Cell Biology Essentials: 250 Structures and Processes

250 definitions, and 145 name the term they get confused with.

250 cardsLast updated 2026-08-21

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Cell theory
Definition: All living things are made of cells, the cell is the basic unit of structure and function, and every cell arises from a pre-existing cell. Why it matters: The third clause is the one with teeth. It rules out spontaneous generation and turns heredity into a question about cell lineage. Often confused with: Germ theory, which says microorganisms cause disease. Both came out of nineteenth-century microscopy, but they answer different questions.
Prokaryotic cell
Definition: A cell with no nuclear envelope and no membrane-bound organelles, whose chromosome sits directly in the cytoplasm. Where: Bacteria and archaea. Why it matters: With no envelope in the way, transcription and translation happen in the same space at the same time, so a ribosome can start on an mRNA that is still being made. Often confused with: The idea that prokaryotic means primitive. Archaea are prokaryotic, yet their transcription and translation machinery resembles the eukaryotic version more than the bacterial one.

Cell biology goes wrong in a predictable place. The terms are easy to recognize and hard to keep apart: smooth against rough ER, the centromere against the centrosome, facilitated diffusion against active transport, anaphase against anaphase I. Each pair reads as familiar right up to the moment a question asks which one you meant. That is what these 250 cards are built for. Every one opens with a clean definition, 209 add the line that makes it usable, and 145 name the term it actually gets confused with and say what separates the two. That last part is the difference between a deck you recognize and a deck you can answer from, because a note that restates the definition teaches nothing you did not already have. Coverage runs from membrane structure and the organelles through transport, signaling, the cell cycle, DNA replication and gene expression, to respiration, photosynthesis and the methods a first course actually uses. Every card carries a topic tag, so you can run just membrane transport, just the cell cycle, or just photosynthesis instead of the whole subject at once. Import it and the deck joins your spaced-repetition schedule. Terms you already know stretch out and drop out of sight, and the ones that keep slipping come back until they stop slipping.

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Showing 100 representative cards from the full 250-card deck.

