General Biology: Cell Structure, Physiology, Metabolism, and Signaling

Comprehensive practice flashcards covering cellular and molecular biology lecture notes, including macromolecular chemistry, cell theory, membrane physiology and transport kinetics, neurophysiology, cellular respiration, cell signaling cascades, and endomembrane protein trafficking.

Major Elements of Life

The four chemical elements—Oxygen, Carbon, Hydrogen, and Nitrogen—that constitute more than $96\%$ of the total body mass of most living organisms.

Covalent Bond

A strong chemical bond formed by the sharing of one or more pairs of electrons between atoms, creating solid, stable molecular structures.

Monosaccharide

A simple single-sugar carbohydrate monomer having the general chemical formula $(CH_2O)_n$, where $3 < n < 7$ for sugars involved in cellular metabolism.

Polysaccharide

A complex polymer composed of numerous monosaccharide units linked together by glycosidic bonds, functioning as energy storage (starch in plants, glycogen in animals) or structural support (cellulose in plants, chitin in animals).

Triglyceride

A lipid composed of a glycerol backbone esterified to three fatty acid chains ($14 < C < 20$), providing an economical energy reserve with more than twice the energy density per gram compared to carbohydrates.

Phospholipid

An amphipathic lipid consisting of a glycerol backbone linked to two hydrophobic fatty acid tails and a hydrophilic head group attached via a phosphate group, forming the fundamental structural unit of biological membranes.

Protein Structure Levels

The structural organization of proteins categorized into primary (linear amino acid sequence), secondary (local foldings such as $\alpha$-helices and $\beta$-pleated sheets), tertiary (complete 3D conformation stabilized by non-covalent interactions), and quaternary (association of multiple polypeptide subunits).

Cell Theory

The foundational biological doctrine stating that all organisms consist of one or more cells, the cell is the structural unit of life, all living cells arise from pre-existing cells through division, and cells contain hereditary information passed from parent to daughter cells.

Prokaryotic Cell

A structurally simple unicellular organism typically measuring $0.3-2\,\mu m$ that lacks a nucleus and internal membrane-bound organelles, possesses circular DNA free in the cytoplasm, contains $70S$ ribosomes, and divides by binary fission.

Eukaryotic Cell

A complex cell type measuring $2-25\,\mu m$ (capable of forming unicellular or multicellular organisms) that features a membrane-enclosed nucleus containing linear DNA, membrane-bound organelles (mitochondria, ER, Golgi, lysosomes), $80S$ ribosomes, and divides by mitosis and meiosis.

Liposome

An artificial spherical vesicle formed by the spontaneous self-assembly of phospholipids in an aqueous solution, commonly used in research and pharmaceutical drug delivery to encapsulate hydrophilic drugs in the aqueous core and hydrophobic drugs in the lipid bilayer.

Integral Membrane Protein

A membrane protein that permanently embeds within or spans the phospholipid bilayer, possessing hydrophobic regions that interact with lipid hydrocarbon tails and hydrophilic regions exposed to aqueous compartments.

Peripheral Membrane Protein

A membrane protein non-covalently attached to integral membrane proteins or phospholipid head groups on either the cytosolic or extracellular surface, often acting as an enzyme or signal transducer.

Freeze-Fracture Technique

A cytological preparation method in which rapidly frozen tissue in liquid nitrogen is split along the hydrophobic interior of the lipid bilayer with a micro-blade, enabling the visualization of integral membrane proteins via electron microscopy or immunogold labeling.

Membrane Phospholipid Movements

Dynamic lipid mobility within the bilayer, including common rapid events (lateral diffusion, flexion, rotation, bobbing) within a single leaflet, and very rare transverse movement between leaflets known as flip-flop.

Membrane Fluidity Buffer (Cholesterol)

An amphipathic steroid intercalated between phospholipid fatty acid chains that restricts phospholipid mobility at elevated temperatures to reduce fluidity and prevents hydrocarbon chain crystallization at low temperatures.

