absorption spectrum
The range of a pigment's ability to absorb various wavelengths of light; also a graph of such a range.
action spectrum
A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process
Autotroph
An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from oxidation of inorganic substances to make organic molecules from inorganic ones
CAM plant
A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, CO2 enter- ing open stomata during the night is converted to organic acids, which release CO2 for the Calvin cycle during the day, when stomata are closed
C3 plant
A plant that uses the Calvin cycle for the initial steps that incorporate CO2 into organic material, forming a three-carbon compound as the first stable intermediate.
C4 plant
A plant in which the Calvin cycle is preceded by reactions that incorporate CO2 into a four-carbon compound, the end product of which supplies CO2 for the Calvin cycle
Calvin cycle
The second of two major stages in photosynthesis (following the light reactions), involving fixation of atmospheric CO2 and reduction of the fixed carbon into carbohydrate
carbon fixation
The initial incorporation of carbon from CO2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic prokaryote).
Carotenoid
An accessory pigment, either yellow or orange, in the chloroplasts of plants and in some prokaryotes. By absorbing wavelengths of light that chlorophyll cannot, carotenoids broaden the spectrum of colors that can drive photosynthesis.
Chlorophyll
A green pigment located in membranes within the chloroplasts of plants and algae and in the membranes of certain prokaryotes. Chlorophyll a participates directly in the light reactions, which convert solar energy to chemical energy.
chlorophyll a
A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.
chlorophyll b
An accessory photosynthetic pigment that transfers energy to chlorophyll a.
cyclic electron flow
A route of electron flow during the light reactions of photosynthesis that involves only one photosystem and that produces ATP but not NADPH or O2.
electromagnetic spectrum
The entire spectrum of electromagnetic radiation, ranging in wavelength from less than a nanometer to more than a kilometer
glyceraldehyde 3-phosphate (G3P)
A three-carbon carbohydrate that is the direct product of the Calvin cycle; it is also an intermediate in glycolysis.
Heterotroph
An organism that obtains organic food molecules by eating other organisms or substances derived from them.
light reactions
The first of two major stages in photosynthesis (preceding the Calvin cycle). These reactions, which occur on the thylakoid membranes of the chloroplast or on membranes of certain prokaryotes, convert solar energy to the chemical energy of ATP and NADPH, releasing oxygen in the process.
light-harvesting complex
A complex of proteins associated with pigment molecules (including chlorophyll a, chlorophyll b, and carotenoids) that captures light energy and transfers it to reaction-center pigments in a photosystem.
linear electron flow
A route of electron flow during the light reactions of photosynthesis that involves both photosystems (I and II) and produces ATP, NADPH, and O2. The net electron flow is from H2O to NADP+
Mesophyll
Leaf cells specialized for photosynthesis. In C3 and CAM plants, mesophyll cells are located between the upper and lower epidermis; in C4 plants, they are located between the bundle-sheath cells and the epidermis.
NADP+
Nicotinamide adenine dinucleotide phosphate, an electron acceptor that, as NADPH, temporarily stores energized electrons produced during the light reactions.
PEP carboxylase
An enzyme that adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate in mesophyll cells of C4 plants. It acts prior to photosynthesis.
Photoautotroph
An organism that harnesses light energy to drive the synthesis of organic compounds from carbon dioxide.
Photon
A quantum, or discrete quantity, of light energy that behaves as if it were a particle.
Photophosphorylation
The process of generating ATP from ADP and phosphate by means of chemiosmosis, using a proton-motive force generated across the thylakoid membrane of the chloroplast or the membrane of certain prokaryotes during the light reactions of photosynthesis.
Photorespiration
A metabolic pathway that consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. Photorespiration generally occurs on hot, dry, bright days, when stomata close and the O2/CO2 ratio in the leaf increases, favoring the binding of O2 rather than CO2 by rubisco.
Photosynthesis
The conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes.
Photosystem
A light-capturing unit located in the thylakoid membrane of the chloroplast or in the membrane of some prokaryotes, consisting of a reaction-center complex surrounded by numerous light-harvesting complexes. There are two types of photosystems, I and II; they absorb light best at different wavelengths.
photosystem I (PS I)
A light-capturing unit in a chloroplast's thylakoid membrane or in the membrane of some prokaryotes; it has two molecules of P700 chlorophyll a at its reaction center.
photosystem II (PS II)
One of two light-capturing units in a chloroplast's thylakoid membrane or in the membrane of some prokaryotes; it has two molecules of P680 chlorophyll a at its reaction center.
primary electron acceptor
In the thylakoid membrane of a chloroplast or in the membrane of some prokaryotes, a specialized molecule that shares the reaction-center complex with a pair of chlorophyll a molecules and that accepts an electron from them.
reaction-center complex
A complex of proteins associated with a special pair of chlorophyll a molecules and a primary electron acceptor. Located centrally in a photosystem, this complex triggers the light reactions of photosynthesis. Excited by light energy, the pair of chlorophylls donates an electron to the primary electron acceptor, which passes an electron to an electron transport chain.
Rubisco
Ribulose bisphosphate (RuBP) carboxylase-oxygenase, the enzyme that normally catalyzes the first step of the Calvin cycle (the addition of CO2 to RuBP). When excess O2 is present or CO2 levels are low, rubisco can bind oxygen, resulting in photorespiration.
Spectrophotometer
An instrument that measures the proportions of light of different wavelengths absorbed and transmitted by a pigment solution.
Stoma
A microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant.
Stroma
The dense fluid within the chloroplast surrounding the thylakoid membrane and containing ribosomes and DNA; involved in the synthesis of organic molecules from carbon dioxide and water.
Thylakoid
A flattened, membranous sac inside a chloroplast. Thylakoids often exist in stacks called grana that are interconnected; their membranes contain molecular "machinery" used to convert light energy to chemical energy.
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