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 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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What's inside

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

Front	Back
Cell theory — what is it?	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. Germ theory, which says microorganisms cause disease. Both came out of nineteenth-century microscopy, but they answer different questions.
Prokaryotic cell — what is it?	A cell with no nuclear envelope and no membrane-bound organelles, whose chromosome sits directly in the cytoplasm. 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 — what is it?	The ratio of a cell's membrane area to the volume that membrane has to serve, which falls as the cell grows. It sets the practical ceiling on cell size. Volume decides how much material has to cross the membrane, area decides how fast it can.
Cytosol — what is it?	The aqueous fluid of the cytoplasm, crowded with enzymes, metabolites, ions, and cytoskeletal filaments. 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 — what is it?	A cell wall of cellulose, chloroplasts, a large central vacuole, and plasmodesmata. Centrioles. Animal cells have them and most plant cells do not, yet plant cells still build a mitotic spindle.
Endosymbiotic theory — what is it?	The proposal that mitochondria and chloroplasts descend from free-living prokaryotes taken up by an ancestral host cell. The idea that the whole eukaryotic cell was assembled this way. Only mitochondria and plastids carry that evidence.
Plasma membrane — what is it?	The selectively permeable phospholipid bilayer that encloses the cell and governs what enters and leaves. The cell wall, a rigid layer outside the membrane. The wall gives shape, the membrane decides permeability.
Phospholipid — what is it?	A lipid with a phosphate-containing hydrophilic head and two hydrophobic fatty acid tails. Triglycerides, which carry three fatty acids and no phosphate group, so they store energy instead of forming membranes.
Phospholipid bilayer — what is it?	Two sheets of phospholipids with their tails facing inward and their heads facing the water on each side. The hydrophobic core is the actual barrier. Small nonpolar molecules slip through it, while ions and large polar molecules effectively cannot.
Integral membrane protein — what is it?	A protein embedded in the hydrophobic core of the bilayer, usually crossing it completely as a transmembrane protein. Peripheral proteins, which sit on a surface and can be stripped off by changing salt or pH without disturbing the bilayer.
Glycocalyx — what is it?	The carbohydrate coat on the outer face of the plasma membrane, made of the sugar chains of glycoproteins and glycolipids. The extracellular matrix, which is secreted material lying outside the cell. The glycocalyx is attached to the membrane's own molecules.
Selective permeability — what is it?	The property of letting some substances cross a membrane while restricting others. Semipermeable in the physics sense, which usually means only the solvent crosses. A cell membrane admits many solutes, but chooses them.
Nucleus — what is it?	The organelle that holds the cell's chromosomes, enclosed by a double membrane. The nucleolus, which is a region inside the nucleus rather than the nucleus itself.
Nuclear pore complex — what is it?	A large protein assembly perforating the nuclear envelope and forming an aqueous channel through both membranes. An open hole. Traffic through it is selective and energy-dependent, driven by a Ran GTPase gradient across the envelope.
Ribosome — what is it?	A two-subunit machine of ribosomal RNA and protein that translates mRNA into a polypeptide. A membrane-bound organelle. It has no membrane, which is why prokaryotes have ribosomes despite having no membrane-bound organelles.
Rough endoplasmic reticulum — what is it?	The region of the endoplasmic reticulum studded with ribosomes on its cytosolic face. 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 — what is it?	A stack of flattened membrane sacs that modifies, sorts, and dispatches proteins and lipids arriving from the endoplasmic reticulum. The endoplasmic reticulum. The ER makes and folds, the Golgi modifies and addresses.
Transport vesicle — what is it?	A small membrane sac that buds from one compartment and fuses with another, carrying cargo in its lumen and in its own membrane. Fusion preserves membrane orientation, so a protein facing the vesicle lumen ends up facing the outside of the cell after exocytosis.
Autophagy — what is it?	The process in which a cell wraps its own damaged organelles or proteins in a membrane and delivers them to a lysosome for digestion. Apoptosis. Autophagy is usually a survival response, apoptosis is programmed self-destruction.
Mitochondrion — what is it?	A double-membraned organelle that carries out pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. The idea that all of respiration happens inside it. Glycolysis runs in the cytosol, before anything enters the mitochondrion.
Cristae — what is it?	The infoldings of the inner mitochondrial membrane. 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 — what is it?	A double-membraned plastid containing chlorophyll, where photosynthesis takes place. The mitochondrion. Both make ATP by chemiosmosis, but the chloroplast's gradient sits across a third internal membrane.
Stroma of the chloroplast — what is it?	The fluid surrounding the thylakoids, inside the chloroplast's inner membrane. The thylakoid lumen, and in writing with a stoma, which is a pore in the leaf surface.