Frye-Edidin Experiment

A 1970 landmark experiment demonstrating membrane fluidity by fusing human cells (red fluorescent markers) and mouse cells (green fluorescent markers); complete mixing of membrane proteins occurred across the hybrid cell surface within $40\,\text{minutes}$.

Partition Coefficient ($K$)

A measure of the lipid solubility of a solute determined by its distribution ratio between oil and water ($K = [S]_A / [S]_B$); solutes with higher $K$ values penetrate the hydrophobic interior of biological membranes more rapidly.

Osmosis

The net passive movement of water molecules across a selectively permeable membrane toward a compartment of higher solute concentration, driven by the solute concentration difference rather than water concentration.

Tonicity

The measure of the effective osmotic pressure gradient determined by non-penetrating solutes across a membrane, categorized as hypotonic (water enters, cell swells), hypertonic (water leaves, cell shrinks), or isotonic (osmotic equilibrium).

Aquaporin

A tetrameric transmembrane protein channel that facilitates the rapid, selective passage of up to billions of water molecules per second across biological membranes without altering solute gradients.

Carrier Protein

A membrane transport protein that binds a specific solute on one side of the bilayer, undergoes an alternating conformational change, and exposes and releases the solute on the opposite side down its concentration gradient.

Transport Saturation Kinetics

The phenomenon whereby mediated transport reaches a maximal velocity ($V_{\max}$) at high solute concentrations because all specific protein binding sites become saturated, unlike simple non-mediated diffusion which increases linearly without plateauing.

Patch-Clamp Technique

An electrophysiological technique utilizing a glass micropipette and microelectrode sealed against a patch of cell membrane to record ionic current conductances flowing through single ion channels.

Primary Active Transport

An active transport mechanism in which the transport protein directly hydrolyzes ATP to generate the energy required to move a solute against its electrochemical or concentration gradient.

$Na^+/K^+$ ATPase Pump

An electrogenic primary active transport pump that hydrolyzes one ATP molecule to export three $Na^+$ ions from the cytosol and import two $K^+$ ions into the cell, establishing and maintaining the resting membrane potential.

Secondary Active Transport

A transport process in which a protein couples the thermodynamically favorable downhill movement of an ion (typically $Na^+$) to the uphill movement of a second solute against its concentration gradient, acting via symport or antiport.

Saltatory Conduction

The rapid propagation of action potentials along myelinated axons, where depolarization jumps from one uninsulated Node of Ranvier to the next, increasing conduction speed approximately $20\,\text{times}$ relative to unmyelinated axons.

Resting Membrane Potential

The steady electrical potential difference (approximately $-70\,mV$) across the plasma membrane of a non-stimulated neuron, created by asymmetric ion distributions, selective $K^+$ leak permeability, and the $Na^+/K^+$ pump.

Action Potential

A transient, rapid all-or-none reversal of membrane polarity along an excitable membrane consisting of depolarization (massive $Na^+$ influx), repolarization ($Na^+$ channel inactivation and $K^+$ efflux), and a transient hyperpolarizing refractory period.

Neuromuscular Junction

A specialized chemical synapse between a motor neuron and a skeletal muscle fiber, where action potentials stimulate voltage-gated $Ca^{2+}$ channels, triggering exocytosis of acetylcholine into the synaptic cleft to induce sarcolemma depolarization.

Pinocytosis

A form of non-specific bulk endocytosis in which the plasma membrane invaginates to take up droplets of extracellular fluid and dissolved solutes into small endocytic vesicles.

Receptor-Mediated Endocytosis

A selective endocytic process whereby extracellular target macromolecules bind to specific cell-surface receptors, which cluster into clathrin-coated pits that bud inward to deliver cargo to endosomes and lysosomes.

Mitochondrion Structure

A double-membrane organelle possessing a permeable outer membrane, an impermeable inner membrane folded into cristae and rich in cardiolipin, an intermembrane space, and an aqueous matrix containing circular mtDNA and independent ribosomes.

