3.1 Biological Molecules

Why Biomolecules Matter on the Praxis

Nearly every cellular process tested on the Praxis Biology exam (5235) traces back to a single class of macromolecules. Enzymes are proteins, genes are nucleic acids, membranes are lipids, and energy is stored in carbohydrates. If you can identify a molecule from a diagram or a structural clue, you can usually predict its function — and that is exactly how the exam writes its stems.

All four major macromolecules are polymers built from smaller monomers. Cells link monomers together by dehydration synthesis (a water molecule is released) and break them apart by hydrolysis (a water molecule is added). Knowing this single mechanism unlocks dozens of question variants.

Carbohydrates

Carbohydrates follow the general formula (CH2O)n — roughly one water per carbon, which is where the name carbo-hydrate comes from. They serve as quick-access energy and as structural material.

Glucose vs. fructose is a classic Praxis distractor. Both share the molecular formula C6H12O6, making them structural isomers. Glucose is an aldose (forms a six-membered pyranose ring); fructose is a ketose (forms a five-membered furanose ring). That ring difference explains why fructose tastes about twice as sweet as glucose despite having identical atoms.

Three polysaccharides made entirely of glucose still behave completely differently because of how the glucose units are linked:

• Starch (plants) — alpha-1,4 linkages, mostly unbranched (amylose) or lightly branched (amylopectin); easily digestible.
• Glycogen (animals) — alpha-1,4 with heavy alpha-1,6 branching; stored in liver and muscle.
• Cellulose (plant cell walls) — beta-1,4 linkages flip every other glucose, producing straight, hydrogen-bonded fibers that humans cannot digest because we lack cellulase.

Lipids

Lipids are defined by what they do not do — they do not dissolve in water — rather than by a common monomer. The exam focuses on three families.

• Triglycerides — one glycerol plus three fatty acids joined by ester bonds. Saturated fatty acids have only single C–C bonds and pack tightly (solid at room temperature; butter, lard). Unsaturated fatty acids contain one or more C=C double bonds that introduce kinks (liquid; olive oil, fish oil).
• Phospholipids — glycerol with two fatty acid tails and one phosphate-containing head. The polar head is hydrophilic and the nonpolar tails are hydrophobic, so phospholipids self-assemble into the lipid bilayer that defines every cell membrane.
• Steroids — four fused carbon rings. Cholesterol is the precursor for testosterone, estrogen, cortisol, and vitamin D, and it also modulates membrane fluidity.

Proteins

Proteins are polymers of amino acids linked by peptide bonds, which form between the carboxyl group of one amino acid and the amino group of the next, releasing water. There are 20 standard amino acids, each defined by a unique R-group (side chain) that may be polar, nonpolar, acidic, or basic.

Proteins fold in four hierarchical levels:

1. Primary — the linear amino-acid sequence (held by peptide bonds).
2. Secondary — local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds along the backbone.
3. Tertiary — the three-dimensional shape of a single polypeptide, driven by R-group interactions (disulfide bridges, hydrogen bonds, ionic bonds, hydrophobic clustering).
4. Quaternary — the assembly of two or more polypeptide subunits (e.g., hemoglobin's four globin chains).

Denaturation is the loss of higher-order structure (secondary, tertiary, quaternary) without breaking the peptide bonds of the primary sequence. Heat, extreme pH, heavy metals, and detergents are common denaturants. Because shape determines function, a denatured enzyme cannot catalyze its reaction.

Nucleic Acids

DNA and RNA are polymers of nucleotides, each containing a five-carbon sugar, a phosphate group, and a nitrogenous base.

The nitrogenous bases sort into two families:

• Purines — adenine (A) and guanine (G); two fused rings.
• Pyrimidines — cytosine (C), thymine (T), and uracil (U); single ring.

Chargaff's rules capture the base-pairing logic that Watson and Crick used: in any double-stranded DNA, A = T and G = C, so a purine always pairs with a pyrimidine. A–T pairs share two hydrogen bonds; G–C pairs share three, which is why GC-rich DNA has a higher melting temperature.

Water: The Solvent of Life

Water is not a macromolecule, but the Praxis routinely tests its emergent properties because they shape every reaction above. Water is polar — the oxygen pulls electrons more strongly than the hydrogens, producing a partial negative charge on O and partial positive charges on H. That polarity creates hydrogen bonds between molecules, and those bonds produce four exam-favorite properties:

• Cohesion — water sticks to itself, producing surface tension and pulling water columns up xylem.
• Adhesion — water sticks to other polar surfaces, enabling capillary action.
• High specific heat — hydrogen bonds resist temperature change, stabilizing cellular and oceanic temperatures.
• Universal solvent behavior — polar and ionic solutes dissolve readily; nonpolar (hydrophobic) molecules cluster away from water, which is the force that drives membrane formation.

Keep these four properties on a mental shortlist; nearly any free-response or multiple-choice item about water can be answered by naming the right one.