Central vacuole — what is it?	A large fluid-filled sac taking up most of the volume of a mature plant cell. The contractile vacuole of a freshwater protist, which pumps water out rather than holding it in.
Plasmodesma — what is it?	A channel through the walls of two adjacent plant cells that joins their cytoplasm. 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 — what is it?	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. Every stage is a membrane-enclosed compartment, so the protein never re-enters the cytosol after it is made.
Endosome — what is it?	A membrane compartment that receives material taken in by endocytosis and sorts it for recycling or for delivery to a lysosome. Its lumen is progressively acidified, which is how receptors let go of their ligands and return to the cell surface.
Cytoskeleton — what is it?	The network of protein filaments that gives a cell its shape, holds organelles in place, and provides tracks for movement. A static scaffold. Microfilaments and microtubules assemble and disassemble constantly, which is what lets a cell change shape, crawl, and divide.
Microtubule — what is it?	A hollow tube about 25 nanometers across, built from alpha- and beta-tubulin dimers. Intermediate filaments. Microtubules grow and shrink from their plus ends within minutes, while intermediate filaments are stable and have no polarity.
Centrosome — what is it?	The main microtubule-organizing center of an animal cell, holding a pair of centrioles surrounded by pericentriolar material. The centriole, which is only one component. The nucleating activity lies in the surrounding material, not in the centriole itself.
Motor protein — what is it?	A protein that hydrolyzes ATP to take directed steps along a cytoskeletal filament. Vesicle transport, chromosome movement, muscle contraction, and ciliary beating are all the same trick, a motor walking on a track.
Dynein — what is it?	A motor protein that walks along microtubules toward the minus end. Kinesin. Same filament, opposite direction, which is how one track carries two-way traffic.
Cilium — what is it?	A short hair-like projection built around a microtubule axoneme, usually present in large numbers and beating with a back-and-forth stroke. The eukaryotic flagellum, which has the same internal structure but is longer, usually solitary, and undulates rather than beating in strokes.
Tight junction — what is it?	A belt of membrane proteins that presses the plasma membranes of neighboring cells together into a continuous seal. 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 — what is it?	A channel formed by aligned connexon pores in two adjacent plasma membranes, joining the cytoplasm of the two cells. The plasmodesma, its counterpart in plants by function but not by structure.
Extracellular matrix — what is it?	The network of proteins and polysaccharides that cells secrete and then live within. It is not just packing material. Signals from the matrix reach the cytoskeleton through integrins and can change which genes a cell expresses.
Integrin — what is it?	A transmembrane receptor that binds extracellular matrix proteins outside the cell and connects to the cytoskeleton inside it. Cadherin, which binds cells to each other rather than to the matrix.
Passive transport — what is it?	Movement of a substance across a membrane down its concentration or electrochemical gradient, with no metabolic energy spent by the cell. Facilitated diffusion, which uses a protein and is still passive. Using a protein is not the same as using energy.
Osmosis — what is it?	Net diffusion of water across a selectively permeable membrane, from the side with lower solute concentration to the side with higher. 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 — what is it?	The combined driving force on an ion, from its concentration gradient and from the membrane potential together. A plain concentration gradient, which is all that acts on an uncharged solute such as glucose.
Hypertonic solution — what is it?	A solution whose concentration of non-penetrating solute is higher than the cell's, so water leaves the cell.
Turgor pressure — what is it?	The pressure the cell contents exert against the cell wall once water has entered the vacuole. It is what holds a non-woody plant upright, and losing it is what wilting is.
Channel protein — what is it?	A transmembrane protein forming a hydrophilic pore that a particular solute passes through, without the protein having to change shape around it. A carrier protein, which binds its solute and changes conformation to release it on the other side.
Carrier protein — what is it?	A transmembrane protein that binds a specific solute and changes shape to release it on the other side. Channel proteins, which never bind and cycle. Carriers serve both facilitated diffusion and active transport.
Active transport — what is it?	Movement of a solute against its electrochemical gradient, powered by an energy source. Facilitated diffusion. The test is direction. Active transport runs uphill, and cutting off the cell's energy supply stops it.
Sodium-potassium pump — what is it?	An ATP-driven pump that exports three sodium ions and imports two potassium ions for each ATP hydrolyzed. A channel. It is a carrier that is phosphorylated and changes shape each cycle, so it moves only hundreds of ions per second.
Endocytosis — what is it?	Uptake of material by folding a patch of plasma membrane inward and pinching it off as a vesicle. 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 — what is it?	Selective uptake in which ligands bind surface receptors that gather in a clathrin-coated pit before it buds inward. Pinocytosis. Both bring in extracellular fluid, but only receptor-mediated uptake selects what comes with it.
