Molecules of Life, Cellular Architecture, and Genetics

A comprehensive vocabulary review of biochemistry, macromolecular structure, cellular respiration, organelle trafficking, cytoskeletal transport, and eukaryotic genetics.

Monosaccharide

A simple sugar consisting of a single ring with one oxygen atom, such as glucose, fructose, or galactose ($\text{C}_6\text{H}_{12}\text{O}_6$), which generally tastes sweet.

Disaccharide

A complex sugar formed by two bonded sugar rings, including lactose (galactose and glucose), sucrose (glucose and fructose), and maltose (glucose and glucose).

Polysaccharide

A complex carbohydrate composed of many linked sugar rings—such as starch and glycogen used for energy storage, or structural fiber—that does not taste sweet.

Triglyceride

A lipid molecule consisting of a glycerol backbone bound to three fatty acid chains, utilized primarily by the body for fat and energy storage.

Phospholipid

A lipid molecule containing a phosphate head and two fatty acid tails that self-assembles into bilayers to form cell membranes by sequestering hydrophobic tails away from water.

Cholesterol

A basic steroid lipid that serves as a precursor to steroid hormones (such as testosterone, estrogens, and cortisol) and stabilizes cell membranes by filling gaps between crooked phospholipids to prevent freezing or melting.

Saturated Fatty Acid

A fatty acid chain lacking double bonds where carbon atoms are fully bonded to hydrogen atoms; typically found in animal fats (dairy, meat) and some plant fats (coconut, palm), and usually solid at room temperature.

Unsaturated Fatty Acid

A fatty acid possessing at least one carbon-carbon double bond; commonly found in fish oils, plant oils, seeds, and nuts, and typically liquid at room temperature.

Partial Hydrogenation

An industrial process where hydrogens are forced into heated oils to break double bonds, causing them to reform randomly into trans configurations rather than natural cis configurations to increase shelf life and stability.

Trans Fat

An unsaturated lipid with trans-configured double bonds stepping across like a staircase; stacks easily in arteries, cannot be efficiently broken down by digestive enzymes, and induces cellular damage and inflammation.

Amino Acid

The monomer building block of proteins consisting of a common backbone and one of $20$ variable $\text{R}$ groups that determine its specific chemical properties.

Peptide Bond

The covalent chemical bond formed between the carboxyl group of one amino acid and the amino group of another through a condensation reaction that links them into a protein chain.

Active Site

The specific region of an enzyme folded to bind a particular target molecule (the substrate) and catalyze a chemical reaction.

Nucleotide

The repeating structural subunit of nucleic acids consisting of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base.

Central Dogma of Molecular Biology

The biological framework describing information flow in cells: $DNA$ stored safely in the nucleus is transcribed into $mRNA$, which is exported and translated by ribosomes into functional proteins.

Codon

A three-base sequence along an $mRNA$ strand that corresponds to a specific amino acid delivered by a matching $tRNA$ during protein synthesis.

RNA World Hypothesis

The theory proposing that $RNA$ originated before $DNA$ and proteins because it can both store genetic information and fold like enzymes to facilitate its own replication.

Prion

An infectious, misfolded protein containing no nucleic acids that replicates by causing normally folded proteins in the host to also misfold, causing conditions such as Mad Cow Disease and variant Creutzfeldt-Jakob Disease ($vCJD$).

Glycocalyx

The sugar-based outer protective coat on the external face of the cell membrane responsible for cellular protection and recognition.

Endocytosis

The cellular process of engulfing large external molecules or fluids by infolding the plasma membrane to form an inward-budding vesicle.

Exocytosis

The process by which an intracellular vesicle fuses with the plasma membrane to expel its contents into the extracellular space.

Endosome

A membrane-bound sorting organelle with an acidic interior that dissociates endocytosed receptors from their ligands, returning receptors to the membrane and routing cargo onward.

Lysosome

An acidic digestive organelle that receives unusable molecules or waste from endosomes and breaks them down using specialized enzymes.

Peroxisome

An organelle separate from the endomembrane system that imports cytoplasmic molecules, breaks down lipids and toxins, generates hydrogen peroxide ($\text{H}_2\text{O}_2$), and neutralizes free radicals.

Adenosine Triphosphate (ATP)

A high-energy molecule acting like a single-use cellular battery that powers cellular work when a high-energy phosphate bond is cleaved to form adenosine diphosphate ($\text{ADP}$).

Glycolysis

A metabolic pathway occurring in the cytosol that cleaves one six-carbon glucose molecule into two three-carbon pyruvates, capturing energy as $\text{ATP}$ and $\text{NADH}$.

Krebs Cycle (Citric Acid Cycle)

A series of reactions in the mitochondrial matrix that processes Acetyl-$CoA$ through cyclical steps, producing $\text{CO}_2$, $\text{ATP}$, $\text{NADH}$, and $\text{FADH}_2$.

