Biology Lecture Series – Key Vocabulary

Vocabulary flashcards covering major terms and definitions from chapters on Cellular Communication, Energy & Metabolism, Cellular Respiration, Cell Cycle, and Meiosis.

Ligand

A signaling molecule that binds specifically to a receptor protein to initiate cellular communication.

Receptor

A protein that recognizes and binds a specific ligand, starting a signal transduction pathway.

Intracellular Receptor

Cytoplasmic or nuclear receptor that binds hydrophobic ligands (e.g., steroid hormones) inside the cell.

Cell-Surface Receptor

Transmembrane protein that binds hydrophilic ligands outside the cell and transmits information across the membrane.

Signal Transduction

Series of intracellular events that convert a ligand–receptor interaction into a cellular response.

Second Messenger

Small intracellular molecule (e.g., cAMP, Ca²⁺) produced by some membrane receptors to relay signals inside the cell.

Protein Kinase

Enzyme that transfers a phosphate group from ATP to a protein, often altering that protein’s activity.

Phosphatase

Enzyme that removes a phosphate group from a protein, reversing kinase action.

Direct Contact Signaling

Cell communication via membrane molecules on adjacent cells or through gap junctions.

Paracrine Signaling

Local signaling in which a cell releases a ligand that affects nearby cells.

Endocrine Signaling

Long-distance signaling in which hormones travel through the bloodstream to target cells.

Synaptic Signaling

Neuronal communication in which neurotransmitters cross a synapse to reach a postsynaptic cell.

Autocrine Signaling

Form of signaling in which a cell secretes a ligand that binds to receptors on its own surface.

Signal Amplification

The multiplication of a signal’s strength inside a cell, often through kinase cascades or second messengers.

Kinase Cascade

Sequential activation of multiple kinases, each phosphorylating the next, to amplify a signal.

Energy

Capacity to do work; appears as kinetic (motion) or potential (stored) energy.

First Law of Thermodynamics

Energy cannot be created or destroyed, only transformed.

Second Law of Thermodynamics

In energy conversions, entropy (disorder) of the universe always increases.

Entropy (S)

Measure of disorder; systems tend toward higher entropy.

Free Energy (G)

Energy in a system available to do work; ΔG predicts reaction spontaneity.

Exergonic Reaction

Chemical reaction with negative ΔG that releases free energy and occurs spontaneously.

Endergonic Reaction

Reaction with positive ΔG that requires energy input and is non-spontaneous.

Activation Energy

Extra energy needed to initiate a chemical reaction by destabilizing existing bonds.

Catalyst

Substance, such as an enzyme, that lowers activation energy without being consumed.

Enzyme

Protein catalyst that speeds up biochemical reactions by lowering activation energy.

Substrate

Specific reactant molecule that an enzyme binds and acts upon.

Active Site

Region on an enzyme where the substrate binds and catalysis occurs.

Induced Fit

Conformational change in an enzyme upon substrate binding that improves catalysis.

Competitive Inhibitor

Molecule that binds an enzyme’s active site, blocking substrate access.

Noncompetitive Inhibitor

Molecule that binds an enzyme at a site other than the active site, changing enzyme shape and function.

Allosteric Site

Regulatory site on an enzyme separate from the active site; binds inhibitors or activators.

Cofactor

Non-protein helper (often a metal ion) required for enzyme activity.

Coenzyme

Organic cofactor (e.g., vitamins, NAD⁺) that assists enzymes, often by carrying electrons.

Anabolic Pathway

Metabolic pathway that builds complex molecules from simpler ones, consuming energy.

Catabolic Pathway

Pathway that breaks down molecules, releasing energy.

Feedback Inhibition

Regulatory mechanism in which the end product of a pathway inhibits an early enzyme in that pathway.

Redox Reaction

Chemical process involving electron transfer; includes oxidation and reduction.

Oxidation

Loss of electrons (often with loss of hydrogen).

Reduction

Gain of electrons (often with gain of hydrogen).

Dehydrogenation

Redox reaction in which a proton and electron (hydrogen atom) are removed from a molecule.

Electron Carrier

Molecule (e.g., NAD⁺, FAD) that transports electrons during cellular respiration.

NAD⁺

Oxidized nicotinamide adenine dinucleotide that accepts electrons to become NADH.

