Cell Biology: Cell Theory, Organelles, Cell Division, and Regulation

Comprehensive vocabulary flashcards covering cell discovery history, cell theory, prokaryotic vs. eukaryotic structures, intracellular organelles, stem cells, cell differentiation, the cell cycle phases, mitosis stages, and mechanisms of cell cycle regulation and cancer.

Robert Hooke (1660's)

The English scientist who was the first to observe and describe cells after viewing thin slices of cork through an early compound microscope.

Anton van Leeuwenhoek (1670's)

The Dutch tradesman and scientist who refined the microscope lens design, allowing him to first observe living microorganisms including bacteria and protozoa.

Matthias Schleiden and Theodor Schwann (1830's)

The German botanist and zoologist who formulated the original principles of cell theory, concluding that all plant and animal tissues are composed of cells.

Rudolf Virchow (1855)

The scientist who proposed the third core tenet of the cell theory: that all existing cells arise only from preexisting cells through cell division.

Cell Theory

A foundational biological concept consisting of three tenets: 1. All living organisms are composed of one or more cells; 2. Cells are the fundamental structural and functional units of life; 3. All cells originate from preexisting cells.

Prokaryotic Cell

A structurally simpler, typically smaller, unicellular cell that lacks a nucleus and membrane-bound organelles, storing its genetic information in a circular chromosome (such as in bacteria and archaea).

Eukaryotic Cell

A complex cell type that can be unicellular or multicellular, containing a membrane-delimited nucleus, linear chromosomes, and membrane-bound organelles (such as in plants, animals, fungi, and protists).

Eukaryotic Linear vs. Prokaryotic Circular Chromosome

The key genomic distinction where eukaryotes package linear DNA strands into discrete rod-like chromosomes within a nucleus, whereas prokaryotes organize double-stranded DNA into a continuous, circular chromosome within the nucleoid.

Unicellular Organism

An organism consisting of only a single cell that executes all necessary life processes, such as bacteria and various protists.

Multicellular Organism

An organism composed of many differentiated, specialized cells that coordinate complex physiological functions, such as animals, land plants, and most fungi.

Microbiome

The collective ecological community of symbiotic, commensal, and pathogenic prokaryotic microorganisms inhabiting a host organism's body, particularly concentrated inside the gut or intestines.

Organelle

A specialized intracellular subunit or structure ('mini organ') within a cell that performs a specific metabolic or structural task.

Cell Membrane (Plasma Membrane)

A selectively permeable boundary composed of a lipid bilayer found in all cells that maintains intracellular homeostasis by regulating which molecules enter and leave.

Cytoplasm (Cytosol)

The jelly-like fluid interior present in all cells that suspends organelles, distributes nutrients, and provides the fluid aqueous environment necessary for biochemical reactions.

Ribosome

A non-membrane-bound molecular complex composed of RNA and protein found in all cells that synthesizes polypeptides by linking amino acids together.

Membrane-Bound Organelle

An intracellular compartment enclosed by its own phospholipid bilayer membrane, allowing specialized biochemical processes to be compartmentalized exclusively within eukaryotic cells.

Nucleus

The double-membrane-bound eukaryotic control center that encloses and protects the cell's genetic material ($\text{DNA}$).

Nucleolus

A prominent dense subregion located within the eukaryotic nucleus dedicated to the synthesis and assembly of ribosomal subunits.

Rough Endoplasmic Reticulum (RER)

A network of membranous cisternae studded on its outer surface with ribosomes that synthesizes, modifies, and folds proteins destined for membranes or secretion.

Smooth Endoplasmic Reticulum (Smooth ER)

A continuous network of tubular eukaryotic membranes devoid of ribosomes that synthesizes lipids and phospholipids, stores calcium ions, and detoxifies chemicals (such as in liver cells).

Golgi Apparatus (Golgi Bodies)

A eukaryotic series of flattened membrane stacks that sorts, chemically modifies, packages, and ships proteins and lipids in vesicles to internal cellular locations or out of the cell.

Mitochondria

Double-membrane eukaryotic organelles that function as the sites of aerobic cellular respiration, converting glucose-derived energy into usable adenosine triphosphate ($\text{ATP}$).

Lysosome

A membrane-bound digestive organelle in animal cells containing hydrolytic enzymes that break down polymers, ingested bacteria, viruses, and worn-out cell parts for molecular recycling.

Centrosome

An animal cell organelle consisting of a pair of centrioles that organizes microtubules and forms the spindle apparatus needed to segregate chromosomes during cell division.

