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 — what is it?	A shared pair of electrons held unequally, so one atom carries a partial negative charge and the other a partial positive charge. An ionic bond, where an electron is transferred outright rather than shared unevenly.
Cohesion — what is it?	The tendency of water molecules to stick to one another through hydrogen bonding. Adhesion, which is water sticking to a different substance.
Specific heat of water — what is it?	The unusually large amount of heat water absorbs for each degree its temperature rises. Hydrogen bonds take up the added energy before molecular motion increases, so oceans and cells resist swings in temperature.
Density of ice — what is it?	The fact that solid water is less dense than liquid water, because hydrogen bonds lock molecules into an open crystal. The general rule for substances, which is that the solid is denser than the liquid. Water is the exception.
pH — what is it?	A logarithmic measure of hydrogen ion concentration, running from 0 to 14 with 7 neutral. 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 — what is it?	A substance that lowers the hydrogen ion concentration of a solution, usually by accepting hydrogen ions. Nitrogenous bases in DNA, which share the word but not the chemistry.
Carbon skeleton — what is it?	The chain or ring of carbon atoms forming the backbone of an organic molecule. Carbon forms four covalent bonds, so skeletons can branch, ring, and vary in length, which is the source of biological molecular diversity.
Hydroxyl group — what is it?	An oxygen bonded to a hydrogen and to the carbon skeleton. It is polar, so it makes a molecule more soluble in water. Alcohols and sugars carry it.
Carboxyl group — what is it?	A carbon double-bonded to one oxygen and single-bonded to a hydroxyl. It donates a hydrogen ion readily, so molecules carrying it act as acids. Amino acids and fatty acids both have one.
Phosphate group — what is it?	A phosphorus bonded to four oxygens, carrying a negative charge at cellular pH. 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 — what is it?	A monomer is a single subunit, and a polymer is a chain of many of them linked together. Lipids, which are not true polymers because their subunits are not joined in a repeating chain.
Monosaccharide — what is it?	A single sugar unit, generally with a formula that is a multiple of CH2O. It is both fuel and raw material, since a cell can oxidize it or link it into a larger carbohydrate.
Glycogen — what is it?	The storage polysaccharide of animals, a heavily branched glucose polymer held in liver and muscle. Starch, which serves the same purpose in plants but is far less branched.
Saturated fatty acid — what is it?	A fatty acid whose carbon chain holds the maximum number of hydrogens, with no double bonds. Straight chains pack tightly, so these fats are solid at room temperature and stiffen membranes.
Triglyceride — what is it?	Three fatty acids joined to a glycerol backbone. Its carbon-hydrogen bonds are highly reduced, so gram for gram it stores about twice the energy of a carbohydrate.
Amino acid — what is it?	A monomer with a central carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain. Twenty of them, differing only in that side chain, are enough to build every protein an organism makes.
Peptide bond — what is it?	The covalent bond joining the carboxyl group of one amino acid to the amino group of the next. 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 — what is it?	Local folding into alpha helices and beta pleated sheets, held by hydrogen bonds along the polypeptide backbone. Tertiary structure. Secondary bonding involves the backbone, not the side chains.
Quaternary structure — what is it?	The arrangement of two or more folded polypeptide subunits into one functional protein.
Purines and pyrimidines — what is it?	Purines are the double-ring bases adenine and guanine, and pyrimidines are the single-ring bases cytosine, thymine, and uracil. A purine always pairs with a pyrimidine, which keeps the width of the double helix constant along its whole length.
Water potential — what is it?	The free energy of water per unit volume, which predicts the direction water will move. It combines solute effects and pressure effects into one number, which osmosis alone cannot do for a walled cell.
Pressure potential — what is it?	The component of water potential contributed by physical pressure, which can be positive or negative. 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 — what is it?	Energy can be transferred and transformed but cannot be created or destroyed. An organism does not make energy. It captures energy already present and converts it into forms it can use.
