Photosynthesis (Chapter 22)

Vocabulary practice flashcards covering the biochemical mechanisms, light-dependent reactions, light-independent reactions (Calvin cycle), photorespiration, and C4 pathways of photosynthesis from Chapter 22.

Chlorophyll

A green pigment whose chemical structure consists of a porphyrin tetrapyrrole ring centered around a $Mg^+$ ion, a fused cyclopentanone ring, and a hydrophobic phytol side chain; it captures photons by absorbing blue ($420\,nm$) and red ($660\,nm$) wavelengths while reflecting green light.

Phytol Group

A long hydrophobic side chain on chlorophyll containing four isoprenoid units that anchors the pigment molecule to the thylakoid membrane via hydrophobic interactions.

Granum

A chloroplast substructure consisting of stacks of membranous sacs called thylakoids that house the chlorophyll pigments and light-harvesting machinery.

Stroma

The fluid-filled intervening space between thylakoid membranes in the chloroplast where carbon fixation and the light-independent reactions (Calvin cycle) take place.

Antenna Complex

A membrane-embedded network of hundreds of chlorophyll and accessory pigment molecules that absorbs photons and transfers excitation energy across pigment molecules to the reaction center.

Reaction Center

A specific complex consisting of chlorophyll $a$ and associated proteins that receives excitation energy from the antenna complex and transfers high-energy excited electrons to a primary electron acceptor molecule.

Oxygen-Evolving Complex (OEC)

An enzyme associated with Photosystem II containing a manganese-calcium ($Mn_4CaO_5$) cluster that catalyzes the photolysis of water ($2H_2O \rightarrow O_2 + 4H^+ + 4e^-$), releasing oxygen and providing replacement electrons.

Photosystem II (P680)

The photosystem with a reaction center absorption maximum at $680\,nm$ that acts first in the light-dependent reactions, splitting water to release $O_2$ and donating energized electrons to an electron transport chain to establish a proton gradient.

Photosystem I (P700)

The photosystem with a reaction center absorption maximum at $700\,nm$ that receives electrons from the electron transport chain and energizes them to reduce $NADP^+$ into NADPH; it lacks water-splitting enzymes and does not evolve $O_2$.

Z Scheme

The pathway of non-cyclic electron transport in photosynthesis, charting electron transfer across energy levels from water through Photosystem II, plastoquinone, the cytochrome $b_6/f$ complex, plastocyanin, Photosystem I, ferredoxin, and ultimately to $NADP^+$.

Bacteriochlorophyll a

A photosynthetic pigment present in photosynthetic bacteria other than cyanobacteria that absorbs light at a characteristic wavelength of $780\,nm$.

RuBisCO

Ribulose-1,5-bisphosphate carboxylase/oxygenase; the stromal enzyme that catalyzes the primary carbon fixation reaction by combining $CO_2$ with ribulose-1,5-bisphosphate to form an unstable $6\text{-carbon}$ intermediate that splits into two molecules of 3-phosphoglycerate.

3-Phosphoglycerate (3-PGA / 3PG)

The first stable $3\text{-carbon}$ intermediate compound generated during the $C_3$ photosynthetic pathway upon the carboxylation of ribulose-1,5-bisphosphate by RuBisCO.

Glyceraldehyde-3-Phosphate (G-3-P / PGAL)

The direct $3\text{-carbon}$ carbohydrate product of the Calvin cycle produced by phosphorylation and reduction using ATP and NADPH; $10$ out of every $12$ molecules regenerate ribulose, while $2$ exit to form glucose and other complex biomolecules.

Photorespiration

A wasteful metabolic process in $C_3$ plants occurring when stomata close and RuBisCO binds $O_2$ instead of $CO_2$, forming 3-phosphoglycerate and 2-phosphoglycolate across the chloroplast, peroxisome, and mitochondrion with no ATP or sugar production.

2-Phosphoglycolate

The $2\text{-carbon}$ product produced alongside 3-phosphoglycerate when RuBisCO binds oxygen ($O_2$) rather than carbon dioxide ($CO_2$) during photorespiration.

C4 Photosynthetic Pathway

An adaptation that bypasses photorespiration by using distinct mesophyll and bundle sheath leaf anatomy to fix $CO_2$ into a $4\text{-carbon}$ intermediate, delivering concentrated $CO_2$ directly to RuBisCO at an energy cost of $30\text{ ATP}$ per glucose compared to $18\text{ ATP}$ in $C_3$.

Phosphoenolpyruvate (PEP) Carboxylase

An enzyme in $C_4$ plant mesophyll cells that fixes $CO_2$ onto phosphoenolpyruvate to produce oxaloacetate; it has a lower $K_m$ and much higher affinity for $CO_2$ than RuBisCO and cannot use $O_2$ as a substrate.

Pyruvate Phosphate Dikinase

The enzyme operating in the mesophyll cells of $C_4$ plants that converts pyruvate, inorganic phosphate ($P_i$), and ATP into phosphoenolpyruvate (PEP), AMP, and inorganic pyrophosphate ($PP_i$).

Bundle Sheath Cells

Specialized vascular vein-surrounding cells in $C_4$ plants where transported malate is decarboxylated to release high concentrations of $CO_2$ for RuBisCO and the Calvin cycle.

NADP-Malic Enzyme

The enzyme in the bundle sheath cells of $C_4$ plants that decarboxylates malate into pyruvate and $CO_2$ while reducing $NADP^+$ to NADPH.