Biology Study Hub

Biology Learning Hub: Free Lessons and 357 Flashcards: a single starting point that brings together the lessons, worksheets, practice tests, and review material for this subject, arranged in a sensible order so you can work through it from start to finish.

All 357 biology terms and definitions

Every flashcard above, written out as a list and grouped by topic. Use it to revise without clicking through the cards, to search the page for a term, or to print the whole set.

Cells, Energy & Chemistry

Acid

a substance that donates hydrogen ions (H^+); in a typical aqueous solution near room temperature, an acid has a pH below 7. Never taste an unknown substance to identify it.

Activation energy

the minimum energy a collision needs to start a reaction.

Active transport

energy-dependent movement across a membrane, often against a concentration or electrochemical gradient.

Amino acid

a small organic molecule that serves as a building block of proteins.

Anion

a negatively charged ion, formed when an atom or group gains electrons.

Aqueous solution

any solution in which water is the solvent.

Atom

the smallest unit that still has the identity of an element.

Atomic number

the number of protons in an atom, which identifies the element.

ATP

the molecule cells spend to power their work.

Base (chemistry)

a substance that accepts hydrogen ions or, in water, may increase hydroxide ions (OH^-); a typical basic aqueous solution near room temperature has a pH above 7.

Buffer

a substance or mixture that resists a sudden change in pH by accepting or releasing hydrogen ions.

Carbohydrate

a family of molecules that includes sugars and polysaccharides and can supply energy, store energy, or provide structural support.

Catalyst

a substance that speeds a reaction by providing a lower-activation-energy pathway and is regenerated by the overall process.

Cation

a positively charged ion, formed when an atom or group loses electrons.

Cell

the smallest unit of life.

Cell membrane

another name for the plasma membrane, the selectively permeable boundary around a cell.

Cell theory

the idea that all living things are made of cells, the cell is life’s basic unit, and cells come from existing cells.

Cell wall

a rigid layer outside the cell membrane in plants, fungi, and many prokaryotes; animal cells do not have one.

Cellular respiration

a series of reactions that transfers energy from fuel molecules into ATP; in typical aerobic eukaryotic cells, most ATP is produced in mitochondria.

Chemical bond

a force that holds two or more atoms together.

Chemical change

a change that rearranges atoms into one or more new substances, such as burning or rusting.

Chemical reaction

a change that rearranges atoms into new substances.

Chloroplast

the plant-cell organelle where photosynthesis captures light energy to build sugar.

Compound

two or more elements chemically bonded into a new substance.

Concentration

how much solute is dissolved compared with the amount of solution.

Conservation of mass

the principle that matter is not created or destroyed in an ordinary chemical reaction, so each type of atom must balance.

Covalent bond

a bond in which atoms share one or more pairs of electrons.

Cytoplasm

the material inside the plasma membrane but outside the nucleus, including cytosol and organelles.

Dehydration synthesis

linking monomers into a polymer by removing a water molecule at each bond.

Denature

to lose a protein’s working shape, often from heat or extreme pH, which reduces or ends its activity.

Diffusion

the net movement arising from random molecular motion down the relevant chemical or electrochemical gradient; for an uncharged solute in a simple solution, this is toward lower concentration.

Electrolyte

a dissolved substance that separates into ions and allows a solution to conduct electric current.

Electron

a negatively charged particle in the space around the nucleus.

Element

a pure substance made of one kind of atom.

Endocytosis

bulk transport into a cell when the plasma membrane surrounds material and pinches off a vesicle.

Endothermic reaction

a reaction that absorbs heat from its surroundings under the stated conditions.

Enzyme

a biological catalyst, usually a protein, that speeds up a chemical reaction.

Exocytosis

bulk transport out of a cell when a vesicle fuses with the plasma membrane and releases its contents.

Exothermic reaction

a reaction that releases heat to its surroundings under the stated conditions.

Facilitated diffusion

passive movement down the relevant concentration or electrochemical gradient through a membrane protein, without direct ATP use.

Fatty acid

a hydrocarbon chain with an acidic end group; fatty acids are components of many lipids.

