Memly's AP® Biology Core Concepts: 300 Free Study Cards

300 cards from water chemistry to ecosystems, tagged by topic.

300 cardsLast updated 2026-08-21

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The hard part of this course is not any single topic. It is that the topics are graded on how they connect, so a definition you can recite still leaves you stuck when a question asks what happens to the recessive phenotype frequency when a population stops mating at random, or why an energy pyramid cannot be inverted. These 300 cards are built for that. Each one gives a clean definition and then a second line that makes it usable, most often why the idea matters in practice, and where two ideas are routinely swapped, the card names the other one instead of restating the first. Coverage runs across water and macromolecules, energetics and enzymes, cell communication and homeostasis, chromosomes and heredity, gene expression and biotechnology, evolution and phylogeny, ecology, and the experimental design and statistics that show up in data questions. Cell structure and the molecular machinery — organelles, membranes, respiration and photosynthesis step by step, mitosis, replication, transcription and translation — live in the Cell Biology Essentials deck instead, so the two are built to be used together. Cards carry a topic tag, so you can drill just heredity, just evolution, or just ecology instead of the whole course. Import it and the deck joins your spaced-repetition schedule, and the concepts that keep slipping keep coming back until they stop slipping.

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Each card gets its own schedule, based on how well you actually recall it.
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What's inside

Showing 100 representative cards from the full 300-card deck.