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Cell theory	Definition: All living things are made of cells, the cell is the basic unit of structure and function, and every cell arises from a pre-existing cell. Why it matters: The third clause is the one with teeth. It rules out spontaneous generation and turns heredity into a question about cell lineage. Often confused with: Germ theory, which says microorganisms cause disease. Both came out of nineteenth-century microscopy, but they answer different questions.
Prokaryotic cell	Definition: A cell with no nuclear envelope and no membrane-bound organelles, whose chromosome sits directly in the cytoplasm. Where: Bacteria and archaea. Why it matters: With no envelope in the way, transcription and translation happen in the same space at the same time, so a ribosome can start on an mRNA that is still being made. Often confused with: The idea that prokaryotic means primitive. Archaea are prokaryotic, yet their transcription and translation machinery resembles the eukaryotic version more than the bacterial one.
Surface-area-to-volume ratio	Definition: The ratio of a cell's membrane area to the volume that membrane has to serve, which falls as the cell grows. Why it matters: It sets the practical ceiling on cell size. Volume decides how much material has to cross the membrane, area decides how fast it can. Example: Microvilli on an intestinal cell and the flattened shape of a red blood cell both raise the ratio without shrinking the cell.
Cytosol	Definition: The aqueous fluid of the cytoplasm, crowded with enzymes, metabolites, ions, and cytoskeletal filaments. Where: Between the organelles, inside the plasma membrane. Why it matters: Glycolysis, the start of most protein synthesis, and many signaling cascades happen here rather than inside an organelle. Often confused with: Cytoplasm, which also takes in the organelles. Anything described as happening in the cytosol is happening outside every organelle.
Plant cell structures absent from animal cells	Definition: A cell wall of cellulose, chloroplasts, a large central vacuole, and plasmodesmata. Why it matters: These four account for most of what separates the two kinds of cell, namely rigidity, the ability to feed on light, turgor, and cytoplasmic continuity between neighbors. Often confused with: Centrioles. Animal cells have them and most plant cells do not, yet plant cells still build a mitotic spindle.
Endosymbiotic theory	Definition: The proposal that mitochondria and chloroplasts descend from free-living prokaryotes taken up by an ancestral host cell. Evidence: Both carry their own circular DNA and 70S ribosomes, both have a double membrane, and both divide by fission independently of the cell. Often confused with: The idea that the whole eukaryotic cell was assembled this way. Only mitochondria and plastids carry that evidence.
Plasma membrane	Definition: The selectively permeable phospholipid bilayer that encloses the cell and governs what enters and leaves. Where: At the cell's outer boundary, and in plants and bacteria just inside the cell wall. Why it matters: It holds the cell's internal chemistry apart from the outside and carries the receptors, channels, and pumps that deal with it. Often confused with: The cell wall, a rigid layer outside the membrane. The wall gives shape, the membrane decides permeability.
Phospholipid	Definition: A lipid with a phosphate-containing hydrophilic head and two hydrophobic fatty acid tails. Why it matters: That two-part structure is why a bilayer assembles by itself in water. No enzyme builds it. Often confused with: Triglycerides, which carry three fatty acids and no phosphate group, so they store energy instead of forming membranes.
Phospholipid bilayer	Definition: Two sheets of phospholipids with their tails facing inward and their heads facing the water on each side. Why it matters: The hydrophobic core is the actual barrier. Small nonpolar molecules slip through it, while ions and large polar molecules effectively cannot.
Integral membrane protein	Definition: A protein embedded in the hydrophobic core of the bilayer, usually crossing it completely as a transmembrane protein. Why it matters: Only a protein reaching both faces can move something from one side to the other, so channels, carriers, and pumps are all integral. Often confused with: Peripheral proteins, which sit on a surface and can be stripped off by changing salt or pH without disturbing the bilayer.
Glycocalyx	Definition: The carbohydrate coat on the outer face of the plasma membrane, made of the sugar chains of glycoproteins and glycolipids. Why it matters: It is the surface other cells actually read, so it carries cell-to-cell recognition, including the blood group antigens. Often confused with: The extracellular matrix, which is secreted material lying outside the cell. The glycocalyx is attached to the membrane's own molecules.
Selective permeability	Definition: The property of letting some substances cross a membrane while restricting others. Crosses readily: Small nonpolar molecules such as oxygen and carbon dioxide, plus small uncharged polar molecules such as water, though slowly without aquaporins. Held back: Ions and large polar molecules such as glucose, which need a transport protein. Often confused with: Semipermeable in the physics sense, which usually means only the solvent crosses. A cell membrane admits many solutes, but chooses them.
Nucleus	Definition: The organelle that holds the cell's chromosomes, enclosed by a double membrane. Where: Usually near the center of a eukaryotic cell. Why it matters: Transcription and RNA processing happen here and translation does not, and that separation is the defining feature of eukaryotic gene expression. Often confused with: The nucleolus, which is a region inside the nucleus rather than the nucleus itself.
