If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Biology library

Welcome to the Biology library!

Biology is the study of life. Here, you can browse videos, articles, and exercises by topic. We keep the library up-to-date, so you may find new or improved content over time.

Intro to biology

Welcome to biology!: Intro to biologyThe science of biology: Intro to biologyWater and life: Intro to biology

Chemistry of life

Elements and atoms: Chemistry of lifeElectron shells and orbitals: Chemistry of lifeChemical bonds and reactions: Chemistry of life

Water, acids, and bases

Hydrogen bonding in water: Water, acids, and basesCohesion and adhesion: Water, acids, and basesTemperature and state changes in water: Water, acids, and bases
Acids, bases, and pH: Water, acids, and bases

Properties of carbon

Carbon: Properties of carbonHydrocarbon structures and functional groups: Properties of carbon

Macromolecules

Introduction to macromolecules: MacromoleculesCarbohydrates: MacromoleculesLipids: Macromolecules
Nucleic acids: MacromoleculesProteins: Macromolecules

Elements of life

More about macromolecules: Elements of life

Energy and enzymes

Energy in metabolism: Energy and enzymesLaws of thermodynamics: Energy and enzymesFree energy: Energy and enzymes
ATP and reaction coupling: Energy and enzymesIntroduction to enzymes: Energy and enzymesEnzyme regulation: Energy and enzymes

Structure of a cell

Introduction to cells: Structure of a cellProkaryotic and eukaryotic cells: Structure of a cellTour of a eukaryotic cell: Structure of a cell
Extracellular structures and cell-cell junctions: Structure of a cell

More about cells

Cell movement: More about cellsCell size: More about cellsProkaryotes and eukaryotes: More about cells

Membranes and transport

The plasma membrane : Membranes and transportDiffusion and osmosis: Membranes and transportPassive transport: Membranes and transport
Active transport: Membranes and transportBulk transport: Membranes and transport

More about membranes

Cell membranes: More about membranesTransport: More about membranes

Cellular respiration

Introduction to cellular respiration: Cellular respirationSteps of cellular respiration: Cellular respirationGlycolysis: Cellular respiration
Pyruvate oxidation and the citric acid cycle: Cellular respirationOxidative phosphorylation: Cellular respirationVariations on cellular respiration: Cellular respiration

Photosynthesis

Intro to photosynthesis: PhotosynthesisThe light-dependent reactions: PhotosynthesisThe Calvin cycle: Photosynthesis
Photorespiration: C3, C4, and CAM plants: Photosynthesis

Cell signaling

How cells signal to each other: Cell signalingCommunication in single-celled organisms: Cell signalingCommunication in multicellular organisms: Cell signaling
Feedback: Cell signaling

Cell division

Introduction to cell division: Cell divisionThe cell cycle and mitosis: Cell divisionMeiosis: Cell division
Cell cycle regulation, cancer, and stem cells: Cell division

Classical and molecular genetics

Mendelian genetics: Classical and molecular geneticsVariations on Mendelian genetics: Classical and molecular geneticsChromosomal basis of genetics: Classical and molecular genetics
Sex linkage, chromosomal mutations, & non-nuclear inheritance: Classical and molecular geneticsMolecular basis of genetics: Classical and molecular genetics

DNA as the genetic material

Structure of DNA: DNA as the genetic materialDiscovery of DNA: DNA as the genetic materialDNA replication: DNA as the genetic material

Central dogma (DNA to RNA to protein)

Central dogma and the genetic code: Central dogma (DNA to RNA to protein)Transcription: Central dogma (DNA to RNA to protein)Translation: Central dogma (DNA to RNA to protein)

Gene regulation

Gene regulation in bacteria: Gene regulationGene regulation in eukaryotes: Gene regulation

Biotechnology

Intro to biotechnology: BiotechnologyDNA cloning: BiotechnologyDNA analysis methods: Biotechnology

More molecular biology

More about DNA and RNA: More molecular biologyMore about genetics: More molecular biologyMutations: More molecular biology
Metabolism: More molecular biologyMore about cell cycle: More molecular biologyMore about biotechnology: More molecular biology

Developmental biology

Development & differentiation: Developmental biologySignaling & transcription factors in development: Developmental biologyApoptosis: Developmental biology

Bacteria and archaea

Prokaryote structure: Bacteria and archaeaProkaryote metabolism and ecology: Bacteria and archaea

Viruses

Viruses: Viruses

Evolution and the tree of life

Evolution and natural selection: Evolution and the tree of lifePopulation genetics: Evolution and the tree of lifeSpeciation and evolutionary trees: Evolution and the tree of life

More about evolution and natural selection

Evidence of evolution: More about evolution and natural selectionArtificial selection: More about evolution and natural selectionMore about populations: More about evolution and natural selection
More evolution and natural selection practice: More about evolution and natural selection

History of life on Earth

Formation of Earth and early life: History of life on EarthThe diversification of life: History of life on EarthRadiometric dating: History of life on Earth

Ecology

Intro to ecology: EcologyIntro to population ecology: EcologyPopulation growth & regulation: EcologyIntro to community ecology: Ecology
Community structure & diversity: EcologyIntro to ecosystems: EcologyBiogeochemical cycles: EcologyBiogeography: Ecology

More about ecology

More about population ecology: More about ecologyLife histories: More about ecologyMore ecology practice: More about ecology

Biodiversity and conservation

Ecosystems & ecosystem services: Biodiversity and conservationGlobal distribution of biodiversity: Biodiversity and conservationThreats to biodiversity: Biodiversity and conservation
Protecting biodiversity: Biodiversity and conservationLevels of biodiversity: Biodiversity and conservation

Behavioral biology

Animal behavior: Behavioral biology

Principles of physiology

Body structure & homeostasis: Principles of physiologyMetabolism & thermoregulation: Principles of physiology

Human biology

Circulatory and pulmonary systems: Human biologyThe neuron and nervous system: Human biologyThe kidney and nephron: Human biology
Muscles: Human biologyImmunology: Human biology

Plant biology

Plant responses to light: Plant biology

AP free response worked examples

2015 Free response worked examples: AP free response worked examples

Crash Course: Biology and Ecology

Crash Course: Biology: Crash Course: Biology and EcologyCrash Course: Ecology: Crash Course: Biology and Ecology

Meet the biology professional

Meet the biology professional: Meet the biology professional
Community questions
Biology is brought to you with support from the Amgen Foundation