Every Biology concept in the middle school Science Bowl study set (grades 6-8), with a short definition for each. Read through them here, then drill them as flashcards or practise real Biology questions.
Double-membraned organelle that produces ATP via aerobic cellular respiration. The Krebs cycle runs in the matrix; the electron transport chain runs on the inner membrane (cristae). Contains its own circular DNA, evidence of endosymbiotic origin.
Nuclear division producing two genetically identical diploid daughter cells. Phases (PMAT): Prophase (chromatin condenses), Metaphase (chromosomes align at cell plate), Anaphase (sister chromatids separate), Telophase (nuclei reform). Used for growth and repair.
Two-stage division producing four haploid gametes with genetic variation. Crossing over (recombination) occurs during Prophase I. Meiosis I separates homologous pairs; Meiosis II separates sister chromatids. Results in genetic diversity.
DNA → RNA → Protein. Transcription: RNA polymerase reads the template DNA strand to produce mRNA in the nucleus. Translation: ribosomes read mRNA codons and tRNA anticodons to assemble polypeptides. rRNA is synthesized in the nucleolus.
Protein catalysts that lower activation energy without being consumed. Substrates bind at the active site. Classes: oxidoreductase (redox), hydrolase (water addition), transferase (group transfer), lyase (bond breaking without water), isomerase, ligase.
Converts light energy to chemical energy in chloroplasts. Light reactions (thylakoid membranes): split water, produce ATP and NADPH, release O₂. Calvin cycle (stroma): fixes CO₂ into G3P using ATP and NADPH. Net: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Breaks down glucose to ATP: Glycolysis (cytoplasm, 2 net ATP), Krebs/Citric Acid Cycle (matrix, 2 ATP), Electron Transport Chain (inner membrane, ~32 ATP). Anaerobic fermentation produces lactic acid (animals) or ethanol + CO₂ (yeast).
Dominant alleles mask recessive ones in heterozygotes. F2 monohybrid cross: 3:1 phenotype ratio. Dihybrid cross (F2): 9:3:3:1 ratio. Law of Segregation: allele pairs separate during gamete formation. Law of Independent Assortment: genes on different chromosomes assort independently.
Mutualism (+/+): both benefit. Commensalism (+/0): one benefits, other unaffected. Parasitism (+/−): parasite benefits, host is harmed. Competition (−/−): both harmed. Predation (+/−): predator benefits. Niche: the ecological role of an organism in its community.
Three domains: Bacteria (no nucleus, peptidoglycan cell walls, single RNA polymerase), Archaea (no nucleus, no peptidoglycan, multiple RNA polymerases, can have introns), Eukarya (nucleus, complex organelles). Six-kingdom system places Monera, Protista, Fungi, Plantae, and Animalia under domains.
Nucleus: DNA storage and transcription. Ribosome: protein synthesis (80S in eukaryotes, 70S in prokaryotes). Rough ER: protein folding with ribosomes. Smooth ER: lipid synthesis. Golgi: modifies and packages proteins. Lysosome: hydrolytic digestion. Vacuole: storage (large in plant cells).
CNS (brain + spinal cord) processes information. PNS: sensory (afferent) carries signals to CNS; motor (efferent) carries signals away. Neurons transmit via action potentials (all-or-nothing); neurotransmitters cross the synaptic cleft. Reflex arcs bypass the brain for fast responses.
Fluid mosaic model: phospholipid bilayer with embedded proteins. Hydrophilic heads face outward; hydrophobic tails face inward. Selectively permeable — small nonpolar molecules pass freely, large or charged molecules require transport proteins. Cholesterol stabilizes membrane fluidity.
Diffusion: net movement of molecules from high to low concentration (passive, no energy). Osmosis: diffusion of water across a semipermeable membrane toward higher solute concentration. Hypertonic solution: cell loses water (crenation in animals, plasmolysis in plants). Hypotonic: cell gains water.
Heart has 4 chambers: right atrium → right ventricle → lungs (pulmonary circuit) → left atrium → left ventricle → body (systemic circuit). Oxygenated blood enters the left atrium via pulmonary veins. Arteries carry blood away from heart; veins return blood to heart.
Innate immunity: non-specific (skin barrier, phagocytes, inflammation). Adaptive immunity: specific responses. B cells produce antibodies (humoral immunity). T cells: helper T cells (CD4+) coordinate response; cytotoxic T cells (CD8+) kill infected cells. Memory cells enable faster secondary responses.