Nuclear pore complex	Definition: A large protein assembly perforating the nuclear envelope and forming an aqueous channel through both membranes. Why it matters: It is the only route in or out. Small molecules diffuse through, but proteins and RNA need a localization or export signal that transport receptors recognize. Often confused with: An open hole. Traffic through it is selective and energy-dependent, driven by a Ran GTPase gradient across the envelope.
Ribosome	Definition: A two-subunit machine of ribosomal RNA and protein that translates mRNA into a polypeptide. Where: Free in the cytosol, bound to the rough ER, and inside mitochondria and chloroplasts. Why it matters: Peptide bond formation is catalyzed by rRNA rather than by protein, which makes the ribosome a ribozyme. Often confused with: A membrane-bound organelle. It has no membrane, which is why prokaryotes have ribosomes despite having no membrane-bound organelles.
Rough endoplasmic reticulum	Definition: The region of the endoplasmic reticulum studded with ribosomes on its cytosolic face. Why it matters: Proteins bound for secretion, the plasma membrane, or a lysosome are threaded into its lumen as they are made, then folded and glycosylated there. Often confused with: Smooth ER. One continuous membrane system doing two jobs, since rough ER makes and processes proteins while smooth ER makes lipids and detoxifies.
Golgi apparatus	Definition: A stack of flattened membrane sacs that modifies, sorts, and dispatches proteins and lipids arriving from the endoplasmic reticulum. Why it matters: It finishes glycosylation and tags proteins for their destination, most famously by adding mannose 6-phosphate to enzymes bound for the lysosome. Often confused with: The endoplasmic reticulum. The ER makes and folds, the Golgi modifies and addresses.
Transport vesicle	Definition: A small membrane sac that buds from one compartment and fuses with another, carrying cargo in its lumen and in its own membrane. Why it matters: Fusion preserves membrane orientation, so a protein facing the vesicle lumen ends up facing the outside of the cell after exocytosis.
Autophagy	Definition: The process in which a cell wraps its own damaged organelles or proteins in a membrane and delivers them to a lysosome for digestion. Why it matters: It is how a cell recycles building blocks during starvation and clears defective mitochondria. Often confused with: Apoptosis. Autophagy is usually a survival response, apoptosis is programmed self-destruction.
Mitochondrion	Definition: A double-membraned organelle that carries out pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. Why it matters: Its inner membrane holds the electron transport chain and ATP synthase, so nearly all the ATP from aerobic respiration is made here. Often confused with: The idea that all of respiration happens inside it. Glycolysis runs in the cytosol, before anything enters the mitochondrion.
Cristae	Definition: The infoldings of the inner mitochondrial membrane. Why it matters: Folding multiplies the area available for electron transport chains and ATP synthase, so ATP output scales with cristae surface. Often confused with: Thylakoids, which serve the same area-maximizing purpose in a chloroplast but form a separate membrane system rather than folds of the inner envelope.
Chloroplast	Definition: A double-membraned plastid containing chlorophyll, where photosynthesis takes place. Where: Plant and algal cells, concentrated in the mesophyll of a leaf. Why it matters: It has three internal compartments rather than two, and the proton gradient is built across the thylakoid membrane, not the inner envelope. Often confused with: The mitochondrion. Both make ATP by chemiosmosis, but the chloroplast's gradient sits across a third internal membrane.
Stroma of the chloroplast	Definition: The fluid surrounding the thylakoids, inside the chloroplast's inner membrane. Why it matters: The Calvin cycle runs here, spending the ATP and NADPH made across the thylakoid membrane. Often confused with: The thylakoid lumen, and in writing with a stoma, which is a pore in the leaf surface.
Central vacuole	Definition: A large fluid-filled sac taking up most of the volume of a mature plant cell. Why it matters: Water entering it presses the cytoplasm against the wall and creates turgor, and it also stores ions, pigments, and waste and carries out lysosome-like digestion. Often confused with: The contractile vacuole of a freshwater protist, which pumps water out rather than holding it in.
Plasmodesma	Definition: A channel through the walls of two adjacent plant cells that joins their cytoplasm. Why it matters: It makes the plant a continuous cytoplasmic network, so water, small solutes, and even some RNA and protein pass without crossing a membrane. Often confused with: The gap junction, the animal equivalent in function. Gap junctions are protein channels in two plasma membranes, while a plasmodesma is lined by continuous membrane running through the wall.
Secretory pathway	Definition: The route a secreted protein takes, from the rough ER lumen into a transport vesicle, through the Golgi, into a secretory vesicle, and out by exocytosis. Why it matters: Every stage is a membrane-enclosed compartment, so the protein never re-enters the cytosol after it is made.
Endosome	Definition: A membrane compartment that receives material taken in by endocytosis and sorts it for recycling or for delivery to a lysosome. Why it matters: Its lumen is progressively acidified, which is how receptors let go of their ligands and return to the cell surface.
Cytoskeleton	Definition: The network of protein filaments that gives a cell its shape, holds organelles in place, and provides tracks for movement. Components: Microfilaments, intermediate filaments, and microtubules. Often confused with: A static scaffold. Microfilaments and microtubules assemble and disassemble constantly, which is what lets a cell change shape, crawl, and divide.
Microtubule	Definition: A hollow tube about 25 nanometers across, built from alpha- and beta-tubulin dimers. Why it matters: It forms the mitotic spindle, the tracks that kinesin and dynein walk along, and the core of cilia and flagella. Often confused with: Intermediate filaments. Microtubules grow and shrink from their plus ends within minutes, while intermediate filaments are stable and have no polarity.
