AP Biology Unit 2: Cell Structure and Function (Sections 2.1–2.10)

Comprehensive vocabulary flashcards covering AP Biology Unit 2: Cell Structure and Function, spanning cell theory, organelle biology, cytoskeletal networks, the endomembrane system, membrane architecture, intercellular junctions, passive/active transport mechanisms, and cellular tonicity.

Cell Theory

The foundational biological principle stating that all living organisms are composed of cells, cells are the basic structural and functional units of life, and all cells arise from pre-existing cells.

Robert Hooke

The scientist who observed thin slices of dead cork tissue under a light microscope in 1665 and coined the term "cells" because the empty chambers reminded him of the rooms where monks lived.

Prokaryotic Cells

Unicellular organisms belonging to the domains Bacteria and Archaea that lack a membrane-bound nucleus and specialized membrane-bound organelles, typically being smaller and less complex than eukaryotes.

Eukaryotic Cells

Organisms that include plants, animals, fungi, and protists, characterized by having genetic material enclosed within a true nucleus and containing complex, membrane-bound organelles.

Ribosomes

Cellular structures composed of two subunits made of RNA and proteins that synthesize polypeptide chains using genetic information transcribed from DNA into RNA.

Cytosol

The semifluid solution consisting of water, inorganic ions, and organic molecules that suspends the organelles of the cell and houses enzymes that facilitate vital metabolic reactions.

Nucleus

The eukaryotic organelle that houses and protects chromosomal DNA, hypothesized to have evolved billions of years ago through the infolding of the ancestral plasma membrane.

Nucleolus

A dense, dark region located within the nucleus dedicated to the synthesis and assembly of ribosomal RNA and ribosomal subunits.

Nuclear Envelope

A double membrane enclosing the eukaryotic nucleus that is perforated with nuclear pores to regulate the passage of proteins, RNA, and other macromolecules between the nucleoplasm and the cytoplasm.

Endosymbiotic Theory

The evolutionary model proposing that energy-converting eukaryotic organelles, specifically mitochondria and chloroplasts, originated as free-living prokaryotic cells that were engulfed by ancestral eukaryotic host cells.

Evidence for Endosymbiosis

Observations supporting endosymbiotic origins, including that mitochondria and chloroplasts possess their own circular DNA, contain prokaryote-like ribosomes, replicate independently via binary fission, exhibit bacterial dimensions, and are surrounded by a double membrane.

Endomembrane System

An integrated network of internal membranes within eukaryotic cells that synthesizes, modifies, and transports proteins and lipids, consisting of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, vesicles, lysosomes, and peroxisomes.

Endoplasmic Reticulum (ER)

A continuous system of interconnected membranous tubules and flattened sacs physically joined to the outer membrane of the nuclear envelope.

Rough Endoplasmic Reticulum

A region of the endoplasmic reticulum studded with ribosomes on its outer cytoplasmic face, functioning in the synthesis and conformational folding of proteins destined for membranes or extracellular export.

Smooth Endoplasmic Reticulum

A region of the endoplasmic reticulum lacking bound ribosomes that synthesizes lipids, metabolizes and breaks down carbohydrates or dangerous compounds, and pinches off transport vesicles directed to the Golgi body.

Vesicle

A small, spherical membrane-bound sac that transports synthesized biomolecules between compartments of the endomembrane system or fuses with the plasma membrane to discharge cargo outside the cell.

Golgi Body

An organelle consisting of a stack of curved, flattened membranous sacs called saccules that receives ER transport vesicles, chemically modifies their contents, and sorts and ships the final products.

Lysosomes

Golgi-derived vesicles packed with hydrolytic digestive enzymes that break down worn-out cellular organelles, digest ingested pathogens, and participate in developmental programmed tissue removal, such as webbing between embryonic digits.

Cutaneous Syndactyly

A developmental abnormality resulting in persistent webbing between human fingers or toes caused by the failure of lysosomes to execute apoptotic tissue breakdown during embryogenesis.

