## Learning Outcomes

- Identify the major biologically relevant molecule classes: amino acids/proteins, carbohydrates, lipids, and nucleic acids.
- Relate functional groups to charge, polarity, solubility, acidity/basicity, and reactivity.
- Explain how dehydration and hydrolysis reactions form and break peptide, glycosidic, ester, and phosphodiester bonds.
- Distinguish structural isomers, stereoisomers, enantiomers, diastereomers, anomers, and epimers in biological contexts.
- Predict how molecular structure affects biological function, including protein folding, membrane formation, and nucleic acid base pairing.
- Apply MCAT-style reasoning to passage data involving biomolecule charge, separation, reactivity, and experimental modification.

## Medical College Admission Test (MCAT) Syllabus

In the Chemical and Physical Foundations of Biological Systems section, you have 59 questions in 95 minutes, with both passage-based and discrete questions. This section draws heavily from chemical principles applied to living systems; general chemistry, organic chemistry, biochemistry, biology, and physics all appear. Foundational Concept 5 accounts for a large fraction of the Chem/Phys section, and subtopic 5D focuses on how the structures of biomolecules explain their functions and chemical behavior.

- Amino acids, peptides, and proteins: side-chain chemistry, charge, peptide bonds, folding, and denaturation.
- Carbohydrates: monosaccharides, stereochemistry, ring formation, glycosidic bonds, and polysaccharide function.
- Lipids: fatty acids, triacylglycerols, phospholipids, steroids, amphipathic structure, and membrane behavior.
- Nucleotides and nucleic acids: bases, sugars, phosphates, phosphodiester bonds, polarity, and base pairing.
- Functional groups in biological molecules: alcohols, amines, carboxylic acids, thiols, phosphates, esters, amides, aldehydes, and ketones.
- Isomerism and stereochemistry: structural isomers, conformational isomers, configurational isomers, chirality, optical activity, and biological specificity.
- Reactivity patterns: condensation, hydrolysis, oxidation-reduction, phosphorylation, acyl transfer, and nucleophilic attack.
- Links between structure and function in enzymes, receptors, cytoskeletal proteins, extracellular matrix proteins, membranes, and genetic material.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What type of bond links amino acids in a protein, and why is this bond relatively planar?
2. How do enantiomers differ from diastereomers, and why does chirality matter in biological systems?
3. What structural feature makes phospholipids form bilayers in water?
4. Which bonds are hydrolyzed when DNA, a protein, and glycogen are broken into monomers?
5. Why can two molecules with the same molecular formula have different melting points, solubilities, or biological effects?

## Introduction

Biological molecules are organic and inorganic chemical systems shaped by the same principles tested across MCAT chemistry: bonding, polarity, acid-base behavior, stereochemistry, thermodynamics, and reaction mechanisms. Subtopic 5D asks you to connect these principles to molecules found in living systems. Rather than memorizing biomolecules as isolated facts, focus on how structure predicts function and reactivity. A phosphate group usually adds negative charge and water solubility; a long hydrocarbon chain adds hydrophobic character; an amide bond is less reactive than an ester because of resonance stabilization.

The MCAT often tests these ideas through passages involving enzymes, protein purification, nucleic acid experiments, carbohydrate metabolism, lipid membranes, or drug-receptor binding. You may be asked to identify an isomer relationship, predict charge at a given pH, infer whether a molecule crosses a membrane, or recognize which bond is formed or broken in a reaction.

> **Key Term: biomolecule**
> A molecule produced by, used by, or closely associated with living systems, including proteins, carbohydrates, lipids, nucleic acids, and their smaller building blocks.

### Functional Groups and Isomerism in Biological Molecules

Functional groups determine much of a biomolecule’s chemical behavior. Physical properties, such as melting point, boiling point, solubility, color, odor, and density, do not require changing the chemical composition of matter. Chemical properties describe reactivity and do involve changes in composition. On the MCAT, this distinction matters because a passage may ask whether a difference between two isomers is due to physical properties, chemical reactivity, or both.

