Photosynthesis: Light Reactions, Gas Exchange, and Regulation

Comprehensive vocabulary flashcards covering the anatomy of photosynthesis, light and dark reactions, electron transport, Rubisco carboxylation and photorespiration, $$CO_2$$ supply-demand economics, stable isotope discrimination, light response curves, and photosynthetic feedback regulation.

Chloroplast

A specialized organelle bounded by a double membrane that contains pigment-bearing thylakoids and stroma, serving as the site of photosynthesis in mesophyll cells.

Light Reactions

The light-driven stage of photosynthesis occurring in thylakoid membranes where photon absorption creates energy to split $H_2O$ and produce $O_2$, protons ($H^+$), $ATP$, and $NADPH$.

Calvin-Benson Cycle

The enzymatic carbon-reduction cycle occurring in the stroma that utilizes $ATP$ and $NADPH$ generated by light reactions to assimilate $CO_2$ and ribulose-1,5-bisphosphate into sugars.

Photosystem II (PS II / $P_{680}$)

A thylakoid pigment-protein complex with an absorption peak at $680\,nm$ that utilizes photon excitation energy to split $H_2O$ into oxygen ($O_2$), protons ($H^+$), and electrons.

Photosystem I (PS I / $P_{700}$)

A thylakoid pigment-protein complex with an absorption peak at $700\,nm$ that accepts electrons from plastocyanin and utilizes additional photon excitation to reduce ferredoxin.

Light Harvesting Complexes (LHC)

Membrane-embedded clusters containing accessory pigments and approximately $100$ chlorophyll $a$ molecules per photosystem that intercept photons and channel excitation energy toward the reaction center.

Chlorophyll $a$ vs. Chlorophyll $b$

Primary photosynthetic pigments where chlorophyll $a$ absorbs red light ($\sim 700\,nm$) and chlorophyll $b$ absorbs blue light ($\sim 500\,nm$); excitation energy transfers unidirectionally from chlorophyll $b$ to chlorophyll $a$.

Fates of Absorbed Photon Energy

The three possible relaxation routes for an electron excited in chlorophyll (which remains stable for only $10^{-9}\,s$): photochemistry (electron transfer), non-photochemical dissipation as heat, or fluorescence (re-emission at a longer wavelength).

Plastoquinone (PQ)

A lipid-soluble mobile electron carrier reduced to $PQH_2$ by Photosystem II upon accepting electrons and stromal protons, subsequently shuttling electrons to the cytochrome complex.

Cytochrome Complex ($b_6f$)

A thylakoid protein complex that oxidizes $PQH_2$, transfers electrons to reduce plastocyanin, and translocates protons from the stroma into the thylakoid lumen.

Plastocyanin (Pc)

A water-soluble lumenal copper-protein carrier that accepts electrons from the cytochrome complex and is oxidized by Photosystem I.

Proton-Motive Force

The electrochemical potential gradient created across the thylakoid membrane by accumulated lumenal $H^+$ during photosynthetic electron transport, driving ATP synthase to phosphorylate $ADP$ to $ATP$.

Ferredoxin (Fd)

An iron-sulfur stromal protein reduced by Photosystem I that can channel electrons to reduce $NADP^+$ to $NADPH$, transfer electrons to $O_2$ to form $H_2O$, or reduce $PQ$ in cyclic electron flow.

Cyclic Electron Transport

An alternative electron pathway wherein reduced ferredoxin passes electrons back to plastoquinone ($PQ$) rather than $NADP^+$, generating additional proton-motive force and $ATP$ without synthesizing $NADPH$.

Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase; the stromal enzyme that catalyzes the primary carboxylation of $RuBP$ with $CO_2$, but can also catalyze oxygenation with $O_2$.

Phosphoglyceric Acid (PGA)

A three-carbon ($C_3$) intermediate produced when Rubisco carboxylates $RuBP$, yielding two $PGA$ molecules per fixed $CO_2$.

Triose-Phosphate (triose-P)

A three-carbon sugar formed by reducing $PGA$ with $ATP$ and $NADPH$; for every $12$ molecules produced, $2$ exit to form sucrose or starch and $10$ regenerate $6$ molecules of $RuBP$.

Photorespiration

A metabolic pathway initiated when Rubisco oxygenates $RuBP$ instead of carboxylating it, consuming $O_2$ and releasing $CO_2$ while diminishing net carbon gain; accounts for roughly $20\%$ of $RuBP$ turnover in ambient C3 leaves.

Phosphoglycolate

A two-carbon ($C_2$) toxic byproduct generated alongside one $PGA$ when Rubisco oxygenates $RuBP$, which must enter the photorespiratory salvage pathway.

Photorespiratory Pathway Organelles

The three compartmentalized organelles that coordinate the salvage of phosphoglycolate: chloroplast (dephosphorylation to glycolate), peroxisome (oxidation to glyoxylate and glycine), and mitochondrion (condensation of two glycines into serine, $CO_2$, and $NH_3$).

Economics of $CO_2$

The framework describing photosynthesis as governed by supply (diffusion of $CO_2$ from bulk air to stromal carboxylation sites) and demand (chloroplast biochemistry, electron transport capacity, and sink utilization).

Diffusion Concentrations ($C_a$, $C_i$, $C_c$)

The inward concentration gradient driving leaf carbon uptake: ambient atmospheric $CO_2$ ($C_a$), $CO_2$ inside the leaf ($C_i$), and chloroplast carboxylation site $CO_2$ ($C_c$), satisfying $C_a > C_i > C_c$.

