The amount of energy that reactants must absorb before a chemical reaction will start; also called activation energy.
The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
The binding of a molecule to a protein that affects the function of the protein at a different site.
A chemical agent that changes the rate of a reaction without being consumed by the reaction.
An organic molecule serving as a cofactor. Most vitamins function as coenzymes in important metabolic reactions.
Any non-protein molecule or ion that is required for the proper functioning of an enzyme. Cofactors can be permanently bound to the active site or may bind loosely with the substrate during catalysis.
A substance that reduces the activity of an enzyme by entering the active site in place of the substrate whose structure it mimics.
A non-spontaneous chemical reaction, in which free energy is absorbed from the surroundings.
A protein serving as a catalyst, a chemical agent that changes the rate of a reaction without being consumed by the reaction.
A method of metabolic control in which the end product of a metabolic pathway acts as an inhibitor of an enzyme within that pathway.
The portion of a system's energy that can perform work when temperature and pressure are uniform throughout the system. The change in free energy of a system is calculated by the equation ΔG = ΔH - T Δs, where T is absolute temperature.
The change in shape of the active site of an enzyme so that it binds more snugly to the substrate, induced by entry of the substrate.
A series of chemical reactions that either builds a complex molecule (anabolic pathway) or breaks down a complex molecule into simpler compounds (catabolic pathway).
The totality of an organism's chemical reactions, consisting of catabolic and anabolic pathways.
A substance that reduces the activity of an enzyme by binding to a location remote from the active site, changing its conformation so that it no longer binds to the substrate.
1st Law of Thermodynamics
The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.
2nd Law of Thermodynamics
The principle whereby every energy transfer or transformation increases the entropy of the universe. Ordered forms of energy are at least partly converted to heat, and in spontaneous reactions, the free energy of the system also decreases.
The uptake of small nutrient molecules by an organism's own body; the third main stage of food processing, following digestion.
The semifluid mass into which food is converted by gastric secretion and which passes from the stomach into the small intestine.
The process in digestion that splits macromolecules from food by the enzymatic addition of water.
A substance that an organism must absorb in preassembled form because it cannot be synthesized from any other material. In humans, there are essential vitamins, minerals, amino acids, and fatty acids.
An extensive pouch that serves as the site of extracellular digestion and a passageway to disperse materials throughout most of an animal's body.
A heterotrophic mode of nutrition in which other organisms or detritus are eaten whole or in pieces.
The joining of food vacuoles and lysosomes to allow chemical digestion to occur within the cytoplasm of a cell.
A tiny lymph vessel extending into the core of an intestinal villus and serving as the destination for absorbed chylomicrons.
(plural, microvilli) One of many fine, fingerlike projections of the epithelial cells in the lumen of the small intestine that increase its surface area.
(1) Rhythmic waves of contraction of smooth muscle that push food along the digestive tract. (2) A type of movement on land produced by rhythmic waves of muscle contractions passing from front to back, as in many annelids.
An animal, such as a cow or a sheep, with an elaborate, multicompartmentalized stomach specialized for an herbivorous diet.
A ringlike valve consisting of modified muscles in a muscular tube, such as a digestive tract; closes off the tube like a drawstring.
An aquatic animal, such as a clam or a baleen whale, that sifts small food particles from the water.
(plural, villi) (1) A finger-like projection of the inner surface of the small intestine. (2) A fingerlike projection of the chorion of the mammalian placenta. Large numbers of villi increase the surface areas of these organs.