Common biological functional groups include:

- Carboxylic acids, which can lose a proton and become negatively charged carboxylates.
- Amines, which can accept a proton and become positively charged ammonium groups.
- Alcohols, which hydrogen bond and can form esters or glycosidic bonds.
- Thiols, which can form disulfide bonds in proteins.
- Phosphates, which often carry negative charge and participate in energy transfer and nucleic acid structure.
- Aldehydes and ketones, which appear in sugars and can react with nucleophiles.
- Esters and amides, which are common linkage groups in lipids and proteins.

> **Key Term: functional group**
> A specific atom or group of atoms within a molecule that gives that molecule characteristic physical and chemical properties.

Isomerism is especially high yield because many biological molecules have the same formula but different structures or spatial arrangements. Isomers have the same molecular formula but different structures. Structural, or constitutional, isomers differ in connectivity. For example, glucose and fructose both have the formula $$C_6H_{12}O_6$$, but glucose is an aldose and fructose is a ketose. Because their atoms are connected differently, they have different chemical properties.

Stereoisomers have the same connectivity but differ in three-dimensional arrangement. Enantiomers are non-superimposable mirror images, while diastereomers are stereoisomers that are not mirror images. In biology, stereochemistry is not a minor detail: enzymes, receptors, transporters, and antibodies bind molecules in three-dimensional active sites. One stereoisomer of a drug may bind strongly to a receptor while another may be inactive or harmful.

> **Key Term: isomer**
> One of two or more compounds with the same molecular formula but different structures or spatial arrangements.
>
> **Key Term: stereoisomer**
> An isomer with the same atom-to-atom connectivity as another molecule but a different three-dimensional arrangement.

Carbohydrates provide several common MCAT stereochemistry examples. Epimers differ at exactly one chiral carbon; glucose and galactose differ at carbon 4, while glucose and mannose differ at carbon 2. Anomers are a special type of epimer found in cyclic sugars; they differ at the anomeric carbon formed when the linear sugar cyclizes. For D-glucose, the alpha anomer has the anomeric hydroxyl trans to the $$CH_2OH$$ group, while the beta anomer has it cis in the common Haworth depiction.

> **Test Tip:**
> When asked whether two biological molecules are structural isomers or stereoisomers, first compare connectivity. If connectivity differs, they are structural isomers. Only consider enantiomers, diastereomers, epimers, or anomers after confirming the same connectivity.

### Amino Acids, Peptides, and Protein Structure

Amino acids contain an amino group, a carboxyl group, a hydrogen atom, and a variable side chain attached to the alpha carbon. With the exception of glycine, standard amino acids are chiral. In proteins, amino acids are usually in the L-configuration. MCAT questions often require side-chain classification: nonpolar, polar uncharged, acidic, basic, aromatic, sulfur-containing, or special structural cases such as proline and glycine.

Amino acids are amphoteric: they can act as acids or bases. At physiological pH, the amino group is usually protonated and the carboxyl group is usually deprotonated, giving a zwitterion. Side chains determine the net charge. Aspartate and glutamate are typically negatively charged near physiological pH; lysine and arginine are positively charged; histidine has a side-chain pKa near physiological pH and often appears in enzyme active sites because it can donate or accept protons.

> **Key Term: zwitterion**
> A molecule with both positive and negative formal charges but an overall net charge that may be zero.

A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing water. This is a condensation reaction. The reverse process, peptide bond hydrolysis, uses water to break the bond. Peptide bonds are amide bonds and have partial double-bond character because the nitrogen lone pair can delocalize into the carbonyl. This resonance restricts rotation and makes the peptide bond relatively planar.

> **Key Term: peptide bond**
> The amide linkage joining amino acids in a peptide or protein, formed by condensation between a carboxyl group and an amino group.

Protein structure is described at several levels:

- Primary structure: amino acid sequence from N-terminus to C-terminus.
- Secondary structure: local folding patterns such as alpha helices and beta sheets, stabilized mainly by backbone hydrogen bonding.
- Tertiary structure: overall three-dimensional folding of one polypeptide, stabilized by hydrophobic effects, hydrogen bonds, ionic interactions, van der Waals forces, and disulfide bonds.
- Quaternary structure: association of multiple polypeptide subunits.

Structure explains function. Collagen, elastin, keratin, actin, and tubulin are structural proteins with repeated patterns that suit their roles in tissues or the cytoskeleton. Collagen provides tensile strength in connective tissue; elastin allows recoil; keratin contributes toughness in hair, skin, and nails; actin and tubulin help form cytoskeletal networks. Motor proteins and binding proteins also depend on precise shape and chemical complementarity.