Glands secrete hormones into the bloodstream. Hypothalamus controls the pituitary (master gland). Key glands: thyroid (thyroxine — metabolism), adrenal (epinephrine, cortisol — stress), pancreas (insulin lowers blood glucose; glucagon raises it), gonads (sex hormones).
Mechanical and chemical breakdown of food. Mouth (amylase), esophagus, stomach (pepsin, HCl, pH ~2), small intestine (site of most absorption; villi and microvilli increase surface area), large intestine (water reabsorption). Liver produces bile; pancreas secretes digestive enzymes and bicarbonate.
Oxygen enters via nose/mouth → trachea → bronchi → bronchioles → alveoli (site of gas exchange). CO₂ diffuses out; O₂ diffuses into capillaries. Hemoglobin in red blood cells carries O₂ as oxyhemoglobin. Diaphragm contraction increases lung volume (inhalation = active process).
Kidneys filter blood to produce urine. Functional unit: nephron. Glomerulus (capillary ball in Bowman's capsule) filters blood by pressure. Loop of Henle concentrates filtrate. Collecting duct reabsorbs water (regulated by ADH). Nitrogenous wastes: urea (mammals), uric acid (birds/reptiles), ammonia (fish).
Non-living infectious agents consisting of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid); some have a lipid envelope. Lytic cycle: virus hijacks host, replicates, lyses cell (releases new viruses). Lysogenic cycle: viral DNA integrates into host genome (prophage) and replicates with host.
Hardy-Weinberg equilibrium: allele frequencies stable if no mutation, migration, genetic drift, natural selection, or non-random mating. Equations: p + q = 1; p² + 2pq + q² = 1. Natural selection acts on phenotypes (determined by genotypes). Genetic drift: random allele frequency change in small populations.
Vascular plants have xylem (water/minerals upward) and phloem (sugars bidirectionally). Angiosperms (flowering plants) produce seeds in fruit; gymnosperms produce naked seeds (cones). Monocots: parallel leaf veins, fibrous roots; dicots: branching veins, taproot. Bryophytes (mosses) lack vascular tissue.
Skeletal muscle: striated, voluntary, attached to bones via tendons, multinucleated. Smooth muscle: non-striated, involuntary, found in organ walls and blood vessels. Cardiac muscle: striated, involuntary, found only in the heart, connected by intercalated discs. Sarcomere is the basic contractile unit.
Maintenance of stable internal conditions despite external changes. Negative feedback: response opposes the stimulus (e.g., insulin lowers high blood glucose). Positive feedback: response amplifies the stimulus (e.g., oxytocin during childbirth, blood clotting). Temperature, pH, and blood glucose are key regulated variables.
Double helix: two antiparallel strands of nucleotides. Each nucleotide: deoxyribose sugar, phosphate group, nitrogenous base. Complementary base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds). 5'→3' directionality. Chargaff's rule: %A = %T and %G = %C in any organism.
Natural selection: heritable variation + differential reproductive success = adaptation over generations. Darwin's observations: overproduction, variation, heritability, differential survival. Speciation: allopatric (geographic isolation) vs sympatric. Evidence: fossil record, homologous structures, molecular phylogenetics.
ATP (adenosine triphosphate) is the universal energy currency of cells. Structure: adenine base + ribose sugar + three phosphate groups. Hydrolysis of ATP → ADP + Pᵢ releases ~7.3 kcal/mol. ATP is regenerated by substrate-level phosphorylation (glycolysis, Krebs) and oxidative phosphorylation (ETC). The cell maintains a high ATP:ADP ratio to drive endergonic reactions.
Process by which cells respond to external signals. Steps: ligand binds receptor → second messenger generated → cascade of phosphorylation events → cellular response. Types: G-protein coupled receptors (GPCRs) use cAMP or IP₃/Ca²⁺ as second messengers; receptor tyrosine kinases (RTKs) phosphorylate proteins directly. Amplification: one ligand triggers many downstream effectors.
Phases: G1 (cell growth), S (DNA replication), G2 (preparation for division), M (mitosis + cytokinesis). Checkpoints: G1/S (DNA damage), G2/M (replication complete), spindle checkpoint (chromosomes attached). Cyclins and CDKs drive progression. p53 is a tumor suppressor that halts the cycle at damage. Mutations disabling checkpoints or activating proto-oncogenes lead to cancer.
