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The smallest unit of a compound; composed of atoms covalently bonded to one another.


A substance composed of atoms that are chemically identical, alike in their number of protons.


The smallest particle of an element that retains the properties of that element.


Referring to small particles that make up atoms, including electrons, protons, and neutrons.


A negatively charged particle that occurs in charge distribution.


A particle bearing a positive electrical charge found in the nuclei of all atoms.


A subatomic particle carrying no electrical charge.

Electron shell

The arrangement of electrons around an atom's nucleus according to the energy they contain; electrons with the least energy are in the shell closest to the nucleus, and those with more are in shells farther from the nucleus.


One of multiple forms of an element having the same atomic number but a different atomic mass.

Chemical bond

The attraction between two atoms resulting from the sharing or transfer of outer electrons.

Chemical reaction

Change in chemical bonds that produces one or more new substances.


The basic living unit.

Law of conservation of matter

The law stating that matter can be neither created nor destroyed; does not hold true at the subatomic level.

Activation energy

The energy necessary to start a chemical reaction.


An atom or a molecule that has either gained or lost one or more electrons, giving it a positive or negative charge.

Ionic bond

A chemical bond formed by the attraction between oppositely charged ions.

Covalent bond

A chemical bond formed by two atoms sharing a pair of electrons.

Hydrogen bond

A weak attraction between hydrogen atoms and oxygen, nitrogen, or fluorine atoms; holds together the strands of DNA in their double helix.

pH scale

A scale from 0 to 14 reflecting the concentration of hydrogen ions in solution; a number less than 7 denotes acidic conditions, and a number greater than 7 denotes basic conditions.


Having a pH of less than 7, reflecting more dissolved hydrogen ions than hydroxide ions.


Alkaline; having a pH greater than 7, reflecting more dissolved hydroxide ions than hydrogen ions.


Refers to compounds that are made up of carbon atoms and other elements; originally thought to be associated only with living things.


A large, complex molecule.


An organic compound made of carbon, hydrogen, and oxygen, with the hydrogen and oxygen atoms in a 2:1 ratio; examples are sugars, starches, and cellulose.


A simple sugar with three to seven carbon atoms in its carbon skeleton.


A double sugar composed of two chemically bonded simple sugars.


A complex carbohydrate composed of many simple sugars bonded in a chain; for example starch and cellulose.


A fat, an oil, a wax, or a fat-like compound that usually has fatty acids in its molecular structure; an important component of the plasma membrane.


An organic compound composed of one or more polypeptide chains of amino acids; most structural materials and enzymes in a cell are these.

Amino acid

An organic compound composed of a central carbon atom to which are bonded a hydrogen atom, an amino group (-NH₂), an acid group (-COOH), and one of a variety of other atoms or groups of atoms; the building block of polypeptides and proteins.

Peptide bond

A covalent chemical bond formed between two amino acids; bonds the amino group of each amino acid to the carboxyl group of the next.


A long chain of chemically bonded amino acids.

Primary structure

The first level of organization of a protein or nucleic acid; refers to the specific sequence of amino acids or nucleotides.

Secondary structure

In proteins, the shape of a folded polypeptide chain; results from hydrogen bonds between adjacent parts of the molecule.

Tertiary structure

The three-dimensional folded structure of a polypeptide or protein molecule.


The tendency to repel water; substances that are hydrophobic are non-polar and cannot hydrogen bond to water.

Quaternary structure

The shape of a complex protein defined by the three-dimensional arrangement of its polypeptide subunits.

Nucleic acid

DNA or RNA; a polymer of nucleotides important in encoding instructions for cell processes.


A subunit of DNA or RNA composed of a 5-carbon sugar, a nitrogen-containing base, and a phosphate group.


Ribonucleic acid; a nucleic acid similar to DNA but having the sugar ribose rather than deoxyribose and uracil rather than thymine as one of the bases.


Deoxyribonucleic acid; the hereditary material of most organisms; a nucleic acid composed of deoxyribose sugar, phosphate groups, and four nitrogen-containing bases.

Chemical energy

Energy stored in the structure of molecules.

Free energy

Energy that is available to do work.


A substance that supports the growth and maintenance of an organism.


An organism that obtains carbon compounds from other organisms.


An organism that forms its own food molecules from abiotic materials.


The process by which cells use light energy to make organic compounds] from inorganic materials.


An organism that derives energy from light and forms its own organic compounds from abiotic carbon sources.


A biochemical pathway that uses energy from the oxidation of inorganic substances to drive the formation of organic molecules.

Cell respiration

The series of chemical reactions by which a living cell breaks down carbohydrates and obtains energy from them.


