Cell Structure & Function: every key term you need (+ practice quiz)
69 flashcard terms for AP Biology Unit 2, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 26-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.
Chloroplast
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.
Peripheral Proteins
Proteins loosely attached to the membrane surface; often involved in signaling and support.
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.
Sodium-Potassium Pump
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.