Fick's First Law (Leaf Gas Exchange)

The principle describing diffusion rate as proportional to the concentration gradient divided by resistance, formulated for net $CO_2$ assimilation as $A_{net} = \frac{C_a - C_i}{r_c}$.

Conductance ($g$) vs. Resistance ($r$)

Reciprocal metrics of diffusive flux ($g = \frac{1}{r}$); conductance measures the ease of gas transport and is typically simpler to evaluate than boundary and stomatal resistance.

Boundary Layer ($r_a$)

A stagnant, unstirred thin cushion of air adjacent to the leaf epidermis possessing higher humidity than bulk air, with resistance modulated by wind velocity and leaf dimensions.

Diffusive Ratio of $H_2O$ to $CO_2$

The factor reflecting the faster diffusion of lighter $H_2O$ relative to $CO_2$: $1.37$ across the boundary layer and $1.6$ through stomatal apertures, giving $r_c = (r_a \times 1.37) + (r_s \times 1.6)$.

Mesophyll Conductance ($g_m$)

The conductance governing $CO_2$ diffusion from the intercellular air spaces ($C_i$) into the chloroplast stroma ($C_c$); at $400\,ppm$ ambient $CO_2$, stomatal conductance reduces concentration by $\sim 100\,ppm$ and $g_m$ reduces it by another $\sim 80\,ppm$.

$CO_2$ Compensation Point ($\Gamma$)

The point on an $A_{net}\text{-}C$ curve where gross photosynthetic $CO_2$ uptake precisely equals $CO_2$ released by photorespiration and dark respiration ($A_{net} = 0$), typically $40\text{--}50\,ppm$ in C3 plants.

RuBP-Limited Range (Demand Curve)

The plateau phase of the $A_{net}\text{-}C_c$ demand curve occurring above $\sim 170\,ppm$, where Rubisco is saturated and assimilation is constrained by $RuBP$ regeneration and Calvin cycle throughput.

Co-Limitation Point

The specific internal $CO_2$ concentration (approximately $170\,ppm$ $C_c$ at a historic $380\,ppm$ $C_a$) where $CO_2$ availability and electron transport capacity co-limit photosynthetic rate.

Isotopic Fractionation

The preferential selection or discrimination against a heavier isotope during physical diffusion or biochemical reactions, enriching or depleting the relative isotopic abundance in plant tissues.

PDB-Belemnite

The universal geological reference standard against which carbon stable isotope differences ($\delta^{13}C$) are compared, assigned a benchmark value of $0\,{‰}$.

$\delta^{13}C$ in C3 and C4 Plants

Stable carbon isotope discrimination values showing that C3 plants typically range from $-27\,{‰}$ to $-30\,{‰}$, whereas C4 plants average approximately $-14.4\,{‰}$.

Photosynthetically Active Radiation (PAR)

The solar waveband ($400\text{--}700\,nm$) absorbed by chlorophyll and accessory pigments, quantified in $\mu mol\,m^{-2}\,s^{-1}$ from $0$ in dark conditions up to $\sim 2{,}000\,\mu mol\,m^{-2}\,s^{-1}$ at the equator.

Canopy Light Extinction Formula ($I = I_0 e^{-k L}$)

The mathematical equation expressing exponential light attenuation through a vegetative canopy, where $I_0$ is irradiance at the canopy top, $k$ is the leaf extinction coefficient ($k=1$ for horizontal foliage), and $L$ is cumulative leaf area.

Sunflecks

Short, fleeting bursts of high irradiance penetrating down to understory or lower canopy foliage caused by fluttering leaves, shifting branches, and changing sun angles.

Light Compensation Point (LCP)

The irradiance level on a light response curve where the rate of photosynthetic $CO_2$ assimilation exactly balances the sum of photorespiration and dark respiration.

Sun Leaves vs. Shade Leaves (Anatomy)

Structural canopy adaptations where sun leaves are thicker with multi-layered palisade mesophyll, more stroma, and higher $A_{max}$, whereas shade leaves are thinner with expanded spongy mesophyll, lower Chl $a$:Chl $b$ ratios, and more grana stacks.

Photoinhibition

A light-induced depression in photosynthetic efficiency occurring when absorbed excitation energy exceeds the capacity of the Calvin cycle and rate of damage to PS II exceeds cellular repair mechanisms.

Xanthophyll Cycle

A photoprotective thermal dissipation mechanism within PS II light-harvesting complexes wherein lumen acidification activates enzymes to de-epoxidize violaxanthin through antheraxanthin into zeaxanthin.

Zeaxanthin

The fully de-epoxidized xanthophyll carotenoid that acts as an excitation 'lightning rod' in PS II antenna complexes, safely dissipating excess energy as heat via non-photochemical quenching (NPQ).

Triose-Phosphate Translocator (TPT)

A membrane antiporter in the inner chloroplast envelope that exports stromal triose-P into the cytosol in strict stoichiometric exchange for cytosolic inorganic phosphate ($P_i$).

Photosynthetic Feedback Inhibition

The inhibition of photosynthesis triggered when cytosolic sucrose accumulates, trapping inorganic phosphate ($P_i$) in phosphorylated intermediates, starving ATP synthase in the stroma and limiting $RuBP$ regeneration.

Compensatory Photosynthesis

An observed increase in the maximum assimilation rate ($A_{max}$) of remaining foliage after partial defoliation, caused by a higher sink-to-source ratio accelerating sucrose export and rapidly releasing free cytosolic $P_i$.