Chapter 10 Photosynthesis

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1

action spectrum

A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process.

2

Absorption Spectrum

The range of a pigment’s ability to absorb various wavelengths of light; also a graph of such a range.

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autotroph (ot′-ō-trōf)

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.

4

bundle-sheath cell

In C 4 plants, a type of photosynthetic cell arranged into tightly packed sheaths around the veins of a leaf.

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c 3 plant

A plant that uses the Calvin cycle for the initial steps that incorporate CO 2 into organic material, forming a three-carbon compound as the first stable intermediate.

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c 4 plant

A plant in which the Calvin cycle is preceded by reactions that incorporate CO 2 into a four-carbon compound, the end product of which supplies CO 2 for the Calvin cycle.

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calvin cycle

The second of two major stages in photosynthesis (following the light reactions), involving fixation of atmospheric CO 2 and reduction of the fixed carbon into carbohydrate.

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cam plant

A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, CO 2 entering open stomata during the night is converted to organic acids, which release CO 2 for the Calvin cycle during the day, when stomata are closed.

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carbon fixation

The initial incorporation of carbon from CO 2 into an organic compound by an autotrophic organism (a plant, another photosynthetic organism, or a chemoautotrophic prokaryote).

10

carotenoid (kuh-rot′-uh-noyd′)

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.

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chlorophyll (klōr′-ō-fil)

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.

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chlorophyll a

(klōr′-ō-fil) A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.

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chlorophyll b

(klōr′-ō-fil) An accessory photosynthetic pigment that transfers energy to chlorophyll a.

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chloroplast (klōr′-ō-plast)

An organelle found in plants and photosynthetic protists that absorbs sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water.

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crassulacean acid metabolism (cam) (crass-yū-lā′-shen)

An adaptation for photosynthesis in arid conditions, first discovered in the family Crassulaceae. In this process, a plant takes up CO 2 and incorporates it into a variety of organic acids at night; during the day, CO 2 is released from organic acids for use in the Calvin cycle.

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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 O 2 .

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electromagnetic spectrum

The entire spectrum of electromagnetic radiation, ranging in wavelength from less than a nanometer to more than a kilometer.

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glyceraldehyde 3-phosphate (g3p) (glis′-er-al′-de-hīd)

A three-carbon carbohydrate that is the direct product of the Calvin cycle; it is also an intermediate in glycolysis.

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heterotroph (het′-er-ō-trōf)

An organism that obtains organic food molecules by eating other organisms or substances derived from them.

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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.

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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.

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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 O 2 . The net electron flow is from H 2 O to NADP+.

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mesophyll (mez′-ō-fil)

Leaf cells specialized for photosynthesis. In C 3 and CAM plants, mesophyll cells are located between the upper and lower epidermis; in C 4 plants, they are located between the bundle-sheath cells and the epidermis.

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nadp +

The oxidized form of nicotinamide adenine dinucleotide phosphate, an electron carrier that can accept electrons, becoming NADPH. NADPH temporarily stores energized electrons produced during the light reactions.

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nadph

The reduced form of nicotinamide adenine dinucleotide phosphate; temporarily stores energized electrons produced during the light reactions. NADPH acts as “reducing power” that can be passed along to an electron acceptor, reducing it.

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pep carboxylase

An enzyme that adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetate in mesophyll cells of C 4 plants. It acts prior to photosynthesis.

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photon (fō′-ton)

A quantum, or discrete quantity, of light energy that behaves as if it were a particle.

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photophosphorylation (fō′-tō-fos′-fōr-uh-lā′-shun)

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.

29

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 O 2 / CO 2 ratio in the leaf increases, favoring the binding of O 2 rather than CO 2 by rubisco.

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photosynthesis (fō′-tō-sin′-thi-sis)

The conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and certain prokaryotes.

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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.

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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.

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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.

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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.

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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.

36

rubisco (rū-bis′-kō)

Ribulose bisphosphate (RuBP) carboxylase-oxygenase, the enzyme that normally catalyzes the first step of the Calvin cycle (the addition of CO 2 to RuBP). When excess O 2 is present or CO 2 levels are low, rubisco can bind oxygen, resulting in photorespiration.

37

spectrophotometer

An instrument that measures the proportions of light of different wavelengths absorbed & transmitted by a pigment solution.

38

stoma (stō′-muh)

(plural, stomata) 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.

39

stroma (strō′-muh)

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.

40

thylakoid (thī′-luh-koyd)

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.

41

visible light

That portion of the electromagnetic spectrum that can be detected as various colors by the human eye, ranging in wavelength from about 380 nm to about 750 nm.

42

wavelength

The distance between crests of waves, such as those of the electromagnetic spectrum.