Key Biological Molecules

Why This Matters

Every cell in every organism depends on a handful of molecular types to store energy, build structures, transmit information, and regulate processes. On the AP Biology exam, you're being tested on how these molecules connect to larger themes: energy transfer, structure-function relationships, information flow, and homeostasis. The College Board wants you to explain why a phospholipid bilayer is selectively permeable, how ATP powers active transport, and what makes water essential for life—not just recite definitions.

Don't just memorize that proteins are made of amino acids. Know that their three-dimensional structure determines function, that enzymes lower activation energy to drive metabolism, and that membrane proteins enable everything from facilitated diffusion to the Na⁺/K⁺ pump. When you understand the underlying principles, you can tackle any FRQ that asks you to connect molecular structure to biological outcomes.

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Energy Storage and Transfer Molecules

Living systems require constant energy input to maintain order. These molecules capture, store, and release energy in forms cells can use—a concept central to cellular respiration and photosynthesis.

Carbohydrates

• Primary energy source—glucose ($C_6H_{12}O_6$) provides immediate fuel through glycolysis and cellular respiration
• Storage polymers differ by organism: starch in plants, glycogen in animals, both easily hydrolyzed to release glucose
• Structural roles include cellulose in plant cell walls, demonstrating how monomer arrangement determines function

Lipids

• Long-term energy storage—lipids yield more than twice the energy per gram compared to carbohydrates due to their high proportion of C–H bonds
• Phospholipids form the hydrophobic core of membranes, creating selective permeability essential for cellular compartmentalization
• Saturated vs. unsaturated fatty acid tails affect membrane fluidity; unsaturated tails with kinks prevent tight packing

ATP (Adenosine Triphosphate)

• Energy currency of the cell—hydrolysis of the terminal phosphate bond releases energy to power cellular work
• Regenerated continuously through oxidative phosphorylation in mitochondria and substrate-level phosphorylation in glycolysis
• Powers active transport, including the Na⁺/K⁺-ATPase that maintains membrane potential and electrochemical gradients

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Information-Carrying Molecules

Genetic information must be stored reliably and expressed precisely. Nucleic acids handle storage and transmission, while the genetic code connects DNA sequence to protein function.

Nucleic Acids

• DNA stores genetic information using a double-helix structure stabilized by hydrogen bonds between complementary base pairs (A-T, G-C)
• RNA mediates gene expression—mRNA carries instructions, tRNA delivers amino acids, rRNA forms ribosome structure
• Nucleotide structure includes a sugar, phosphate group, and nitrogenous base; the sequence of bases encodes the genetic code

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Structural and Functional Proteins

Proteins perform more diverse functions than any other macromolecule class. Their structure-function relationship—from primary sequence to three-dimensional shape—is a core AP Biology concept.

Proteins

• Amino acid sequence determines structure—the R-groups of amino acids dictate folding patterns and ultimately protein function
• Diverse roles include structural support (collagen), transport (hemoglobin), immune defense (antibodies), and membrane receptors
• Peptide bonds link amino acids through dehydration synthesis; protein shape can be disrupted by changes in pH or temperature (denaturation)

Enzymes

• Biological catalysts that lower activation energy, dramatically increasing reaction rates without being consumed
• Substrate specificity results from the precise fit between enzyme active site and substrate (induced fit model)
• Regulated by environmental factors—temperature, pH, and inhibitors affect enzyme shape and function, connecting to homeostasis

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Water and Its Unique Properties

Water's molecular structure creates emergent properties essential for life. The AP exam frequently tests how hydrogen bonding explains water's biological roles.

Water

• Polarity and hydrogen bonding—oxygen's electronegativity creates partial charges ($\delta^-$ on O, $\delta^+$ on H), enabling hydrogen bonds between molecules
• High specific heat buffers temperature changes in cells and ecosystems; high heat of vaporization enables evaporative cooling
• Cohesion and adhesion drive capillary action and the cohesion-tension theory of water transport in xylem

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Signaling and Regulatory Molecules

Cells must communicate and respond to changing conditions. These molecules transmit signals and fine-tune metabolic pathways to maintain homeostasis.

Hormones

• Chemical messengers that travel through blood to target cells, binding to specific receptors to trigger responses
• Two major classes: lipid-soluble steroid hormones cross membranes to bind intracellular receptors; water-soluble peptide hormones bind surface receptors
• Feedback loops regulate hormone release—insulin lowers blood glucose, while glucagon raises it, maintaining homeostasis

Vitamins

• Organic coenzymes or cofactors required in small amounts to assist enzyme function in metabolic pathways
• Water-soluble vitamins (B complex, C) are not stored and must be consumed regularly; fat-soluble vitamins (A, D, E, K) accumulate in tissues
• Deficiencies impair specific pathways—scurvy results from vitamin C deficiency affecting collagen synthesis

Minerals

• Inorganic cofactors essential for enzyme activity and structural components like bone (calcium, phosphorus)
• Electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻) maintain electrochemical gradients critical for nerve impulses and muscle contraction
• Trace elements like iron (hemoglobin) and zinc (enzyme function) are required in small but essential amounts

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Quick Reference Table

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Self-Check Questions

1. Which two molecules both store energy but differ in their accessibility and energy density? Explain why organisms maintain both types.

2. How does the structure of a phospholipid relate to the selective permeability of cell membranes? Connect your answer to the movement of specific molecule types.

3. Compare and contrast DNA and RNA in terms of structure, stability, and function in gene expression.

4. An FRQ asks you to explain how water's properties support transpiration in plants. Which specific properties would you discuss, and how do they work together?

5. A mutation changes one amino acid in an enzyme's active site. Using your understanding of protein structure-function relationships, predict and explain the possible effects on enzyme activity.