Denaturation disrupts secondary, tertiary, or quaternary structure without necessarily breaking primary peptide bonds. Heat, extreme pH, detergents, organic solvents, or reducing agents can denature proteins. If disulfide bonds are reduced, covalent cross-links between cysteine residues are broken.

> **Exam Warning:**
> Denaturation usually does not hydrolyze peptide bonds. A denatured protein may lose function because it loses shape, even though its amino acid sequence remains intact.

A useful MCAT skill is predicting protein charge. If pH is below a group’s pKa, that group tends to be protonated. If pH is above pKa, it tends to be deprotonated. For acidic groups, protonation gives a neutral carboxylic acid; deprotonation gives a negative carboxylate. For basic groups, protonation gives a positive ammonium group; deprotonation gives a neutral amine.

For an amino acid with no ionizable side chain, the isoelectric point can be estimated as the average of the two pKa values surrounding the neutral zwitterion:

$$pI=\frac{pK_{a,\text{carboxyl}}+pK_{a,\text{amino}}}{2}$$

For acidic or basic amino acids, use the two pKa values that border the neutral form.

### Carbohydrates, Lipids, and Membrane-Relevant Structure

Carbohydrates contain multiple hydroxyl groups and either an aldehyde or ketone in their open-chain form. Monosaccharides can cyclize when a hydroxyl group attacks the carbonyl carbon, forming a hemiacetal or hemiketal. This creates the anomeric carbon, a major site of glycosidic bond formation.

> **Key Term: glycosidic bond**
> A covalent linkage between a sugar’s anomeric carbon and another group, often another sugar, formed through condensation.

Glycosidic bonds determine polysaccharide function. Alpha linkages in starch and glycogen are readily used for energy storage in humans. Glycogen is highly branched, allowing rapid glucose release from multiple ends. Cellulose contains beta-1,4 linkages between glucose units; humans lack the digestive enzyme needed to hydrolyze these bonds efficiently, so cellulose functions as dietary fiber rather than a major energy source.

MCAT carbohydrate questions often ask you to track reducing ends. A reducing sugar has a free anomeric carbon that can open to the aldehyde or ketone form and reduce mild oxidizing agents. If the anomeric carbon is locked in a glycosidic bond, it is nonreducing at that end.

Lipids are defined more by hydrophobicity than by one single structural pattern. Fatty acids contain a long hydrocarbon chain and a carboxyl group. Saturated fatty acids have no carbon-carbon double bonds; unsaturated fatty acids have one or more double bonds, often cis in natural lipids. Cis double bonds introduce kinks, reduce tight packing, and lower melting point. This is why unsaturated fats tend to be more fluid than saturated fats.

Triacylglycerols contain three fatty acids esterified to glycerol and function in energy storage. Phospholipids contain hydrophobic fatty acid tails and a polar phosphate-containing head group. This dual character drives phospholipids to form bilayers in water: hydrophilic heads face aqueous environments, and hydrophobic tails cluster away from water.

> **Key Term: amphipathic molecule**
> A molecule with both hydrophilic and hydrophobic regions, such as a phospholipid.

Membrane fluidity depends on lipid composition. Shorter fatty acid chains and more cis double bonds increase fluidity. Cholesterol can buffer membrane fluidity: it restrains phospholipid movement at higher temperatures but prevents tight packing at lower temperatures.

Steroids have four fused rings and are largely hydrophobic, though specific substituents can add polar character. Cholesterol is the parent molecule for steroid hormones and also affects membrane properties. Because steroid hormones are lipid-soluble, they can cross membranes and bind intracellular receptors more readily than peptide hormones.

> **Test Tip:**
> For membrane permeability questions, rank small nonpolar molecules as most permeable, followed by small uncharged polar molecules. Large polar molecules and ions generally need channels, carriers, or vesicular transport.

### Nucleic Acids, Phosphates, and Reaction Patterns

Nucleotides contain three parts: a nitrogenous base, a pentose sugar, and one or more phosphate groups. Nucleosides contain only the base and sugar. DNA uses deoxyribose and the bases A, G, C, and T; RNA uses ribose and replaces thymine with uracil. Purines, adenine and guanine, have two rings. Pyrimidines, cytosine, thymine, and uracil, have one ring.

