Cell Biology: Cellular Chemistry, Diversity, Microscopy, and Membrane Structure

Comprehensive practice vocabulary flashcards covering the chemical foundations of life, cellular diversity, microscopy techniques, and membrane structure.

Modern Cell Theory

The foundational biological principle stating that all living organisms are composed of one or more cells; the cell is the structural and functional unit of life; all cells arise from pre-existing cells through division; all cells contain hereditary information as DNA; all cells share fundamentally similar chemical composition; and all energy flow (metabolism and biochemistry) occurs within cells.

NASA's Definition of Life

A formal definition characterizing life as "a self-contained chemical entity capable of undergoing Darwinian evolution."

"What Is Life? The Physical Aspect of the Living Cell"

A seminal 1944 work published by physicist Erwin Schrödinger (based on lectures delivered at Trinity College, Dublin in 1943) exploring how the physical and chemical principles of molecular structures relate to living systems.

Octet Rule

The chemical principle that an atom is most stable when its outermost electron shell is fully populated with electrons, such as oxygen sharing electrons via two covalent bonds with hydrogen to form water.

Electronegativity

The measure of an atom's ability to attract shared electrons in a covalent bond; unequal sharing between atoms of differing electronegativities generates polar covalent bonds.

Van der Waals Forces

Weak, noncovalent attractive electrostatic forces ($0.1\text{ to }0.3\,\text{kcal/mol}$) operating at optimum intermolecular distances, generated by transient asymmetric electron distributions (temporary dipoles) in nonpolar molecules.

Amphoteric Molecule

A molecule capable of acting as either a proton donor (acid) or a proton acceptor (base), exemplified by water in the dissociation equilibrium $\text{H}_3\text{O}^+ \rightleftharpoons \text{H}^+ + \text{H}_2\text{O} \rightleftharpoons \text{OH}^- + \text{H}^+$.

Ion Product Constant of Water ($K_w$)

The equilibrium constant for the self-ionization of water, defined as $K_w = [\text{H}^+][\text{OH}^-] = 10^{-14}$ at $25\,^{\circ}\text{C}$.

Bicarbonate Blood Buffer System

The physiological buffering mechanism in the bloodstream governed by the equilibrium $\text{HCO}_3^- + \text{H}^+ \rightleftharpoons \text{H}_2\text{CO}_3$, mitigating changes in blood $\text{pH}$.

Glycosidic Bond

The covalent linkage formed between the hydroxyl groups of two monosaccharide units via a dehydration condensation reaction.

Glycogen

A highly branched nutritional storage polysaccharide composed of linked glucose monomers found in animal cells.

Chitin

A structural polysaccharide comprising unbranched chains of $N$-acetylglucosamine monomers, providing mechanical rigidity to the exoskeletons of insects and crustaceans.

Glycosaminoglycans (GAGs)

Structural polysaccharides found in the extracellular space composed of repeating disaccharide units made of two different sugars.

Triacylglycerol

A lipid storage molecule composed of a single glycerol moiety esterified to three fatty acid tails.

Micelle

A spherical aggregate formed by amphipathic lipid molecules (such as soaps) in water, wherein hydrophobic hydrocarbon tails congregate inward and polar heads interface with the aqueous environment.

Amino Acid Structure

The fundamental monomer of proteins, containing an $\alpha$-carbon bonded to a hydrogen atom, an amino group ($-\text{NH}_2$ or $-\text{NH}_3^+$), a carboxyl group ($-\text{COOH}$ or $-\text{COO}^-$), and a variable side chain (R-group).

Peptide Bond

The planar covalent amide bond formed when the carboxyl group of one amino acid reacts with the amino group of another via dehydration synthesis.

Primary Structure of Proteins

The linear sequence of amino acids linked together by peptide bonds from the amino terminus (N-terminus) to the carboxyl terminus (C-terminus).

Secondary Structure of Proteins

Regular local polypeptide conformations—primarily $\alpha$-helices and $\beta$-pleated sheets—stabilized by hydrogen bonds between the carbonyl oxygen and amide hydrogen of the polypeptide backbone.

Tertiary Structure of Proteins

The overall three-dimensional folded conformation of a single polypeptide chain, stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds (salt bridges), van der Waals forces, and covalent disulfide linkages between side chains.

Quaternary Structure of Proteins

The structural arrangement that arises when two or more independent polypeptide subunits assemble into a multi-subunit complex, exemplified by hemoglobin and collagen.

Sickle-Cell Hemoglobin Mutation

A genetic substitution in the $\beta$-globin chain where the charged glutamic acid (Glu) at position 6 is replaced by the nonpolar valine (Val), creating an exposed hydrophobic patch that causes hemoglobin tetramers to polymerize into fibers.

Nucleotide

The monomeric subunit of nucleic acids consisting of a nitrogenous base, a five-carbon pentose sugar (ribose or deoxyribose), and one or more phosphate groups attached at the $5'$ carbon.

Purines vs. Pyrimidines

The two classes of nitrogenous bases: purines are double-ring structures (adenine and guanine), whereas pyrimidines are single-ring structures (cytosine, thymine in DNA, and uracil in RNA).

Ribozyme

An RNA molecule possessing catalytic activity capable of performing biochemical reactions, such as the catalytic RNA core of ribosomes.

Three-Domain System (Woese, 1990)

A phylogenetic classification system constructed by Carl Woese based on $16\text{S}$ ribosomal RNA sequence comparisons that divides cellular life into three domains: Bacteria, Archaea, and Eucarya.

LUCA (Last Universal Common Ancestor)

The hypothetical single-celled common ancestor of all extant life on Earth; inferred to have possessed a cell boundary, utilized the central dogma, encoded hereditary information using four nucleotides, synthesized RNA, and utilized ATP as chemical energy.