Glucose

a simple sugar that cells can use as a fuel and as a building material.

Glycogen

the branched storage form of glucose found mainly in liver and skeletal muscle cells.

Golgi apparatus

the organelle that modifies, packages, and ships materials, like a mailroom.

Hydrolysis

breaking a polymer apart by adding a water molecule at each bond.

Hypertonic solution

a solution with a greater effective concentration of nonpenetrating solute than the comparison solution; a cell placed in it tends to lose water.

Hypotonic solution

a solution with a lower effective concentration of nonpenetrating solute than the comparison solution; a cell placed in it tends to gain water.

Ion

an atom or bonded group of atoms with a net electric charge.

Ionic bond

the electrostatic attraction between oppositely charged ions.

Isotonic solution

a solution with the same effective concentration of nonpenetrating solute as a cell, causing no net water gain or loss.

Isotopes

atoms of one element that have different numbers of neutrons.

Lipid

a mostly water-insoluble biological molecule, including fats, oils, phospholipids, and steroids.

Lysosome

the organelle that breaks down and recycles waste and worn-out parts.

Macromolecule

a very large biological molecule; proteins, nucleic acids, and many polysaccharides are major examples.

Mitochondria

organelles that produce most ATP during aerobic respiration in typical eukaryotic cells.

Mixture

substances physically combined in variable proportions, separable by physical means.

Molecule

two or more atoms held together by covalent bonds.

Monomer

the small building-block unit that a polymer is made from.

Neutralization

a reaction in which an acid and a base consume each other’s acidic and basic properties; in the common acidu2013hydroxide model, the products are water and a salt.

Neutron

a neutral particle in the nucleus of an atom.

Nucleus (atom)

the dense center of an atom, holding the protons and neutrons.

Nucleus (cell)

the compartment that holds most of a cell’s DNA and directs the cell.

Octet rule

a useful main-group pattern in which many atoms gain, lose, or share electrons to reach eight valence electrons; important exceptions exist.

Organelle

a small structure inside a cell with a specific job.

Osmosis

the net movement of water across a selectively permeable membrane, often toward the side with the higher effective concentration of nonpenetrating solute.

Passive transport

net movement down the relevant electrochemical or concentration gradient without direct cellular energy input.

pH scale

a logarithmic measure related to hydrogen-ion activity; for typical dilute aqueous solutions near room temperature, 7 is neutral, below 7 is acidic, and above 7 is basic.

Photosynthesis

using light energy to build sugar; in plant cells it happens in the chloroplasts.

Physical change

a change in form only, with no new substance formed, such as melting or boiling.

Plasma membrane

the boundary that controls what enters and leaves the cell.

Polymer

a long chain of many monomers linked together.

Precipitate

a solid that forms or separates from a solution.

Product

a new substance that forms in a chemical reaction, written to the right of the arrow.

Prokaryotic cell

a cell with no membrane-bound nucleus, found in bacteria and archaea.

Protein

a polymer of amino acids folded into a shape that supports a specific structural, transport, signaling, or catalytic function.

Proton

a positively charged particle in the nucleus; its count sets the element.

Reactant

a starting substance in a chemical reaction, written to the left of the arrow.

Reaction rate

how fast reactants turn into products.

Ribosome

the cell structure that builds proteins.

Salt

an ionic compound; a salt is one product of many acidu2013base neutralization reactions.

Saturated solution

a solution at equilibrium with the maximum dissolved solute possible under the stated conditions.

Solubility

the maximum amount of a solute that dissolves in a set amount of solvent at a set temperature.

Solute

the substance that is dissolved in a solution.

Solution

a homogeneous mixture in which one or more substances are dispersed at the molecular or ionic level.

Solvent

the substance that does the dissolving in a solution.

Subscript

a small number within a chemical formula that shows how many atoms of the preceding element are in one unit; balancing never changes subscripts.

Substrate

the reactant molecule that binds to an enzyme’s active site.

Supersaturated solution

an unstable solution containing more dissolved solute than the usual equilibrium maximum at that temperature.

