Calvin Cycle

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.

Thylakoid

A flattened membranous sac inside a chloroplast. They exist in an interconnected system in the chloroplast and contain the molecular "machinery" used to convert light energy to chemical energy.

Chlorophyll

A green pigment located within the chloroplasts of plants and algae and in the membranes of certain prokaryotes.

chlorophyll b

An accessory photosynthetic pigment that transfers energy to chlorophyll a.

crassulacean acid metabolism (CAM)

An adaptation for photosynthesis in arid conditions, first discovered in the family Crassulaceae. In this process, a plant takes up CO2 and incorporates it into a variety of organic acids at night; during the day, CO2 is released from organic acids for use in the Calvin cycle.

PEP carboxylase

An enzyme that adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate in C4 plants. It acts prior to photosynthesis.

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. Two types of photosystems, I and II; they absorb light best at different wavelengths.

Photorespiration

A metabolic pathway that consumes oxygen and ATP, releases carbon dioxide, and decreases photosynthetic output. This process generally occurs on hot, dry, bright days, when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide.

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.

chlorophyll a

A photosynthetic pigment that participates directly in the light reactions, which convert solar energy to chemical energy.

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.

CAM plant

A plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. In this process, carbon dioxide entering 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.

Photon

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

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+.

cyclic electron flow

A route of electron flow during the light reactions of photosynthesis that involves only photosystem I and that produces ATP but not NADPH or O2.

glyceraldehyde 3-phosphate (G3P)

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

mesophyll cell

In C4 plants, a type of loosely arranged photosynthetic cell located between the bundle sheath and the leaf surface.

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.

NADP+

Nicotinamide adenine dinucleotide phosphate, An electron acceptor that, as NADPH, temporarily stores energized electrons produced during the light reactions.

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.

photosystem I (PS I)

One of two light-capturing units 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.

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.

Wavelength

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

Rubisco

The enzyme that catalyzes the first step of the Calvin cycle (the addition of CO2 to RuBP).

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.

Mesophyll

The ground tissue of a leaf, sandwiched between the upper and lower epidermis and specialized for photosynthesis.

Photophosphorylation

The process of generating ATP from ADP and phosphate by means of a proton-motive force generated across the thylakoid membrane of the chloroplast or the membrane of certain prokaryotes during the light reactions of photosynthesis.

absorption spectrum

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

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.

bundle sheath cell

in C4 plants, a type of photosynthetic cell arranged into tightly packed sheaths around the veins of A leaf.

Stroma

Within the chloroplast, the dense fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water.