Photosynthesis and Carbohydrate Pathways Vocabulary

Vocabulary flashcards covering light-dependent reactions, the Calvin cycle, carbohydrate biosynthesis, C4/CAM pathways, and light regulation mechanisms.

Photorespiration

A wasteful side reaction that happens in light when rubisco binds $O_2$ instead of $CO_2$. It consumes $O_2$, releases $CO_2$, costs $ATP$, and lowers the amount of carbon the plant retains.

CO2 Fixation

The process of covalently attaching inorganic $CO_2$ to an organic molecule, such as adding $CO_2$ to ribulose 1,5-bisphosphate (RuBP) in the Calvin cycle to form 3-phosphoglycerate.

Light-Dependent Reactions

Photosynthetic reactions occurring in thylakoid membranes requiring light, where water is split to release $O_2$, electrons move through Photosystem II and Photosystem I, a proton gradient forms, and $ATP$ and $NADPH$ are produced.

Dark Reactions of Photosynthesis

The $CO_2$-assimilation reactions (primarily the Calvin cycle) occurring in the chloroplast stroma that utilize $ATP$ and $NADPH$ to convert $CO_2$ into triose phosphates.

Z-Scheme Order of Electron Carriers

The sequence of electron flow in plant chloroplasts: $H_2O \rightarrow \text{PSII (P680)} \rightarrow \text{pheophytin} \rightarrow P_{QA} \rightarrow P_{QB}\text{/plastoquinone pool} \rightarrow \text{cytochrome } b_6f \rightarrow \text{plastocyanin} \rightarrow \text{PSI (P700)} \rightarrow A_0 \rightarrow \text{phylloquinone } (A_1/Q_K) \rightarrow \text{Fe-S centers} \rightarrow \text{ferredoxin} \rightarrow \text{ferredoxin:NADP}^+ \text{ reductase} \rightarrow NADPH$.

Chlorophyll

A green, light-absorbing, isoprene-based pigment featuring a flat, conjugated cyclic tetrapyrrole ring with $Mg^{2+}$ coordinated at the center and a hydrophobic phytol tail anchoring it in membrane protein complexes.

Fate of Absorbed Light Energy in Pigments

The four pathways for light energy absorbed by a pigment: (1) release as heat, (2) release as fluorescence, (3) transfer to a nearby pigment by exciton transfer, or (4) utilization for photochemistry (electron transfer and charge separation).

Ribulose 1,5-bisphosphate (RuBP)

A five-carbon acceptor molecule that condenses with $CO_2$ in the first reaction of carbon dioxide assimilation.

UDP-Glucose

An activated sugar donor formed by sucrose synthase from sucrose and $UDP$; it is utilized by cellulose synthase to extend cellulose chains and is also used for glycogen and sucrose synthesis.

Glycine Decarboxylase Complex

An enzyme complex localized in the mitochondrial matrix in pea or spinach leaves.

Irreversible RuBP Regeneration Reactions

The three irreversible steps in Calvin cycle sugar regeneration, catalyzed by fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and ribulose 5-phosphate kinase.

Rubisco Reaction Products

Rubisco forms two molecules of 3-phosphoglycerate (3-PGA) from one $CO_2$ and one RuBP. When using $O_2$ instead, it yields one molecule of 3-PGA and one molecule of 2-phosphoglycolate.

Energy Requirement for Hexose Synthesis

Synthesis of one hexose molecule from $6\text{ }CO_2$ requires $12\text{ }NADPH$ and $18\text{ }ATP$ (or $6\text{ }NADPH$ and $9\text{ }ATP$ per triose phosphate).

Fructose 2,6-bisphosphate (F2,6BP)

A major regulator of sucrose biosynthesis in plants whose concentration decreases during active photosynthesis to favor fructose 6-phosphate and sucrose formation.

