Unit 2: Cell Structure and Function

Comprehensive vocabulary flashcards covering Unit 2 AP Biology topics: history of cell theory, prokaryotic and eukaryotic organelles, the endomembrane system, cytoskeleton, cell size constraints, membrane structure, passive and active transport, tonicity, water potential, and bulk transport.

Robert Hooke (1665)

The English scientist who first observed cork tissue under a microscope and coined the discovery that cells exist.

Anton Leeuwenhoek (1674)

The Dutch microscopist who discovered that living unicellular organisms exist.

Matthias Schleiden (1838)

The botanist who contributed to cell theory by discovering that all plants are composed of cells.

Theodor Schwann (1839)

The zoologist who contributed to cell theory by concluding that all animals are made of cells.

Cell Theory

The fundamental biological theory stating that all living organisms are made of one or more cells, the cell is the structural and functional unit of life, all cells arise from pre-existing cells by division, and cells pass hereditary material to offspring.

Universal Cellular Components

The four structural features present in all cells: a plasma membrane, cytosol/cytoplasm, genetic material (chromosomes), and ribosomes.

Prokaryotic Cell

A cell type (from "pro" meaning before and "karyon" meaning kernel/nucleus) lacking a membrane-bound nucleus and internal membrane-bound organelles.

Capsule

A sticky external polysaccharide or protein layer in prokaryotes that promotes surface adhesion and offers additional protection.

Prokaryotic Cell Wall

A rigid exterior structure that supports cell shape, provides protection, and remains permeable to water and dissolved solutes.

Pili

Surface appendages in prokaryotes that aid in adhesion to surfaces or facilitate the exchange of plasmid DNA between bacterial cells.

Prokaryotic Flagella

Appendages used for locomotion that rotate and whip around like a propeller to move the cell.

Plasmid

A small, circular, double-stranded DNA molecule separate from chromosomal DNA in bacteria that carries accessory genes easily transferred to other cells.

Nucleoid

The non-membrane-enclosed region in a prokaryotic cell where the single, large circular chromosome of DNA is located.

Eukaryotic Cell

A cell type (from "eu" meaning true and "karyon" meaning kernel/nucleus) characterized by a membrane-bound nucleus and endomembrane-bound organelles.

Organelle Compartmentalization

The structural organization of eukaryotic cells using internal membranes to isolate metabolic functions, specialize microenvironments (such as pH and chemical concentrations), and increase internal surface area for reactions.

Autotrophic Cells

Plant-like eukaryotic cells, including plant cells and certain algal/protist cells, that synthesize organic nutrients from inorganic substances.

Heterotrophic Cells

Animal-like eukaryotic cells, including animal, fungal, and protozoan cells, that must ingest or absorb organic molecules for energy.

Cell Differentiation

The process by which genetically identical cells in a multicellular organism express distinct sets of genes to develop specialized structures and functions.

Endomembrane System

The coordinated network of internal eukaryotic membranes that synthesizes, modifies, packages, and transports proteins and lipids to their cellular destinations.

Nucleus

The double-membrane organelle that permanently houses the eukaryotic genome and serves as the site for DNA replication and RNA transcription.

Nucleolus

A dense specialized compartment located within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosomal subunits are assembled.

Ribosome

A non-membrane-bound molecular complex composed of ribosomal RNA and protein that translates mRNA transcripts into polypeptide chains.

Free Ribosomes

Ribosomes suspended in the cytosol that synthesize soluble proteins that function within the cytoplasm, nucleus, mitochondria, chloroplasts, and peroxisomes.

Bound Ribosomes

Ribosomes attached to the cytosolic face of the rough endoplasmic reticulum that synthesize membrane-inserted proteins, lysosomal enzymes, and secretory proteins.

Rough Endoplasmic Reticulum (RER)

A region of the endoplasmic reticulum studded with bound ribosomes that compartmentalizes protein synthesis and initiates protein folding and trafficking.

Smooth Endoplasmic Reticulum (SER)

A tubular membrane network lacking ribosomes that synthesizes membrane phospholipids, packages proteins into transport vesicles, and contains enzymes for macromolecule breakdown and detoxification.

Golgi Apparatus

A stack of flattened membranous sacs (cisternae) that receives vesicles from the ER at its cis face, modifies products (e.g., adding phosphate groups), and sorts them at its trans face for export or organelle targeting.