Front	Back
Polar covalent bond	Definition: A shared pair of electrons held unequally, so one atom carries a partial negative charge and the other a partial positive charge. Why it matters: Oxygen pulls harder than hydrogen, which is what makes water polar and gives every other property of water its cause. Often confused with: An ionic bond, where an electron is transferred outright rather than shared unevenly.
Cohesion	Definition: The tendency of water molecules to stick to one another through hydrogen bonding. Why it matters: It lets a column of water in a xylem vessel be pulled upward as one thread rather than breaking apart. Often confused with: Adhesion, which is water sticking to a different substance.
Specific heat of water	Definition: The unusually large amount of heat water absorbs for each degree its temperature rises. Why it matters: Hydrogen bonds take up the added energy before molecular motion increases, so oceans and cells resist swings in temperature.
Density of ice	Definition: The fact that solid water is less dense than liquid water, because hydrogen bonds lock molecules into an open crystal. Why it matters: Ice floats and insulates the water below, so a pond freezes from the top down and life continues underneath. Often confused with: The general rule for substances, which is that the solid is denser than the liquid. Water is the exception.
pH	Definition: A logarithmic measure of hydrogen ion concentration, running from 0 to 14 with 7 neutral. Why it matters: Because the scale is logarithmic, a change of one unit is a tenfold change in hydrogen ion concentration, which is why small pH shifts denature proteins.
Base	Definition: A substance that lowers the hydrogen ion concentration of a solution, usually by accepting hydrogen ions. Often confused with: Nitrogenous bases in DNA, which share the word but not the chemistry.
Carbon skeleton	Definition: The chain or ring of carbon atoms forming the backbone of an organic molecule. Why it matters: Carbon forms four covalent bonds, so skeletons can branch, ring, and vary in length, which is the source of biological molecular diversity.
Hydroxyl group	Definition: An oxygen bonded to a hydrogen and to the carbon skeleton. Why it matters: It is polar, so it makes a molecule more soluble in water. Alcohols and sugars carry it.
Carboxyl group	Definition: A carbon double-bonded to one oxygen and single-bonded to a hydroxyl. Why it matters: It donates a hydrogen ion readily, so molecules carrying it act as acids. Amino acids and fatty acids both have one.
Phosphate group	Definition: A phosphorus bonded to four oxygens, carrying a negative charge at cellular pH. Why it matters: Adding one to a protein changes its charge and shape, which is the basis of regulation by phosphorylation, and chains of them store energy in ATP.
Monomer and polymer	Definition: A monomer is a single subunit, and a polymer is a chain of many of them linked together. Why it matters: Three of the four macromolecule classes are polymers, so the same two reactions build and break nearly everything in a cell. Often confused with: Lipids, which are not true polymers because their subunits are not joined in a repeating chain.
Monosaccharide	Definition: A single sugar unit, generally with a formula that is a multiple of CH2O. Example: Glucose, fructose, and galactose, all six-carbon sugars with the same formula but different arrangements. Why it matters: It is both fuel and raw material, since a cell can oxidize it or link it into a larger carbohydrate.
Glycogen	Definition: The storage polysaccharide of animals, a heavily branched glucose polymer held in liver and muscle. Why it matters: Branching gives many free ends for enzymes to attack at once, so glucose can be released quickly. Often confused with: Starch, which serves the same purpose in plants but is far less branched.
Saturated fatty acid	Definition: A fatty acid whose carbon chain holds the maximum number of hydrogens, with no double bonds. Why it matters: Straight chains pack tightly, so these fats are solid at room temperature and stiffen membranes.
Triglyceride	Definition: Three fatty acids joined to a glycerol backbone. Why it matters: Its carbon-hydrogen bonds are highly reduced, so gram for gram it stores about twice the energy of a carbohydrate.
Amino acid	Definition: A monomer with a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain. Why it matters: Twenty of them, differing only in that side chain, are enough to build every protein an organism makes.
Peptide bond	Definition: The covalent bond joining the carboxyl group of one amino acid to the amino group of the next. Why it matters: It gives every polypeptide a direction, from an amino end to a carboxyl end, which is why sequence is always written one way.
Secondary structure	Definition: Local folding into alpha helices and beta pleated sheets, held by hydrogen bonds along the polypeptide backbone. Often confused with: Tertiary structure. Secondary bonding involves the backbone, not the side chains.
Quaternary structure	Definition: The arrangement of two or more folded polypeptide subunits into one functional protein. Example: Hemoglobin, built from four subunits whose cooperation is what makes its oxygen binding cooperative.
Purines and pyrimidines	Definition: Purines are the double-ring bases adenine and guanine, and pyrimidines are the single-ring bases cytosine, thymine, and uracil. Why it matters: A purine always pairs with a pyrimidine, which keeps the width of the double helix constant along its whole length.
Water potential	Definition: The free energy of water per unit volume, which predicts the direction water will move. Rule: Water always moves from higher water potential to lower water potential, and pure water at standard pressure is defined as zero. Why it matters: It combines solute effects and pressure effects into one number, which osmosis alone cannot do for a walled cell.
Pressure potential	Definition: The component of water potential contributed by physical pressure, which can be positive or negative. Why it matters: In a turgid plant cell the wall pushes back with positive pressure, which raises water potential and eventually stops further water entry.
First law of thermodynamics	Definition: Energy can be transferred and transformed but cannot be created or destroyed. Why it matters: An organism does not make energy. It captures energy already present and converts it into forms it can use.
Gibbs free energy	Definition: The portion of a system's energy that can do work at constant temperature and pressure. Why it matters: The sign of its change tells you whether a reaction runs on its own, which is more useful than knowing the heat released.