Cell signaling — what is it?	The process by which a cell detects a chemical or physical signal and converts it into a change in its own behavior. The signal molecule usually never enters the cell. Its message is relayed inward by proteins.
Ligand — what is it?	A molecule that binds specifically to a receptor and changes its shape. A substrate. A ligand is not chemically altered by the protein it binds to.
Autocrine signaling — what is it?	Signaling in which a cell responds to a molecule it secreted itself.
Endocrine signaling — what is it?	Long-distance signaling in which hormones travel through the circulation to distant target cells. The signal reaches every cell, so specificity comes entirely from which cells carry the receptor.
G protein — what is it?	A relay protein that is active while bound to GTP and inactive once it has hydrolyzed that GTP to GDP. Built-in hydrolysis makes it a self-timing switch, so the signal shuts itself off without needing a separate off signal.
Second messenger — what is it?	A small diffusible molecule inside the cell whose concentration changes when a receptor is activated. The first messenger, the extracellular ligand, which usually stays outside the cell.
Calcium as a second messenger — what is it?	The use of a rise in cytosolic calcium concentration as an internal signal. 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 — what is it?	Programmed cell death, in which a cell dismantles itself in an orderly way and is packaged for phagocytosis. Necrosis, which is uncontrolled death from injury. There the cell swells, bursts, and provokes inflammation.
Cell cycle — what is it?	The ordered sequence a cell runs through from its own formation to its division, made of interphase followed by the mitotic phase. Almost all of it is interphase. Mitosis is the short, visually dramatic minority of the time.
G1 phase — what is it?	The first gap phase, in which the cell grows, makes proteins and organelles, and decides whether to divide at all. The commitment point for the whole cycle sits here. Past it, a cell normally goes on to finish the cycle.
G2 phase — what is it?	The second gap phase, in which the cell keeps growing and assembles the machinery for division. A checkpoint at its end verifies that replication finished and that DNA damage has been repaired before mitosis can start.
Mitosis — what is it?	Division of one duplicated nucleus into two genetically identical nuclei. 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 — what is it?	The stage in which the chromosomes are lined up at the metaphase plate, each attached to both poles. Metaphase I of meiosis, where pairs of homologous chromosomes line up side by side instead of single chromosomes lining up in file.
Cytokinesis — what is it?	Division of the cytoplasm into two separate cells. 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 — what is it?	The bipolar array of microtubules and associated proteins that moves chromosomes during mitosis.
Sister chromatids — what is it?	The two identical copies of a replicated chromosome, held together along their length and most tightly at the centromere. Homologous chromosomes, which are the maternal and paternal copies of the same chromosome, carrying the same genes but not identical sequences.
Cell cycle checkpoint — what is it?	A control point at which internal signals can halt the cycle until conditions are right. Stopping is the default when something is wrong, which is why a checkpoint is described as a stop signal that is normally overridden.
Cyclin — what is it?	A regulatory protein whose concentration rises and falls across the cell cycle. Cyclin-dependent kinase, which is present at a steady level throughout. The cyclin is the part that cycles.
Meiosis — what is it?	Two consecutive divisions following one round of DNA replication, turning one diploid cell into four haploid cells. Mitosis. Mitosis makes two identical diploid cells in one division, meiosis makes four genetically distinct haploid cells in two.
Meiosis II — what is it?	The second meiotic division, in which sister chromatids separate with no DNA replication before it. It is mechanically like mitosis but starts from haploid cells, which is why it is called the equational division.
Oncogene — what is it?	A gene whose overactive product drives cell division, formed from a normal proto-oncogene by mutation, amplification, or relocation in the genome. A tumor suppressor gene, which normally restrains division and causes trouble when it is lost rather than when it is overactive.
Nucleotide — what is it?	The building block of a nucleic acid, made of a five-carbon sugar, a phosphate group, and a nitrogenous base. 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 — what is it?	Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. Pairing in RNA, where uracil takes the place of thymine opposite adenine.
Chromatin — what is it?	The complex of DNA and protein that makes up a eukaryotic chromosome. The chromosome. Chromatin is the material, a chromosome is one continuous DNA molecule packaged in it.
Histone — what is it?	A small, positively charged protein that DNA wraps around. 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 — what is it?	The copying of a DNA molecule, in which the helix is unwound and each strand serves as a template for a new complementary strand. The result is two molecules that each hold one old strand and one new one.