Transport Vesicles

Small, fluid-filled membranous spheres that shuttle synthesized proteins and lipids between organelles of the endomembrane system.

Secretory Vesicles

Membrane-bound spheres derived from the trans-Golgi that fuse with the plasma membrane to release secretory proteins or incorporate membrane proteins via exocytosis.

Large Central Vacuole

A large plant organelle that stores water, ions, and waste, exerting hydrostatic turgor pressure against the cell wall to maintain structural rigidity.

Contractile Vacuole

A specialized organelle in freshwater unicellular eukaryotes (such as $Paramecium$) that periodically accumulates and expels excess water to prevent osmotic lysis.

Lysosome

A membrane-bound digestive organelle in animal cells containing over 50 kinds of hydrolytic enzymes that digest macromolecules, recycle worn-out organelles, and destroy engulfed pathogens.

Peroxisome

A metabolic vesicle formed by the fusion of ER and mitochondrial vesicles that breaks down fatty acids; abundant in animal liver cells and germinating plant seeds.

Mitochondria

The double-membrane energy organelle that generates $ATP$ through aerobic respiration; contains its own circular DNA (mtDNA inherited maternally) and ribosomes within its central matrix.

Cristae

The infoldings of the inner mitochondrial membrane that significantly increase surface area for the chemical reactions of aerobic ATP synthesis.

Mitochondrial Matrix

The fluid-filled innermost compartment enclosed by the inner mitochondrial membrane that contains metabolic enzymes, ribosomes, and circular mitochondrial DNA.

Chloroplast

A photosynthetic plastid organelle found in plants and algae that captures light energy and carbon from $CO_2$ to produce glucose.

Thylakoid

A flattened, chlorophyll-containing membranous sac within a chloroplast where the light-dependent reactions of photosynthesis take place.

Grana

Stacks of thylakoid discs inside chloroplasts that maximize membrane surface area for light absorption.

Stroma

The fluid surrounding thylakoids inside the chloroplast where the second half of photosynthesis occurs; contains chloroplast DNA and ribosomes.

Endosymbiotic Theory

The evolutionary theory stating that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells, establishing an obligate mutualistic relationship.

Cytoskeleton

A dynamic intracellular network of protein filaments that reinforces cell shape, anchors and organizes organelles, and directs cellular motility.

Centrosome

The microtubule-organizing center of animal cells, containing a pair of centrioles, responsible for organizing the mitotic spindle during cell division.

Extracellular Matrix (ECM)

A supportive external scaffold in animal tissues composed of cross-linked connective proteins like collagen that aids in structural integrity and intercellular adhesion.

Cell Wall Polysaccharides

Cross-linked structural polymers providing rigidity: cellulose in plants, chitin in fungi, and peptidoglycan in bacteria.

Surface Area-to-Volume Ratio ($\text{SA:V}$)

The geometric ratio between a cell's outer surface and interior volume; as cell volume expands, this ratio declines, limiting the efficiency of membrane exchange.

Microvilli

Tiny, finger-like membrane protrusions on epithelial cells that increase surface area to maximize nutrient absorption without adding substantial volume.

Fluid Mosaic Model

The structural model describing biological membranes as a fluid, dynamic phospholipid bilayer embedded with a mosaic of proteins, steroids, and carbohydrate markers.

Amphipathic

A chemical characteristic describing molecules, such as phospholipids, that possess both a hydrophilic (polar) region and a hydrophobic (nonpolar) region.

Cholesterol (Membrane Buffer)

A steroid lipid in animal membranes that prevents excessive fluidity at high temperatures by restraining phospholipid movement and prevents solidification at low temperatures by disrupting close packing.

Peripheral Membrane Proteins

Proteins that associate temporarily with the hydrophilic exterior or interior surface of the lipid bilayer, functioning in signal transduction and communication.

Integral Membrane Proteins

Proteins permanently embedded within the lipid bilayer, including transmembrane proteins that span the entire membrane to mediate transport and signal reception.

Glycoproteins and Glycolipids

Carbohydrate chains covalently attached to membrane proteins or lipids facing the extracellular environment, functioning in cell-cell recognition and immune identification.