Programmed cell death — an orderly, energy-dependent process essential for development and homeostasis. Intrinsic pathway: mitochondria release cytochrome c in response to DNA damage → activates caspase cascade. Extrinsic pathway: death receptors (e.g., Fas) activated by external signals. Apoptosis eliminates damaged, infected, or unneeded cells without triggering inflammation. Defective apoptosis → cancer or autoimmune disease.
Undifferentiated cells capable of self-renewal and differentiation into specialized cell types. Totipotent: can form any cell (fertilized egg). Pluripotent: most cell types (embryonic stem cells, iPSCs). Multipotent: limited types (bone marrow stem cells → blood cells). Induced pluripotent stem cells (iPSCs): adult cells reprogrammed to pluripotency by Yamanaka factors (Oct4, Sox2, Klf4, c-Myc).
PCR (polymerase chain reaction): amplifies DNA exponentially using primers and Taq polymerase. Gel electrophoresis: separates DNA fragments by size. Restriction enzymes cut DNA at specific sequences; ligase joins fragments (recombinant DNA). CRISPR-Cas9: guide RNA directs Cas9 nuclease to cut target DNA; allows precise gene editing. Southern blot: detects specific DNA sequences; Northern: RNA; Western: proteins.
Primary: sequence of amino acids linked by peptide bonds (determined by DNA). Secondary: local folding into α-helices and β-sheets via hydrogen bonds between backbone atoms. Tertiary: overall 3-D shape stabilized by hydrophobic interactions, disulfide bonds, ionic bonds, H-bonds between R groups. Quaternary: multiple polypeptide subunits (e.g., hemoglobin has 4). Denaturation disrupts 3-D structure without breaking peptide bonds.
Hydrophobic biomolecules. Triglycerides: glycerol + 3 fatty acids; energy storage (9 kcal/g). Saturated fats: no double bonds, solid at room temp. Unsaturated (mono/poly): one or more double bonds, liquid at room temp. Phospholipids: form cell membranes (hydrophilic head, hydrophobic tails). Steroids: four-ring structure; cholesterol is membrane component and precursor to steroid hormones (testosterone, estrogen, cortisol).
Monosaccharides: simplest sugars (glucose, fructose, galactose — C₆H₁₂O₆). Disaccharides: two monosaccharides joined by glycosidic bond (sucrose = glucose + fructose; lactose = glucose + galactose; maltose = glucose + glucose). Polysaccharides: starch and glycogen (branched glucose polymers for energy storage); cellulose (β-1,4 linkages; structural; humans cannot digest); chitin (structural in fungi/arthropods).
Nitrogen fixation: N₂ → NH₃ by Rhizobium (in legume root nodules) and free-living bacteria (Azotobacter). Nitrification: NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter). Assimilation: plants absorb NH₄⁺ or NO₃⁻ to make amino acids and nucleic acids. Ammonification: decomposers convert organic N back to NH₄⁺. Denitrification: NO₃⁻ → N₂ by anaerobic bacteria (Pseudomonas). Humans disrupt the cycle via fertilizers and combustion.
The gradual process of community change over time. Primary succession: begins on bare rock with no soil (volcanic island, glacier retreat); pioneer species (lichens, mosses) weather rock and build soil. Secondary succession: disturbance of an existing ecosystem (fire, flood); soil remains so recovery is faster. Climax community: stable endpoint determined by climate. Facilitation: early species modify the environment enabling later species.
Innate (fixed action patterns, FAPs): genetically determined, triggered by specific stimuli (releaser). Learned: habituation (decreased response to repeated stimulus), classical conditioning (Pavlov), operant conditioning (Skinner), imprinting (Lorenz — critical period), insight learning. Altruism explained by kin selection (Hamilton's rule: rb > c, where r = relatedness, b = benefit, c = cost). Foraging theory: optimal energy gain per unit time.
Heterotrophic decomposers with cell walls of chitin. Body plan: hyphae (filaments) forming a mycelium. Reproduction: asexual (conidia spores) and sexual (zygospores, ascospores, basidiospores). Major phyla: Zygomycota (bread molds), Ascomycota (yeasts, morels, Penicillium), Basidiomycota (mushrooms, rusts, smuts). Mycorrhizae: mutualistic symbiosis with plant roots (provide phosphorus; receive sugars). Lichens: mutualism with algae/cyanobacteria.
