The Fundamental Unit of Life

Comprehensive vocabulary flashcards covering the structural organization, cell organelles, biomolecules, membrane transport, and cell division mechanisms described in the lecture notes.

Cell

The basic structural and functional unit of life in all living organisms.

Cell Biology

The branch of science that studies the cell in all aspects of its structure, composition, and function.

Periderm (Bark)

The dead outer protective tissue of a plant in which Robert Hooke first observed honeycomb-like compartments termed 'cells' in 1665.

Longitudinal Section

A slice of an anatomical structure prepared by cutting along its lengthwise axis.

Transverse Section

A slice of an anatomical structure prepared by cutting crosswise across the structure.

Magnification (Microscope)

The degree of enlargement of an object's image, calculated as $\text{Magnification} = \frac{\text{Observed size}}{\text{Actual size}}$.

Compound Microscope

An optical instrument in which light is reflected by a mirror through a condenser and specimen into objective and eyepiece lenses to produce a magnified image up to $1500\times$.

Cell Theory

A biological principle formulated by M.J. Schleiden (1838) and T. Schwann (1839) stating that all organisms consist of cells and cell products, and all cells arise from pre-existing cells.

PPLO (Pleuropneumonia-like Organism)

The smallest known cell, measuring between $0.1\,\mu\text{m}$ and $0.5\,\mu\text{m}$ ($1\,\mu\text{m} = 10^{-6}\,\text{m}$).

Nerve Cell

The longest animal cell, characterized by an elongated, branched morphology that can reach lengths of up to $1\,\text{meter}$.

Virus

A biological entity considered living only inside a host cell that lacks a cellular structure and consists of genetic material (DNA or RNA) enclosed in a protein capsule.

Protoplasm

The entire living substance of a cell, composed of the cytoplasm and nucleus bounded by the plasma membrane.

Plasma Membrane

A selectively permeable boundary less than $1/10{,}000\,\text{mm}$ ($70\text{ \AA}$) thick, composed of proteins scattered within a phospholipid bilayer.

Fluid Mosaic Model

The membrane model proposed by Singer and Nicolson in 1972 describing membrane proteins floating in or associated with a fluid phospholipid bilayer.

Cell-to-Cell Recognition

A mechanism mediated by polysaccharides attached to membrane proteins or lipids that allows cells to identify each other, such as during fertilization.

Passive Transport

The movement of substances across a cell membrane along a concentration gradient without the expenditure of metabolic energy.

Diffusion

The spontaneous movement of substance particles from a region of higher concentration to a region of lower concentration.

Osmosis

The net diffusion of water molecules across a selectively permeable membrane toward a region of higher solute concentration.

Turgor Pressure

The hydrostatic pressure exerted outward on the cell wall by the water content accumulated within the cell due to endosmosis.

Endosmosis

The inward osmotic flow of water into a cell when the surrounding solution is hypotonic relative to the cell interior.

Exosmosis

The outward osmotic passage of water from a cell into the exterior medium when placed in a hypertonic solution.

Active Transport

The rapid, typically unidirectional movement of substances across a cell membrane against a concentration gradient utilizing energy supplied by ATP.

Middle Lamella

The cementing intercellular layer common to adjacent plant cells, composed primarily of calcium and magnesium pectate.

Primary Cell Wall

The thin, flexible, first-formed plant cell wall layer composed of cellulose microfibrils and pectin polysaccharides.

Secondary Cell Wall

A thicker, rigid layer formed on the inner surface of the primary wall in mature, non-dividing plant cells.

Glycocalyx

An extra protective surface layer outside the bacterial cell wall, occurring either as a loose slime layer or as a dense, rigid capsule.

Plasmolysis

The shrinkage of the plant cell protoplasm away from its cell wall caused by the extensive exosmosis of water from the central vacuole in a hypertonic medium.

Cytosol

The transparent, structureless, aqueous fluid portion of the cytoplasm that surrounds the organelles.

