AP Biology: Macromolecules – Structure and Function (Carbs, Lipids, Proteins, Nucleic Acids)

Macromolecules – Structure and Function (Carbs, Lipids, Proteins, Nucleic Acids)

Concept Summary

• Macromolecules: Large organic polymers essential for life, built from smaller monomers via dehydration synthesis and broken down by hydrolysis.
• Carbohydrates: Sugars and starches (C:H:O = 1:2:1) serving as quick energy (monosaccharides) and structural support (polysaccharides like cellulose).
• Lipids: Hydrophobic molecules (fats, phospholipids, steroids) storing long-term energy, forming membranes, and acting as hormones.
• Proteins: Polymers of amino acids folded into 3D shapes, performing functions like catalysis (enzymes), transport (hemoglobin), and structure (collagen).
• Nucleic Acids: DNA/RNA polymers of nucleotides storing/transmitting genetic information and enabling protein synthesis.

Core Questions

WHAT (definitional)

Q: What is a monomer? A: A small, repeating subunit that bonds to form polymers (e.g., glucose for starch, amino acids for proteins). Trap/Clarification: Monomers-functional units; polymers (e.g., enzymes) perform biological roles.

Q: What is a glycosidic linkage? A: A covalent bond between monosaccharides (e.g., glucose + fructose = sucrose) formed via dehydration synthesis. Trap/Clarification: Not all carbs have the same linkage (e.g., ?-1,4 in starch vs. ?-1,4 in cellulose).

Q: What is denaturation? A: Loss of a protein’s 3D shape (and function) due to heat, pH, or salinity disrupting hydrogen/ionic bonds. Trap/Clarification: Primary structure (peptide bonds) remains intact; only secondary/tertiary/quaternary levels unfold.

WHY (causal/explanatory)

Q: Why are phospholipids amphipathic? A: Their hydrophilic phosphate heads and hydrophobic fatty acid tails enable spontaneous bilayer formation in aqueous environments (cell membranes). Trap/Clarification: Amphipathic-soluble; phospholipids form micelles/bilayers, not true solutions.

Q: Why is cellulose indigestible by humans? A: Humans lack cellulase enzymes to break ?-1,4 glycosidic bonds in cellulose; cows use gut microbes for this. Trap/Clarification: Cellulose is a carb, but its structure (linear, H-bonded fibers) makes it indigestible, not its composition.

Q: Why do unsaturated fats stay liquid at room temperature? A: Kinked fatty acid tails (from C=C double bonds) prevent tight packing, reducing van der Waals forces. Trap/Clarification: "Unsaturated" refers to double bonds, not fewer hydrogens (though that’s a consequence).

HOW (process/application)

Q: How do you identify a reducing sugar? A: Use Benedict’s reagent (blue-red precipitate) if the sugar has a free aldehyde/ketone group (e.g., glucose, fructose; not sucrose). Trap/Clarification: Sucrose is non-reducing because its glycosidic bond locks both anomeric carbons.

Q: How is protein primary structure determined? A: Amino acid sequence is encoded by DNA-transcribed to mRNA-translated by ribosomes (peptide bonds form between amino/carboxyl groups). Trap/Clarification: Primary structure-3D shape; it’s just the linear sequence.

Q: How do you calculate the number of peptide bonds in a protein? A: # peptide bonds = # amino acids – 1 (e.g., 100 amino acids = 99 bonds). Trap/Clarification: Count bonds, not amino acids; the first amino acid has a free amino group.

CAN (conditions/possibilities)

Q: Can lipids dissolve in water? A: No; lipids are nonpolar/hydrophobic, but phospholipids can form emulsions (e.g., micelles) in water. Trap/Clarification: "Dissolve"-"form structures"; lipids don’t dissolve but can self-assemble.

Q: Under what conditions does a protein refold after denaturation? A: Only if the primary structure is intact and conditions (pH, temperature) return to optimal; chaperonins may assist. Trap/Clarification: Not all proteins refold; some aggregate irreversibly (e.g., cooked egg whites).

Quick Facts & Traps

• Fact: Chitin is a structural polysaccharide in fungi/exoskeletons (?-1,4 N-acetylglucosamine linkages).
• Trap: "All carbs are sugars"-Reality: Polysaccharides (e.g., starch) are polymers, not sweet or soluble like monosaccharides.
• Fact: Steroids (e.g., cholesterol) are lipids with 4 fused carbon rings; they’re hydrophobic but not fats.
• Trap: "Saturated fats are unhealthy"-Reality: Trans fats (artificial unsaturated fats) are worse; saturated fats are natural but overconsumed.
• Fact: Peptide bonds form via dehydration synthesis between amino acids (C-N bond); resonance makes them planar.
• Trap: "DNA is the only nucleic acid"-Reality: RNA (single-stranded, uracil, ribose) is critical for protein synthesis and regulation.

Rapid-Fire True/False

• Statement: All proteins have quaternary structure. Answer: FALSE Why the common mistake happens: Students confuse potential for requirement; only multi-subunit proteins (e.g., hemoglobin) have quaternary structure.

• Statement: Starch and glycogen are both ?-glucose polymers, but glycogen is more branched. Answer: TRUE Why the common mistake happens: Students assume all glucose polymers are identical; branching affects solubility and energy release speed.

• Statement: A triglyceride with 3 unsaturated fatty acids is called a "polyunsaturated fat." Answer: FALSE Why the common mistake happens: "Polyunsaturated" refers to multiple double bonds in one fatty acid, not the number of unsaturated fatty acids in a triglyceride.