Herbicides That Act Through Photosynthesis Glossary

amino acids

The basic building blocks of proteins. The sequence of amino acids in a protein and protein function are determined by the genetic code.


A plant pigment that protects chlorophyll from being photodegraded; enzymes that aid in the production of carotenoids are the target site of herbicides belonging to the pigment synthesis inhibitor mode of action.


An organelle within the plant cell in which the photosynthetic reactions are compartmentalized. The thylakoid membranes within the chloroplast are the site of the photosynthetic pigments and electron transfer components used to make energy from photosynthesis. The non-membrane space within the chloroplast is called the stroma; this is where photosynthetic energy is used to convert CO2 into sugars.

electron transfer

The transfer of electrons between a series of components in a controlled fashion. Examples include the mitochondrial electron transfer chain involved in oxidative phosphorylation and the chloroplast electron transfer chain carrying out photophosphorylation. Electrons are transferred from molecules with a higher free energy to those with a lower free energy.

free radical

A molecule with an unpaired electron. These are extremely reactive species and are capable of damaging a wide variety of cellular molecules.

mechanism of action

The specific process inhibited by a herbicide.

modes of action

The method by which an organism is affected, damaged or killed. The mode of action usually involves a chemical reaction instigated by another source. (herbicide, etc.).


The basic unit of light. Although light has some properties consistent with a wave, light from the sun is in individual packages. Each photon is a particle of electromagnetic radiation traveling with the speed of light (3 X 108 m sec-1). The energy in each photon is dependent on the frequency with which the electromagnetic field vibrates.


The coupling of photosystems I and II with an electron transfer chain that moves electrons from water (which is oxidized to form O2) to NADP+ (which is reduced to form NADPH). The transfer of electrons between photosystem II and photosystem I releases energy, which is conserved in the form of a trans-membrane proton gradient and used to synthesize ATP.


An array of pigment-protein complexes and electron transfer components that function together to harvest light energy, transfer the energy to photochemical reaction centers, and move the excited electrons in a controlled fashion to produce usable biochemical energy. Each photosystem contains hundreds of chlorophyll and carotenoid molecules functioning as antennae, while only a few chlorophyll molecules are employed in the reaction centers.


Compounds produced by the plant to regulate growth and development of the plant.


An intermediate in the biosynthetic pathways for chlorophyll and heme. This molecule is a strong photosensitizer and causes much cellular damage if it accumulates in tissues exposed to light.


The process of causing the de-excitation of a molecule from an excited singlet or triplet state back down to the ground state. Quenching of chlorophyll is an important protective role of carotenoids. If a triplet molecule is not rapidly quenched, it can react with oxygen to generate singlet oxygen. Singlet oxygen can cause much cellular damage.


The opposite of oxidation. When a molecule acquires one or two electrons it becomes more reduced.

triplet state

A normally forbidden excited electron orbital that is reserved for an electron with the same spin as the electron remaining in the ground state. Upon excitation to an excited singlet state, there is some possibility that the electron will reverse spin. When this happens, the electron then drops down to the triplet state, which is at a lower energy level than the excited singlet state. Once in the triplet state, the electron must remain there until its spin is again reversed. Molecular oxygen is unusual in that it is normally in a triplet state.