Cell Structure & Function: every key term you need (+ practice quiz)
103 flashcard terms for AP Biology Unit 2, written to match the course framework. Read them here, drill them as flashcards, or take the 52-question quiz. Free, no account needed.
All living things are made of cells; the cell is the basic unit of life; and all cells come from pre-existing cells. A foundational theory of biology.
Prokaryotic Cell
A cell lacking a membrane-bound nucleus and organelles (bacteria and archaea). DNA sits in a nucleoid region; smaller and simpler than eukaryotic cells.
Eukaryotic Cell
A cell with a true nucleus and membrane-bound organelles (animals, plants, fungi, protists). Larger and compartmentalized.
Compartmentalization
Membrane-bound organelles separate incompatible chemical reactions into distinct spaces, increasing efficiency — a key advantage of eukaryotic cells.
Surface Area–to–Volume Ratio
As a cell grows, volume increases faster than surface area. A high ratio (small cells, folds, microvilli) speeds exchange of materials — a limit on cell size.
Nucleus
The eukaryotic control center; houses DNA (as chromatin), directs protein synthesis, and is bounded by a double membrane (nuclear envelope) with pores.
Nucleolus
A dense region inside the nucleus where ribosomal RNA is made and ribosome subunits are assembled.
Nuclear Envelope & Pores
The double membrane around the nucleus; pores control passage of RNA and proteins between nucleus and cytoplasm.
Ribosomes
Sites of protein synthesis (translation). Made of rRNA and protein; found free in the cytosol or bound to the rough ER. Present in all cells.
Endomembrane System
An interconnected set of membranes (nuclear envelope, ER, Golgi, lysosomes, vesicles, plasma membrane) that synthesizes, modifies, and ships molecules.
Rough ER
Endoplasmic reticulum studded with ribosomes; synthesizes and folds proteins destined for membranes, secretion, or organelles.
Smooth ER
ER without ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies drugs and poisons (abundant in liver cells).
Golgi Apparatus
The cell's 'post office': modifies, sorts, tags, and packages proteins and lipids from the ER into vesicles for delivery.
Vesicle
A small membrane sac that transports materials between organelles and to/from the plasma membrane.
Lysosome
A membrane sac of digestive (hydrolytic) enzymes that breaks down worn organelles, food, and waste; functions best at acidic pH.
Vacuole
A storage sac. The large central vacuole of plant cells stores water and maintains turgor pressure; contractile vacuoles pump out water in protists.
Mitochondria
The 'powerhouse': site of cellular respiration, making ATP. Has a double membrane (inner folds = cristae) and its own DNA and ribosomes.
Cristae
The folds of the inner mitochondrial membrane that increase surface area for the reactions of ATP production.
The site of photosynthesis in plants and algae; captures light to make sugar. Has a double membrane, stacked thylakoids, and its own DNA.
Thylakoids & Grana
Thylakoids are membrane discs (stacked into grana) inside chloroplasts where the light reactions occur; the surrounding fluid is the stroma.
Endosymbiotic Theory
Mitochondria and chloroplasts arose when free-living prokaryotes were engulfed by a host cell. Evidence: double membranes, own circular DNA, own ribosomes, and binary-fission-like division.
Peroxisome
An organelle that breaks down fatty acids and detoxifies harmful substances, producing hydrogen peroxide that it then converts to water.
Cytoskeleton
A protein framework (microtubules, microfilaments, intermediate filaments) that gives shape and support, anchors organelles, and drives movement.
Microtubules
Thick, hollow tubes of tubulin; form tracks for transport, the mitotic spindle, and the core of cilia and flagella.
Microfilaments (Actin)
Thin actin fibers responsible for cell shape, muscle contraction, and cytoplasmic streaming.
Cilia and Flagella
Microtubule-based projections that move cells or move fluid across cell surfaces (e.g., flagella of sperm, cilia lining airways).