Nucleic acid strands are joined by phosphodiester bonds between the 3′ hydroxyl of one sugar and the 5′ phosphate of the next nucleotide. This gives nucleic acids directionality: sequences are written 5′ to 3′. The sugar-phosphate backbone is negatively charged because phosphate groups are deprotonated under physiological conditions.

> **Key Term: phosphodiester bond**
> A covalent linkage in nucleic acids connecting the 3′ hydroxyl of one nucleotide sugar to the 5′ phosphate of the next nucleotide.

Base pairing depends on hydrogen bonding and shape complementarity. In DNA, adenine pairs with thymine, and guanine pairs with cytosine. G-C pairs have three hydrogen bonds, while A-T pairs have two. As a result, DNA with higher G-C content usually has a higher melting temperature, all else equal. Base stacking interactions also contribute to double-helix stability.

Several reaction patterns recur across biomolecules:

- Condensation forms larger molecules by releasing water, as in peptide, glycosidic, ester, and phosphodiester bond formation.
- Hydrolysis breaks those bonds by adding water.
- Phosphorylation transfers a phosphate group, often changing charge, shape, or activity.
- Oxidation-reduction changes electron density; oxidation commonly increases bonds to oxygen or decreases bonds to hydrogen in organic molecules.
- Acyl transfer moves a carbonyl-containing group, common in metabolism and lipid chemistry.
- Nucleophilic attack occurs when an electron-rich species attacks an electron-poor center, such as a carbonyl carbon or phosphorus atom.

Adenosine triphosphate, ATP, links structure to function through its phosphate groups. Hydrolysis of ATP is favorable because products are stabilized by resonance, reduced electrostatic repulsion, and improved solvation. In biological systems, ATP hydrolysis is often coupled to nonspontaneous processes such as biosynthesis, transport, or mechanical work.

> **Exam Warning:**
> Do not say ATP “stores energy in high-energy bonds” as if breaking a bond releases energy by itself. Bond breaking requires energy; the overall hydrolysis is favorable because forming the products and solvating them gives a net decrease in free energy.

Nucleic acids also highlight the link between structure and separation methods. Because DNA has a negatively charged phosphate backbone, it migrates toward the positive electrode during gel electrophoresis. Smaller DNA fragments generally move farther through a gel. Proteins are more variable because their charge depends on amino acid composition and pH; SDS-PAGE gives proteins a roughly uniform negative charge so separation depends mainly on size.

## Key Point Checklist

This article has covered the following key knowledge points:

- Biomolecule structure predicts function through functional groups, shape, charge, polarity, and intermolecular interactions.
- Physical properties do not change chemical composition; chemical properties describe reactivity and changes in composition.
- Structural isomers have the same molecular formula but different connectivity; stereoisomers have the same connectivity but different three-dimensional arrangement.
- Biological systems are highly stereospecific because enzymes, receptors, transporters, and binding proteins have three-dimensional binding sites.
- Amino acids form peptide bonds through condensation; peptide bonds are planar because of amide resonance.
- Protein folding depends on hydrophobic effects, hydrogen bonding, ionic interactions, van der Waals forces, and disulfide bonds.
- Denaturation disrupts protein shape and function but usually does not break peptide bonds.
- Carbohydrates form cyclic hemiacetals or hemiketals and link through glycosidic bonds at anomeric carbons.
- Phospholipids are amphipathic and form bilayers because hydrophobic tails avoid water while polar heads interact with water.
- Nucleic acids have negatively charged sugar-phosphate backbones joined by phosphodiester bonds and are read 5′ to 3′.
- Condensation and hydrolysis reactions are recurring patterns in proteins, carbohydrates, lipids, and nucleic acids.
- ATP hydrolysis is favorable because the products are more stable overall, not because bond breaking alone releases energy.

## Key Terms and Concepts

- biomolecule
- functional group
- isomer
- stereoisomer
- zwitterion
- peptide bond
- glycosidic bond
- amphipathic molecule
- phosphodiester bond


Foundational concept 5: Chemical principles - 5D: Structure, function, and reactivity of biologically relevant molecules