DNA polymerase — what is it?	The enzyme that adds nucleotides to the 3' end of a growing strand, working only in the 5' to 3' direction. RNA polymerase, which also reads a template but needs no primer and makes a single-stranded product.
Okazaki fragment — what is it?	One of the short pieces of DNA that make up the lagging strand. 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 — what is it?	The use of the information in a gene to build a functional product, usually a protein. Every cell in a body carries the same genes, so what makes cells different is which genes are expressed and how strongly.
Transcription — what is it?	The synthesis of an RNA copy of a gene, in which RNA polymerase reads one DNA strand and builds a complementary RNA strand. Translation. Transcription copies one nucleic acid into another, while translation changes languages, from nucleotides to amino acids.
RNA processing — what is it?	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. Prokaryotic transcription, where the transcript is translated while it is still being made and none of this happens.
Intron — what is it?	A non-coding stretch within a gene that is transcribed and then cut out of the pre-mRNA. Exons. The usual hook is that introns stay in the nucleus and exons exit it.
Genetic code — what is it?	The set of rules assigning each of the 64 nucleotide triplets to an amino acid or to a stop signal. 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) — what is it?	A small folded RNA carrying an amino acid at one end and an anticodon at the other. 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 — what is it?	A protein that binds DNA and raises or lowers the transcription of particular genes. 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 — what is it?	The complete set of chemical reactions in a cell, organized into pathways in which each step is catalyzed by a specific enzyme. Because every step needs its own enzyme, a pathway can be regulated by controlling a single one of them.
Enzyme — what is it?	A biological catalyst, almost always a protein, that speeds up a specific reaction without being consumed by it. It lowers activation energy and does not shift the reaction's equilibrium, only how quickly equilibrium is reached.
Induced fit — what is it?	The change in an enzyme's shape when a substrate binds, tightening the active site around it. 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 — what is it?	The catabolic pathway that oxidizes organic fuel to make ATP. Glucose is not burned in one step. Its energy is released gradually and captured mostly as electrons carried by NADH and FADH2.
Glycolysis — what is it?	The pathway that splits one glucose into two molecules of pyruvate through ten enzyme-catalyzed steps. 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 — what is it?	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. A step of the citric acid cycle. It is a separate reaction that happens before the cycle begins.
NADH — what is it?	The reduced form of NAD+, an electron carrier that picks up two electrons and one proton at an oxidation step. 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 — what is it?	A membrane enzyme that lets protons flow back down their gradient and uses the rotation this drives to phosphorylate ADP. A proton pump. This runs a pump in reverse, since protons move downhill through it and ATP is the output.
Fermentation — what is it?	The regeneration of NAD+ from NADH without oxygen, so that glycolysis can keep running. Anaerobic respiration, which does use an electron transport chain but ends it on an acceptor other than oxygen, such as sulfate or nitrate.
Chlorophyll — what is it?	The green pigment that absorbs light energy for photosynthesis. The chloroplast. Chlorophyll is a molecule, held in the thylakoid membrane inside the chloroplast.
Photolysis of water — what is it?	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. All the oxygen in the atmosphere came from this step, not from the carbon dioxide.
Calvin cycle — what is it?	The stage in which carbon dioxide is fixed onto a five-carbon acceptor, reduced using ATP and NADPH, and the acceptor is regenerated. 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 — what is it?	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. It becomes significant on hot dry days, when stomata close, carbon dioxide inside the leaf falls, and oxygen builds up.
Light microscope — what is it?	A microscope that focuses visible light through a specimen using glass lenses. 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) — what is it?	A protein from a jellyfish that fluoresces green, used as a genetically encoded tag by fusing its gene to a gene of interest. 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 — what is it?	Breaking cells open and separating their components so that one organelle can be studied on its own. Assigning a function to an organelle usually means purifying it and showing that the activity travels with it.
Western blot — what is it?	A method that separates proteins by gel electrophoresis, transfers them onto a membrane, and detects one of them with a specific antibody. 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.

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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.

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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.

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Editorial method and sources

Memly's editors use the primary sources below to check scope and terminology, then independently write and review the cards.

NCBI Bookshelf: Molecular Biology of the Cell, 4th edition