Centrosome	Definition: The main microtubule-organizing center of an animal cell, holding a pair of centrioles surrounded by pericentriolar material. Where: Beside the nucleus, duplicating during S phase. Why it matters: Microtubules nucleate here with their minus ends anchored, so the spindle radiates outward from the two centrosomes at mitosis. Often confused with: The centriole, which is only one component. The nucleating activity lies in the surrounding material, not in the centriole itself.
Motor protein	Definition: A protein that hydrolyzes ATP to take directed steps along a cytoskeletal filament. Why it matters: Vesicle transport, chromosome movement, muscle contraction, and ciliary beating are all the same trick, a motor walking on a track.
Dynein	Definition: A motor protein that walks along microtubules toward the minus end. Why it matters: It hauls cargo inward toward the cell center, and axonemal dynein sliding between microtubule doublets is what bends a cilium. Often confused with: Kinesin. Same filament, opposite direction, which is how one track carries two-way traffic.
Cilium	Definition: A short hair-like projection built around a microtubule axoneme, usually present in large numbers and beating with a back-and-forth stroke. Why it matters: Motile cilia sweep fluid across an epithelial surface, and a single non-motile primary cilium acts as a signaling antenna on many cell types. Often confused with: The eukaryotic flagellum, which has the same internal structure but is longer, usually solitary, and undulates rather than beating in strokes.
Tight junction	Definition: A belt of membrane proteins that presses the plasma membranes of neighboring cells together into a continuous seal. Why it matters: It blocks leakage between cells, so an epithelium can hold different fluids on its two sides. Often confused with: The desmosome. Tight junctions seal, desmosomes fasten, so a leaky epithelium is a tight junction problem and a torn one is a desmosome problem.
Gap junction	Definition: A channel formed by aligned connexon pores in two adjacent plasma membranes, joining the cytoplasm of the two cells. Why it matters: Ions and small molecules pass directly, so cardiac muscle and many embryonic tissues are electrically and chemically coupled. Often confused with: The plasmodesma, its counterpart in plants by function but not by structure.
Extracellular matrix	Definition: The network of proteins and polysaccharides that cells secrete and then live within. Components: Collagen fibers, proteoglycans, and adhesive glycoproteins such as fibronectin and laminin. Why it matters: It is not just packing material. Signals from the matrix reach the cytoskeleton through integrins and can change which genes a cell expresses.
Integrin	Definition: A transmembrane receptor that binds extracellular matrix proteins outside the cell and connects to the cytoskeleton inside it. Why it matters: It carries force and information in both directions, so being attached to the matrix is itself a signal that keeps many cells alive and dividing. Often confused with: Cadherin, which binds cells to each other rather than to the matrix.
Passive transport	Definition: Movement of a substance across a membrane down its concentration or electrochemical gradient, with no metabolic energy spent by the cell. Includes: Simple diffusion, facilitated diffusion, and osmosis. Often confused with: Facilitated diffusion, which uses a protein and is still passive. Using a protein is not the same as using energy.
Osmosis	Definition: Net diffusion of water across a selectively permeable membrane, from the side with lower solute concentration to the side with higher. Why it matters: Water responds to total solute concentration, not to the identity of the solute. Often confused with: The direction. Water moves toward the higher solute concentration, which is the lower water concentration, so the two descriptions are the same movement.
Electrochemical gradient	Definition: The combined driving force on an ion, from its concentration gradient and from the membrane potential together. Why it matters: An ion can move against its concentration gradient if the charge difference pulls hard enough, so predicting ion movement from concentration alone fails. Often confused with: A plain concentration gradient, which is all that acts on an uncharged solute such as glucose.
Hypertonic solution	Definition: A solution whose concentration of non-penetrating solute is higher than the cell's, so water leaves the cell. Result: An animal cell shrivels. A plant cell plasmolyzes as its membrane pulls away from the wall.
Turgor pressure	Definition: The pressure the cell contents exert against the cell wall once water has entered the vacuole. Why it matters: It is what holds a non-woody plant upright, and losing it is what wilting is.
Channel protein	Definition: A transmembrane protein forming a hydrophilic pore that a particular solute passes through, without the protein having to change shape around it. Why it matters: Because nothing has to cycle, channels move solutes far faster than carriers, up to millions of ions per second. Often confused with: A carrier protein, which binds its solute and changes conformation to release it on the other side.
Carrier protein	Definition: A transmembrane protein that binds a specific solute and changes shape to release it on the other side. Why it matters: Each cycle takes time and the binding sites are finite, so transport saturates at high solute concentration. Often confused with: Channel proteins, which never bind and cycle. Carriers serve both facilitated diffusion and active transport.
Active transport	Definition: Movement of a solute against its electrochemical gradient, powered by an energy source. Why it matters: It is what lets a cell hold an internal composition unlike its surroundings. Often confused with: Facilitated diffusion. The test is direction. Active transport runs uphill, and cutting off the cell's energy supply stops it.