Sucrose Phosphate Synthase Regulation

Regulated allosterically (activated by glucose 6-phosphate and inhibited by inorganic phosphate $P_i$) and covalently (inactivated by phosphorylation of a Ser residue and activated by dephosphorylation).

PEP Carboxylase

An enzyme that fixes carbon with high efficiency and high affinity for $HCO_3^-$ without using $O_2$ as an alternative substrate, avoiding competition from oxygen.

Photochemical Reaction Center

A complex where light excites a special chlorophyll pair, causing it to donate an electron to a nearby acceptor and creating charge separation (oxidized chlorophyll and reduced acceptor).

Plastocyanin and Ferredoxin Roles

Plastocyanin carries an electron from cytochrome $b_6f$ to oxidized $P700^+$ in Photosystem I, while ferredoxin accepts electrons at the end of PSI and transfers them to ferredoxin:$NADP^+$ reductase.

Unique Glyoxylate Cycle Reactions

Two unique enzyme steps that bypass $CO_2$-releasing steps of the citric acid cycle: isocitrate lyase (splits isocitrate into succinate and glyoxylate) and malate synthase (combines glyoxylate and acetyl-CoA to make malate).

Glyoxylate Cycle

A metabolic pathway in germinating seeds that converts carbon from stored fats into four-carbon compounds, providing precursors for glucose and sucrose synthesis before photosynthesis begins.

Fatty Acid to Carbohydrate Pathway

The pathway sequence in plants: beta oxidation converts fatty acids to acetyl-CoA, the glyoxylate cycle converts acetyl-CoA to succinate, the citric acid cycle converts succinate to oxaloacetate, and gluconeogenesis converts oxaloacetate to sugars.

ADP-Glucose vs. UDP-Glucose

Activated sugar donors in plants; $ADP\text{-glucose}$ is used for starch synthesis, whereas $UDP\text{-glucose}$ is used for glycogen, sucrose, and cellulose synthesis.

Cyclic Electron Flow

Electron transport around Photosystem I only, where electrons from ferredoxin cycle back through plastoquinone, cytochrome $b_6f$, and plastocyanin to $P700$, generating a proton gradient and $ATP$ without making $NADPH$ or $O_2$.

Action Spectrum

A graph showing the relative biological efficiency of different light wavelengths in driving a process like photosynthesis when equal photon counts are provided.

Rubisco Activation Mechanism

Rubisco activase uses $ATP$ to clear tightly bound RuBP from inactive rubisco; an activator $CO_2$ carbamylates Lys201, and $Mg^{2+}$ binds at the active site to turn rubisco on.

Thylakoid Light-Induced Ion Movement

When light pumps $H^+$ from the stroma into the thylakoid lumen, $Mg^{2+}$ moves in the opposite direction from the lumen to the stroma to balance electrical charge.

Ubiquinone (Coenzyme Q)

A mobile, lipid-soluble mitochondrial electron carrier that performs a function in mitochondria analogous to plastoquinone in chloroplasts.

Photon Requirement for O2 Evolution

Linear electron flow requires $8\text{ photons}$ total ($4\text{ photons}$ absorbed by PSII and $4\text{ photons}$ by PSI) to oxidize $2\text{ }H_2O$ and produce $1\text{ }O_2$.

C4 Carbon Fixation

A spatial carbon-fixation pathway where PEP carboxylase in mesophyll cells fixes $HCO_3^-$ into oxaloacetate (converted to malate or aspartate), which moves to bundle-sheath cells to release concentrated $CO_2$ for rubisco and the Calvin cycle.

NADP-linked Malic Enzyme

An enzyme in bundle-sheath cells of C4 plants that oxidatively decarboxylates malate into pyruvate, releasing $CO_2$ and reducing $NADP^+$ to $NADPH$.

Crassulacean Acid Metabolism (CAM)

A temporal carbon-fixation adaptation in dry environments where stomata open at night to fix $CO_2$ via PEP carboxylase into malate stored in vacuoles, and close during the day to release $CO_2$ for rubisco.