AP Biology Unit 2: Cell Biology

Comprehensive vocabulary flashcards covering the molecular origins of life, cell theory, membrane transport dynamics, water potential equations, cytoskeletal filaments, endomembrane trafficking, and organelle structures from AP Biology Unit 2.

Dehydration Synthesis

An anabolic reaction in which two monomer building blocks are covalently linked together with an input of energy (such as $ATP$) and the concurrent release of a water molecule ($H_2O$).

Hydrolysis

A catabolic chemical process where a covalent bond between subunits is cleaved by the addition of a water molecule ($H_2O$), resulting in the release of energy.

Prokaryotic Cell

A simple single-celled organism belonging to Bacteria or Archaea, typically measuring $1\text{--}10\,\mu\text{m}$, characterized by the absence of membrane-bound organelles or a nucleus, possessing circular DNA in a nucleoid region, and containing $70\text{S}$ ribosomes.

Eukaryotic Cell

A structurally complex cell measuring $10\text{--}100\,\mu\text{m}$, derived from archaeal lineages, characterized by membrane-bound compartments, linear chromosomes compartmentalized within a membrane-bound nucleus, and $80\text{S}$ ribosomes.

Plasma Membrane

A semipermeable phospholipid bilayer embedded with proteins and carbohydrates that forms an outer physical boundary separating a cell's internal cytoplasm from the external environment, crucial for transport and homeostasis.

Amphipathic

A property describing molecules that possess both hydrophilic (polar or water-attracting) and hydrophobic (nonpolar or water-repelling) regions, such as phospholipids.

Fluid Mosaic Model

The membrane model proposed by Seymour Singer and Garth Nicolson in 1972 describing cellular membranes as dynamic, two-dimensional solutions of oriented globular proteins freely diffusing within a fluid phospholipid bilayer.

Membrane Cholesterol

A steroid lipid interspersed among fatty acid tails that functions as a fluidity buffer: it restrains excessive phospholipid movement at warm temperatures and prevents tight lipid packaging and solidification at cold temperatures.

Integral Membrane Protein

A protein permanently embedded within the phospholipid bilayer that spans across one or both layers, containing hydrophobic amino acid regions that interact with the fatty acid core.

Peripheral Membrane Protein

A protein temporarily or non-covalently attached to the hydrophilic exterior or interior surface of the phospholipid bilayer, frequently bound to integral proteins or membrane lipids.

Selective Permeability

A membrane characteristic allowing small, nonpolar molecules ($O_2$, $CO_2$, $N_2$) to diffuse unassisted, while restricting large polar molecules (glucose, sucrose) and charged ions ($Na^+$, $K^+$, $Cl^-$) unless transported by specific transport proteins.

Aquaporin

A specialized transmembrane channel protein that selectively facilitates the rapid passive transport (facilitated diffusion) of water molecules across the hydrophobic core of the plasma membrane.

Simple Diffusion

The passive, spontaneous movement of small nonpolar solutes directly across a lipid bilayer down their concentration gradient without the use of cellular energy ($ATP$) or transport proteins.

Facilitated Diffusion

The passive movement of polar or charged substances down their electrochemical gradient via transmembrane channels (gated or non-gated) or carrier proteins without the expenditure of metabolic energy.

Water Potential ($\Psi$)

The measure of free potential energy of water per unit volume relative to pure water under standard conditions, calculated as the sum of pressure potential and solute potential: $\Psi = \Psi_p + \Psi_s$.

Solute Potential ($\Psi_s$)

The osmotic component of water potential determined by dissolved solute molecules, calculated as $\Psi_s = -iCRT$; pure water has $\Psi_s = 0$, and adding solutes decreases $\Psi_s$ to negative values.

Pressure Potential ($\Psi_p$)

The physical hydrostatic pressure exerted on a liquid system or by a cell membrane against a cell wall, which is assigned a value of $0\,\text{bars}$ in an open container.