Surface area

the total amount of an object’s or membrane’s outside surface that is exposed.

Unsaturated solution

a solution containing less than the maximum amount of dissolved solute possible under the stated conditions.

Vacuole

a membrane-bound storage compartment; the large central vacuole in a plant cell also helps maintain pressure and shape.

Valence electrons

the electrons in an atom’s outermost shell, the ones involved in bonding.

Ecology & Evolution

Are all introduced species invasive?

No. Invasive species are introduced organisms that spread and cause harm in the relevant context.

Are all the oak trees of one species in a park a population or community?

A population, with the park and time interval defining the boundary.

Can carrying capacity change after a drought?

Yes. Changed water or food supplies can change what the habitat supports.

Does a populationu2019s decline prove predation caused it?

No. Births, deaths, movement, sampling, and several possible causes must be examined.

Does immigration increase or decrease population size?

It increases size by adding arriving individuals.

Does one snowy winter disprove global warming?

No. Weather at one place and time is not the same as a global climate trend.

Is reducing emissions mitigation or adaptation?

Mitigation, because it reduces a driver of climate change.

What distinguishes a community from an ecosystem?

An ecosystem explicitly includes the physical environment as well as the community.

What process can consume oxygen after a bloom?

Respiration by decomposers and other organisms can increase oxygen demand.

Which two climate factors commonly help describe terrestrial biomes?

Temperature and precipitation, including their seasonal patterns.

Why include dates with a warming estimate?

The estimate refers to a specified observation interval and baseline.

Why might a habitat corridor fail?

Its location or habitat conditions may not support the target speciesu2019 movement or survival.

Genetics & Information

Allele

one version of a gene, such as B or b.

Base (DNA)

one of the four “letters” of DNA (A, T, G, C) that pair up to form the rungs of the ladder.

Can a cell change enzyme activity without changing its gene sequence?

Yes. Regulatory binding or chemical modification can change an existing enzymeu2019s activity.

Carrier (genetics)

in a simple autosomal-recessive model, a heterozygous person who usually does not show the condition but can pass the recessive allele on.

Cell cycle

the ordered sequence in which a cell grows, copies its DNA, and divides.

Chromosome

a DNAu2013protein structure that carries many genes; after DNA replication, one chromosome consists of two sister chromatids until they separate.

Codon

a three-base sequence in messenger RNA that specifies an amino acid or a stop signal during translation.

Cytokinesis

division of a cell’s cytoplasm to form separate daughter cells.

Diploid

carrying two sets of chromosomes (2n), with one homolog of each pair inherited from each parent; most human body cells are diploid.

DNA

the molecule that stores a cell’s genetic instructions.

Do equal-size gel bands prove identical sequences?

No. Different sequences can have the same length.

Does a successful edit in one cell prove all cells were edited?

No. Editing outcomes can differ among cells and require verification.

Does a three-base deletion necessarily shift the downstream reading frame?

No. It preserves the frame, although it can remove an amino acid and change protein function.

Dominant

in a simple complete-dominance model, an allele whose associated phenotype appears in a heterozygote; it is commonly written as a capital letter.

Double helix

the twisted-ladder shape formed by two strands of DNA.

Fertilization

the fusion of haploid gametes, including fusion of their nuclei, to form a zygote.

Gamete

a haploid reproductive cell that can fuse with another gamete during sexual reproduction; sperm and egg cells are animal examples.

Gene

a DNA sequence used to make a functional RNA or protein product and thereby influence cell function or traits.

Genotype

the allele composition of a cell or organism at one or more loci; at a simple diploid locus it may be written as BB, Bb, or bb.

Haploid

carrying a single set of chromosomes (n); sperm and egg cells are haploid.

Heterozygous

having two different alleles for a gene, such as Bb.

Homologous chromosomes

a matching chromosome pair, one inherited from each parent, carrying the same kinds of genes in the same locations.

Homozygous

having two identical alleles for a gene, such as BB or bb.