Centrosome & Centrioles
The microtubule-organizing center in animal cells; centrioles help organize the spindle during cell division.
Cell Wall
A rigid outer layer outside the plasma membrane in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan) that protects and maintains shape.
Plasmodesmata
Channels through plant cell walls that connect adjacent cells' cytoplasm, allowing transport and communication.
Plant vs. Animal Cells
Plant cells uniquely have a cell wall, chloroplasts, and a large central vacuole; animal cells uniquely have centrioles and lysosomes (typically) and no wall.
Plasma Membrane
The selectively permeable boundary of every cell, controlling what enters and exits. Built from a phospholipid bilayer with embedded proteins.
Fluid Mosaic Model
Describes the membrane as a fluid phospholipid bilayer with a 'mosaic' of proteins, cholesterol, and carbohydrates that drift laterally.
Selective Permeability
The membrane lets some substances cross freely (small, nonpolar) while controlling others (large, polar, charged) — the basis of homeostasis.
Phospholipid Bilayer (Membrane)
Hydrophilic heads face the watery inside and outside; hydrophobic tails face inward, creating a barrier to polar and charged molecules.
Cholesterol in Membranes
Wedged among phospholipids, cholesterol buffers membrane fluidity — keeping it fluid when cold and stable when warm.
Integral (Transport) Proteins
Proteins embedded in the bilayer, including channels and carriers that move hydrophilic substances across the hydrophobic core.
Carbohydrate tags on the outer membrane surface that serve as cell-identity markers for recognition.
Passive Transport
Movement of substances across a membrane down their concentration gradient (high → low) without energy input.
Diffusion
The passive movement of particles from high to low concentration until evenly spread (equilibrium), driven by their random thermal motion.
Concentration Gradient
A difference in concentration between two regions. Substances tend to move down it; maintaining gradients requires energy.
Simple Diffusion
Small, nonpolar molecules (O2, CO2) pass directly through the bilayer, high to low concentration, no proteins or energy needed.
Facilitated Diffusion
Passive transport of polar/charged substances through channel or carrier proteins, down the gradient — no energy, but protein-assisted.
Aquaporins
Channel proteins that dramatically speed the facilitated diffusion of water across membranes.
Osmosis
The passive diffusion of water across a selectively permeable membrane, from higher water potential (low solute) to lower (high solute).
Tonicity
How a solution's solute concentration affects water movement into or out of a cell: hypertonic, hypotonic, or isotonic.
Hypertonic Solution
Higher solute (lower water) outside the cell; water leaves the cell, which shrinks (crenates). Plant cells plasmolyze.
Hypotonic Solution
Lower solute (higher water) outside the cell; water enters, and the cell swells or bursts (lyses). Plant cells become firm (turgid).
Isotonic Solution
Equal solute concentration inside and out; no net water movement — the stable state for animal cells.
Osmoregulation
How organisms control water and solute balance (e.g., contractile vacuoles, kidneys) to survive in different environments.
Turgor Pressure
The pressure of water pushing the plant cell membrane against its wall in a hypotonic environment; keeps plants rigid. Wilting = loss of turgor.
Water Potential (Ψ)
Predicts the direction of water movement; water flows from higher to lower water potential. Ψ = pressure potential + solute potential.
Solute Potential
The component of water potential lowered by dissolved solutes (always negative in a solution); more solute means lower (more negative) water potential.
Active Transport
Movement of substances against their concentration gradient (low → high), requiring energy (ATP) and carrier proteins.
An active-transport protein that pumps 3 Na+ out and 2 K+ in per ATP, maintaining gradients vital for nerve and muscle function.
Electrochemical Gradient
The combined concentration and charge gradient across a membrane that stores energy and drives ion movement.
Bulk Transport
Movement of large particles or large amounts across the membrane using vesicles and energy — endocytosis and exocytosis.