Plasmolysis

The phenomenon in plant cells placed into a hypertonic external solution where water leaves the cell via osmosis, causing the protoplast to shrink and the plasma membrane to pull away from the rigid cell wall.

Turgidity

The firm, swollen state of a walled plant cell resulting from water entering via osmosis in a hypotonic medium, causing internal pressure to press the plasma membrane tightly against the cell wall.

Primary Active Transport

A transmembrane transport mechanism that directly consumes metabolic energy, typically by hydrolyzing $ATP$, to induce conformational changes that move solutes across a membrane against their concentration gradient.

$Na^+/K^+$ ATPase

An electrogenic primary active transport pump that hydrolyzes one $ATP$ molecule to export three $Na^+$ ions and import two $K^+$ ions against their respective electrochemical gradients.

Secondary Active Transport

A transport process in which a cotransporter harnesses the favorable downhill electrochemical gradient established by a primary pump (like $Na^+$) to drive the uphill movement of another solute (like glucose) against its gradient without directly consuming $ATP$.

Symport

A secondary active cotransport mechanism where the driving ion and the cotransported substance are translocated across the membrane simultaneously in the exact same direction.

Antiport

A secondary active cotransport mechanism in which the driving ion moves in one direction across the membrane while driving the translocation of another solute in the opposite direction.

Phagocytosis

A form of endocytosis ('cell eating') wherein the plasma membrane surrounds and internalizes large solid particulate matter or microbes into an internal vesicle called a phagosome.

Pinocytosis

A non-selective bulk transport endocytic process ('cell drinking') through which a cell continually invaginates its plasma membrane to engulf droplets of extracellular fluid containing dissolved solutes.

Receptor-Mediated Endocytosis

A highly selective form of endocytosis in which specific extracellular ligands bind to membrane-bound surface receptors, triggering vesicle invagination and internalization (e.g., $LDL$ cholesterol uptake).

Exocytosis

An energy-dependent bulk transport mechanism in which intracellular secretory vesicles fuse with the plasma membrane, releasing their contents outside the cell and adding vesicle membrane to the cell surface.

Surface Area-to-Volume Ratio

The geometric ratio ($SA:V$) determining cellular efficiency; as cell dimensions increase, volume scales with the cube of length while surface area scales with the square, resulting in a decreased ratio that constrains cell size based on diffusion rates.

Cell Wall

A rigid extracellular layer located outside the plasma membrane that provides mechanical support and prevents osmotic lysis in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan); it is freely permeable to water and small solutes.

Central Vacuole

A large, fluid-filled organelle bounded by the tonoplast in mature plant cells that stores nutrients, water, pigments, and wastes while generating hydrostatic turgor pressure against the cell wall.

Tonoplast

The selectively permeable cytoplasmic membrane that delimits the central vacuole of a plant cell, separating its contents (cell sap) from the cytoplasm.

Microfilaments

The thinnest cytoskeletal components, measuring approximately $7\,\text{nm}$ in diameter, composed of two intertwined strands of actin monomers; they exhibit structural polarity ($+/-\text{ ends}$) and function in muscle contraction, cell division, and cell shape.

Intermediate Filaments

Rope-like cytoskeletal protein cables measuring $8\text{--}12\,\text{nm}$ in diameter composed of varied proteins (keratin, vimentin, desmin, lamin); they lack polarity, provide high tensile strength, anchor organelles, and construct the nuclear lamina.

Microtubules

Hollow cytoskeletal cylinders measuring roughly $25\,\text{nm}$ in diameter assembled from $\alpha$ and $\beta$ tubulin heterodimers; they exhibit polarity ($+/-\text{ ends}$), form the mitotic spindle, build the core of cilia and flagella, and act as tracks for motor proteins.

Axoneme

The internal structural core of eukaryotic motile cilia and flagella, characterized by a $9+2$ microtubule arrangement (nine outer doublets surrounding two central single microtubules) held together by nexin links, radial spokes, and dynein arms.