Selective Permeability

The membrane property of regulating solute traffic, allowing small nonpolar molecules ($O_2$, $CO_2$) and small uncharged molecules to pass easily while blocking ions and large polar molecules.

Passive Transport

The net movement of substances down their concentration gradient (from high to low concentration) without the expenditure of metabolic cellular energy ($ATP$).

Simple Diffusion

The unassisted passive movement of small, nonpolar or uncharged molecules directly across the phospholipid bilayer driven by thermal motion.

Facilitated Diffusion

The passive transport of polar molecules or charged ions across a biological membrane through transmembrane channel or carrier proteins.

Aquaporins

Specialized transmembrane channel proteins that facilitate the rapid passive diffusion of water molecules across cell membranes.

Osmosis

The passive net diffusion of free water across a selectively permeable membrane toward the region with a higher solute concentration.

Tonicity

The capability of an extracellular solution to cause water to move into or out of a cell by osmosis, determined only by non-penetrating solutes.

Isotonic Solution

A solution whose non-penetrating solute concentration is equal to that of the cytoplasm, resulting in dynamic equilibrium with equal rates of water flux.

Hypotonic Solution

A solution with a lower solute concentration (and higher free water) than the interior of a cell, driving water influx and causing animal cells to lyse while plant cells become turgid.

Hypertonic Solution

A solution with a higher solute concentration (and lower free water) than the interior of a cell, pulling water out and causing animal cells to crenate and plant cells to plasmolyze.

Turgor Pressure

The outward hydrostatic pressure exerted by a fluid-filled central vacuole against the plant cell wall, providing structural support to keep non-woody tissues upright.

Plasmolysis

The shrinking of a plant cell's protoplast away from its rigid cell wall caused by substantial osmotic water loss in a hypertonic environment.

Osmoregulation

The active physiological management of internal salt and water concentrations to maintain osmotic equilibrium and protect cells from lysis or dehydration.

Osmolarity

The total solute concentration expressed as osmoles of solute particles per liter of solvent, measuring all dissolved solutes regardless of membrane permeability.

Water Potential ($\Psi$)

The physical measure of the potential energy of water per unit volume relative to pure water, calculated by $\Psi = \bar{\Psi}_s + \Psi_p$; water moves spontaneously from higher to lower water potential.

Solute Potential ($\Psi_s$)

The component of water potential proportional to the concentration of dissolved solutes, calculated via $\Psi_s = -iCRT$; pure water equals $0\,\text{bars}$, and adding solutes decreases it to negative values.

Pressure Potential ($\Psi_p$)

The physical hydrostatic pressure exerted on or within a system; equals $0\,\text{bars}$ in an open container at atmospheric pressure and is typically positive in living plant cells.

Active Transport

The energy-requiring movement of a solute against its concentration gradient (from lower to higher concentration) powered by ATP hydrolysis.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ Pump)

An electrogenic transport protein in animal cells that hydrolyzes ATP to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions, generating resting membrane potential.

Electrochemical Gradient

The combined dual force acting on an ion across a membrane, consisting of a chemical concentration gradient and an electrical charge gradient.

Electrogenic Pump

An active transport protein that moves net electrical charge across a membrane, generating a voltage difference (membrane potential) used for cellular work.

Proton Pump

The primary electrogenic pump in plants, fungi, and bacteria that actively translocates hydrogen ions ($\text{H}^+$) across membranes using ATP.

Cotransport (Coupled Transport)

A mechanism in which an active transport pump indirectly drives the transport of a second solute down an established electrochemical gradient (e.g., $\text{H}^+/\text{sucrose}$ cotransporter).

Exocytosis

The form of bulk transport where intracellular transport vesicles fuse with the plasma membrane to secrete macromolecules into the extracellular space.

Endocytosis

The active process by which a cell internalizes extracellular material through the invagination and pinching off of the plasma membrane into vesicles.

Phagocytosis

A type of endocytosis ("cell eating") in which a cell projects pseudopodia to engulf large solid particles or microorganisms into a food vacuole.

Pinocytosis

A type of endocytosis ("cell drinking") involving the nonspecific uptake of extracellular fluid and dissolved solutes within coated vesicles.

Receptor-Mediated Endocytosis

A highly selective form of endocytosis in which target molecules bind to specific plasma membrane receptors, triggering coated pit invagination to form coated vesicles.