Photosynthesis: every key term you need (+ practice quiz)
25 flashcard terms for General Biology I Topic 6, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
The conversion of light energy into chemical bonds, using water as an electron source and carbon dioxide as the carbon source, releasing oxygen and storing carbon in sugar.
Van Niel hypothesis
The insight, drawn from sulfur bacteria that make sulfur instead of oxygen, that photosynthesis splits the hydrogen donor rather than carbon dioxide, predicting that released oxygen comes from water.
Ruben oxygen labeling
The isotope experiment in which heavy oxygen was placed first in water and then in carbon dioxide, and the released gas was labeled only in the first case, confirming water as the oxygen source.
Engelmann light experiment
The classic demonstration in which oxygen-seeking bacteria clustered around a filamentous alga where red and blue light struck it, matching the action spectrum to chlorophyll absorption.
Absorption spectrum
A plot of how strongly a pigment absorbs each wavelength. Chlorophyll absorbs strongly in blue and red and poorly in green, which is why leaves reflect green light back to our eyes.
Chlorophyll a
The pigment that carries out the actual photochemistry, with a magnesium-centered ring that absorbs light and a long hydrocarbon tail anchoring it in the thylakoid membrane.
Accessory pigment
A pigment such as chlorophyll b or a carotenoid that widens the usable spectrum by absorbing wavelengths the main pigment misses and passing the energy inward, some also quenching harmful excited states.
Antenna complex
The array of pigment molecules surrounding a reaction center that captures photons and funnels excitation energy inward by resonance transfer, so many pigments feed one chemical event.
Reaction center
The site where a special pair of chlorophyll molecules loses an excited electron to a primary acceptor, converting light energy into a separated charge, the first stable chemical step of the process.
Photosystem two
The complex that oxidizes water, replacing the electrons it loses. It is named second but acts first in the linear flow, and its manganese cluster performs the difficult water-splitting chemistry.
Photosystem one
The complex that re-energizes electrons arriving from the transport chain and passes them to a soluble carrier, which ultimately reduces the coenzyme used to build sugar.
Water splitting
The oxidation of two water molecules to yield four electrons, four protons and one molecule of oxygen. The protons are released inside the thylakoid, adding directly to the gradient.
Linear electron flow
The path in which electrons travel from water through both photosystems to reduce the sugar-building coenzyme, producing oxygen, a proton gradient and reducing power in fixed proportions.
Cyclic electron flow
A route in which electrons from the first-listed photosystem return to the transport chain instead of leaving, pumping protons and making extra ATP without producing reducing power or oxygen.
Plastoquinone and plastocyanin
The mobile carriers of the thylakoid chain, one moving within the membrane and one within the lumen, connecting the two photosystems through the proton-pumping cytochrome complex between them.
Photophosphorylation
ATP synthesis powered by a light-driven proton gradient. Protons accumulate in the thylakoid lumen and return to the stroma through a synthase, exactly as in the mitochondrion.
Stroma
The fluid surrounding the thylakoids, holding the carbon-fixing enzymes, chloroplast DNA and ribosomes. Its slightly alkaline pH during illumination helps activate the fixation enzymes.
Calvin cycle
The stromal pathway that fixes carbon dioxide, reduces it using energy carriers from the light reactions, and regenerates its acceptor. Three turns are needed to yield one three-carbon export molecule.
Rubisco
The enzyme that attaches carbon dioxide to a five-carbon acceptor. It is abundant but slow and imprecise, since it can bind oxygen instead, which sets a ceiling on plant productivity.
Carbon fixation cost
The accounting for the cycle: making one three-carbon sugar requires three carbon dioxides, nine ATP and six reduced coenzyme molecules, most of the ATP spent regenerating the acceptor.
Photorespiration
The wasteful route begun when the fixing enzyme binds oxygen instead of carbon dioxide, consuming energy and releasing previously fixed carbon. It worsens on hot dry days when stomata close.
Four-carbon pathway
An adaptation in which an oxygen-insensitive enzyme first traps carbon dioxide in mesophyll cells as a four-carbon acid, which is shipped to interior cells and released around the fixing enzyme.
Crassulacean acid metabolism
A desert strategy separating the two steps in time rather than space: stomata open at night to store carbon dioxide as acid, and the acid is released for fixation behind closed stomata by day.
Stomatal trade-off
The unavoidable compromise in which the pores that admit carbon dioxide are also the main route of water loss, so plants must balance carbon gain against dehydration.
Triose phosphate export
The three-carbon product that leaves the chloroplast on a dedicated translocator and becomes the starting point for sucrose in the cytosol, while some carbon stays behind as leaf starch.