Gibbs free energy — what is it?	The portion of a system's energy that can do work at constant temperature and pressure. 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 — what is it?	A reaction that absorbs free energy from its surroundings and does not proceed on its own.
Cofactor — what is it?	A non-protein inorganic helper, usually a metal ion, that an enzyme needs in order to work.
Optimal pH for an enzyme — what is it?	The pH at which an enzyme's reaction rate is highest, set by the charges its side chains need to carry.
Allosteric regulation — what is it?	A change in an enzyme's activity caused by a molecule binding at a site away from the active site. 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 — what is it?	The relationship in which rate rises with substrate concentration and then levels off. The plateau means every active site is occupied, so at saturation only adding enzyme raises the rate further.
Autotroph — what is it?	An organism that builds its own organic molecules from inorganic carbon. 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 — what is it?	The total energy an organism uses per unit time.
Ectotherm — what is it?	An animal whose body temperature is governed chiefly by its surroundings. 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 — what is it?	An absorption spectrum plots how much light a pigment absorbs at each wavelength, while an action spectrum plots how effectively each wavelength drives photosynthesis. 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 — what is it?	The maintenance of a relatively stable internal environment despite changes outside. 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 — what is it?	A control loop in which the response amplifies the original stimulus. Being harmful by definition. It is not a malfunction, it is the mechanism used when a process needs to run to completion quickly.
Hormone — what is it?	A chemical messenger released by one tissue that acts on cells elsewhere in the body carrying the matching receptor. Hormones are slow and long-lasting compared with nerve signals, which is why they govern growth, development, and metabolism rather than reflexes.
Pheromone — what is it?	A chemical released into the environment that affects the behavior or physiology of another individual of the same species.
Action potential — what is it?	A rapid, self-propagating reversal of membrane voltage that travels along a neuron. It is all-or-none, so a stronger stimulus is encoded as a higher firing frequency rather than as a larger signal.
Signal amplification — what is it?	The growth in the number of activated molecules at each step of a transduction pathway. 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 — what is it?	The fast, non-specific defenses present from birth, including barriers, phagocytes, and inflammation. It recognizes broad molecular patterns shared by many pathogens, so it responds within minutes but does not improve with repeated exposure.
Homologous chromosomes — what is it?	The maternal and paternal copies of the same chromosome, carrying the same genes at the same loci but not necessarily the same alleles. Their pairing and separation in the first meiotic division is what produces haploid cells and what independent assortment acts on.
Nondisjunction — what is it?	The failure of homologous chromosomes or sister chromatids to separate during meiosis. It produces gametes with one chromosome too many or too few, and it is the origin of most changes in chromosome number.
Binary fission — what is it?	Prokaryotic cell division, in which a circular chromosome is replicated and the two copies are drawn apart as the cell elongates and divides. Mitosis, which needs a spindle and a nuclear envelope to break down. Binary fission uses neither.
Fertilization — what is it?	The fusion of two haploid gametes to form a diploid zygote. It restores the chromosome number that meiosis halved and adds a third source of variation, since which two gametes meet is itself random.
Gene — what is it?	A stretch of DNA that specifies a functional product and occupies a particular place on a chromosome. An allele. A gene is the locus and its general function, an allele is one of the sequence versions found there.
Locus — what is it?	The fixed position a gene occupies on a chromosome. Homologous chromosomes carry the same loci in the same order, which is what makes pairing and crossing over meaningful.
Phenotype — what is it?	The observable traits of an individual. 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 — what is it?	The allele whose effect appears only when no dominant allele is present. 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 — what is it?	A grid that pairs every possible gamete from one parent with every possible gamete from the other. It shows probabilities for each offspring independently, not a guaranteed distribution among a small number of actual offspring.