Ribosome

A non-membrane-bound granular organelle ('protein factory') composed of r-RNA and proteins whose two subunits assemble in the presence of $\text{Mg}^{2+}$ ions during protein synthesis.

Polysome (Polyribosome)

A functional translation cluster consisting of two to six ribosomes attached simultaneously along a single molecule of messenger RNA.

Endoplasmic Reticulum (ER)

An interconnecting three-dimensional network of membrane-bound tubules, cisternae, and vesicles continuous with the nuclear envelope and plasma membrane.

Smooth Endoplasmic Reticulum (SER)

The agranular region of the ER responsible for synthesizing glycogen, lipids, and steroids, as well as detoxifying drugs and poisons.

Rough Endoplasmic Reticulum (RER)

The ribosome-bearing portion of the ER that specializes in protein synthesis and processing.

Golgi Body

A single-membrane-bound organelle discovered in 1898 by Camillo Golgi that modifies, packages, and secretes glycoproteins, cell wall precursors, and lysosomes.

Dictyosome

The term used for the discrete, unconnected Golgi apparatus sub-units found dispersed in plant cells.

Cis Face (Golgi)

The convex, forming face of a Golgi cisterna oriented toward the nucleus that receives transport vesicles.

Trans Face (Golgi)

The concave, maturing face of a Golgi cisterna directed toward the plasma membrane from which secretory vesicles pinch off.

Lysosome

A membrane-bound cytoplasmic vesicle containing hydrolytic enzymes, discovered by Christian de Duve in 1955, that performs intracellular digestion.

Autophagy

The cellular self-digestion process where lysosomes engulf and digest old, damaged, or unneeded cell organelles.

Mitochondrion

A semi-autonomous, double-membrane organelle ('powerhouse of the cell') that replicates independently via circular DNA and produces ATP through aerobic respiration.

Cristae

Infoldings of the inner mitochondrial membrane that extend into the matrix, expanding surface area for ATP-generating oxysomes.

Oxysomes ($F_0\text{–}F_1$ Particles)

Tennis-racket-shaped elementary particles situated on mitochondrial cristae that contain ATP synthetase for synthesizing ATP.

Leucoplasts

Colourless plastids that store nutrients, subdivided into amyloplasts (starch), elaioplasts (oil/fat), and aleuroplasts (protein).

Chromoplasts

Plastids that synthesize and store non-green pigments, giving flowers and fruits their characteristic colours.

Chloroplast

A green, double-membrane, semi-autonomous plastid whose thylakoids are stacked into grana within a fluid stroma to carry out photosynthesis.

Tonoplast

The semi-permeable single membrane enclosing the central sap vacuole in plant cells.

Contractile Vacuole

A specialized osmoregulatory organelle in freshwater unicellular organisms like Amoeba that collects and expels excess water and waste.

Glyoxysomes

Specialized microbodies found in oil-rich seeds and unripe fruits that convert stored fatty acids into carbohydrates.

Sphaerosomes

Single-membrane-bound plant organelles that synthesize and store lipids and exhibit lysosome-like enzymatic activity ('plant lysosomes').

Peroxisomes (Uricosomes)

Microbodies housing oxidative enzymes like catalase and urate oxidase to metabolize and decompose hydrogen peroxide ($\text{H}_2\text{O}_2$).

Cytoskeleton

A structural network of proteinaceous filaments comprising microtubules, microfilaments, and intermediate filaments that maintains cell shape and enables motility.

Microtubules

Hollow protein cylinders composed of tubulin that maintain plant cell wall organisation and form spindle fibres during cell division.

Microfilaments

Solid, slender cytoskeletal protein fibres composed of actin that participate in cell movement and muscle contraction.

Centriole

A non-membrane-bound organelle showing a $9 + 0$ arrangement of peripheral microtubule triplets that organizes mitotic spindle asters in animal cells.

Centrosome

A juxtanuclear cytoplasmic region consisting of two centrioles oriented at right angles surrounded by an amorphous centrosphere.

Nucleotide

The basic building block of nucleic acids, formed by the condensation of a pentose sugar, phosphoric acid, and a nitrogenous base.