Sodium-potassium pump	Definition: An ATP-driven pump that exports three sodium ions and imports two potassium ions for each ATP hydrolyzed. Why it matters: The unequal exchange makes the cell interior more negative and stores the sodium gradient that secondary active transport spends. Often confused with: A channel. It is a carrier that is phosphorylated and changes shape each cycle, so it moves only hundreds of ions per second.
Endocytosis	Definition: Uptake of material by folding a patch of plasma membrane inward and pinching it off as a vesicle. Why it matters: It is how a cell takes in anything too large to cross a membrane, and it removes membrane from the surface, which exocytosis puts back.
Receptor-mediated endocytosis	Definition: Selective uptake in which ligands bind surface receptors that gather in a clathrin-coated pit before it buds inward. Why it matters: It concentrates one substance, so a cell can take up a rare molecule such as an LDL particle efficiently. Often confused with: Pinocytosis. Both bring in extracellular fluid, but only receptor-mediated uptake selects what comes with it.
Cell signaling	Definition: The process by which a cell detects a chemical or physical signal and converts it into a change in its own behavior. Why it matters: The signal molecule usually never enters the cell. Its message is relayed inward by proteins.
Ligand	Definition: A molecule that binds specifically to a receptor and changes its shape. Why it matters: Binding is reversible and non-covalent, so the signal fades as the ligand concentration falls. Often confused with: A substrate. A ligand is not chemically altered by the protein it binds to.
Autocrine signaling	Definition: Signaling in which a cell responds to a molecule it secreted itself. Example: Growth factors released by some tumor cells, which drive the very cell that produced them.
Endocrine signaling	Definition: Long-distance signaling in which hormones travel through the circulation to distant target cells. Why it matters: The signal reaches every cell, so specificity comes entirely from which cells carry the receptor.
G protein	Definition: A relay protein that is active while bound to GTP and inactive once it has hydrolyzed that GTP to GDP. Why it matters: Built-in hydrolysis makes it a self-timing switch, so the signal shuts itself off without needing a separate off signal.
Second messenger	Definition: A small diffusible molecule inside the cell whose concentration changes when a receptor is activated. Example: Cyclic AMP, calcium ions, and inositol trisphosphate, plus diacylglycerol, which acts in the membrane rather than in the cytosol. Why it matters: Being small and water-soluble, it spreads the signal quickly through the cytosol and amplifies it, since one activated enzyme makes many copies. Often confused with: The first messenger, the extracellular ligand, which usually stays outside the cell.
Calcium as a second messenger	Definition: The use of a rise in cytosolic calcium concentration as an internal signal. Why it matters: The cell holds cytosolic calcium thousands of times lower than the level outside and in the endoplasmic reticulum, so opening a channel briefly gives a large, fast signal. Often confused with: Calcium's structural roles, in bone and in cadherin binding. As a messenger, what matters is the steepness of the gradient the cell pays to maintain.
Apoptosis	Definition: Programmed cell death, in which a cell dismantles itself in an orderly way and is packaged for phagocytosis. Why it matters: The cell shrinks and fragments without leaking, so no inflammation follows, and this is how a developing hand loses the webbing between its fingers. Often confused with: Necrosis, which is uncontrolled death from injury. There the cell swells, bursts, and provokes inflammation.
Cell cycle	Definition: The ordered sequence a cell runs through from its own formation to its division, made of interphase followed by the mitotic phase. Why it matters: Almost all of it is interphase. Mitosis is the short, visually dramatic minority of the time.
G1 phase	Definition: The first gap phase, in which the cell grows, makes proteins and organelles, and decides whether to divide at all. Why it matters: The commitment point for the whole cycle sits here. Past it, a cell normally goes on to finish the cycle.
G2 phase	Definition: The second gap phase, in which the cell keeps growing and assembles the machinery for division. Why it matters: A checkpoint at its end verifies that replication finished and that DNA damage has been repaired before mitosis can start.
Mitosis	Definition: Division of one duplicated nucleus into two genetically identical nuclei. Stages: Prophase, prometaphase, metaphase, anaphase, and telophase. Often confused with: Cytokinesis, the division of the cytoplasm. They overlap but are separate events, and a cell can finish mitosis without dividing, which produces a cell with many nuclei.
Metaphase	Definition: The stage in which the chromosomes are lined up at the metaphase plate, each attached to both poles. Why it matters: The plate is a plane, not a structure, and reaching it is what the spindle assembly checkpoint is waiting for. Often confused with: Metaphase I of meiosis, where pairs of homologous chromosomes line up side by side instead of single chromosomes lining up in file.
Cytokinesis	Definition: Division of the cytoplasm into two separate cells. Why it matters: It works differently in the two kingdoms. An actin and myosin ring pinches an animal cell, while a plant builds a new wall outward from the middle.
Mitotic spindle	Definition: The bipolar array of microtubules and associated proteins that moves chromosomes during mitosis. Parts: Kinetochore microtubules attach to chromosomes, while non-kinetochore microtubules overlap at the equator and push the poles apart.
Sister chromatids	Definition: The two identical copies of a replicated chromosome, held together along their length and most tightly at the centromere. Why it matters: They count as one chromosome until anaphase. Once cohesin is cut and they part, each is a chromosome in its own right. Often confused with: Homologous chromosomes, which are the maternal and paternal copies of the same chromosome, carrying the same genes but not identical sequences.