Endergonic reaction	Definition: A reaction that absorbs free energy from its surroundings and does not proceed on its own. Example: Building a polypeptide from amino acids, which is why protein synthesis has a running energy cost.
Cofactor	Definition: A non-protein inorganic helper, usually a metal ion, that an enzyme needs in order to work. Example: Zinc, iron, and magnesium ions bound in the active sites of many enzymes.
Optimal pH for an enzyme	Definition: The pH at which an enzyme's reaction rate is highest, set by the charges its side chains need to carry. Example: Pepsin works near pH 2 in the stomach while most cytosolic enzymes work near pH 7, so an optimum is a property of the enzyme rather than a universal value.
Allosteric regulation	Definition: A change in an enzyme's activity caused by a molecule binding at a site away from the active site. Why it matters: It works in both directions, since an allosteric activator stabilizes the active shape while an inhibitor stabilizes the inactive one.
Substrate concentration and reaction rate	Definition: The relationship in which rate rises with substrate concentration and then levels off. Why it matters: The plateau means every active site is occupied, so at saturation only adding enzyme raises the rate further.
Autotroph	Definition: An organism that builds its own organic molecules from inorganic carbon. Why it matters: Autotrophs are the entry point for both energy and carbon into an ecosystem, which is why they occupy the base of every food web.
Metabolic rate	Definition: The total energy an organism uses per unit time. Measured by: Oxygen consumption, carbon dioxide production, or heat released, all of which track the same underlying process.
Ectotherm	Definition: An animal whose body temperature is governed chiefly by its surroundings. Why it matters: A low resting cost lets it survive long fasts, but its activity level tracks the environment, which is why a lizard basks before hunting.
Absorption spectrum and action spectrum	Definition: An absorption spectrum plots how much light a pigment absorbs at each wavelength, while an action spectrum plots how effectively each wavelength drives photosynthesis. Why it matters: The two curves resemble each other but do not match exactly, and the gap is evidence that pigments other than chlorophyll a also contribute.
Homeostasis	Definition: The maintenance of a relatively stable internal environment despite changes outside. Often confused with: Something unchanging. The internal value oscillates around a set point rather than holding still, which is why measurements fluctuate in a healthy organism.
Positive feedback	Definition: A control loop in which the response amplifies the original stimulus. Example: Childbirth contractions, fruit ripening, and blood clotting, each of which needs an outside event to end it. Often confused with: Being harmful by definition. It is not a malfunction, it is the mechanism used when a process needs to run to completion quickly.
Hormone	Definition: A chemical messenger released by one tissue that acts on cells elsewhere in the body carrying the matching receptor. Why it matters: Hormones are slow and long-lasting compared with nerve signals, which is why they govern growth, development, and metabolism rather than reflexes.
Pheromone	Definition: A chemical released into the environment that affects the behavior or physiology of another individual of the same species. Example: Ant trail markers and moth mating attractants, both of which can act at very low concentrations.
Action potential	Definition: A rapid, self-propagating reversal of membrane voltage that travels along a neuron. Why it matters: It is all-or-none, so a stronger stimulus is encoded as a higher firing frequency rather than as a larger signal.
Signal amplification	Definition: The growth in the number of activated molecules at each step of a transduction pathway. Why it matters: It is why a hormone present at very low concentration can produce a large cellular response, and why receptor number rather than ligand number often limits the effect.
Innate immune response	Definition: The fast, non-specific defenses present from birth, including barriers, phagocytes, and inflammation. Why it matters: It recognizes broad molecular patterns shared by many pathogens, so it responds within minutes but does not improve with repeated exposure.
Homologous chromosomes	Definition: The maternal and paternal copies of the same chromosome, carrying the same genes at the same loci but not necessarily the same alleles. Why it matters: Their pairing and separation in the first meiotic division is what produces haploid cells and what independent assortment acts on.
Nondisjunction	Definition: The failure of homologous chromosomes or sister chromatids to separate during meiosis. Why it matters: It produces gametes with one chromosome too many or too few, and it is the origin of most changes in chromosome number.
Binary fission	Definition: Prokaryotic cell division, in which a circular chromosome is replicated and the two copies are drawn apart as the cell elongates and divides. Often confused with: Mitosis, which needs a spindle and a nuclear envelope to break down. Binary fission uses neither.
Fertilization	Definition: The fusion of two haploid gametes to form a diploid zygote. Why it matters: It restores the chromosome number that meiosis halved and adds a third source of variation, since which two gametes meet is itself random.
Gene	Definition: A stretch of DNA that specifies a functional product and occupies a particular place on a chromosome. Often confused with: An allele. A gene is the locus and its general function, an allele is one of the sequence versions found there.
Locus	Definition: The fixed position a gene occupies on a chromosome. Why it matters: Homologous chromosomes carry the same loci in the same order, which is what makes pairing and crossing over meaningful.
Phenotype	Definition: The observable traits of an individual. Often confused with: Genotype. Two organisms with different genotypes can share a phenotype, which is why a dominant phenotype never tells you the genotype by itself.
Recessive allele	Definition: The allele whose effect appears only when no dominant allele is present. Why it matters: It can be carried invisibly through generations, which is why a recessive condition can appear in a child whose parents both lack it.
Punnett square	Definition: A grid that pairs every possible gamete from one parent with every possible gamete from the other. Why it matters: It shows probabilities for each offspring independently, not a guaranteed distribution among a small number of actual offspring.