Dihybrid cross — what is it?	A cross following two genes at once. 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 — what is it?	The phenotype ratio among the offspring of two individuals heterozygous for two independently assorting genes. 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 — what is it?	The probability of an event that can happen in more than one mutually exclusive way is the sum of the separate probabilities.
Incomplete dominance — what is it?	An inheritance pattern in which the heterozygote shows a phenotype intermediate between the two homozygotes. Blending inheritance. The alleles are not mixed, since crossing two pink plants restores red and white offspring.
Multiple alleles — what is it?	The situation in which a population contains more than two alleles of one gene, though each individual still carries only two. It expands the number of possible genotypes sharply while leaving Mendelian segregation untouched.
Polygenic inheritance — what is it?	A pattern in which several genes each add a small effect to one trait. 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 — what is it?	An interaction in which one gene masks or modifies the expression of another gene. 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 — what is it?	A chromosome that differs between the sexes and carries the genes determining sex, as opposed to an autosome. 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 — what is it?	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. It is far more common in males, and an affected male inherits the allele from his mother rather than from his father.
Linked genes — what is it?	Genes close together on the same chromosome, which therefore tend to be inherited as a unit. They violate independent assortment, so a dihybrid cross gives an excess of parental combinations and a shortfall of recombinants.
Map unit — what is it?	A unit of genetic distance equal to one percent recombination frequency between two genes. 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 — what is it?	A change affecting a single nucleotide pair in a gene. Its consequences range from nothing at all to a completely nonfunctional protein, depending entirely on where in the codon it falls.
Frameshift mutation — what is it?	An insertion or deletion of a number of bases not divisible by three, which shifts the reading frame. Every codon downstream is misread, so a frameshift is usually far more damaging than a substitution.
Enhancer — what is it?	A DNA sequence that increases transcription of a gene when the right regulatory proteins bind it. It can sit far from the promoter and still act, because the DNA loops to bring the two together.
Histone modification — what is it?	The chemical alteration of histone tails, which loosens or tightens the grip of the histones on DNA. Acetylation generally opens chromatin and raises transcription, so gene accessibility is regulated without changing the sequence.
Capsid — what is it?	The protein shell enclosing a viral genome. 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 — what is it?	A viral cycle in which the genome integrates into the host chromosome and is copied along with it. The virus is replicated for generations without killing anything, and stress can trigger a switch to the lytic cycle later.
Horizontal gene transfer — what is it?	The movement of genetic material between organisms other than from parent to offspring. It lets a trait such as antibiotic resistance spread across a bacterial population, and even between species, far faster than inheritance could.
Restriction enzyme — what is it?	A bacterial enzyme that cuts DNA at a specific short sequence. Many leave staggered single-stranded ends, so fragments from two different sources can be joined by base pairing.
Natural selection — what is it?	The process in which heritable variation that improves survival and reproduction becomes more common in a population over generations. Individuals adapting during their lifetime. Selection acts on individuals but the population is what changes.
Directional selection — what is it?	Selection favoring one extreme of a trait range, shifting the population mean toward it.
Genetic drift — what is it?	Change in allele frequencies from generation to generation caused by random sampling rather than by selection. Its effect is far stronger in a small population, where chance alone can eliminate an allele that is not harmful.
Gene flow — what is it?	The movement of alleles between populations through migration and interbreeding. It makes populations more similar to one another, so it works against the divergence that speciation requires.
Hardy-Weinberg equilibrium — what is it?	The condition in which allele and genotype frequencies stay constant from generation to generation. 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 — what is it?	The process by which one population splits into two that can no longer interbreed successfully. It is what turns gradual change within a lineage into the branching pattern that phylogenies record.
Postzygotic barrier — what is it?	A barrier acting after fertilization, so that the hybrid fails to develop, fails to survive, or is sterile.
Gradualism — what is it?	The view that evolutionary change accumulates slowly and steadily over long spans of time. A claim about mechanism. Both gradualism and punctuated equilibrium use the same mechanisms and differ over tempo.