Interphase

the G1, S, and G2 portions of the cell cycle; the cell grows and functions, and DNA is replicated specifically during S phase.

Is high mRNA proof of rapid transcription?

No. Reduced RNA degradation can also raise mRNA abundance.

Meiosis

a division process that reduces ploidy, commonly producing haploid products from a diploid starting cell. In animals it contributes to gamete formation, including polar bodies during egg development; in plants it produces spores.

Mitosis

nuclear division that usually produces two genetically similar nuclei with the same chromosome number as the parent nucleus.

Mutation

a change in a DNA sequence; its effect may be harmful, neutral, or beneficial depending on the change and context.

Nucleotide

the monomer of DNA or RNA, made of a sugar, a phosphate group, and a nitrogenous base.

Phenotype

an organism’s observable characteristics, shaped by genotype and often by the environment.

Recessive

in the simple diploid complete-dominance model used here, an allele whose associated phenotype appears when no dominant allele is present; it is commonly written as a lowercase letter.

RNA

a nucleic acid that is usually single-stranded; different RNAs help carry, interpret, regulate, or use genetic information.

Sister chromatids

the two usually near-identical replicated copies of a chromosome, joined until they separate during cell division.

Transcription

making an RNA copy from a DNA template.

Translation

using the codons in messenger RNA to assemble amino acids into a polypeptide.

What directly binds the lac repressor in the standard induction model?

Allolactose; it changes the repressoru2019s ability to bind the operator.

What does a band in a no-template PCR control suggest?

Possible contamination or an unwanted product; the sample result needs investigation.

What sets the boundaries of a standard PCR target?

The primer pair, together with the template sequence.

Which electrode attracts DNA in a typical agarose gel?

The positive electrode because DNA is negatively charged.

Which usually travels farther: a 200-base-pair or a 900-base-pair linear fragment?

The 200-base-pair fragment under the same suitable gel conditions.

Why can two cell types make different proteins with similar DNA?

They express different sets and amounts of genes.

Why is a promoter mutation potentially important?

It may alter transcription and the amount of gene product even when the protein-coding sequence is unchanged.

Zygote

The cell formed by fusion of gametes, such as sperm and egg in animals.

Health & Microbes

Adaptive immunity

a specific defense carried out by lymphocytes; the first response takes time, and memory can make later responses faster and stronger.

Antibiotic

a medicine that kills susceptible bacteria or slows their growth; antibiotics do not treat viral infections.

Antibiotic resistance

the ability of bacteria to survive an antibiotic that once killed them or stopped their growth.

Antibody

a protein secreted by plasma cells that binds a specific antigen and can help neutralize or mark a target.

Antigen

a molecule or molecular feature that can be specifically recognized by an antibody or immune-cell receptor.

B cell

an adaptive-immune lymphocyte that can develop into an antibody-secreting plasma cell or a memory cell.

Bacteria

single-celled prokaryotes; many are harmless or helpful, while some cause diseases that may be treatable with an appropriate antibiotic.

Fever

a regulated rise in body temperature caused by an increase in the hypothalamic temperature set point, often during infection.

Fungi

eukaryotic organisms that include yeasts and molds; some can cause superficial or systemic infections.

Inflammation

a protective response to injury or infection that commonly produces redness, warmth, swelling, and pain.

Innate immunity

the rapid, broad defense that recognizes common signs of injury or infection without antigen-specific memory.

Memory cell

a long-lived B or T lymphocyte that helps the adaptive immune system respond faster after later exposure to the same antigen.

Parasite

an organism that lives in or on a host and obtains resources at the host’s expense.

Pathogen

a microbe or agent, such as a virus, bacterium, fungus, or parasite, that can cause disease.

T cell

an adaptive-immune lymphocyte that can coordinate immune responses or destroy infected and abnormal cells.

Vaccination

exposing the adaptive immune system safely to target antigens so a later response can be faster and stronger.

Virus

an acellular infectious agent containing DNA or RNA in a protein coat, sometimes with a lipid envelope, that must use a host cell to replicate.