Paraphyletic group of mostly unicellular eukaryotes. Algae (photoautotrophic): diatoms (silica shells), dinoflagellates (red tides, bioluminescence), green algae (ancestor of plants). Animal-like protozoa: amoeba (pseudopods), paramecium (cilia), trypanosoma (flagella, causes sleeping sickness), Plasmodium (causes malaria — transmitted by Anopheles mosquito). Slime molds: form multicellular fruiting bodies for reproduction.
Auxin (IAA): cell elongation, phototropism (shoots toward light), apical dominance (suppresses lateral buds); made in shoot tips. Gibberellins: stem elongation, seed germination, fruit development (seedless fruits). Cytokinins: promote cell division, delay senescence; synthesized in roots. Ethylene (gas): fruit ripening, leaf/flower/fruit abscission, response to mechanical stress. Abscisic acid (ABA): stomatal closure during drought, seed dormancy, stress response.
Angiosperms (flowering plants): flowers contain stamens (anther produces pollen = male gametophyte) and pistil (stigma, style, ovary with ovules). Pollination: transfer of pollen to stigma (wind, insects, birds). Double fertilization (unique to angiosperms): one sperm + egg → zygote (2n); second sperm + polar nuclei → endosperm (3n, nutrient tissue). Fruit = mature ovary. Seed dispersal: wind, water, animals (ingestion or attachment).
Regulation of water and solute balance. Kidney nephron: glomerular filtration (pressure-driven), tubular reabsorption (glucose, amino acids, Na⁺ reabsorbed; loop of Henle concentrates filtrate), tubular secretion (H⁺, K⁺ added). ADH (vasopressin): increases water permeability of collecting duct → concentrated urine. Aldosterone: increases Na⁺ reabsorption → retains water. Freshwater fish: hypotonic environment; lose salts, gain water — excrete dilute urine.
Ectotherms (poikilotherms): body temperature varies with environment; rely on behavioral regulation (basking, burrowing). Endotherms (homeotherms): maintain constant core temperature via metabolic heat; include birds and mammals. Mechanisms: shivering (muscle thermogenesis), sweating/panting (evaporative cooling), vasodilation (radiates heat) vs. vasoconstriction (conserves heat). Brown adipose tissue (BAT): non-shivering thermogenesis. Countercurrent heat exchange in extremities.
External fertilization: sperm and eggs released into water (fish, amphibians); requires aquatic environment. Internal fertilization: sperm deposited inside female; required for terrestrial life. Oviparous: eggs develop outside (birds, reptiles, fish). Ovoviviparous: eggs retained internally (some sharks, snakes). Viviparous: live birth with placenta (most mammals). r-strategy: many small offspring, little parental care (insects). K-strategy: few offspring, heavy parental investment (elephants, humans).
Prokaryotes — operons: lac operon (glucose absent + lactose present → CAP activates; lac repressor inactivated → transcription on). Eukaryotes: transcription factors bind enhancers/promoters; histone acetylation opens chromatin (active); methylation silences genes; RNA processing (alternative splicing → multiple proteins from one gene); miRNA: small RNAs that base-pair with mRNA → degradation or translation inhibition. Epigenetics: heritable changes in gene expression without DNA sequence change.
Transgenic organisms: GMO crops (Bt toxin gene from Bacillus thuringiensis inserted into corn/soy). Gene therapy: correcting defective genes (e.g., AAV vectors for SCID, hemophilia). Monoclonal antibodies: produced by hybridoma cells; used in cancer treatment (trastuzumab) and diagnostics. DNA fingerprinting: uses STR (short tandem repeat) analysis; used in forensics and paternity testing. Human Genome Project (completed 2003): sequenced ~3 billion base pairs across 23 chromosome pairs.
Mechanoreceptors: detect pressure/vibration (Meissner's, Pacinian corpuscles in skin; hair cells in cochlea for hearing). Photoreceptors: rods (dim light, monochromatic, peripheral) and cones (color vision, S/M/L for blue/green/red, fovea for acuity). Chemoreceptors: olfactory neurons (smell, ~400 receptor types) and taste buds (sweet, salty, sour, bitter, umami). Nociceptors: pain. Vestibular system: semicircular canals detect rotational acceleration; utricle/saccule detect linear acceleration.
Allele frequencies remain stable in a population across generations when: no mutation, no migration (gene flow), no genetic drift, random mating, no natural selection. Equations: p + q = 1 (allele frequencies); p² + 2pq + q² = 1 (genotype frequencies). Hardy-Weinberg is used as a null model to detect evolution: any deviation from expected frequencies indicates one of the five forces is acting on the population.
