Plant Pigments and Photosynthesis Glossary


A pigment with the primary function of capturing the energy from photons and transferring that energy to other pigments within the photosystem. Most chlorophyll molecules function as antennae, with relatively few of the hundreds of chlorophyll molecules carrying out photochemistry in the reaction center. Carotenoids also function as antennae, but additionally play an important role in quenching triplet chlorophyll and singlet oxygen molecules that would cause damage to the plant cell.


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.

conjugated double bonds

Double bonds on alternating pairs of carbon molecules in a linear or cyclic organic molecule. An example is as follows -C=C-C=C-C=C- The electrons in conjugated double bonds form a resonance structure that is smeared over the double bond system. These electrons are responsible for light absorption.

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.


Also known as terpenes, this is a diverse class of biomolecules. They have in common a five carbon branched molecule as the basic building block.


The reaction center of photosystem II which best absorbs light at a 680 nm wavelength. P-680 is a chlorophyll a molecule.


The reaction center of photosystem I which best absorbs light at a 700 nm wavelength. P-700 is a chlorophyll a molecule.


The development of a plant cell or organ that is triggered by exposure to light. For example, if a corn plant is germinated in the dark, it will produce an etiolated shoot, lacking chlorophyll or the ability to carry out photosynthesis. Exposing an etiolated shoot to light will initiate development of pigment biosynthesis and the production of cellular components necessary for photosynthesis.


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.


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.

reaction center

A specialized component of the photosystem that actually carries out the photochemistry with the excitation energy harvested by antenna pigments.


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

singlet states

An orbital to which an electron may move after being excited by a photon. Singlet state orbitals require that the excited electron have a spin opposite to that of the spin on the electron remaining in the ground state.


A class of molecules characterized by a series of four, five-membered rings containing four carbon atoms and one nitrogen. Tetrapyrrole molecules include hemes, chlorophylls, vitamin B12 and phytochrome.

thylakoid membranes

A series of sealed membrane compartments found inside the chloroplast of plant cells. The membranes contain the photosynthetic apparatus and all of the chlorophyll in the leaf. Their structure as closed 'sacks' permits functional 'inside' and 'outside' spaces within the chloroplast across which photophosphorylation can generate the proton gradient necessary for ATP synthesis.