Peroxisomes

Membrane-bound metabolic compartments whose oxidase enzymes break down fatty acids and toxic chemicals, producing hydrogen peroxide ($H_2O_2$), which is subsequently degraded into water ($H_2O$) and oxygen ($O_2$) by the enzyme catalase.

Zellweger Syndrome

A severe congenital metabolic disorder caused by a defective peroxisome gene, leading to the absence or loss of functional peroxisomes and a failure to oxidize fatty acids.

Vacuoles

Large, membrane-bound storage sacs that hold water, pigments, nutrients, and waste products; plant cells typically possess a dominant large central vacuole, whereas animal cells possess numerous smaller vacuoles.

Chloroplasts

Photosynthetic, pigment-containing plant and algal organelles that capture radiant solar energy to convert carbon dioxide and water into energy-rich carbohydrates.

Mitochondria

Double-membrane eukaryotic organelles responsible for cellular respiration, where carbohydrate derivatives are systematically catabolized to produce cellular energy in the form of $ATP$.

Cytoskeleton

A dynamic, changeable internal meshwork of filamentous protein fibers that provides mechanical structural support, maintains cell architecture, and assists in organelle transport and whole-cell movement.

Actin Filaments

Cytoskeletal elements approximately $7\,\text{nm}$ in diameter made of two twisted chains of actin globular monomers that work alongside myosin motor proteins to support cell shape and drive intracellular contractions.

Intermediate Filaments

Tough, rope-like cytoskeletal proteins measuring approximately $8\,\text{nm}$ to $12\,\text{nm}$ across (e.g., keratin) that represent the most stable and permanent cytoskeletal elements, structurally reinforcing the plasma membrane and nuclear envelope.

Microtubules

Hollow cylindrical protein cylinders approximately $25\,\text{nm}$ wide assembled from tubulin subunits that maintain cell shape, serve as conveyor tracks for motor proteins, and facilitate chromosomal separation during cell division.

Cell Wall

A rigid, protective extracellular barrier external to the plasma membrane in plants and algae, composed primarily of structural carbohydrates (such as cellulose) and associated structural proteins.

Phospholipid Bilayer

The core framework of biological membranes consisting of two opposing leaflets of phospholipids with polar, hydrophilic heads interacting with exterior aqueous phases and nonpolar, hydrophobic fatty acid tails sequestered inside.

Amphipathic

A chemical property referring to molecules, such as phospholipids, that possess both a distinct hydrophilic (water-soluble/polar) domain and a hydrophobic (water-insoluble/nonpolar) domain.

Cholesterol (Membrane)

A steroid lipid interspersed within animal cell membranes that modulates bilayer fluidity and structural flexibility across temperature fluctuations; notably absent from plant cell membranes.

Glycolipids and Glycoproteins

Lipids and membrane-embedded proteins bearing attached branching carbohydrate chains that project into the extracellular space to serve as molecular identity markers for cellular recognition and immune surveillance.

Selective Permeability

A property of biological membranes that allows nonpolar, hydrophobic molecules to cross the hydrophobic core easily by diffusion, while blocking or strictly regulating the passage of polar molecules, large solutes, and ions.

Integral Proteins

Membrane proteins that are permanently integrated within the hydrophobic interior of the phospholipid bilayer, often spanning both faces as transmembrane proteins.

Peripheral Proteins

Membrane-associated proteins located non-covalently on either the intracellular or extracellular hydrophilic surfaces of the phospholipid bilayer that can reversibly attach and detach.

Aquaporins

Specialized integral channel proteins that facilitate the rapid and selective passive diffusion of water molecules across biological membranes.

Transport Proteins

Transmembrane channel or carrier proteins that permit and regulate the passage of specific ions, polar molecules, and other solutes across the phospholipid bilayer.

Receptor Proteins

Membrane proteins exhibiting specific extracellular binding pockets designed to recognize and bind signaling molecules, triggering conformational changes that elicit specific cellular responses.