Three principles: (1) All living things are made of cells. (2) The cell is the basic unit of life. (3) All cells come from pre-existing cells. Proposed by Schleiden (plants), Schwann (animals), and Virchow (cell division). Prokaryotic cells (bacteria) lack a membrane-bound nucleus; eukaryotic cells (animals, plants, fungi) have a nucleus.
Cell membrane: controls what enters and exits. Nucleus: contains DNA and directs cell activities. Cytoplasm: jelly-like fluid inside cell. Mitochondria: produce energy (ATP). Ribosomes: make proteins. Plant cells also have: cell wall (rigid support), chloroplasts (photosynthesis), and a large central vacuole (water storage).
Process by which plants, algae, and some bacteria convert sunlight, water, and carbon dioxide into glucose and oxygen. Occurs in the chloroplasts. Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Chlorophyll is the green pigment that absorbs light. Plants use glucose for energy and to build plant matter.
Process that breaks down glucose to release energy (ATP) that cells can use. Aerobic respiration requires oxygen: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Occurs in the mitochondria. Anaerobic respiration (fermentation) occurs without oxygen and produces less ATP. Animals, plants, and most organisms use aerobic respiration.
Producers (plants) make their own food via photosynthesis. Primary consumers (herbivores) eat producers. Secondary consumers eat herbivores. Tertiary consumers are top predators. Decomposers (bacteria, fungi) break down dead matter and recycle nutrients. Only about 10% of energy transfers between each trophic level.
Traits are controlled by genes, which are segments of DNA. Each organism has two copies of each gene (alleles), one from each parent. Dominant alleles (capital letter) are expressed when present; recessive alleles (lowercase) are only expressed when two copies exist. Punnett squares predict the probability of offspring traits.
Organisms are classified into a hierarchy: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Three domains: Bacteria, Archaea, Eukarya. Six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, Animalia. Scientific names use two-part (binomial) nomenclature: Genus species (e.g., Homo sapiens).
Skeletal (support/protection), muscular (movement), nervous (signal coordination), circulatory (blood transport), respiratory (gas exchange), digestive (nutrient absorption), excretory (waste removal), endocrine (hormone regulation), immune (disease defense), integumentary (skin barrier). Each system works with others to maintain homeostasis.
DNA (deoxyribonucleic acid) carries genetic information in the nucleus. DNA is made of nucleotides with four bases: adenine (A), thymine (T), guanine (G), cytosine (C). A pairs with T; G pairs with C. Genes on chromosomes code for proteins. Mutations are changes in DNA sequence. Humans have 46 chromosomes (23 pairs).
Process by which individuals with favorable traits survive and reproduce more than others, passing those traits to offspring. Four conditions: (1) variation exists in a population, (2) traits are heritable, (3) more offspring are produced than can survive, (4) some traits increase survival. Over time, adaptations accumulate in a population.
Cell division that produces two identical daughter cells with the same number of chromosomes as the parent cell. Used for growth, repair, and asexual reproduction. Phases: Prophase (chromatin condenses into chromosomes), Metaphase (chromosomes line up in the middle), Anaphase (chromosomes pulled to opposite poles), Telophase (nuclear membranes reform). Cytokinesis: division of the cytoplasm. Result: 2 genetically identical diploid cells.
Diffusion: movement of particles from high concentration to low concentration (passive transport; no energy needed). Osmosis: diffusion of water across a semipermeable membrane from a region of low solute concentration to high solute concentration. Hypotonic solution: cell gains water and may burst (lysis). Hypertonic solution: cell loses water and shrinks (crenation in animals; plasmolysis in plants). Isotonic: no net water movement.
Large regions of Earth characterized by similar climate, vegetation, and wildlife. Major terrestrial biomes: tropical rainforest (hot, wet, high biodiversity), savanna (warm, seasonal rainfall, grassland), desert (low rainfall, hot or cold), temperate forest (seasonal, deciduous trees), taiga/boreal forest (cold, conifers), tundra (very cold, permafrost, treeless), grassland/prairie. Aquatic biomes: freshwater (lakes, rivers, wetlands) and marine (ocean, coral reef, estuary).
Traits that increase an organism's ability to survive and reproduce in its environment. Structural adaptations: physical features (thick fur for cold climates, webbed feet for swimming, camouflage coloring). Behavioral adaptations: actions that aid survival (migration, hibernation, nocturnal behavior). Physiological adaptations: internal processes (producing antifreeze proteins, efficient water retention in desert animals). Adaptations evolve through natural selection over many generations.
