The minimum amount of energy required to start a reaction.
Activation energy
A molecule that is produced by a reaction.
Product
A molecule that enters a reaction and is changed by participating in it.
Reactant
Pocket in an enzyme where substrates bind and a reaction occurs.
Active site
Of an enzyme, a reactant that is specifically acted upon by the enzyme.
Substrate
An organic cofactor.
Coenzyme
A molecule or metal ion that associates with a protein and is necessary for its function.
Cofactor
Series of enzyme-mediated reactions by which cells build, remodel, or break down an organic molecule.
Metabolic pathway
A phospate-group transfer
Phosphorylation
The spontaneous spreading of molecules or atoms.
Diffusion
Series of enzymes and other molecules in a cell membrane that accept and give up electrons, thus releasing the energy of the electrons in steps.
Electron transfer chain
The mechanism by which a change that results from some activity decreases or stops the activity.
Feedback inhibition
Describes a fluid that has a high overall solute concentration relative to another fluid.
Hypertonic
Describes two fluids with identical solute concentrations.
Isotonic
Diffusion of water across a selectively permeable membrane; occurs when there is a difference in solute concentration between the fluids on either side of the membrane.
Osmosis
Passive transport mechanism in which a solute follows its concentration gradient across a membrane by moving through a transport protein.
Facilitated diffusion
The membrane-crossing mechanism that requires no energy input.
Passive transport
Pressure that a fluid exerts against a membrane, wall or other structure that contains it.
Turgor
Energy requiring mechanism in which a transport protein pumps a solute across a cell membrane against the solute's concentration gradient.
Active transport
Process by which a cell takes in a small amount of extracellular fluid (and its contents) by the ballooning inward of the plasma membrane.
Endocytosis
Process by which a cell expels a vesicle's contents to extracellular fluid.
Exocytosis
"Cell eating," an endocytic pathway by which a cell engulgs large particles such as microbes or cellular debris.
Phagocytosis
An organic molecule that can absorb light of certain wavelengths.
Pigment
Distance between the crests of two successive waves.
Wavelength
Main photosynthetic piment in plants.
Chlorophyll a