NADH

Reduced form of NAD⁺ that carries high-energy electrons to the electron transport chain.

FAD/FADH₂

Flavin adenine dinucleotide and its reduced form that shuttle electrons in respiration.

Substrate-Level Phosphorylation

Direct formation of ATP by transferring a phosphate from a substrate to ADP.

Oxidative Phosphorylation

ATP synthesis powered by the proton gradient created by the electron transport chain.

Glycolysis

Ten-step pathway in the cytoplasm that splits glucose into two pyruvate, yielding 2 ATP and 2 NADH.

Pyruvate Oxidation

Conversion of pyruvate to acetyl-CoA, producing NADH and CO₂; occurs in mitochondria.

Krebs Cycle (Citric Acid Cycle)

Mitochondrial matrix pathway that oxidizes acetyl-CoA, generating CO₂, NADH, FADH₂, and ATP.

Electron Transport Chain (ETC)

Series of membrane protein complexes that pass electrons and pump protons to create a gradient.

Chemiosmosis

Use of a proton gradient across a membrane to drive ATP synthesis via ATP synthase.

Proton Gradient

Electrochemical difference in proton concentration across a membrane; stores potential energy.

ATP Synthase

Membrane enzyme that uses proton flow to phosphorylate ADP to ATP.

Fermentation

Anaerobic pathway that regenerates NAD⁺ by reducing an organic molecule (e.g., lactate, ethanol).

β-Oxidation

Sequential removal of two-carbon units from fatty acids, producing acetyl-CoA and reduced carriers.

Deamination

Removal of an amino group from an amino acid before it enters respiratory pathways.

Chromatin

Complex of DNA and associated proteins (histones) forming eukaryotic chromosomes.

Nucleosome

Basic chromatin unit: DNA wrapped around an octamer of histone proteins.

Sister Chromatids

Identical copies of a chromosome held together at the centromere after DNA replication.

Homologous Chromosomes

Pair of chromosomes—one maternal, one paternal—that carry the same genes in the same order.

Centromere

Chromosome region where sister chromatids remain attached and kinetochore forms.

Kinetochore

Protein structure on the centromere where spindle microtubules attach during division.

Cohesin

Protein complex holding sister chromatids together until anaphase.

Cell Cycle

Ordered series of events in a cell’s life: G₁, S, G₂, M phases and cytokinesis.

Cyclin

Regulatory protein whose levels rise and fall to control progression through the cell cycle.

Cyclin-Dependent Kinase (Cdk)

Protein kinase activated by cyclin binding; drives cell cycle transitions.

Checkpoint

Control point in the cell cycle where progression can be halted if conditions are unfavorable.

Mitosis

Nuclear division producing two genetically identical daughter nuclei; phases: prophase, metaphase, anaphase, telophase.

Cytokinesis

Division of the cytoplasm, forming two separate daughter cells after mitosis.

Binary Fission

Asexual reproduction in bacteria where the single chromosome replicates and the cell splits.

Tumor-Suppressor Gene

Gene whose normal product inhibits cell division; loss-of-function mutations can lead to cancer (e.g., p53).

Proto-Oncogene

Normal gene that promotes cell division; gain-of-function mutations convert it to an oncogene (e.g., Ras).

Oncogene

Mutated proto-oncogene that drives unregulated cell proliferation and can contribute to cancer.

Meiosis

Two consecutive nuclear divisions in germ cells producing four non-identical haploid gametes.

Synapsis

Pairing of homologous chromosomes during prophase I of meiosis.

Crossing-Over

Exchange of genetic material between non-sister chromatids, creating recombinant chromosomes.

Chiasma

Visible site where crossing-over has occurred between homologous chromatids.

Independent Assortment

Random orientation of homologous chromosome pairs at metaphase I, generating genetic diversity.

Nondisjunction

Failure of homologues or sister chromatids to separate properly during meiosis, producing aneuploid gametes.

Haploid (n)

Cell with one complete set of chromosomes, typical of gametes.

Diploid (2n)

Cell with two sets of chromosomes—one from each parent; typical of somatic cells.

Gamete

Haploid reproductive cell (sperm or egg) produced by meiosis.

Zygote

Diploid cell formed by the fusion of two haploid gametes during fertilization.