Mitochondrial DNA (mtDNA)

A circular, double-stranded genome present in the mitochondrial matrix comprising $37\,\text{genes}$ in humans that encode $13\,\text{polypeptides}$ of the electron transport chain, $2\,\text{rRNAs}$, and $22\,\text{tRNAs}$.

Hexokinase

The regulatory glycolytic enzyme that catalyzes Step 1: the irreversible transfer of a phosphate group from ATP to the C6 position of glucose, forming glucose 6-phosphate (G6P) and ADP in the presence of $Mg^{2+}$.

Phosphofructokinase-1 (PFK-1)

The principal allosteric regulatory enzyme of glycolysis that catalyzes Step 3: the irreversible phosphorylation of fructose 6-phosphate into fructose 1,6-bisphosphate using one ATP; it is inhibited by high levels of ATP, NADH, and citrate.

Triose Phosphate Isomerase

The enzyme that catalyzes Step 5 of glycolysis: the rapid, reversible isomerization of dihydroxyacetone phosphate (DHAP) into glyceraldehyde 3-phosphate (G3P), concluding the endergonic preparatory phase.

Substrate-Level Phosphorylation

The direct enzymatic transfer of a high-energy phosphate group from a phosphorylated metabolic intermediate (such as 1,3-bisphosphoglycerate or phosphoenolpyruvate) directly to ADP to generate ATP.

Pyruvate Kinase

The regulatory enzyme that catalyzes Step 10 of glycolysis: the irreversible transfer of a phosphate group from phosphoenolpyruvate (PEP) to ADP, producing pyruvate and ATP.

Net Glycolysis Equation

The overall stoichiometric reaction converting one molecule of glucose into two pyruvate molecules: $\text{Glucose} + 2\,\text{ATP} + 4\,\text{NAD}^+ \rightarrow 2\,\text{Pyruvate} + 4\,\text{ATP} + 2\,\text{NADH} + 2\,H_2O + 2\,H^+$, yielding a net gain of $2\,\text{ATP}$ and $2\,\text{NADH}$.

Pyruvate Oxidation

The oxidative decarboxylation occurring in the mitochondrial matrix wherein pyruvate loses a carboxyl group as $CO_2$ and its remaining two-carbon acetyl group is transferred to Coenzyme A to form acetyl-CoA while reducing $NAD^+$ to NADH.

Citric Acid Cycle (Krebs Cycle)

An eight-step cyclic metabolic pathway in the mitochondrial matrix that oxidizes acetyl-CoA, producing $2\,CO_2$, $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP/GTP}$ per acetyl unit oxidized.

Mitochondrial Electron Transport Chain

A series of multi-protein inner mitochondrial membrane complexes (I, II, III, IV) and mobile carriers (ubiquinone, cytochrome c) that conduct electrons from NADH and $\text{FADH}_2$ to oxygen while pumping protons ($H^+$) into the intermembrane space.

ATP Synthase

A rotary nanomachine composed of an oligomycin-sensitive membrane-embedded proton channel ($F_o$) and a catalytic headpiece ($F_1$) in the mitochondrial matrix that synthesizes ATP using the electrochemical proton-motive force.

Paracrine Signaling

A form of local cell communication where signaling molecules secreted by a regulatory cell diffuse through extracellular fluid to act upon adjacent target cells in close proximity.

Autocrine Signaling

A signaling mechanism where a cell secretes extracellular chemical messengers that bind to and activate specific receptors situated on its own surface, such as in tumor cell proliferation.

Receptor Tyrosine Kinase (RTK)

A cell-surface transmembrane receptor that, upon ligand binding (such as insulin, EGF, PDGF, or NGF), undergoes dimerization and autophosphorylation on cytoplasmic tyrosine residues to recruit downstream intracellular signaling proteins.