Central Dogma of Molecular Biology

The biological framework describing the directional flow of genetic information: DNA replication followed by transcription into messenger RNA (mRNA) and subsequent translation into functional proteins.

Pluripotent Stem Cells (PSCs)

Self-renewing, undifferentiated cells capable of differentiating into all cell types of the three embryonic germ layers: mesoderm (e.g., muscle, kidney, RBCs), endoderm (e.g., lung, thyroid, pancreatic cells), and ectoderm (e.g., neurons, skin epidermis).

Endosymbiotic Theory

The evolutionary model explaining organelle origins wherein an ancestral anaerobic eukaryote engulfed an aerobic Alphaproteobacteria that evolved into mitochondria, and later engulfed a photosynthetic cyanobacterium that evolved into plastids (chloroplasts).

Cluster of Differentiation (CD Markers)

A standardized nomenclature of cell surface proteins (e.g., CD4 for T helper cells, CD8 for cytotoxic T cells) used to identify, classify, and track the differentiation lineages of hematopoietic and immune cells.

H&E Stain (Hematoxylin and Eosin)

A standard histological staining method using hematoxylin, a basic dye that stains acidic/basophilic cellular components (such as DNA/nuclei) purple/blue, and eosin, an acidic dye that stains basic/acidophilic components (cytoplasmic proteins) pink.

Inverted Microscope

A light microscope configuration where the light source and condenser sit above the stage pointing downward while the objective lenses are situated below the stage, allowing visualization of living adherent cells in culture vessels without removing lids.

Fluorophore

A fluorescent chemical compound that absorbs light at a specific excitation wavelength and subsequently emits light at a longer, lower-energy emission wavelength.

Green Fluorescent Protein (GFP)

A naturally fluorescent protein containing an internal fluorophore that absorbs blue/UV light and emits green light; routinely fused to recombinant proteins to visualize their localization and dynamics in living cells.

Confocal Fluorescence Microscopy

An optical imaging technique that employs pinhole apertures in the conjugate focal plane to eliminate out-of-focus background fluorescence, enabling high-resolution optical sectioning and 3D reconstructions.

Transmission Electron Microscopy (TEM)

An electron microscopy modality that passes a focused beam of electrons through ultra-thin specimen slices to image internal cellular ultrastructure and organelles at nanometer-scale resolution.

Scanning Electron Microscopy (SEM)

An electron microscopy technique that sweeps a beam of electrons across the surface of a specimen, collecting scattered electrons to yield high-depth, three-dimensional topographical images.

Trilaminar Appearance of Membranes

The characteristic "three-layered" image of cellular membranes seen by TEM (approximately $5\text{ to }10\,\text{nm}$ thick), consisting of two dark-staining outer bands (osmium-binding hydrophilic polar head groups) sandwiching a light unstained center (hydrophobic fatty acid tails).

Fluid Mosaic Model

The model of biological membrane structure proposing that proteins float as distinct components within or on a fluid, two-dimensional lipid bilayer matrix.

Liposome

A spherical, artificially prepared vesicle consisting of a continuous lipid bilayer enclosing an internal aqueous compartment, formed spontaneously when phospholipids disperse in water.

Membrane Transition Temperature ($T_m$)

The specific melting temperature at which a lipid bilayer shifts between a rigid, ordered gel-like consistency and a fluid, disordered liquid-crystalline state; determined by fatty acid chain length and degree of saturation.

Phospholipid Translocators (Flippase, Floppase, Scramblase)

Membrane transport enzymes that regulate lipid distribution: Flippases move specific phospholipids from the outer/extracellular leaflet to the inner/cytosolic leaflet (ATP-dependent); Floppases move lipids from inner to outer leaflet (ATP-dependent); and Scramblases randomly bidirectionalize lipids non-specifically without ATP.

Membrane Lipid Asymmetry

The non-uniform distribution of phospholipid species between the inner and outer bilayer leaflets: sphingomyelin (SM) and phosphatidylcholine (PC) reside predominantly on the exoplasmic leaflet, while phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI) predominate on the cytosolic leaflet.

Phosphatidylserine (PS)

A negatively charged inner-leaflet phospholipid that binds basic amino acids (Lys/Arg) on intracellular peripheral proteins; when translocated to the outer surface on aging or apoptotic cells, it acts as an "eat-me" signal for phagocytosis by macrophages.

Phosphatidylethanolamine (PE)

A phospholipid concentrated in the cytosolic leaflet whose conical shape promotes membrane curvature necessary for vesicle budding and membrane fusion events.

Lipid Rafts

Dynamic, specialized microdomains ($10\text{ to }200\,\text{nm}$) within biological membranes enriched in cholesterol, sphingolipids, and GPI-anchored proteins that function as organizing centers for cellular signaling, endocytosis, and pathogen entry.

ABO Blood-Group Antigens

Carbohydrate antigens on red blood cell glycolipids where the O antigen is a core oligosaccharide chain, the A antigen has an added terminal $N$-acetylgalactosamine, the B antigen has an added terminal galactose, and AB individuals synthesize both terminal modifications.

GPI Anchor (Glycosylphosphatidylinositol)

A complex glycolipid anchor covalently attached to the carboxyl terminus of certain proteins, anchoring them strictly to the external (exoplasmic) leaflet of the plasma membrane and facilitating their lateral mobility.

Cell Cortex

A specialized framework of cytoskeletal proteins (including spectrin, actin complexes, and ankyrin) lining the cytosolic face of the plasma membrane, providing mechanical strength and restricting lateral diffusion of membrane proteins.