Nucleoside

A chemical compound formed by the linkage of a purine or pyrimidine nitrogenous base to a pentose sugar without a phosphate group.

Chargaff's Rule

The law of base equivalence stating that in double-stranded DNA, total purines equal total pyrimidines ($A + G = C + T$).

Deoxyribonucleic Acid (DNA)

A double-helical polymer of deoxyribonucleotides having a diameter of $20\text{ \AA}$ and completing a full helical turn of 10 base pairs every $34\text{ \AA}$.

Nucleus

The master eukaryotic organelle discovered in 1831 by Robert Brown that is enclosed by a double-layered porous envelope and regulates cellular metabolism and heredity.

Nucleolus

A dense, non-membrane-bound structure in the nucleoplasm that acts as the primary site for ribosomal RNA (rRNA) transcription and ribosome assembly.

Chromatin Reticulum

The interphase network of uncoiled, overlapping nucleoprotein fibres composed of DNA and histone proteins.

Prokaryote

An organism whose cell lacks a true membrane-bound nucleus and membrane-bound organelles, with DNA localized in an open nucleoid.

Eukaryote

An organism characterized by cells containing a distinct, membrane-enclosed true nucleus and compartmentalized membrane-bound organelles.

Centromere

The primary constriction of a chromosome that holds two sister chromatids together and bears kinetochore proteins.

Kinetochore

A disc-like proteinaceous structure located on the centromere that anchors spindle microtubules during nuclear division.

Telomere

The repetitive, non-coding terminal end of a eukaryotic linear chromosome that prevents chromosomal end-to-end fusion.

Nucleosome

The basic repeating subunit of chromatin, consisting of a core of histone proteins wrapped by DNA and linked by spacer DNA.

Metacentric Chromosome

A chromosome with its centromere located centrally, dividing the chromosome into two equal arms.

Submetacentric Chromosome

A chromosome with its centromere situated slightly off-center, producing one slightly shorter arm and one longer arm.

Acrocentric Chromosome

A chromosome whose centromere is positioned sub-terminally near one end, producing one very short arm and one very long arm.

Telocentric Chromosome

A chromosome whose centromere is located at the extreme terminal tip, leaving only one chromosomal arm visible.

Homologous Chromosomes

A pair of maternal and paternal chromosomes possessing identical size, morphology, and linear gene loci in a diploid ($2n$) cell.

Genome

The complete, single set ($n$) of chromosomes and associated genetic information found in a gamete.

Allosomes

Sex chromosomes that determine the biological sex of an organism, differentiated from the non-sex autosomes.

Gene

A short, linear segment of DNA positioned at a specific locus on a chromosome that codes for a hereditary trait or functional product.

Interphase

The extended metabolic growth phase of the cell cycle comprising $G_1$, $S$, and $G_2$ periods that precedes mitosis.

Mitosis

Equational somatic cell division producing two genetically identical diploid ($2n$) daughter cells with conserved chromosome numbers.

Metaphase (Mitosis)

The mitotic stage where highly condensed chromosomes align along the equatorial metaphase plate; ideal for examining chromosome morphology.

Anaphase (Mitosis)

The shortest phase of mitosis, characterized by the cleavage of centromeres and poleward migration of sister chromatids.

Cytokinesis (Animal)

The division of animal cytoplasm occurring centripetally through the constriction of an actin-myosin contractile ring forming a cleavage furrow.

Phragmoplast

A complex of microtubules and ER fragments that aligns Golgi vesicles at the spindle equator to direct centrifugal cell plate formation in plant cytokinesis.

Meiosis

A two-stage reduction division producing four genetically varied haploid ($n$) gametes from a diploid ($2n$) germ cell.

Synapsis

The lengthwise pairing of homologous chromosomes during prophase I of meiosis to establish a bivalent or tetrad.

Crossing-Over

The reciprocal exchange of genetic material between non-sister chromatids of homologous chromosomes occurring at chiasmata during prophase I of meiosis.