An autotroph; any organism that produces its own food.


A heterotroph; an organism that feeds on other organisms or on their organic wastes.


An organism that lives on decaying organic material, from which it obtains energy and nutrients.

Food web

The overlapping food chains of an ecosystem.


Referring to a physical or nonliving component of an ecosystem.


Relating to a living component of an ecosystem.


A biological community and its abiotic environment.


Type of place where an organism lives.


The outer portion of Earth-air, water, and soil-where life is found.

First law of thermodynamics

The law derived from the principle of the conservation of energy stating that energy can be neither created nor destroyed, but it can be transferred or transformed.

Law of conservation of energy

The law stating that energy can be neither created nor destroyed, only changed from one form into another.

Second law of thermodynamicds

The law stating that energy transfers and transformations increase the entropy of the universe.


A measure of the degree of disorganization of a system, that is, how much energy in a system has become so dispersed that it is no longer available to do work.


A protein or part-protein molecule made by an organism and used as a catalyst in a specific biochemical reaction.


A chemical that promotes a reaction between other chemicals by reducing the energy required to activate the reaction; may take part in the reaction but emerges in its original form.

Active site

The portion of an enzyme that attaches to the substrate through weak chemical bonds.


A molewcule on which enzymes act.


The sum of all the chemical changes taking place in an organism.


The process of building chemical compounds from smaller components by means of chemical reactions.


The process of breaking substances down into smaller chemical units.


The loss of electrons from a substance in a chemical reaction.


Adenosine triphosphate; a compound that has three phosphate groups and is used by cells to store energy and to fuel many metabolic processes.


Adenosine diphosphate; the compound that remains when a phosphate group is removed from ATP, releasing energy.


The process by which food breaks down into molecules that an organism can absorb or use.

Extracellular digestion

The breakdown of nutrient molecules outside of cells.

Intracellular digestion

The breakdown of nutrients within a cell.


The process of taking a substance from the environment, usually food, into the body.


Liquid secreted in the mouth; begins mechanical and chemical digestion.


Flap of cartilaginous tissue at the base of the tongue in mammals; prevents food from entering the trachea, the airway to the lungs, during swallowing.


The rhythmic waves of contraction of the smooth muscle that pushes food through the digestive tract.

Salivary amylase

An enzyme in saliva that begins digestion of starch; converts starch to disaccharides.


A digestive hormone secreted by the stomach lining; stimulates the secretion of fluid by gastric glands in the stomach.


A protein-splitting enzyme secreted by the gastric glands of the stomach.


The inactive form of pepsin.


An enzyme in pancreatic juice that breaks down protein molecules.


A secretion of the liver stored in the gallbladder and released through a duct to the small intestine; breaks large fat droplets into smaller ones that enzymes can act on more efficiently.


A fat-digesting enzyme.


A fingerlike projection of the small intestine that increases surface area for absorption of digested foods.


A microscopic blood vessel penetrating the tissues and consisting of a single layer of cells that allows exchange between the blood and tissue fluids.


The entire contents of the cell, except the nucleus, bounded by the plasma membrane.

Transport protein

A protein that plays a role in the active or passive movement of specific substances through cell membranes.

Selectively permeable

Of membranes, allowing some substances to cross and preventing others from crossing.


A protein linked to a sugar or polysaccharide; component of receptor molecules on the outer surface of cells.


A lipid covalently linked to a sugar or polysaccharide; an important part of animal cell membranes.


The movement of a substance down its concentration gradient from a more concentrated area to a less concentrated area.

Concentration gradient

A difference in the concentration of a substance over a distance.


The movement of water (or another solvent) through a selectively permeable membrane from a solution with a lower concentration of solutes to one with a higher concentration of solutes.


A cell's swelling against its cell wall caused by the pressure of the cell's contents.

Passive transport

The diffusion of a substance through a biological membrane.

Active transport

The movement of a substance through a biological membrane against a concentration gradient.

Facilitated diffusion

The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradients.


The cellular uptake of materials in which the plasma membrane surrounds and engulfs extracellular materials.


The release of macromolecules from a cell by the fusion of vesicles with the plasma membrane.


Air sacs in a lung.


The waxy outer layer covering the surfaces of most land-dwelling plants, animals, and fungi.













Urinary system




Urinary bladder




Glomerular capsule






Feedback regulation


Antidiuretic hormone










Light reactions


Calvin cycle


ATP synthetase




Limiting factors




Bundle sheath


Cell respiration








Krebs cycle


Electron transport system


Pyruvie acid




Lactic-acid fermentation




Coenzyme A




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