Long-term interactions between two different species living in close association. Mutualism: both species benefit (clownfish and sea anemone; bees and flowers; nitrogen-fixing bacteria in legume roots). Commensalism: one species benefits, the other is unaffected (barnacles on whales; birds eating insects stirred up by cattle). Parasitism: one species (parasite) benefits at the expense of the other (host) (tapeworms, fleas, mistletoe). Endoparasites live inside the host; ectoparasites live outside.
Single-celled prokaryotes (no membrane-bound nucleus). Shapes: cocci (spherical), bacilli (rod-shaped), spirilla (spiral). Reproduce by binary fission (asexual). Some cause disease (pathogens): Streptococcus (strep throat), Salmonella (food poisoning), Mycobacterium tuberculosis. Most are beneficial: decomposers, nitrogen fixers, used to make yogurt and cheese. Antibiotics target bacterial cell walls, ribosomes, or DNA replication. Bacteria can develop antibiotic resistance.
Non-living particles consisting of genetic material (DNA or RNA) surrounded by a protein coat (capsid). Not cells — cannot reproduce on their own; must infect a host cell. Lytic cycle: virus injects DNA into host, hijacks cell machinery to make new viruses, lyses (bursts) cell to release them. Examples: influenza (RNA virus), HIV (retrovirus, uses reverse transcriptase), chickenpox, COVID-19. Vaccines stimulate immune system to recognize viral proteins without causing disease.
Eukaryotic organisms with cell walls made of chitin. Heterotrophs: absorb nutrients from decomposing organic matter (decomposers). Not plants — they do not photosynthesize. Body made of thread-like hyphae forming a mycelium. Reproduce by spores. Types: mushrooms (Basidiomycota), molds (Zygomycota, Ascomycota), yeasts (unicellular Ascomycota). Useful fungi: yeast (bread, beer), Penicillium (penicillin antibiotic). Harmful: athlete's foot, ringworm (actually fungi, not worms).
Mostly unicellular eukaryotes that don't fit neatly into other kingdoms. Three types: animal-like (protozoa), plant-like (algae), and fungus-like (slime molds). Amoeba: moves with pseudopods, engulfs food. Paramecium: moves with cilia. Euglena: has chloroplasts (photosynthesize) AND a flagellum (can move). Algae: produce much of Earth's oxygen via photosynthesis. Plasmodium: protist that causes malaria, transmitted by Anopheles mosquito.
Multicellular, photosynthetic eukaryotes with cell walls of cellulose. Major groups: non-vascular plants (bryophytes: mosses, liverworts — lack xylem/phloem, need moisture to reproduce), seedless vascular plants (ferns — have xylem/phloem but reproduce by spores), gymnosperms (conifers, produce naked seeds in cones), angiosperms (flowering plants, produce seeds in fruits — most diverse and widespread). Xylem transports water up; phloem transports sugars.
Multicellular, heterotrophic eukaryotes without cell walls. Two major groups: invertebrates (no backbone; ~97% of animal species) and vertebrates (have a backbone/vertebral column; includes fish, amphibians, reptiles, birds, mammals). Key features: movement, nervous system for sensing/responding, reproduce sexually. Major invertebrate phyla: Porifera (sponges), Cnidaria (jellyfish), Platyhelminthes (flatworms), Annelida (earthworms), Mollusca (snails, clams, squid), Arthropoda, Echinodermata (sea stars).
Fish: aquatic vertebrates; breathe through gills; ectothermic. Cartilaginous fish (Chondrichthyes): sharks, rays — skeleton of cartilage, no swim bladder. Bony fish (Osteichthyes): most fish — swim bladder for buoyancy, scales, lateral line. Amphibians: first terrestrial vertebrates; ectothermic; moist, glandular skin (no scales); breathe through gills (larvae) and lungs/skin (adults); must return to water to reproduce (external fertilization, jelly-coated eggs). Examples: frogs, salamanders, caecilians.
Reptiles: ectothermic; dry, scaly skin (waterproof); breathe with lungs; internal fertilization; amniotic eggs (leathery shell; can be laid on land). Examples: snakes, lizards, turtles, crocodilians. Birds (Aves): endothermic (warm-blooded); feathers; hollow bones; wings; four-chambered heart; hard-shelled amniotic eggs; descended from theropod dinosaurs. Adaptations for flight: keeled sternum (muscle attachment), fused clavicles (wishbone), air sacs for efficient respiration.