G Protein-Coupled Receptor (GPCR)

A large superfamily of integral membrane receptors characterized by seven transmembrane $\alpha$-helical domains coupled to intracellular heterotrimeric G proteins that exchange GDP for GTP upon activation.

cAMP Second Messenger Pathway

A signal transduction cascade initiated by GPCR activation of adenylyl cyclase, which converts ATP to cyclic AMP (cAMP); cAMP then activates protein kinase A (PKA) until degraded into AMP by phosphodiesterase.

IP3 / DAG Pathway

A signaling pathway where an activated effector cleaves membrane phosphoinositides into inositol 1,4,5-trisphosphate (IP3), which mobilizes $Ca^{2+}$ release from the endoplasmic reticulum, and diacylglycerol (DAG), which together activate protein kinase cascades.

Ras-MAP Kinase Cascade

A downstream signal transduction cascade in which activated RTK recruits adaptor proteins to activate the small G protein Ras (exchanging GDP for GTP), which sequentially phosphorylates and activates a cascade of MAP kinases that translocate into the nucleus to stimulate mitogenic gene transcription.

Co-Translational Translocation

The mechanism by which secretory and lysosomal proteins containing an N-terminal signal sequence are recognized by the Signal Recognition Particle (SRP), arresting translation until bound to the SRP receptor and directed into the translocon channel on the rough endoplasmic reticulum.

GET Pathway

The post-translational targeting pathway (Guided Entry of Tail-anchored proteins) that captures proteins containing a single C-terminal transmembrane domain in the cytosol using Get3/4/5 complexes and inserts them into the ER membrane via Get1/2 receptors.

Dolichol Phosphate

A membrane lipid carrier on which the 14-sugar core oligosaccharide (comprising 2 NAG, 9 mannoses, and 3 glucoses) is assembled before being flipped into the ER lumen and transferred en bloc to an asparagine residue of a nascent polypeptide.

Unfolded Protein Response (UPR)

A cellular stress response activated when misfolded proteins accumulate in the ER lumen; activated ER transmembrane sensors recruit chaperones, suppress general protein translation, and induce gene expression for ER quality control.

Golgi Complex

An endomembrane organelle organized into stacked, flattened membranous cisternae displaying polarized cis, medial, and trans compartments that sequentially process, glycosylate, and sort proteins arriving from the ER.

COPII Coated Vesicles

Vesicles coated by the Sar1 GTPase and Sec23/Sec24 and Sec13/Sec31 protein complexes that mediate anterograde transport from the rough ER to the ERGIC and cis-Golgi network.

COPI Coated Vesicles

Retrograde transport vesicles coated with a 7-protein coatomer complex that recycle escaped ER-resident proteins carrying a C-terminal KDEL retrieval signal from the Golgi and ERGIC back to the endoplasmic reticulum.

Clathrin-Coated Vesicles

Vesicles composed of triskelion-shaped clathrin lattices and adaptor complexes (like GGA) that bud from the trans-Golgi network (TGN) toward endosomes/lysosomes or from the plasma membrane during endocytosis.

Mannose-6-Phosphate (M6P) Lysosomal Targeting Pathway

A protein sorting pathway where lysosomal enzymes undergo phosphorylation of mannose residues in the Golgi, are recognized by M6P receptors (MPRs) in the trans-Golgi network, and are packaged into clathrin-coated vesicles destined for lysosomes.

SNARE Hypothesis

The molecular mechanism of vesicle fusion wherein a v-SNARE in the transport vesicle membrane pairs specifically with t-SNAREs in the target membrane to form a four-stranded $\alpha$-helical bundle that drives bilayer apposition and fusion pore formation.

Lysosome

A membrane-enclosed digestive organelle maintained at an acidic internal $pH \approx 4.6$ by active proton ($H^+$) pumps, containing approximately $50\,\text{acid hydrolases}$ that digest endocytosed macromolecules, cellular debris, and autophagosomes.