Cell cycle checkpoint	Definition: A control point at which internal signals can halt the cycle until conditions are right. Where: Late in G1, at the G2 to M transition, and during metaphase. Why it matters: Stopping is the default when something is wrong, which is why a checkpoint is described as a stop signal that is normally overridden.
Cyclin	Definition: A regulatory protein whose concentration rises and falls across the cell cycle. Why it matters: Its periodic destruction by the proteasome is what makes the cycle run one way instead of oscillating back. Often confused with: Cyclin-dependent kinase, which is present at a steady level throughout. The cyclin is the part that cycles.
Meiosis	Definition: Two consecutive divisions following one round of DNA replication, turning one diploid cell into four haploid cells. Why it matters: Halving the chromosome number is what stops fertilization from doubling it every generation. Often confused with: Mitosis. Mitosis makes two identical diploid cells in one division, meiosis makes four genetically distinct haploid cells in two.
Meiosis II	Definition: The second meiotic division, in which sister chromatids separate with no DNA replication before it. Why it matters: It is mechanically like mitosis but starts from haploid cells, which is why it is called the equational division.
Oncogene	Definition: A gene whose overactive product drives cell division, formed from a normal proto-oncogene by mutation, amplification, or relocation in the genome. Why it matters: One overactive copy is enough to push the cell, so oncogenes act in a dominant way at the level of the single cell. Often confused with: A tumor suppressor gene, which normally restrains division and causes trouble when it is lost rather than when it is overactive.
Nucleotide	Definition: The building block of a nucleic acid, made of a five-carbon sugar, a phosphate group, and a nitrogenous base. Why it matters: The sugar is what separates the two nucleic acids, deoxyribose in DNA against ribose in RNA, and that extra hydroxyl is part of why RNA is the less stable of the two.
Complementary base pairing	Definition: Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. Why it matters: Each strand specifies the other, which is what makes replication and transcription possible at all. Often confused with: Pairing in RNA, where uracil takes the place of thymine opposite adenine.
Chromatin	Definition: The complex of DNA and protein that makes up a eukaryotic chromosome. Why it matters: About two meters of DNA fits into a nucleus a few micrometers across only because it is packaged in stages. Often confused with: The chromosome. Chromatin is the material, a chromosome is one continuous DNA molecule packaged in it.
Histone	Definition: A small, positively charged protein that DNA wraps around. Why it matters: The positive charge comes from lysine and arginine and grips the negatively charged DNA backbone, and chemical modification of the histone tails loosens or tightens that grip.
DNA replication	Definition: The copying of a DNA molecule, in which the helix is unwound and each strand serves as a template for a new complementary strand. Where: In the nucleus, during S phase. Why it matters: The result is two molecules that each hold one old strand and one new one.
DNA polymerase	Definition: The enzyme that adds nucleotides to the 3' end of a growing strand, working only in the 5' to 3' direction. Why it matters: It cannot start a strand from nothing, so an RNA primer laid down by primase has to come first. Often confused with: RNA polymerase, which also reads a template but needs no primer and makes a single-stranded product.
Okazaki fragment	Definition: One of the short pieces of DNA that make up the lagging strand. Why it matters: Each starts with an RNA primer that is later replaced with DNA, and DNA ligase seals the remaining nicks into one continuous strand.
Gene expression	Definition: The use of the information in a gene to build a functional product, usually a protein. Why it matters: Every cell in a body carries the same genes, so what makes cells different is which genes are expressed and how strongly.
Transcription	Definition: The synthesis of an RNA copy of a gene, in which RNA polymerase reads one DNA strand and builds a complementary RNA strand. Where: The nucleus in eukaryotes, the cytoplasm in prokaryotes. Often confused with: Translation. Transcription copies one nucleic acid into another, while translation changes languages, from nucleotides to amino acids.
RNA processing	Definition: The modifications that turn a eukaryotic pre-mRNA into a mature mRNA, namely adding a 5' cap, adding a poly-A tail, and splicing out introns. Where: In the nucleus, before the transcript is exported. Often confused with: Prokaryotic transcription, where the transcript is translated while it is still being made and none of this happens.
Intron	Definition: A non-coding stretch within a gene that is transcribed and then cut out of the pre-mRNA. Why it matters: Because introns are removed at the RNA stage, the gene on the chromosome is longer than the sequence that ends up specifying the protein. Often confused with: Exons. The usual hook is that introns stay in the nucleus and exons exit it.
Genetic code	Definition: The set of rules assigning each of the 64 nucleotide triplets to an amino acid or to a stop signal. Why it matters: It is redundant but not ambiguous. Several codons can specify one amino acid, but no codon specifies two. Often confused with: The idea that redundancy makes it sloppy. Redundancy buffers mutation, since many changes in the third position leave the amino acid unchanged.
Transfer RNA (tRNA)	Definition: A small folded RNA carrying an amino acid at one end and an anticodon at the other. Why it matters: It is the adaptor that makes the code physical, and accuracy depends on the enzyme that attaches the correct amino acid rather than on the ribosome.