Dihybrid cross	Definition: A cross following two genes at once. Why it matters: The classic 9 to 3 to 3 to 1 ratio appears only when the two genes assort independently, so a departure from it is evidence of linkage or gene interaction.
Nine to three to three to one ratio	Definition: The phenotype ratio among the offspring of two individuals heterozygous for two independently assorting genes. Why it matters: It is the product of two separate 3 to 1 ratios, which is why the same logic scales to three genes without a bigger grid.
Rule of addition	Definition: The probability of an event that can happen in more than one mutually exclusive way is the sum of the separate probabilities. Example: A heterozygote can arise two ways in a cross, so the two routes are added rather than multiplied.
Incomplete dominance	Definition: An inheritance pattern in which the heterozygote shows a phenotype intermediate between the two homozygotes. Example: A red-flowered and a white-flowered parent producing pink offspring. Often confused with: Blending inheritance. The alleles are not mixed, since crossing two pink plants restores red and white offspring.
Multiple alleles	Definition: The situation in which a population contains more than two alleles of one gene, though each individual still carries only two. Why it matters: It expands the number of possible genotypes sharply while leaving Mendelian segregation untouched.
Polygenic inheritance	Definition: A pattern in which several genes each add a small effect to one trait. Why it matters: It produces continuous variation and a bell-shaped distribution rather than discrete categories, which is why height cannot be charted in a simple square.
Epistasis	Definition: An interaction in which one gene masks or modifies the expression of another gene. Why it matters: It distorts the expected dihybrid ratio, so a cross yielding 9 to 7 or 12 to 3 to 1 is a signal that two genes act in the same pathway.
Sex chromosome	Definition: A chromosome that differs between the sexes and carries the genes determining sex, as opposed to an autosome. Why it matters: In the human system the Y is much smaller, so many genes on the X have no counterpart in an XY individual.
X-linked recessive pattern	Definition: The inheritance pattern in which a recessive allele on the X shows in every XY individual that carries it and only in homozygous XX individuals. Why it matters: It is far more common in males, and an affected male inherits the allele from his mother rather than from his father.
Linked genes	Definition: Genes close together on the same chromosome, which therefore tend to be inherited as a unit. Why it matters: They violate independent assortment, so a dihybrid cross gives an excess of parental combinations and a shortfall of recombinants.
Map unit	Definition: A unit of genetic distance equal to one percent recombination frequency between two genes. Often confused with: A physical distance in base pairs. Recombination is not equally likely everywhere on a chromosome, so the two measures do not scale together.
Point mutation	Definition: A change affecting a single nucleotide pair in a gene. Why it matters: Its consequences range from nothing at all to a completely nonfunctional protein, depending entirely on where in the codon it falls.
Frameshift mutation	Definition: An insertion or deletion of a number of bases not divisible by three, which shifts the reading frame. Why it matters: Every codon downstream is misread, so a frameshift is usually far more damaging than a substitution.
Enhancer	Definition: A DNA sequence that increases transcription of a gene when the right regulatory proteins bind it. Why it matters: It can sit far from the promoter and still act, because the DNA loops to bring the two together.
Histone modification	Definition: The chemical alteration of histone tails, which loosens or tightens the grip of the histones on DNA. Why it matters: Acetylation generally opens chromatin and raises transcription, so gene accessibility is regulated without changing the sequence.
Capsid	Definition: The protein shell enclosing a viral genome. Why it matters: It is built from a few repeated subunits, which is how a very small genome can encode a container much larger than itself.
Lysogenic cycle	Definition: A viral cycle in which the genome integrates into the host chromosome and is copied along with it. Why it matters: The virus is replicated for generations without killing anything, and stress can trigger a switch to the lytic cycle later.
Horizontal gene transfer	Definition: The movement of genetic material between organisms other than from parent to offspring. Why it matters: It lets a trait such as antibiotic resistance spread across a bacterial population, and even between species, far faster than inheritance could.
Restriction enzyme	Definition: A bacterial enzyme that cuts DNA at a specific short sequence. Why it matters: Many leave staggered single-stranded ends, so fragments from two different sources can be joined by base pairing.
Natural selection	Definition: The process in which heritable variation that improves survival and reproduction becomes more common in a population over generations. Requires: Variation, heritability, differential reproductive success, and more offspring than the environment can support. Often confused with: Individuals adapting during their lifetime. Selection acts on individuals but the population is what changes.
Directional selection	Definition: Selection favoring one extreme of a trait range, shifting the population mean toward it. Example: Antibiotic resistance rising in a bacterial population under continuous exposure.
Genetic drift	Definition: Change in allele frequencies from generation to generation caused by random sampling rather than by selection. Why it matters: Its effect is far stronger in a small population, where chance alone can eliminate an allele that is not harmful.
Gene flow	Definition: The movement of alleles between populations through migration and interbreeding. Why it matters: It makes populations more similar to one another, so it works against the divergence that speciation requires.
Hardy-Weinberg equilibrium	Definition: The condition in which allele and genotype frequencies stay constant from generation to generation. Why it matters: It is a null model rather than a description of reality, so its value is that a departure from it is evidence that something is acting on the population.
Speciation	Definition: The process by which one population splits into two that can no longer interbreed successfully. Why it matters: It is what turns gradual change within a lineage into the branching pattern that phylogenies record.
