Chapter 8

antenna complex

A complex of hundreds of pigment molecules in a photosystem that collects photons and feeds the light energy to a reaction center.

action spectrum

A measure of the efficiency of different wavelengths of light for photosynthesis. In plants it corresponds to the absorption spectrum of chlorophylls.

crassulacean acid metabolism (CAM)

A mode of carbon dioxide fixation by which CO2 enters open leaf stomata at night and is used in photosynthesis during the day, when stomata are closed to prevent water loss.

pigments

A molecule that absorbs light

photon

A particle of light having a discrete amount of energy. The wave concept of light explains the different colors of the spectrum, whereas the particle concept of light explains the energy transfers during photosynthesis.

C3 plants

A plant that utilizes the C3 carbon fixation pathway as the sole mechanism to convert CO2 into an organic compound

C4 plants

A plant that utilizes the C4 carbon fixation pathway in which the CO2 is first bound to a phosphoenolpyruvate in mesophyll cell resulting in the formation of four-carbon compound (oxaloacetate) that is shuttled to the bundle sheath cell where it will be decarboxylated to liberate the CO2 to be utilized in the C3 pathway.

b6-f complex

A proton pump found in the thylakoid membrane. This complex uses energy from excited electrons to pump protons from the stroma into the thylakoid compartment.

accessory pigment

A secondary light-absorbing pigment used in photosynthesis, including chlorophyll b and the carotenoids, that complement the absorption spectrum of chlorophyll a

reaction center

A transmembrane protein complex in a photosystem that receives energy from the antenna complex exciting an electron that is passed to an acceptor molecule.

phycobiloproteins

A type of accessory pigment found in cyanobacteria and some algae. Complexes of phycobiloprotein are able to absorb light energy in the green range.

chlorophyll a

A type of chlorophyll that is most common and predominant in all oxygen-evolving photosynthetic organisms such as higher plants, red and green algae. It is best at absorbing wavelength in the 400-450 nm and 650-700 nm of the electromagnetic spectrum. Its molecular formula is C55H72O5N4Mg.

chlorophyll b

A yellow-green chlorophyll pigment which occurs only in plants and green algae. It functions as a light harvesting pigment that pass on the light excitation to chlorophyll a. It absorbs well at wavelength of 450-500 nm and 600-650 nm of the electromagnetic spectrum. Its molecular formula is C55H70O6N4Mg.

photoresperation

Action of the enzyme rubisco, which catalyzes the oxidization of RuBP, releasing CO2; this reverses carbon fixation and can reduce the yield of photosynthesis.

photosystems

An organized complex of chlorophyll, other pigments, and proteins that traps light energy as excited electrons. Plants have two linked photosystems in the thylakoid membrane of chloroplasts. Photosystem II passes an excited electron through an electron transport chain to photosystem I to replace an excited electron passed to NADPH. The electron lost from photosystem II is replaced by the oxidation of water.

carotenoids

Any of a group of accessory pigments found in plants; in addition to absorbing light energy, these pigments act as antioxidants, scavenging potentially damaging free radicals.

cytochrome

Any of several iron-containing protein pigments that serve as electron carriers in transport chains of photosynthesis and cellular respiration.

stroma

In chloroplasts, the semiliquid substance that surrounds the thylakoid system and that contains the enzymes needed to assemble organic molecules from CO2.

light dependant reactions

In photosynthesis, the reactions in which light energy is captured and used in production of ATP and NADPH. In plants this involves the action of two linked photosystems.

light independant reactions

In photosynthesis, the reactions of the Calvin cycle in which ATP and NADPH from the light-dependent reactions are used to reduce CO2 and produce organic compounds such as glucose. This involves the process of carbon fixation, or the conversion of inorganic carbon (CO2) to organic carbon (ultimately carbohydrates).

ribulose1,5 bisphosphate (RuBP)

In the Calvin cycle, the five-carbon sugar to which CO2 is attached, accomplishing carbon fixation. This reaction is catalyzed by the enzyme rubisco.

CAM plants

Plants that use C4 carbon fixation at night, then use the stored malate to generate CO2 during the day to minimize dessication.

photoelectric effect

The ability of a beam of light to excite electrons, creating an electrical current.

carbon fixation

The conversion of CO2 into organic compounds during photosynthesis; the first stage of the dark reactions of photosynthesis, in which carbon dioxide from the air is combined with ribulose 1,5-bisphosphate.

calvin cycle

The dark reactions of C3 photosynthesis; also called the Calvin-Benson cycle.

ribulose 1,5 bisphosphate carboxylase/oxygenase (rubisco)

The four-subunit enzyme in the chloroplast that catalyzes the carbon fixation reaction joining CO2 to RuBP.

C3 Photosynthesis

The main cycle of the dark reactions of photosynthesis, in which CO2 binds to ribulose 1,5-bisphosphate (RuBP) to form two 3-carbon phosphoglycerate (PGA) molecules.

photosystem II

The photosystem that absorbs light for use to drive the oxidation of water and the reduction of plastoquinone, and whose reaction center chlorophyll is P680.

photosystem I

The photosystem that makes use of light to transfer electron particualrly from plastocyanin to ferredoxin, and whose reaction center chlorophyll is P700.

chlorophyll

The primary type of light-absorbing pigment in photosynthesis. Chlorophyll a absorbs light in the violet-blue and the red ranges of the visible light spectrum; chlorophyll b is an accessory pigment to chlorophyll a, absorbing light in the blue and red-orange ranges. Neither pigment absorbs light in the green range, 500-600 nm.

absorbtion spectrum

The relationship of absorbance vs. wavelength for a pigment molecule. This indicates which wavelengths are absorbed maximally by a pigment. For example, chlorophyll a absorbs most strongly in the violet-blue and red regions of the visible light spectrum.

noncyclic photophosphorylation

The set of light-dependent reactions of the two plant photosystems, in which excited electrons are shuttled between the two photosystems, producing a proton gradient that is used for the chemiosmotic synthesis of ATP. The electrons are used to reduce NADP to NADPH. Lost electrons are replaced by the oxidation of water producing O2.