Endocytosis
The cell takes in material by engulfing it in a membrane vesicle. Includes phagocytosis (solids), pinocytosis (liquids), and receptor-mediated uptake.
Exocytosis
Vesicles fuse with the plasma membrane to release their contents outside the cell (e.g., secreting hormones or neurotransmitters).
Phagocytosis
'Cell eating' — endocytosis of large solid particles; how some white blood cells engulf pathogens.
Receptor-Mediated Endocytosis
Specific molecules bind receptors that cluster and are taken in — a selective form of endocytosis (e.g., cholesterol uptake).
Homeostasis
The maintenance of a stable internal environment. Selective membrane transport is central to keeping conditions within tolerable limits.
Protein Synthesis & Secretion Pathway
Rough ER makes and folds a protein → Golgi modifies and packages it into a vesicle → the vesicle exocytoses it — an integrated endomembrane journey.
Cytosol vs. Cytoplasm
Cytosol is the fluid; cytoplasm is everything between the nucleus and membrane, including organelles suspended in cytosol.
Origin of Eukaryotes
Eukaryotic complexity arose via membrane infolding (nucleus, ER) and endosymbiosis (mitochondria, chloroplasts) — increasing compartmentalization.
Free vs. Bound Ribosomes
Free ribosomes make proteins for use in the cytosol; bound ribosomes (on rough ER) make proteins for membranes, secretion, or organelles.
Why Cells Stay Small
Small size keeps a high surface-area-to-volume ratio, ensuring the membrane can exchange enough materials to supply the whole cell.
Membrane Fluidity Factors
Temperature, cholesterol, and fatty-acid saturation adjust how fluid a membrane is — unsaturated tails and cholesterol maintain fluidity in cold.
Structure–Function of Organelles
Each organelle's structure fits its job — cristae maximize respiratory surface, microvilli maximize absorption — echoing the course's central theme.
Fluid Mosaic Model
Membranes are a fluid phospholipid bilayer with embedded and peripheral proteins, cholesterol, and carbohydrates (glycoproteins/glycolipids) that drift laterally.
Selective Permeability
Small nonpolar molecules (O₂, CO₂) cross the bilayer freely; small polar molecules cross slowly; ions and large polar molecules need transport proteins.
Channel Protein
Forms a hydrophilic pore for specific ions or water (aquaporins) to diffuse down their gradient — facilitated diffusion, no ATP required.
Binds a solute and changes shape to move it across the membrane. Used in both facilitated diffusion (e.g., GLUT glucose transporters) and active transport.
Primary Active Transport
Uses ATP directly to pump solutes against their gradient; the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP, generating membrane potential.
Secondary Active Transport (Cotransport)
An ion gradient built by a primary pump powers uphill transport of another solute — e.g., the sucrose–H⁺ symporter in plants.
Water Potential (Ψ)
Ψ = Ψs + Ψp. Water moves from higher to lower water potential; pure water at atmospheric pressure has Ψ = 0, and solutes make Ψ negative.
Solute Potential Formula
Ψs = −iCRT, where i is the ionization constant, C molar concentration, R = 0.0831 L·bar/mol·K, T in kelvin. NaCl has i = 2, sucrose i = 1.
Tonicity in Plant Cells
In hypotonic solution the cell becomes turgid (wall resists lysis); in hypertonic solution the membrane pulls from the wall — plasmolysis.
Endocytosis Types
Phagocytosis engulfs solids, pinocytosis takes in fluid, and receptor-mediated endocytosis concentrates specific ligands (e.g., LDL cholesterol) in coated pits.
Exocytosis
Vesicles fuse with the plasma membrane to release contents (neurotransmitters, hormones) and add membrane and proteins to the cell surface.
Surface Area-to-Volume Consequences
As a cell grows, volume rises faster than surface area (V ∝ r³ vs SA ∝ r²), limiting exchange; folding (microvilli, cristae) raises effective surface area.