Cell Wall

A rigid, protective structural layer located outside the plasma membrane in plant cells, prokaryotes, fungi, and algae that provides mechanical support, shape, and defense.

Chloroplast

A specialized, chlorophyll-containing double-membrane organelle found in plant and algal cells that carries out photosynthesis by capturing solar energy to produce glucose.

Central Vacuole

A prominent, fluid-filled organelle occupying the center of mature plant cells that stores water and solutes to generate internal turgor pressure against the cell wall.

Vacuole

A membrane-bound storage vesicle found in eukaryotic cells that isolates wastes, stores nutrients or water, and assists with osmotic balance.

Cytoskeleton

A dynamic intracellular network of protein filaments extending through the cytoplasm of all cells (eukaryotes and prokaryotes) providing structural architecture, mechanical support, and tracks for transport.

Microtubule

A hollow, cylindrical cytoskeletal protein polymer in eukaryotic cells that maintains cell shape, forms tracks for organelle motility, and constitutes the core structure of cilia, flagella, and mitotic spindle fibers.

Microfilament

A thin, solid helical rod composed of actin in eukaryotic cells that supports cell shape, enables muscle contraction with myosin, forms the cleavage furrow contractile ring in animal cytokinesis, and guides cell plate formation in plants.

Intermediate Filament

A durable, fibrous cytoskeletal protein class found in animal cells that provides permanent structural reinforcement to bear mechanical stress and anchor organelles securely in place.

Peroxisome

A eukaryotic metabolic organelle housing oxidative enzymes, such as catalase, which breaks down toxic compounds including ethanol and degrades hydrogen peroxide ($H_2O_2$) into water and oxygen.

Plasmodesmata

Microscopic open channels traversing the rigid cell walls of adjacent plant cells that allow direct cytoplasmic communication and continuous intercellular transport of nutrients and signaling molecules.

Flagella

Long, lash-like cellular appendages that generate propeller-like rotational or undulating motion to propel cells through liquid media; composed of membrane-encased microtubules in eukaryotes.

Cilia

Short, numerous hair-like projections composed of membrane-covered microtubules on the surface of animal cells that move back and forth to propel single cells or move fluid over tissue layers.

Nucleoid

The irregularly shaped, non-membrane-enclosed central region of a prokaryotic cell containing the primary circular genomic DNA molecule.

Capsule

A slippery, gelatinous outer protective envelope situated exterior to the bacterial cell wall that prevents phagocytosis and enables bacteria to adhere firmly to environmental surfaces.

Plasmid

A small, extrachromosomal circular DNA molecule found within prokaryotic cells capable of autonomous replication, often conferring adaptive traits such as antibiotic resistance.

Pili

Short, filamentous protein projections covering the exterior of certain prokaryotes that mediate adherence to surfaces or stabilize bacteria during horizontal gene transfer (conjugation).

Stem Cells

Undifferentiated biological cells that lack specialized structures or functions, possessing the distinct capacity for self-renewal and the potential to differentiate into specialized cell types.

Cell Differentiation

The developmental process by which generalized, unspecialized precursor cells transition into mature, structurally distinct cells designed to carry out specific physiological roles.

Pluripotent Stem Cells

Embryonic stem cells derived from early blastocysts that retain the developmental capacity to differentiate into essentially any specialized cell type in the adult body.

Multipotent Stem Cells

Adult tissue stem cells (such as hematopoietic stem cells in bone marrow) restricted to differentiating into a limited lineage of related mature cell types within a given tissue.

Binary Fission

A rapid mechanism of asexual reproduction in prokaryotes in which a single cell copies its circular DNA, elongates, and divides down the middle into two genetically identical clone daughter cells.

Cell Cycle

An ordered, repeating progression of growth, genomic replication, and division occurring in eukaryotic cells, consisting of interphase, mitosis, and cytokinesis.

Interphase

The extended growth and metabolic phase of the eukaryotic cell cycle comprising the $G_1$, $S$, and $G_2$ phases, occupying approximately $90\%$ of a typical cell's total life span.

Gap 1 ($G_1$) Phase

The initial subphase of interphase following division, during which the cell grows in volume, carries out metabolic functions, and synthesizes necessary structural proteins and enzymes.

Synthesis ($S$) Phase

The intermediate subphase of interphase where the eukaryotic cell carries out complete and faithful replication of its genomic DNA.

Gap 2 ($G_2$) Phase

The final subphase of interphase during which the cell completes protein synthesis, duplicates necessary organelles, and ensures sufficient growth prior to entering mitosis.

