The Molecules of Cells: Carbohydrates, Lipids, Proteins, and Nucleic Acids

Comprehensive practice flashcards defining key terminology, functional groups, and structural concepts of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids).

Polymer

A long molecule consisting of many similar or identical building blocks linked by covalent bonds.

Monomer

A small repeating unit that serves as the building block of a polymer.

Dehydration Reaction

A chemical reaction in which two monomers become covalently bonded to each other with the simultaneous loss of a water molecule ($H_2O$).

Hydrolysis

A chemical reaction that breaks bonds between monomers in a polymer through the addition of a water molecule ($H_2O$).

Enzyme

A specialized biological macromolecule (almost always a protein) that acts as a catalyst to speed up chemical reactions without being consumed.

Isomer

Compounds that possess the same molecular formula but have different structural arrangements of atoms, resulting in distinct physical and chemical properties.

Hydroxyl Group

A chemical functional group consisting of a hydrogen atom bonded to an oxygen atom ($-OH$), which imparts polarity and makes organic molecules hydrophilic alcohols.

Carbonyl Group

A chemical group consisting of a carbon atom linked by a double bond to an oxygen atom ($C=O$), found in aldoses when at the end of a skeleton and ketoses when within a skeleton.

Carboxyl Group

A functional group consisting of a carbon atom double-bonded to an oxygen and single-bonded to a hydroxyl group ($-COOH$); it acts as an acid because it donates an $H^+$ in aqueous solution.

Amino Group

A functional group consisting of a nitrogen atom bonded to two hydrogen atoms ($-NH_2$); it acts as a base by picking up an $H^+$ from solution to form $-NH_3^+$ in ionized form.

Phosphate Group

A functional group consisting of a phosphorus atom bonded to four oxygen atoms ($-OPO_3^{2-}$), imparting a negative charge and participating in cellular energy transfers such as in ATP.

Methyl Group

A nonpolar, hydrophobic functional group consisting of a carbon atom bonded to three hydrogen atoms ($-CH_3$), commonly involved in chemical modifications that affect molecular shape.

Monosaccharide

The simplest carbohydrate monomer, possessing molecular formulas that are generally multiples of $CH_2O$ (such as glucose, $C_6H_{12}O_6$).

Disaccharide

A double sugar formed when two monosaccharides are joined by a dehydration reaction through a covalent glycosidic linkage (e.g., maltose, sucrose, and lactose).

Glycosidic Linkage

A covalent bond formed between two monosaccharides by a dehydration reaction, such as the 1–4 linkage in maltose or the 1–2 linkage in sucrose.

Starch

A plant storage polysaccharide consisting entirely of glucose monomers linked by $\alpha$ glycosidic linkages; unbranched forms are known as amylose, while branched forms are known as amylopectin.

Glycogen

An extensively branched animal storage polysaccharide composed of glucose monomers joined by $\alpha\text{ 1-4}$ and $\alpha\text{ 1-6}$ glycosidic linkages, stored predominantly in liver and muscle cells.

Cellulose

An unbranched structural polysaccharide composed of glucose monomers connected by $\beta\text{ 1-4}$ glycosidic linkages that form hydrogen-bonded parallel microfibrils in plant cell walls.

Chitin

A structural polysaccharide found in arthropod exoskeletons and fungal cell walls, built from modified glucose monomers carrying a nitrogen-containing appendage.

Triacylglycerol

A fat molecule constructed from three fatty acid molecules covalently joined to one glycerol molecule by ester linkages.

Ester Linkage

A covalent bond formed between the hydroxyl group of glycerol and the carboxyl group of a fatty acid via a dehydration reaction.

Saturated Fatty Acid

A fatty acid hydrocarbon chain containing no double bonds, maximizing the number of hydrogen atoms bound to carbons and resulting in a straight chain that is solid at room temperature.

Unsaturated Fatty Acid

A fatty acid whose hydrocarbon chain contains one or more double bonds, often creating a kink due to a cis arrangement that prevents tight packing and keeps it liquid at room temperature.

Trans Fat

An unsaturated fat containing trans double bonds produced artificially by the industrial hydrogenation of vegetable oils, associated with an elevated risk of cardiovascular disease.