Basal Body

A centriole-derived anchoring structure at the base of eukaryotic cilia and flagella composed of a $9+0$ microtubule pattern (nine outer microtubule triplets with no central microtubules).

Dynein

A retrograde motor protein that uses $ATP$ hydrolysis to walk along microtubules toward their minus ($-$) end, generating the sliding force responsible for the bending motion of cilia and flagella.

Kinesin

An anterograde motor protein that hydrolyzes $ATP$ to transport vesicular cargo and organelles along microtubules toward their plus ($+\text{ end}$).

Nuclear Envelope

A double-bilayer lipid barrier surrounding the eukaryotic nucleus, comprising an inner and outer membrane separated by a perinuclear lumen that remains continuous with the endoplasmic reticulum.

Nuclear Lamina

A dense meshwork of intermediate filaments composed of lamin proteins lining the inner nuclear membrane that provides mechanical structural support, anchors chromatin, and helps regulate the cell cycle.

Nuclear Pore

A proteinaceous channel complex traversing the double membrane of the nuclear envelope, mediating the passive diffusion of small molecules and the selective active transport of macromolecules like proteins and RNA.

Nucleolus

A dense, non-membrane-bound droplet inside the nucleus maintained by liquid-liquid phase separation that serves as the site for ribosomal RNA ($rRNA$) synthesis and ribosome subunit assembly.

Chromatin

The eukaryotic complex of genomic $DNA$ wound around histone protein complexes; it remains in an unwound, accessible state during interphase to facilitate transcription and replication.

Rough Endoplasmic Reticulum (RER)

A convoluted membrane network of flattened sacs continuous with the nuclear envelope, studded on its cytosolic surface with ribosomes that translate proteins targeted for secretion, membrane insertion, or lysosomes.

Smooth Endoplasmic Reticulum (SER)

A tubular membrane organelle continuous with the RER that lacks bound ribosomes; it specializes in lipid and steroid synthesis, carbohydrate metabolism, calcium ($Ca^{2+}$) storage, and enzymatic detoxification.

Golgi Apparatus

An endomembrane organelle formed of flattened cisternae that receives proteins and lipids at its cis face, modifies and sorts them through sequential enzyme actions, and dispatches them from its trans face inside transport vesicles.

Cisternae

The flattened, stacked membrane-bound disks or tubules that comprise the functional subunits of both the endoplasmic reticulum and the Golgi apparatus.

Lysosome

A membrane-bound catabolic eukaryotic organelle maintained at an acidic $\text{pH} \sim 4.5\text{--}5.0$ that contains hydrolytic digestive enzymes capable of breaking down biomolecules, cellular debris, and ingested pathogens.

Autophagy

A cellular self-degradation and recycling process in which damaged or redundant intracellular organelles are enclosed in double-membrane autophagosomes and delivered to lysosomes for enzymatic breakdown.

Apoptosis

A programmed, orderly pathway of cellular self-destruction that eliminates damaged or unnecessary cells through orchestrated biochemical signaling, involving organellar leakage and death pathways.

Peroxisome

A eukaryotic membrane-enclosed metabolic organelle that utilizes oxidases to perform $\beta$-oxidation on very-long-chain fatty acids ($VLCFAs$), generating toxic hydrogen peroxide ($H_2O_2$) as a byproduct.

Catalase

An antioxidant enzyme concentrated in peroxisomes that rapidly decomposes potentially damaging hydrogen peroxide into water and molecular oxygen: $2H_2O_2 \rightarrow 2H_2O + O_2$.

Endosymbiotic Theory

The evolutionary model explaining that mitochondria and chloroplasts originated when an ancestral eukaryotic cell engulfed aerobic bacteria and photosynthetic cyanobacteria respectively, establishing a permanent symbiotic relationship supported by double membranes, circular DNA, and $70\text{S}$ ribosomes.

Cellular Compartmentalization

The structural segregation of a eukaryotic cell's internal volume into separate membrane-delimited organelles, which enhances metabolic efficiency by isolating incompatible reactions, increasing surface area, and establishing specialized local microenvironments.