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.

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.

wavelength

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

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 740 nm.

Bundle-sheath cells

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

photons

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

Photosystem 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.

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.

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.

Carotenoids

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.

Spectrophotometer

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

Autotrophs

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.

C4 plants

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

Photosystem 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.

Chlorophyll a

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

Rubisco

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

PEP carboxylase

An enzyme that adds carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate in mesophyll cells of C4 plants. It acts prior to photosynthesis.

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.

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 oxygen.

CAM plants

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 carbon dioxide for the Calvin cycle during the day, when stomata are closed.

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.

C3 plants

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

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.

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.

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.

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.

Action spectrum

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

Calvin cycle

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

Carbon fixation

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

Crassulacean acid metabolism

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

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 the stomata close and the oxygen to carbon dioxide ratio in the leaf increases, favoring the binding of oxygen rather than carbon dioxide by rubisco.

Electromagnetic spectrum

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

Chlorophyll b

An accessory photosynthetic pigment that transfers energy to chlorophyll a.

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 oxygen. The net electron flow is from water to NADP plus.

Glyceraldehyde 3-phosphate

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

Cyclic AMP

its ring structure, is a common chemical signal that has a diversity of roles, including as a second messenger in many eukaryotic cells, and as a regulator of some bacterial operons.

Receptor tyrosine kinases

A receptor protein spanning the plasma membrane, the cytoplasmic (intracellular) part of which can catalyze the transfer of a phosphate group from ATP to a tyrosine on another protein. Receptor tyrosine kinases often respond to the binding of a signaling molecule by dimerizing and then phosphorylating a tyrosine on the cytoplasmic portion of the other receptor in the dimer.

Hormones

In multicellular organisms, one of many types of secreted chemicals that are formed in specialized cells, travel in body fluids, and act on specific target cells in other parts of the organism, changing the target cells’ functioning.

Protein phosphatases

An enzyme that removes phosphate groups from (dephosphorylates) proteins, often functioning to reverse the effect of a protein kinase.

Apoptosis

A type of programmed cell death, which is brought about by activation of enzymes that break down many chemical components in the cell.

Diacylglycerol

A second messenger produced by the cleavage of the phospholipid PIP2 in the plasma membrane.

Signal transduction pathway

A series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response.

Second messengers

A molecule that relays messages in a cell from a receptor to a target where an action within the cell takes place.

Inositol trisphosphate

A second messenger that functions as an intermediate between certain signaling molecules and a subsequent second messenger, a calcium ion (Ca superscript 2-plus), causing a rise in cytoplasmic calcium ion concentration.

Ligand

A molecule that binds specifically to another molecule, usually a larger one.

G proteins

A GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G protein-coupled receptor, to other signal transduction proteins inside the cell.

G protein-coupled receptors

A signal receptor protein in the plasma membrane that responds to the binding of a signaling molecule by activating a G protein.

Signal reception

In cellular communication, the first step of a signaling pathway in which a signaling molecule is detected by a receptor molecule on or in the cell.

Ligand-gated ion channels

A transmembrane protein containing a pore that opens or closes as it changes shape in response to a signaling molecule, allowing or blocking the flow of specific ions; also called an ionotropic receptor.

Scaffolding proteins

A type of large relay protein to which several other relay proteins are simultaneously attached, increasing the efficiency of signal transduction.

Phosphorylation cascade

A series of chemical reactions during cell signaling mediated by enzymes (kinases), in which each kinase in turn phosphorylates and activates another, ultimately leading to phosphorylation of many proteins.

Protein kinase

An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein.

Adenylyl cyclase

An enzyme that converts ATP to cyclic AMP in response to an extracellular signal.