White blood cell

an immune-system cell in blood and tissues that helps defend the body against infection and abnormal cells.

Human Biology & Core Ideas

Abdominopelvic cavity

the large body cavity below the diaphragm that contains many digestive, urinary, and reproductive organs.

Absorption

the movement of digested nutrients from the digestive tract into the blood or lymph.

Alveoli

the tiny air sacs at the end of the airways, where gases cross between the lungs and the blood.

Anatomical position

the standard reference pose: standing, facing forward, arms at the sides, palms forward.

Anterior

toward the front of the body; the breastbone is anterior to the spine.

Aorta

the large artery that carries oxygen-rich blood from the left ventricle to the body.

Appendicular skeleton

the bones of the arms and legs plus the shoulder and pelvic girdles that attach them to the trunk.

Are all antigens parts of pathogens?

No. Antigens are recognized molecular features and can come from pathogens or other sources.

Artery

a blood vessel that carries blood away from the heart.

Atrium

an upper heart chamber that receives blood arriving at the heart.

Autonomic nervous system

the division of the peripheral nervous system that controls involuntary functions such as heart rate and digestion.

Axial skeleton

the bones along the body’s central axis: the skull, vertebral column, ribs, and sternum.

Bile

a fluid made by the liver that emulsifies fat into smaller droplets; it is stored and concentrated in the gallbladder.

Bladder

the muscular sac that stores urine until it leaves the body.

Blood pressure

the force per unit area that blood exerts against vessel walls, usually reported for systemic arteries as systolic over diastolic pressure.

Bone marrow

soft tissue inside certain bones; red marrow produces blood cells, while yellow marrow stores mainly fat.

Bronchi

airways that branch from the trachea; the right and left main bronchi enter the lungs and divide into smaller branches.

Bronchioles

small branching airways that carry air from the bronchi toward the alveoli.

Calcium

a mineral element stored mainly in bones and teeth; calcium ions also support muscle contraction, nerve signaling, and blood clotting.

Capillary

a tiny vessel whose wall consists chiefly of a single endothelial cell layer, allowing substances to move between blood and tissues.

Cardiac muscle

striated, involuntary muscle found only in the heart.

Cartilage

firm, flexible connective tissue that cushions joints and supports structures such as the nose and trachea.

Central nervous system

the brain and spinal cord.

Chemical digestion

the breakdown of large food molecules by enzymes.

Connective tissue

a tissue that supports, connects, and cushions other tissues.

Deep

farther from the body surface; the lungs are deep to the ribs.

Dermis

the thick middle layer of skin that holds vessels, nerves, hair follicles, and glands.

Diaphragm

the dome-shaped muscle below the lungs that is the main muscle of quiet inhalation.

Diastole

the part of the cardiac cycle when a heart chamber relaxes and fills with blood.

Distal

farther from the trunk or point of attachment; the fingers are distal to the wrist.

Do actin filaments become shorter during contraction?

No. Filaments slide so the sarcomere shortens.

Do arteries always contain oxygen-rich blood?

No. Arteries carry blood away from the heart; pulmonary arteries normally carry oxygen-poor blood.

Does a stronger stimulus normally make each action potential taller?

No. Stimulus intensity can be represented through firing patterns and recruited neurons rather than larger individual action potentials.

Does differentiation usually require removing unused genes?

No. Regulation changes which genes are expressed and how strongly.

Does everything filtered by the kidney leave in urine?

No. Much filtered water and many useful solutes are reabsorbed.

Does immune memory guarantee that later infection cannot happen?

No. It can improve the response, but protection is not absolute and can change over time.

Does negative feedback always stop the controlled process entirely?

No. It counteracts a deviation and adjusts activity toward regulated conditions.

Effector

the tissue, organ, or cell that carries out the response directed by a control center.

Endocrine gland

a ductless gland that releases hormones into the blood or nearby tissue fluid.

Epidermis

the thin, outer layer of skin, which has no blood vessels of its own.

Epiglottis

a flap that helps cover the airway during swallowing so food is directed toward the esophagus.