Phospholipid

A lipid composed of a glycerol backbone bonded to two hydrophobic fatty acid tails and one hydrophilic phosphate head, forming bilayers that construct biological membranes.

Steroid

A lipid characterized by a hydrophobic carbon skeleton made of four fused rings with attached chemical groups (e.g., cholesterol, testosterone, and estradiol).

Cholesterol

A crucial steroid that acts as a membrane fluidity buffer in animal cells and serves as the precursor from which other steroids, including sex hormones, are synthesized.

Amino Acid

An organic monomer containing a central $\alpha$ carbon bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain ($R$ group).

Peptide Bond

A covalent bond formed between the carboxyl group of one amino acid and the amino group of an adjacent amino acid through a dehydration reaction.

Primary Structure

The precise, linear sequence of amino acids in a polypeptide chain, encoded by inherited genetic information and maintained exclusively by covalent peptide bonds.

Secondary Structure

Repetitive coiling or folding in a polypeptide chain ($\alpha$ helices or $\beta$ pleated sheets) maintained exclusively by hydrogen bonds between constituents of the polypeptide backbone.

$\alpha$ Helix

A delicate coil-shaped secondary protein structure stabilized by hydrogen bonding between the backbone oxygen and hydrogen atoms of every fourth amino acid.

$\beta$ Pleated Sheet

A sheet-like secondary protein structure stabilized by hydrogen bonding between parallel or antiparallel segments of the polypeptide backbone.

Tertiary Structure

The overall three-dimensional shape of a polypeptide, stabilized by interactions among amino acid $R$ groups, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

Disulfide Bridge

A strong covalent bond formed when the sulfhydryl groups ($-SH$) of two cysteine monomers oxidize to cross-link their sulfur atoms ($-S-S-$).

Quaternary Structure

The overall functional protein structure formed by the association of two or more individual polypeptide chains (e.g., hemoglobin or collagen).

Denaturation

The process by which a protein loses its native shape and becomes biologically inactive due to the disruption of weak chemical interactions caused by changes in pH, salt concentration, or temperature.

Chaperonin

A specialized protein complex consisting of a hollow cylinder and cap that provides a hydrophilic folding environment to shield newly synthesized polypeptides from misfolding.

Gene Expression

The cellular process whereby genetic information encoded in DNA is transcribed into mRNA, which is subsequently translated into a functional polypeptide ($\text{DNA} \rightarrow \text{RNA} \rightarrow \text{protein}$).

Nucleotide

The monomer of a nucleic acid, consisting of a nitrogenous base, a five-carbon pentose sugar, and one or more phosphate groups.

Nucleoside

The structural portion of a nucleotide that consists only of a nitrogenous base covalently linked to a pentose sugar, lacking any phosphate group.

Pyrimidine

A family of nitrogenous bases characterized by a single six-membered ring; includes cytosine (C), thymine (T), and uracil (U).

Purine

A family of nitrogenous bases characterized by a six-membered ring fused to a five-membered ring; includes adenine (A) and guanine (G).

Phosphodiester Linkage

The covalent link in a polynucleotide that joins the phosphate group attached to the $5'$ carbon of one sugar to the hydroxyl group on the $3'$ carbon of the neighboring sugar.

Antiparallel

The spatial arrangement of the two sugar-phosphate backbones in a DNA double helix, where they run parallel to each other but in opposite orientations (one $5' \rightarrow 3'$ and the other $3' \rightarrow 5'$).

Complementary Base Pairing

The predictable hydrogen-bonding rule between nitrogenous bases across polynucleotide strands, where adenine (A) pairs with thymine (T) via two hydrogen bonds and guanine (G) pairs with cytosine (C) via three hydrogen bonds.

Genomics

The scientific discipline and approach that sequences, analyzes, and compares whole sets of genes or entire genomes among various organisms and species.

Proteomics

The large-scale, systematic study and analysis of the complete set of proteins expressed by an organism or genome, including their amino acid sequences, structures, and activities.

Lactase

A digestive enzyme that breaks down lactose into glucose and galactose; regulatory mutations keeping its gene active in adults evolved as a recent adaptation to dairy farming.