Homologous structures — what is it?	Structures shared by different species because both inherited them from a common ancestor.
Phylogenetic tree — what is it?	A branching diagram representing hypothesized evolutionary relationships among groups. 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 — what is it?	The proposal that simple organic molecules formed on the early Earth from inorganic precursors before any life existed. 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 — what is it?	The nested scales of ecological study, from individual organism to population, community, ecosystem, and biosphere. Each level adds one kind of interaction, since a community adds other species and an ecosystem adds the nonliving surroundings.
Population density — what is it?	The number of individuals of a species per unit area or volume.
Density-dependent factor — what is it?	A limiting factor whose effect grows stronger as population density increases.
Species with K-selected traits — what is it?	A species favored by slow reproduction, few large offspring, heavy parental investment, and long life. This strategy pays in stable habitats near carrying capacity, where competitive ability matters more than reproductive speed.
Interspecific competition — what is it?	Competition between individuals of different species for the same limited resource. Both populations are harmed, which distinguishes it from interactions where one side gains.
Predation — what is it?	An interaction in which one organism kills and consumes another. 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 — what is it?	A species whose effect on community structure is far larger than its abundance would suggest. Removing it changes the whole community, which is how the concept is demonstrated experimentally.
Trophic level — what is it?	A position in a food chain defined by how many steps separate an organism from the producers. Energy is lost at every transfer, which is what limits the number of levels a food chain can support.
Energy pyramid — what is it?	A diagram showing the energy available at each trophic level, narrowing sharply toward the top. The shape cannot invert, because a level can never hold more energy than the one that feeds it.
Carbon cycle — what is it?	The movement of carbon among the atmosphere, living organisms, soils, oceans, and rock. Photosynthesis and respiration move it in opposite directions on a short timescale, while burial and combustion move it on a much longer one.
Eutrophication — what is it?	The enrichment of a body of water with nutrients, leading to algal blooms and then to oxygen depletion as the blooms decay. The fish die from lack of oxygen rather than from the nutrients, so the damage arrives one step after the cause.
Hypothesis — what is it?	A testable, falsifiable proposed explanation that predicts what should be observed. A guess or a question. A hypothesis has to rule something out, so a statement no result could contradict is not one.
Control group — what is it?	The group that does not receive the experimental treatment, giving a baseline to compare against. Without it, a change cannot be attributed to the treatment rather than to time, handling, or the setup itself.
Mean — what is it?	The sum of the values divided by the number of values. It is pulled by extreme values, so a single outlier can move it well away from the typical measurement.
Null hypothesis — what is it?	The statement that there is no real difference or association, and that any observed one is due to chance. A statistical test evaluates this statement rather than the interesting one, so the outcome is a decision to reject it or not.
Xylem — what is it?	The plant tissue that carries water and dissolved minerals upward from the roots. Its conducting cells are dead and hollow at maturity, so transport happens through empty tubes rather than through living cytoplasm.
Stomata — what is it?	Adjustable pores in the leaf surface through which gases enter and leave. They cannot admit carbon dioxide without also losing water, so every stomatal setting is a trade-off between photosynthesis and dehydration.
Gas exchange surface — what is it?	The thin, moist, highly folded surface across which an animal exchanges oxygen and carbon dioxide. Gases must dissolve to diffuse, so the surface has to stay wet, which is why terrestrial animals keep it internal.
Antigen — what is it?	A molecule the immune system recognizes as foreign and responds to. A pathogen. The antigen is usually one molecule on a pathogen's surface rather than the organism itself.
Sliding filament model — what is it?	The explanation of muscle contraction in which thick and thin filaments slide past one another without changing length. The sarcomere shortens while its filaments do not, which is why the model is stated in terms of overlap rather than shrinkage.
Circadian rhythm — what is it?	A biological cycle of roughly twenty-four hours that continues even without external cues. 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 does not use your AI allowance, 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.