaccharides (glucose, fructose, galactose) are single sugars. Disaccharides join two: sucrose = glucose + fructose; lactose = glucose + galactose. Polysaccharides are long chains with strikingly different functions depending on how the glucose units are linked:
- Starch — plant energy storage (potatoes, grains).
- Glycogen — animal energy storage, held in liver and muscle.
- Cellulose — plant cell wall structure; humans cannot digest it, which is why it functions as dietary fiber.
- Chitin — fungal cell walls and arthropod exoskeletons.
All four are built from glucose. That single fact is a strong "structure determines function" teaching example: identical monomers, different bonding geometry, completely different biological roles.
Lipids, Proteins, and Nucleic Acids
Lipids are nonpolar and therefore hydrophobic. Saturated fats have no carbon-carbon double bonds, pack tightly, and are solid at room temperature; unsaturated fats have one or more double bonds that kink the chain, so they remain liquid. Phospholipids have a hydrophilic phosphate head and hydrophobic tails, which is exactly why they self-assemble into the bilayer that forms every cell membrane.
Proteins are chains of amino acids joined by peptide bonds. Each amino acid has an amino group, a carboxyl group, and a variable R group; the sequence of R groups causes the chain to fold into a specific three-dimensional shape. Function follows that shape, which is the reason denaturation matters.
Nucleic acids are chains of nucleotides, each consisting of a five-carbon sugar, a phosphate group, and a nitrogenous base. DNA uses deoxyribose and the bases A, T, C, G; RNA uses ribose and substitutes U for T. ATP is a modified nucleotide that carries usable chemical energy in the bonds between its phosphate groups.
Building Up and Breaking Down
Dehydration synthesis (condensation) joins two monomers and releases one molecule of water per bond formed. Hydrolysis does the reverse: it adds a water molecule to split a bond.
Digestion is hydrolysis on a large scale. Salivary and pancreatic amylase hydrolyze starch to maltose and glucose; protease enzymes hydrolyze proteins to amino acids; lipase hydrolyzes triglycerides to glycerol and fatty acids. This is why the digestive system is described as breaking large molecules into small ones that can cross the intestinal lining — and why "hydro-lysis" literally means water-splitting.
Enzymes
An enzyme is a protein catalyst. It lowers the activation energy of a specific reaction and is not consumed, so a single enzyme molecule can process substrate repeatedly.
- Specificity. The enzyme's active site has a shape and chemistry complementary to its substrate, described by the induced-fit model. Lactase acts on lactose and not on sucrose.
- Temperature. Reaction rate rises with temperature up to an optimum (about 37 °C for most human enzymes), then falls sharply as the protein denatures — loses its three-dimensional shape.
- pH. Each enzyme has an optimal pH: pepsin works in the stomach near pH 2, while trypsin works in the small intestine near pH 8.
Denaturation explains everyday observations students can connect: a fried egg white turns opaque and solid because heat denatures its proteins irreversibly, and a fever above about 40 °C is dangerous because enzyme function degrades.
Water: The Inorganic Molecule Life Depends On
Water is a polar molecule — oxygen pulls electron density away from the hydrogens, giving the molecule partial negative and partial positive ends. Hydrogen bonding between water molecules produces five consequences tested across the exam:
| Property | Cause | Biological or earth-science consequence |
|---|---|---|
| Universal solvent | Polarity dissolves ionic and polar solutes | Blood, cytoplasm, and sap are water-based transport media |
| High specific heat | Hydrogen bonds absorb energy | Organisms and coastal climates resist rapid temperature swings |
| Cohesion and adhesion | Hydrogen bonds; attraction to polar surfaces | Water columns rise in xylem; surface tension supports insects |
| Expands on freezing | Crystal lattice holds molecules apart | Ice floats and insulates ponds so aquatic life survives winter |
| High heat of vaporization | Much energy needed to break bonds | Sweating and transpiration cool organisms efficiently |
A biochemist reports that an unknown molecule contains carbon, hydrogen, oxygen, nitrogen, and sulfur, and that heating it to 80 °C permanently destroys its function. Which biomolecule family is it most likely from, and why?
Starch, glycogen, and cellulose are all polymers of glucose, yet humans can digest the first two and not the third. What best explains the difference?
Ice floats on liquid water, allowing fish to survive under a frozen pond surface. Which property of water explains this, and what causes it?