Key Terms

absorption spectrum

range of wavelengths of electromagnetic radiation absorbed by a given substance

antenna pigment

pigment molecule that directly absorbs light and transfers the energy absorbed to other pigment molecules

Calvin cycle

light-independent reactions of photosynthesis that convert carbon dioxide from the atmosphere into carbohydrates using the energy and reducing power of ATP and NADPH

carbon fixation

process of converting inorganic CO₂ gas into organic compounds

carotenoid

photosynthetic pigment that functions to dispose of excess energy

chemoautotroph

organism that can build organic molecules using energy derived from inorganic chemicals instead of sunlight

chlorophyll a

form of chlorophyll that absorbs violet-blue and red light and consequently has a bluish-green color; the only pigment molecule that performs the photochemistry by getting excited and losing an electron to the electron transport chain

chlorophyll b

accessory pigment that absorbs blue and red-orange light and consequently has a yellowish-green tint

chloroplast

organelle in which photosynthesis takes place

cytochrome complex

group of reversibly oxidizable and reducible proteins that forms part of the electron transport chain between photosystem II and photosystem I

electromagnetic spectrum

range of all possible frequencies of radiation

electron transport chain

group of proteins between PSII and PSI that pass energized electrons and use the energy released by the electrons to move hydrogen ions against their concentration gradient into the thylakoid lumen

granum

stack of thylakoids located inside a chloroplast

heterotroph

organism that consumes organic substances or other organisms for food

light harvesting complex

complex that passes energy from sunlight to the reaction center in each photosystem; it consists of multiple antenna pigments that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids

light-dependent reaction

first stage of photosynthesis where certain wavelengths of the visible light are absorbed to form two energy-carrying molecules (ATP and NADPH)

light-independent reaction

second stage of photosynthesis, though which carbon dioxide is used to build carbohydrate molecules using energy from ATP and NADPH

mesophyll

middle layer of chlorophyll-rich cells in a leaf

P680

reaction center of photosystem II

P700

reaction center of photosystem I

photoact

ejection of an electron from a reaction center using the energy of an absorbed photon

photoautotroph

organism capable of producing its own organic compounds from sunlight

photon

distinct quantity or “packet” of light energy

photosystem

group of proteins, chlorophyll, and other pigments that are used in the light-dependent reactions of photosynthesis to absorb light energy and convert it into chemical energy

photosystem I

integral pigment and protein complex in thylakoid membranes that uses light energy to transport electrons from plastocyanin to NADP⁺ (which becomes reduced to NADPH in the process)

photosystem II

integral protein and pigment complex in thylakoid membranes that transports electrons from water to the electron transport chain; oxygen is a product of PSII

pigment

molecule that is capable of absorbing certain wavelengths of light and reflecting others (which accounts for its color)

primary electron acceptor

pigment or other organic molecule in the reaction center that accepts an energized electron from the reaction center

reaction center

complex of chlorophyll molecules and other organic molecules that is assembled around a special pair of chlorophyll molecules and a primary electron acceptor; capable of undergoing oxidation and reduction

reduction

gain of electron(s) by an atom or molecule

spectrophotometer

instrument that can measure transmitted light and compute the absorption

stoma

opening that regulates gas exchange and water evaporation between leaves and the environment, typically situated on the underside of leaves

stroma

fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of photosynthesis take place

thylakoid

disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of photosynthesis take place; stacks of thylakoids are called grana

thylakoid lumen

aqueous space bound by a thylakoid membrane where protons accumulate during light-driven electron transport

wavelength

distance between consecutive points of equal position (two crests or two troughs) of a wave in a graphic representation; inversely proportional to the energy of the radiation