Epithelial tissue

a tissue that covers body surfaces and lines hollow organs.

Estrogen

a group of steroid hormones important in reproductive development, the menstrual cycle, and several other body functions.

Eukaryotic cell

a cell that keeps most of its DNA in a true nucleus and has membrane-bound organelles.

Evidence

the observations or measurements offered to support a claim.

Filtration

movement of water and small solutes across a membrane because of pressure, as in the renal glomerulus.

Frontal plane

a vertical plane that divides the body into front and back portions; also called the coronal plane.

Gland

a secretory cell, tissue, or organ that makes and releases a substance such as a hormone, enzyme, sweat, or mucus.

Glomerulus

a knot of capillaries in a nephron where blood pressure drives filtration into Bowman’s capsule.

Glucagon

a pancreatic hormone that raises blood glucose, especially by promoting the release of stored fuel.

Hemoglobin

the iron-containing protein in red blood cells that carries most oxygen in the blood.

Homeostasis

dynamic regulation of internal variables within workable ranges despite internal or external disturbances.

Hormone

a chemical messenger released by endocrine cells and carried to target cells, often through the blood.

Hyper- (prefix)

above normal, excessive, or high, as in hypertension.

Hypo- (prefix)

below normal, deficient, or low, as in hypoglycemia.

Hypodermis

the fatty layer below the skin that cushions, insulates, stores energy, and anchors the skin.

Hypothalamus

a brain region that links nervous and endocrine control and helps regulate temperature, hunger, thirst, and pituitary activity.

Inferior

below another structure; the stomach is inferior to the heart.

Insulin

a pancreatic hormone that lowers blood glucose by promoting glucose uptake and storage.

Integumentary system

the skin, hair, nails, sweat glands, and oil glands, which protect the body and help regulate temperature.

Interstitial fluid

the extracellular fluid that directly surrounds and bathes tissue cells.

Is breathing identical to cellular respiration?

No. Breathing ventilates lungs; cellular respiration includes cellular pathways that transfer energy from fuels.

Keratin

a strong structural protein that helps form the outer epidermis, hair, and nails.

Kidney

a bean-shaped organ that filters plasma and modifies tubular fluid to form urine.

Lateral

farther from the body’s midline; the arms are lateral to the chest.

Ligament

a strong band of connective tissue that connects bone to bone.

Liver

a large organ that processes nutrients, detoxifies many substances, stores glycogen, and makes bile.

Lymph

interstitial fluid once it has been collected into the lymphatic vessels.

Lymph node

an organized immune organ where lymph-borne material is sampled and adaptive immune responses can be initiated.

Mechanical digestion

the physical breakdown of food by chewing and churning.

Medial

closer to the body’s midline; the nose is medial to the eyes.

Motor neuron

a neuron that carries a command from the central nervous system to a muscle or gland.

Must every reflex pass through the spinal cord?

No. Some reflex circuits involve the brainstem or other locations.

Myelin

a fatty insulating covering around many axons that helps nerve signals travel quickly.

Negative feedback

a response that counteracts a change and moves a regulated condition back toward its usual range.

Nephron

the kidney’s microscopic functional unit; it filters plasma and modifies tubular fluid.

Nerve

a bundle of peripheral axons together with supporting cells, connective tissue, and blood vessels.

Nervous system

the brain, spinal cord, nerves, and sensory receptors that detect information, process it, and direct responses.

Neuron

a single nerve cell that carries signals.

Neurotransmitter

a chemical messenger released by a neuron that carries a signal across a synapse.

Organ

two or more tissues working together as one structure, such as the heart or stomach.

Organ system

a group of organs that work together in major body functions, such as circulation.

Ovulation

the release of a secondary oocyte from an ovary.

Pancreas

an organ that releases digestive enzymes into the small intestine and hormones such as insulin and glucagon into the blood.

Partial pressure

one gas’s contribution to a mixture’s total pressure; respiratory gases diffuse from higher toward lower partial pressure.

Peripheral nervous system

all nerves and ganglia outside the brain and spinal cord.