Endothermic vertebrates with hair or fur; females produce milk from mammary glands to feed young; most have a diaphragm for breathing; four-chambered heart. Three groups: monotremes (egg-laying; platypus, echidna), marsupials (young born early, develop in pouch; kangaroo, opossum), eutherians/placental mammals (young develop in uterus via placenta; most mammals). Adaptations: homeothermy allows activity in cold, specialized teeth (incisors, canines, premolars, molars).
Largest animal phylum (~1 million+ species). Characteristics: exoskeleton (chitin), jointed appendages, segmented body, open circulatory system. Must molt (ecdysis) to grow. Classes: Insecta (6 legs, 3 body segments, wings; bees, beetles, butterflies), Arachnida (8 legs, 2 body segments; spiders, scorpions, ticks), Crustacea (2 pairs antennae; crabs, lobsters, shrimp), Myriapoda (many legs; centipedes, millipedes). Insects are the most diverse group of animals on Earth.
Closed double-loop system. Pulmonary circuit: right heart → lungs (gas exchange) → left heart. Systemic circuit: left heart → body → right heart. Heart: four chambers (right/left atria and ventricles); valves prevent backflow; sinoatrial (SA) node is the pacemaker. Arteries carry blood away from heart (oxygenated, except pulmonary); veins carry blood toward heart; capillaries are site of gas/nutrient exchange. Blood components: red blood cells (oxygen transport), white blood cells (immune), platelets (clotting), plasma.
Brings oxygen into the body and removes carbon dioxide. Air path: nose/mouth → trachea (windpipe) → bronchi → bronchioles → alveoli. Alveoli: tiny air sacs surrounded by capillaries; site of gas exchange (O₂ in, CO₂ out) by diffusion. Diaphragm: dome-shaped muscle below lungs; contracts to inhale (lung volume increases, pressure decreases). Breathing rate increases with exercise. Hemoglobin in red blood cells carries oxygen as oxyhemoglobin.
Breaks food down mechanically and chemically for absorption. Path: mouth (chewing + amylase breaks starch) → esophagus (peristalsis) → stomach (pepsin + HCl breaks proteins; churning) → small intestine (most digestion and absorption; villi increase surface area; bile from liver, enzymes from pancreas) → large intestine (water reabsorption; bacteria produce vitamins) → rectum/anus. Liver: produces bile, detoxifies blood. Pancreas: produces digestive enzymes and bicarbonate.
206 bones in adult human body. Functions: support, protection of organs, movement (with muscles), blood cell production (red bone marrow), mineral storage (calcium, phosphorus). Bone composition: compact bone (dense outer layer) and spongy bone (inner lattice). Types of joints: ball-and-socket (hip, shoulder — full rotation), hinge (knee, elbow — one plane), pivot (neck — rotation), gliding (wrist). Cartilage: flexible connective tissue at joint surfaces; no blood supply. Ligaments: bone to bone. Tendons: muscle to bone.
CNS: brain + spinal cord; processes information. PNS: all nerves outside CNS; sensory (carries signals to CNS) and motor (carries signals from CNS to muscles/glands). Neurons: basic unit; dendrites receive signals, cell body integrates, axon transmits, synapse releases neurotransmitters. Reflex arc: stimulus → sensory neuron → interneuron (spinal cord) → motor neuron → response; bypasses brain for speed. Somatic: voluntary muscle control. Autonomic: involuntary (sympathetic = fight-or-flight; parasympathetic = rest-and-digest).
First line of defense: physical barriers (skin, mucus, cilia in respiratory tract, stomach acid). Second line: nonspecific innate responses (phagocytes engulf pathogens, inflammation, fever — higher temperature slows bacterial growth, interferons signal neighboring cells). Third line: specific adaptive immunity. B cells produce antibodies (proteins that bind specific antigens). T cells: helper T cells coordinate response; cytotoxic T cells destroy infected cells. Memory cells enable faster secondary immune response (basis of vaccination).
Biodiversity: variety of life — genetic diversity (variation within species), species diversity (number of species), ecosystem diversity (variety of habitats). Biodiversity hotspots: regions with exceptionally high species richness and endemism. Extinction: permanent disappearance of a species; background extinction rate vs. mass extinctions (five major events; sixth ongoing due to human activities). Threatened species: vulnerable, endangered, critically endangered. IUCN Red List classifies species by extinction risk. Habitat loss is the primary cause.