Postzygotic barrier	Definition: A barrier acting after fertilization, so that the hybrid fails to develop, fails to survive, or is sterile. Example: The mule, which is vigorous but almost always sterile because its parent chromosome sets cannot pair in meiosis.
Gradualism	Definition: The view that evolutionary change accumulates slowly and steadily over long spans of time. Often confused with: A claim about mechanism. Both gradualism and punctuated equilibrium use the same mechanisms and differ over tempo.
Homologous structures	Definition: Structures shared by different species because both inherited them from a common ancestor. Example: The same bone arrangement in a human arm, a whale flipper, and a bat wing, despite very different uses.
Phylogenetic tree	Definition: A branching diagram representing hypothesized evolutionary relationships among groups. Often confused with: A ladder of progress. Living species sit at the tips, so no living group is ancestral to another, and rotating a branch changes nothing about the relationships shown.
Abiotic synthesis hypothesis	Definition: The proposal that simple organic molecules formed on the early Earth from inorganic precursors before any life existed. Why it matters: Laboratory experiments passing energy through mixtures of simple gases produce amino acids and other monomers, showing the step is chemically possible.
Levels of ecological organization	Definition: The nested scales of ecological study, from individual organism to population, community, ecosystem, and biosphere. Why it matters: Each level adds one kind of interaction, since a community adds other species and an ecosystem adds the nonliving surroundings.
Population density	Definition: The number of individuals of a species per unit area or volume. Measured by: Direct counts for small populations, and sampling methods such as quadrats or mark and recapture for large ones.
Density-dependent factor	Definition: A limiting factor whose effect grows stronger as population density increases. Example: Competition for food, predation, disease transmission, and accumulation of waste.
Species with K-selected traits	Definition: A species favored by slow reproduction, few large offspring, heavy parental investment, and long life. Why it matters: This strategy pays in stable habitats near carrying capacity, where competitive ability matters more than reproductive speed.
Interspecific competition	Definition: Competition between individuals of different species for the same limited resource. Why it matters: Both populations are harmed, which distinguishes it from interactions where one side gains.
Predation	Definition: An interaction in which one organism kills and consumes another. Why it matters: Predator and prey numbers often cycle out of phase, because a rise in prey supports a later rise in predators that then drives the prey down again.
Keystone species	Definition: A species whose effect on community structure is far larger than its abundance would suggest. Why it matters: Removing it changes the whole community, which is how the concept is demonstrated experimentally.
Trophic level	Definition: A position in a food chain defined by how many steps separate an organism from the producers. Why it matters: Energy is lost at every transfer, which is what limits the number of levels a food chain can support.
Energy pyramid	Definition: A diagram showing the energy available at each trophic level, narrowing sharply toward the top. Why it matters: The shape cannot invert, because a level can never hold more energy than the one that feeds it.
Carbon cycle	Definition: The movement of carbon among the atmosphere, living organisms, soils, oceans, and rock. Why it matters: Photosynthesis and respiration move it in opposite directions on a short timescale, while burial and combustion move it on a much longer one.
Eutrophication	Definition: The enrichment of a body of water with nutrients, leading to algal blooms and then to oxygen depletion as the blooms decay. Why it matters: The fish die from lack of oxygen rather than from the nutrients, so the damage arrives one step after the cause.
Hypothesis	Definition: A testable, falsifiable proposed explanation that predicts what should be observed. Often confused with: A guess or a question. A hypothesis has to rule something out, so a statement no result could contradict is not one.
Control group	Definition: The group that does not receive the experimental treatment, giving a baseline to compare against. Why it matters: Without it, a change cannot be attributed to the treatment rather than to time, handling, or the setup itself.
Mean	Definition: The sum of the values divided by the number of values. Why it matters: It is pulled by extreme values, so a single outlier can move it well away from the typical measurement.
Null hypothesis	Definition: The statement that there is no real difference or association, and that any observed one is due to chance. Why it matters: A statistical test evaluates this statement rather than the interesting one, so the outcome is a decision to reject it or not.
Xylem	Definition: The plant tissue that carries water and dissolved minerals upward from the roots. Why it matters: Its conducting cells are dead and hollow at maturity, so transport happens through empty tubes rather than through living cytoplasm.
Stomata	Definition: Adjustable pores in the leaf surface through which gases enter and leave. Why it matters: They cannot admit carbon dioxide without also losing water, so every stomatal setting is a trade-off between photosynthesis and dehydration.
Gas exchange surface	Definition: The thin, moist, highly folded surface across which an animal exchanges oxygen and carbon dioxide. Why it matters: Gases must dissolve to diffuse, so the surface has to stay wet, which is why terrestrial animals keep it internal.
Antigen	Definition: A molecule the immune system recognizes as foreign and responds to. Often confused with: A pathogen. The antigen is usually one molecule on a pathogen's surface rather than the organism itself.
Sliding filament model	Definition: The explanation of muscle contraction in which thick and thin filaments slide past one another without changing length. Why it matters: The sarcomere shortens while its filaments do not, which is why the model is stated in terms of overlap rather than shrinkage.
Circadian rhythm	Definition: A biological cycle of roughly twenty-four hours that continues even without external cues. Why it matters: Because it persists in constant conditions, it is generated internally and merely reset by light rather than driven by it.