Nuclear Pore Complex
Regulates traffic of mRNA and ribosomal subunits out of, and proteins such as transcription factors into, the nucleus.
Protein Trafficking Pathway
Secreted proteins go rough ER → transport vesicle → cis Golgi → trans Golgi → secretory vesicle → plasma membrane; the sequence can be traced with pulse-chase labeling.
Evidence for Endosymbiosis
Mitochondria and chloroplasts have double membranes, circular DNA, 70S ribosomes, and divide by binary fission — features shared with bacteria.
Cell Wall Comparison
Plants use cellulose, fungi use chitin, and bacteria use peptidoglycan; walls provide structural support and prevent osmotic lysis.
Osmoregulation by Contractile Vacuole
Freshwater protists such as Paramecium constantly pump out water that enters osmotically, an ATP-costly adaptation to a hypotonic environment.
Cholesterol as a Fluidity Buffer
At warm temperatures cholesterol restrains phospholipid movement; at cold temperatures it wedges between tails and prevents tight packing. It narrows the range over which fluidity changes.
Homeoviscous Adaptation
Organisms adjust membrane composition to their environment — cold-water fish and winter-hardened plants make more unsaturated tails so membranes stay fluid at low temperature.
Glycocalyx and Cell Recognition
Glycoproteins and glycolipids project sugar chains from the outer membrane face. Their patterns act as identity tags in immune recognition, ABO blood typing, and tissue sorting.
Aquaporins
Channel proteins that let water cross the bilayer far faster than simple diffusion allows, without using ATP. Kidney collecting ducts regulate them to concentrate urine.
Diffusion rate rises with surface area, concentration gradient, and temperature, and falls with distance and molecule size. Alveoli and villi maximize area while minimizing diffusion distance.
Tight Junctions
Membranes of adjacent animal cells are fused into a watertight seal, blocking leakage between cells. Found in the intestinal lining and the blood-brain barrier.
Desmosomes
Rivet-like anchoring junctions tied to intermediate filaments that let sheets of cells withstand stretching, as in skin and heart muscle.
Gap Junctions and Plasmodesmata
Cytoplasmic channels that let ions and small molecules pass directly between neighbors — gap junctions in animals (coordinating heart contraction), plasmodesmata through plant cell walls.
Signal Peptide and the SRP
An N-terminal signal sequence is bound by the signal-recognition particle, which docks the ribosome on the rough ER so the growing chain enters the lumen. Sorting begins during translation.
Oxidizing ER Lumen
The ER interior favors disulfide-bond formation, so secreted and membrane proteins get cross-linked cysteines while cytosolic proteins usually do not.
Lysosomal Storage Disorders
A missing lysosomal hydrolase lets its substrate accumulate — in Tay-Sachs disease undigested lipid swells nerve-cell lysosomes. Evidence that organelle enzymes are gene products.
Peroxisomes
Organelles that transfer hydrogen to O2, generating H2O2 during fatty-acid breakdown and detoxification; catalase inside immediately converts H2O2 to water and O2.
Motor Proteins on the Cytoskeleton
Kinesin and dynein walk along microtubules in opposite directions, and myosin walks along actin. They power vesicle transport, chromosome movement, and muscle contraction.
Nuclear Lamina
A meshwork of intermediate filaments lining the inner nuclear envelope that maintains nuclear shape and organizes chromatin at the periphery.
70S vs. 80S Ribosomes
Bacteria (and mitochondria and chloroplasts) use smaller 70S ribosomes; eukaryotic cytosol uses 80S. Antibiotics such as tetracycline exploit the difference.
Cell Fractionation
Homogenized cells spun at increasing speeds pellet organelles by size and density — nuclei first, then mitochondria, then microsomes and ribosomes — allowing biochemical assay of each.
Incipient Plasmolysis
The solute concentration at which exactly half the cells in a tissue show plasmolysis; it estimates the tissue's internal solute potential in a water-potential lab.