Peristalsis

wave-like contractions of smooth muscle that push material through the digestive tract.

Pituitary gland

a small endocrine gland below the hypothalamus whose hormones regulate several other glands and body processes.

Platelet

a small cell fragment in blood that helps begin clot formation and reduce bleeding.

Posterior

toward the back of the body; the spine is posterior to the breastbone.

Progesterone

a steroid hormone that helps regulate the menstrual cycle and supports the uterine lining.

Proximal

closer to the trunk or point of attachment; the elbow is proximal to the wrist.

Pulmonary circulation

blood flow from the right side of the heart to the lungs and back to the left side of the heart.

Punnett square

a grid that models the possible allele combinations from a genetic cross.

Reabsorption

movement of useful substances from kidney tubules back into the blood.

Receptor

a structure that detects a signal or change, such as a protein a hormone binds to.

Red blood cell

a flexible blood cell packed with hemoglobin that transports oxygen and helps carry carbon dioxide.

Red marrow

bone-marrow tissue that produces red blood cells, white blood cells, and platelets.

Sagittal plane

a vertical plane that divides the body into left and right portions.

Secretion (kidney)

movement of selected substances from the blood into kidney tubules for removal in urine.

Sensory neuron

a neuron that carries information from a receptor toward the central nervous system.

Septum

a wall that separates two spaces, such as the wall dividing the right and left sides of the heart.

Set point

the reference value or range around which a regulated condition is controlled.

Skeletal muscle

striated muscle under somatic motor control, often voluntarily controlled, that commonly attaches to bones and moves the body.

Small intestine

the digestive organ where most chemical digestion and nutrient absorption occur.

Smooth muscle

non-striated, involuntary muscle in the walls of hollow organs.

Superficial

closer to the body surface; the skin is superficial to the muscles.

Superior

above another structure; the head is superior to the chest.

Synapse

the junction where a neuron communicates with another cell.

Systemic circulation

blood flow from the left side of the heart to body tissues and back to the right side.

Systole

the part of the cardiac cycle when a heart chamber contracts and ejects blood.

Target cell

a cell that has the receptor needed to respond to a particular hormone or signal.

Tendon

a strong band of connective tissue that connects muscle to bone.

Testosterone

a steroid hormone important in reproductive development, sperm production, and several other body functions.

Tissue

an organized group of cells and, in many tissues, extracellular material that work together in related functions.

Trachea

the windpipe, a cartilage-ringed tube that carries air toward the lungs.

Transverse plane

a horizontal plane that divides the body into upper and lower portions.

Urea

the main nitrogen waste from breaking down protein.

Ureter

the tube that carries urine from a kidney down to the bladder.

Urethra

the tube that carries urine from the bladder to the outside of the body.

Valve

a structure that opens and closes to promote forward flow and help prevent backflow.

Vein

a blood vessel that carries blood toward the heart.

Ventilation

the movement of air into and out of the lungs.

Ventricle

a lower heart chamber that pumps blood out of the heart.

Villi

tiny fingerlike projections in the small intestine that greatly increase surface area for nutrient absorption.

What carries most oxygen in blood?

Hemoglobin in red blood cells.

What drives oxygen diffusion from alveoli into blood?

A suitable oxygen partial-pressure gradient across the thin exchange barrier.

What links skeletal muscle to bone?

A tendon.

What makes adaptive memory specific?

The receptors and cell populations involved recognize particular antigen features.

What normally restores the diploid chromosome complement in human sexual reproduction?

Fertilization, when haploid gamete contributions come together.

Where does most nutrient absorption occur?

In the small intestine, through a specialized absorptive surface.

Which cells secrete antibodies after B-cell activation?

Plasma cells, which develop from activated B cells.

Why might a cell ignore a circulating hormone?

It may lack the appropriate receptor or response machinery.

Scientific Practice & Data

Accuracy

how close a measurement is to the true, accepted value.

Baseline

the starting measurement or comparison condition used to judge later change.

Bias

a systematic influence that pushes measurements, sampling, interpretation, or conclusions in one direction.