Transcription factor	Definition: A protein that binds DNA and raises or lowers the transcription of particular genes. Why it matters: It is the main way a cell controls gene expression, and it is how a signal arriving at the cell surface ends up changing which proteins the cell makes.
Metabolism	Definition: The complete set of chemical reactions in a cell, organized into pathways in which each step is catalyzed by a specific enzyme. Why it matters: Because every step needs its own enzyme, a pathway can be regulated by controlling a single one of them.
Enzyme	Definition: A biological catalyst, almost always a protein, that speeds up a specific reaction without being consumed by it. Why it matters: It lowers activation energy and does not shift the reaction's equilibrium, only how quickly equilibrium is reached.
Induced fit	Definition: The change in an enzyme's shape when a substrate binds, tightening the active site around it. Often confused with: The older lock and key picture, which imagined a rigid site. Induced fit explains how binding itself helps strain the substrate's bonds toward the reaction.
Cellular respiration	Definition: The catabolic pathway that oxidizes organic fuel to make ATP. Stages: Glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. Why it matters: Glucose is not burned in one step. Its energy is released gradually and captured mostly as electrons carried by NADH and FADH2.
Glycolysis	Definition: The pathway that splits one glucose into two molecules of pyruvate through ten enzyme-catalyzed steps. Where: The cytosol. Net yield per glucose: 2 ATP, 2 NADH, and 2 pyruvate. Why it matters: It needs neither oxygen nor an organelle, which is why essentially every cell has it and why it is thought to be very old.
Pyruvate oxidation	Definition: The link step in which each pyruvate loses a carbon as carbon dioxide, is oxidized to an acetyl group, and is joined to coenzyme A. Where: The mitochondrial matrix in eukaryotes. Yield per pyruvate: 1 NADH. Often confused with: A step of the citric acid cycle. It is a separate reaction that happens before the cycle begins.
NADH	Definition: The reduced form of NAD+, an electron carrier that picks up two electrons and one proton at an oxidation step. Why it matters: It delivers its electrons to the first complex of the electron transport chain, which is why it supports more proton pumping than FADH2 does.
ATP synthase	Definition: A membrane enzyme that lets protons flow back down their gradient and uses the rotation this drives to phosphorylate ADP. Where: The inner mitochondrial membrane, the thylakoid membrane, and the prokaryotic plasma membrane. Often confused with: A proton pump. This runs a pump in reverse, since protons move downhill through it and ATP is the output.
Fermentation	Definition: The regeneration of NAD+ from NADH without oxygen, so that glycolysis can keep running. Why it matters: It produces no extra ATP of its own. Its purpose is to recycle the carrier, and the ATP still comes from glycolysis. Often confused with: Anaerobic respiration, which does use an electron transport chain but ends it on an acceptor other than oxygen, such as sulfate or nitrate.
Chlorophyll	Definition: The green pigment that absorbs light energy for photosynthesis. Why it matters: It absorbs blue and red wavelengths and reflects green, which is why leaves look green and why green light is the least useful part of the spectrum for a plant. Often confused with: The chloroplast. Chlorophyll is a molecule, held in the thylakoid membrane inside the chloroplast.
Photolysis of water	Definition: The splitting of water that supplies electrons to replace those photosystem II lost, releasing protons into the thylakoid lumen and oxygen as a by-product. Why it matters: All the oxygen in the atmosphere came from this step, not from the carbon dioxide.
Calvin cycle	Definition: The stage in which carbon dioxide is fixed onto a five-carbon acceptor, reduced using ATP and NADPH, and the acceptor is regenerated. Where: The stroma of the chloroplast. Often confused with: Being called the dark reactions. It does not require darkness and it stops in the dark anyway, because it depends on what the light reactions supply.
Photorespiration	Definition: The pathway that follows when rubisco binds oxygen instead of carbon dioxide, producing a two-carbon compound the cell has to salvage at a cost in ATP and fixed carbon. Why it matters: It becomes significant on hot dry days, when stomata close, carbon dioxide inside the leaf falls, and oxygen builds up.
Light microscope	Definition: A microscope that focuses visible light through a specimen using glass lenses. Why it matters: Its resolution stops at about 200 nanometers because of the wavelength of light, which is enough for cells and large organelles but not for ribosomes or a membrane. Often confused with: Magnification being the limit. A light microscope can magnify further, but past the resolution limit the image only gets larger and blurrier.
Green fluorescent protein (GFP)	Definition: A protein from a jellyfish that fluoresces green, used as a genetically encoded tag by fusing its gene to a gene of interest. Why it matters: The cell makes the label itself, so a protein can be watched in a living cell over time rather than in a fixed specimen.
Cell fractionation	Definition: Breaking cells open and separating their components so that one organelle can be studied on its own. Why it matters: Assigning a function to an organelle usually means purifying it and showing that the activity travels with it.
Western blot	Definition: A method that separates proteins by gel electrophoresis, transfers them onto a membrane, and detects one of them with a specific antibody. Why it matters: It reports both whether a protein is present and roughly how much, since position on the blot gives size and band intensity gives amount. Often confused with: Southern and northern blots, which use the same transfer idea for DNA and RNA and detect them with a nucleic acid probe instead of an antibody.