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.

How is it organized for studying one topic at a time?

Every card carries a topic tag and a finer one below it, across chemistry of life, cells and transport, energetics, cell communication, chromosomes, heredity, gene expression, biotechnology and viruses, evolution, ecology, experimental design and organismal systems. Filter by a tag to drill one block.

How does this differ from the Cell Biology Essentials deck?

That deck goes deep on one subject, covering organelles, membranes, transport and the molecular machinery in detail. This one is broad, putting its largest share of cards on heredity, evolution, ecology and data analysis, which that deck does not cover at all. No card here repeats a card there.

Which topics get the most cards?

Chemistry of life 48, evolution 44, ecology 44, heredity 34, energetics 26, cell communication 20, organismal systems 20, gene expression 18, biotechnology and viruses 16, experimental design 16, chromosomes 10, cells and transport 4. The weighting is heaviest where a course asks you to connect ideas rather than recall one, and the cell material sits in the Cell Biology Essentials deck instead.

Does it cover the data and statistics questions?

Yes, as its own block of 16 cards rather than scattered through the biology: 6 on designing an experiment, 5 on describing data, and 5 on inference including the chi-square test, degrees of freedom and what a significance threshold does and does not mean. The four Hardy-Weinberg cards sit in evolution, where the calculation actually gets used.

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.