Population size changes due to: births, deaths, immigration, emigration. Exponential growth: occurs when resources are unlimited (J-shaped curve). Logistic growth: population growth slows as it approaches carrying capacity K (S-shaped curve). Limiting factors: food, space, disease, predation (density-dependent) or floods, fires (density-independent). Predator-prey cycles: as prey increases, predator population rises; predator overhunts prey → prey drops → predator drops (e.g., lynx-snowshoe hare). Invasive species disrupt native population dynamics.
Transpiration: evaporation of water from plant leaves through stomata; creates a tension (negative pressure) that pulls water up from roots through xylem (cohesion-tension theory). Stomata: pores on leaf surface controlled by guard cells; open in light (CO₂ enters, O₂ exits, water exits); close in drought. Xylem transports water and minerals upward (passive). Phloem transports sugars (sucrose) bidirectionally via pressure-flow (source to sink). Root hairs increase surface area for water/mineral absorption.
Biological catalysts (usually proteins) that speed up chemical reactions without being consumed. Enzymes are specific: each enzyme has an active site that fits only certain substrates (lock-and-key model; induced fit model). Temperature: enzyme activity increases with temperature until the optimal temperature; above this, the enzyme denatures (shape changes, active site lost). pH: each enzyme has an optimal pH (stomach enzymes work best at ~pH 2; intestinal enzymes at ~pH 7–8). Inhibitors can slow enzyme activity.
Carbon cycle: CO₂ removed from air by photosynthesis → stored in biomass → released by respiration, decomposition, combustion. Fossil fuel burning increases atmospheric CO₂. Nitrogen cycle: N₂ in atmosphere → fixed to ammonia by bacteria (Rhizobium in legume roots) → converted to nitrates by nitrification → absorbed by plants → returned to atmosphere by denitrification. Both cycles are essential for life; human activities (fertilizers, combustion) disrupt natural balances.
Blood components: red blood cells (RBCs/erythrocytes — carry O₂ via hemoglobin; no nucleus in mammals), white blood cells (WBCs/leukocytes — immune defense), platelets (thrombocytes — blood clotting), plasma (55% of blood; water, proteins, hormones, nutrients, waste). Blood types: ABO system (A, B, AB, O) and Rh factor (+/−). Universal donor: O−. Universal recipient: AB+. Clotting cascade: platelets + fibrin form clot; vitamin K is required. Lymph: fluid that returns from tissues to blood via lymphatic system.
Water pollution: sewage, fertilizer runoff (eutrophication → algal blooms → oxygen depletion → dead zones), heavy metals, oil spills. Air pollution: particulate matter, SO₂, NOₓ, CO, ozone (ground level — bad), VOCs. Soil pollution: pesticides, heavy metals, industrial waste. Bioaccumulation: toxins increase in concentration in organism's tissue. Biomagnification: toxin concentration increases up the food chain (e.g., DDT in birds → thin eggshells). Point source pollution: specific location. Non-point source: runoff from large areas.
Biotic factors: living components of an ecosystem (producers, consumers, decomposers, competitors, predators, pathogens). Abiotic factors: non-living physical and chemical components (temperature, light, water/precipitation, soil type, pH, salinity, dissolved oxygen, wind, humidity). Biotic and abiotic factors interact to determine what organisms can survive in an ecosystem. Tolerance range: range of abiotic conditions an organism can survive; optimal range for best growth; limits of tolerance define distribution.
Decomposers (bacteria and fungi) break down dead organic matter into simpler inorganic molecules, returning nutrients to the soil. Without decomposers, nutrients would remain locked in dead organisms and ecosystems would collapse. Detritivores (earthworms, millipedes, pill bugs) shred dead material, increasing surface area for decomposers. Decomposition rate depends on temperature, moisture, and oxygen availability (fastest in warm, moist, aerobic conditions). Composting uses decomposers to recycle organic waste.
Plants respond to environmental stimuli. Phototropism: growth toward light (positive) or away from light (negative); caused by unequal distribution of auxin — more auxin on shaded side → faster growth → bending toward light. Gravitropism: roots grow toward gravity (positive); shoots grow away (negative). Thigmotropism: growth in response to touch (climbing plants, tendrils). Photoperiodism: response to day length — short-day plants flower when nights are long; long-day plants flower when nights are short.