Frequently asked

Can I import the whole deck on the free plan?

Yes. Importing a saved deck runs no new AI generation and spends no AI credits, so the free plan imports every card. You can study, edit and delete them afterwards.

Will importing it twice create duplicates?

No. Cards you already have are skipped and only cards added in a revision come through. Including re-imports after deleting it, one official deck can be imported three times per account.

Does it work on the web and in the mobile app?

Yes. The deck is added to your account, so the web app, the iOS app and the Android app all show the same cards and the same progress.

Can I edit the cards after importing?

Yes. Once imported they are your cards: edit either side, delete the ones your course does not cover, retag them, or move them into another deck.

What is on each card?

A term or process on the front. On the back, a definition on all 250, where in the cell it belongs on 35 where location is the point, why it matters on 209, and on 145 the term it is most often confused with. Each line is labeled, so the back reads as a short structured answer rather than a paragraph.

How is the deck organized for studying one topic at a time?

Every card carries a topic tag and a finer one below it, covering cell basics, membranes, organelles, the cytoskeleton, cell junctions, membrane transport, cell signaling, the cell cycle, DNA and chromosomes, gene expression, metabolism, cellular respiration, photosynthesis and lab methods. Filter by a tag to drill just that block.

What is the level, and what is deliberately left out?

It is written for an introductory or first-year college course. It leaves out exam questions, clinical guidance and advanced research protocols, and it covers laboratory methods only as concepts, not as procedures you could run.

What if a card does not match the way my course puts it?

Conventions differ between courses and textbooks, especially over which stage a process belongs to and which name is preferred. Once imported the cards are yours, so edit the back to match the way your course puts it, and study from that.