Chromatin

The loose, relaxed, uncoiled state of nuclear genomic DNA wrapped around histone proteins present throughout interphase.

Chromosome

The highly compacted, condensed, and ordered thread-like packaging of DNA and structural proteins that condenses at prophase to prevent genomic damage during nuclear division.

Duplicated Chromosome

A replicated chromosome formed during the $S$ phase consisting of two identical sister chromatids attached together along a centromere.

Sister Chromatids

Two identical copies of a single replicated chromosome joined by a centromere that separate during anaphase to become independent daughter chromosomes.

Centromere

The specialized, constricted central chromosome region that holds two sister chromatids together and serves as the attachment point for spindle fibers.

Spindle Fibers

Dynamic microtubule structures radiating from centrosomes that attach to kinetochores/centromeres and physically coordinate the alignment and segregation of chromosomes during mitosis.

Mitosis

The eukaryotic process of nuclear division consisting of prophase, metaphase, anaphase, and telophase, which evenly partitions duplicated chromosomes into two identical daughter nuclei.

Prophase

The initial stage of mitosis characterized by the condensation of chromatin into visible sister chromatids, disintegration of the nuclear envelope, migration of centrosomes to opposite poles, and spindle emergence.

Metaphase

The mitotic stage during which spindle fibers guide all duplicated chromosomes to align linearly along the equatorial metaphase plate in the center of the cell.

Anaphase

The mitotic stage where spindle fibers shorten and exert tension to separate sister chromatids at the centromere, pulling them toward opposite cellular poles as independent chromosomes.

Telophase

The final stage of mitosis wherein segregated sets of chromosomes reach opposite poles, nuclear envelopes reform around them, and chromosomes decondense back into diffuse chromatin.

Cytokinesis

The physical cleavage and partitioning of the cytoplasm following karyokinesis, dividing the parent cell into two separate, genetically identical daughter cells; begins during telophase.

Cleavage Furrow

A shallow contractile indentation formed by actin microfilaments at the equator of an animal cell membrane that pinches the cell inward until it splits during cytokinesis.

Cell Plate

A membranous, midline partition formed in dividing plant cells through coalescing Golgi-derived vesicles carrying cell wall materials, which develops outward to establish a new dividing cell wall.

Somatic Cells

Any biological cell forming the body of an organism (body cells) that divide mitotically to produce genetically identical diploid cells, excluding germline gametes.

Gametes

Specialized reproductive haploid sex cells (eggs and sperm) produced via meiosis that do not divide by somatic mitosis.

Cell Cycle Checkpoints

Built-in molecular monitoring stages ($G_1$, $G_2$, and Metaphase/Spindle Assembly) that assess cell size, nutrient sufficiency, DNA replication completeness, DNA damage, and chromosome spindle attachment before proceeding.

Growth Factors

Extracellular regulatory signaling proteins that bind to specific surface receptors on target cells to trigger intracellular biochemical cascades that stimulate cell division and growth.

$G_0$ (G-Zero) Phase

A resting state of the cell cycle where a cell exits active division; cells in $G_0$ perform normal physiological functions and maintain metabolism but do not replicate their DNA or prepare for division.

Apoptosis

A controlled, genetically programmed mechanism of cellular suicide triggered by internal or external signals, wherein a damaged or unneeded cell cleanly fragments so its molecular components can be recycled without tissue damage.

Sonic Hedgehog ($\text{SHH}$) Protein

A regulatory protein that orchestrates digit separation during human fetal development at approximately $6\text{ weeks}$ of gestation by inducing targeted apoptosis in the interdigital tissue between fingers and toes.

Cancer

A disease state marked by unregulated, uncontrollable cell division caused by accumulating mutations in genes responsible for regulating the cell cycle and checkpoints.

Tumor

A disorganized, abnormal mass or clump of tissue formed by rapidly and inappropriately proliferating cells.

Benign Tumor

A non-invasive, localized neoplasm where abnormal cells remain clustered together at their initial tissue site and can typically be removed surgically without invading surrounding tissue.

Malignant Tumor

An aggressive, cancerous tumor whose cells invade neighboring healthy tissue and can break away to enter the bloodstream or lymphatic system to establish secondary tumors.

Metastasis

The pathological dissemination of malignant cancer cells breaking away from a primary tumor, traveling via the bloodstream or lymphatic vessels, and colonizing distant anatomical sites.

Carcinogen

Any physical, chemical, or environmental agent known to cause DNA damage or mutations that disrupt cell-cycle regulation and promote cancer development (such as UV rays, air pollutants, and tobacco smoke).