Enzymatic Proteins

Membrane-bound proteins that catalyze specific metabolic and chemical reactions directly along the cytoplasmic or extracellular surface of the bilayer.

Extracellular Matrix (ECM)

An intricate meshwork of fibrous proteins and complex polysaccharides secreted outside animal cells that anchors tissues, regulates cellular adhesion, and facilitates intercellular signaling.

Integrins

Transmembrane adhesion proteins that physically span the plasma membrane to link extracellular matrix proteins outside the cell directly to the internal actin cytoskeleton.

Collagen

The primary tensile glycoprotein of the animal extracellular matrix and the most abundant protein in mammals, constituting approximately $25\%$ to $30\%$ of all mammalian body protein.

Desmosomes

Mechanically robust anchoring junctions that behave like microscopic "spot welds," fastening adjacent plasma membranes together in animal tissues subjected to severe stretching, such as the heart, stomach, skin, and bladder.

Tight Junctions

Continuous protein seals fused between adjacent epithelial cells that form an impermeable barrier preventing fluids and solutes from leaking between cells in structures like the urinary bladder and kidneys.

Gap Junctions

Intercellular channel junctions composed of lined protein tunnels that permit direct cytoplasmic exchange of ions and small signaling molecules, coordinating electrical coupling in cardiac and nervous tissue.

Passive Transport

The net movement of chemical substances across a biological membrane down their respective concentration gradients from high to low concentration without the expenditure of cellular metabolic energy ($ATP$).

Concentration Gradient

A physical measurement describing a difference in solute concentration between two adjacent regions or across opposite sides of a semipermeable membrane.

Diffusion

The passive net dispersion of molecules from an area of higher concentration to an area of lower concentration, commonly describing the spontaneous crossing of small nonpolar molecules and dissolved gases across a membrane.

Facilitated Diffusion

A form of passive membrane transport where polar or charged solutes diffuse down their concentration gradient through designated transmembrane channel or carrier proteins without cellular energy expenditure.

Osmosis

The passive diffusion of free water molecules across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration).

Hypotonic Solution

An external solution that has a lower solute concentration (and thus a higher free water concentration) than the cell interior, causing net osmotic water influx that makes the cell swell.

Isotonic Solution

A bathing solution exhibiting equal concentrations of non-penetrating solutes and free water relative to the cell's cytoplasm, resulting in dynamic equilibrium where water moves inward and outward at identical rates.

Hypertonic Solution

An external solution with a higher concentration of solutes (and a lower water concentration) relative to the cell's interior, drawing water out of the cell via osmosis and causing cellular shrinkage.

Plasmolysis

The biological process wherein a plant cell loses water to a hypertonic environment, causing its cytoplasm and internal plasma membrane to shrivel and pull away from the rigid cell wall.

Active Transport

The energy-requiring transport mechanism that pumps ions or molecules across a cellular membrane against their concentration gradient (from an area of low concentration to high concentration), requiring $ATP$ input.

Protein Pump

A specialized transmembrane transport protein that couples the hydrolysis of $ATP$ to the mechanical translocation of solutes across the membrane against their established concentration gradient.

Endocytosis

An active bulk-transport process during which the plasma membrane invaginates around external matter, pinching off internally to enclose the material within an endocytic vesicle.

Exocytosis

An active bulk transport mechanism where intracellular membrane-bound secretory vesicles migrate to and fuse with the plasma membrane, releasing their contents into the extracellular matrix.

Phagocytosis

A specialized form of endocytosis, often termed "cellular eating," in which a cell surrounds, engulfs, and packages large solid particulate matter, cell debris, or microbial invaders into large internal vacuoles.

Pinocytosis

A non-specific type of endocytosis, often termed "cellular drinking," wherein small invaginations of the cell membrane take up extracellular fluid droplets along with their dissolved solutes into tiny vesicles.