Caption

text beside or below a figure that explains what the image shows and may provide essential evidence.

Causation

one thing actually bringing about another.

Claim

a statement or proposed answer that should be supported by relevant evidence and reasoning.

Coefficient

a number placed before a chemical formula to show how many molecules or formula units participate in a reaction.

Collision theory

the idea that particles react only when they collide with enough energy and a suitable orientation.

Column

a vertical set of cells in a table; read its heading before using a value.

Conclusion

a judgment that summarizes what the data show, and no more.

Confounding variable

a factor related to both the condition being studied and the outcome, offering a competing explanation for an observed relationship.

Control group

the group assigned to a comparison condition, such as no treatment, a placebo, or a standard treatment.

Controlled variable

a factor kept the same in every trial to keep a test fair (also called a constant).

Correlation

two things changing together; by itself it is not proof that one causes the other.

Data

the recorded measurements or observations from a study.

Density

mass per unit volume; for example, grams per milliliter.

Dependent variable

the outcome that is measured in an experiment.

Experimental group

the group that receives the treatment or planned change being studied.

Extrapolation

an estimate beyond the measured data range; it is generally less certain than interpolation.

Falsifiable

stated so that some possible observation could count against it.

Graph

a visual display that uses axes, scales, points, bars, or lines to show values and relationships.

Hypothesis

a tentative, testable explanation or claim; it can be used to generate a specific prediction.

Independent variable

A factor a researcher varies or uses as a predictor. A simple experiment changes one factor; factorial experiments can vary several independently.

Indicator

a substance whose visible change, often a color change, gives information about pH or another chemical condition.

Inference

a reasonable interpretation drawn from evidence.

Interpolation

an estimate between measured data points within the observed range.

Legend

the key on a graph or diagram that explains what colors, line styles, patterns, or symbols represent.

Mass number

the number of protons plus neutrons in an atom.

Mean

the arithmetic average: add all values and divide by the number of values.

Median

the middle value after data are placed in order; for an even number of values, average the two middle values.

Meniscus

the curved surface of liquid in a narrow container; for water, read the bottom of the curve at eye level.

Metric system

the decimal measuring system used in science, scaled by tens.

Mode

the value that occurs most often in a data set.

Observation

something you notice with your senses or instruments.

Outlier

a data value unusually far from the rest; investigate it before deciding whether it should be excluded.

Placebo

an inactive or comparison treatment designed to resemble the experimental treatment.

Precision

how close repeated measurements are to one another.

Prediction

the specific result you expect if a hypothesis holds.

Qualitative data

descriptive, nonnumerical observations such as color, texture, or behavior.

Quantitative data

numerical measurements or counts that include values and, when appropriate, units.

Random assignment

using chance to place participants into experimental groups, helping distribute other factors more evenly.

Random sample

a sample selected by chance so each member of the target population has a known opportunity to be chosen.

Range

the highest value minus the lowest value in a data set.

Reliability

the consistency of a measurement or result when a procedure is repeated.

Repeated trials

performing the same procedure more than once to reveal variation and improve confidence in a pattern.

Replication

copying DNA before cell division; the term also describes an independent repeat of a study.

Row

a horizontal set of cells in a table; use the row label to identify what the values describe.

Sample

the subset of individuals, objects, or measurements actually studied.

Sample size

the number of independent observations or participants in a sample.

Table

data arranged in labeled rows and columns for organized comparison.

Testable

able to be checked by an experiment or observation.

Theory

a broad, well-supported explanation backed by a large body of evidence.

Trend

the overall direction or pattern in a set of data, such as increasing, decreasing, or remaining stable.

True value

the value consistent with the definition of a quantity; in practice, a certified reference or best accepted estimate is often used.

Validity

how well a method measures or tests what it claims to measure or test.

Variable

anything in an experiment that can change or be changed.

Volume

the amount of three-dimensional space occupied by a substance or object.

X-axis

the horizontal axis of a graph; the independent variable is commonly plotted here.

Y-axis

the vertical axis of a graph; the dependent variable is commonly plotted here.