Cristae

Inward foldings of the inner mitochondrial membrane that significantly increase the surface area available for the electron transport chain and $\text{ATP}$ synthase.

Electron Transport Chain

A series of protein complexes embedded in the inner mitochondrial membrane that use high-energy electrons from $\text{NADH}$ to pump protons ($\text{H}^+$) into the intermembrane space.

ATP Synthase

A multi-part protein machine containing an axle, stator, and rotor components ($F_0$ and $F_1$) that spins like a dam turbine as protons ($\text{H}^+$) flow down their electrochemical gradient to generate $\text{ATP}$.

Metabolic Water

Water formed at the terminus of the electron transport chain when oxygen ($\text{O}_2$) acts as the final electron acceptor and binds with electrons and free protons ($\text{H}^+$).

Endosymbiotic Theory

The scientific explanation that mitochondria originated billions of years ago when an ancestral host cell engulfed free-living bacteria without digesting them, resulting in their double membrane, independent circular genome, and autonomous replication.

Mitochondrial DNA (mtDNA)

Non-recombining genetic material located exclusively inside mitochondria that is inherited $100\%$ maternally from the mother via the egg cell and used to track matrilines and calculate molecular clocks.

Free Radical

A highly reactive molecule missing an electron (such as reactive oxygen species produced during electron transport) that strips electrons from proteins, membranes, and $DNA$, leading to oxidative stress.

Antioxidant

A molecule possessing spare outer electrons capable of safely donating an electron to neutralize a free radical without becoming destabilized itself.

Brown Fat

A specialized heat-generating adipose tissue abundant in infants and restricted to areas such as the neck and back in adults; it bypasses $\text{ATP}$ synthase by uncoupling the mitochondrial inner membrane to release proton gradient energy directly as heat.

2,4-Dinitrophenol (DNP)

A synthetic chemical originally used in World War I explosives and sold as a diet pill in $1934$ before being banned in $1938$; it uncouples the mitochondrial proton gradient, causing fatal hyperthermia.

Endomembrane System

The interconnected cellular system comprising the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles that coordinates the synthesis, processing, and transit of proteins and lipids.

Rough Endoplasmic Reticulum

A membrane-bound organelle studded with ribosomes where membrane-bound and secreted proteins are translated and threaded into channels for folding and vesicular dispatch.

Smooth Endoplasmic Reticulum

A tubular organelle lacking ribosomes that is responsible for lipid manufacturing and membrane synthesis.

Golgi Apparatus

The central sorting and distribution hub ('Grand Central Station') of the cell that receives vesicles from the endoplasmic reticulum, refines proteins and lipids, and routes them to their destinations.

Microtubule

A hollow cytoskeletal cylinder assembled from $\alpha$-tubulin and $\beta$-tubulin heterodimers that radiates from the centrosome to maintain cellular structure and act as a highway for motor protein transport.

Centrosome

The primary microtubule-organizing center located near the nucleus from which microtubules radiate outward throughout the cell.

Actin Filaments

Cytoskeletal fibers made of actin subunits arranged in patchy networks concentrated near the plasma membrane that reinforce cell shape and serve as local roads for short-distance transport.

Intermediate Filaments

Rope-like cytoskeletal cords (such as keratin) distributed throughout the cell that anchor organelles in place and resist mechanical stretching, without functioning in transport.

Nucleosome

The basic structural unit of chromatin, consisting of a length of $DNA$ wound around an octamer core of histone proteins.

Heterochromatin

Tightly condensed, transcriptionally inactive chromatin that appears dark under an electron microscope.

Euchromatin

Loosely packed, open chromatin that appears light under an electron microscope and is accessible for active gene transcription.

Promoter

A regulatory $DNA$ sequence positioned immediately adjacent to a gene where the transcription machinery binds to initiate $mRNA$ synthesis.

Enhancer

A regulatory $DNA$ sequence located far from a gene that increases transcription rates when brought into proximity with the promoter via chromatin looping.

Trisomy

The presence of three copies of a specific chromosome instead of the normal two; non-lethal survivable forms in humans are restricted to smaller autosomes such as chromosomes $13$, $18$, and $21$ (Down syndrome).

Human Karyotype

The complete set of $23\text{ pairs}$ of chromosomes ($46\text{ total}$) within a human somatic cell, numbered $1$ through $22$ from longest to shortest plus the $23\text{rd}$ pair of sex chromosomes ($XX$ or $XY$).

Transposon

A 'jumping gene' sequence of $DNA$ coding for an enzyme that cuts or copies and pastes the sequence into new genomic positions, functioning as an evolutionary genomic element.

Barbara McClintock

The scientist who discovered transposons ('jumping genes') in the $1950\text{s}$, facing decades of skepticism before receiving the Nobel Prize in $1983$.