Water Relations, Transport, Stomata, Nitrogen Metabolism, Photosynthesis and Respiration in Plants
2. Uptake and transport of water, ions and solutes
Water and ions cross the root cortex by three routes. The apoplastic route runs through cell walls and intercellular spaces without crossing a membrane; the symplastic route runs from cytoplasm to cytoplasm through plasmodesmata; the transmembrane route crosses membranes repeatedly through aquaporins (PIPs in the plasma membrane, TIPs in the tonoplast), which close when the cytosol acidifies, one reason root water uptake falls under flooding. At the endodermis the Casparian strip, a band of lignin and suberin in the radial and transverse walls, blocks the apoplast, so everything entering the stele must cross a plasma membrane at least once. That membrane step is where the root selects its ions.
Xylem water rises by cohesion–tension: evaporation from mesophyll walls curves the menisci in the wall pores, and the resulting negative pressure is transmitted down continuous water columns held by cohesion (hydrogen bonding) and adhesion to the walls, with no metabolic energy spent in the xylem. Root pressure causes guttation through hydathodes at night but cannot supply a transpiring tree. High tension risks cavitation and embolism, which pit membranes confine to one conduit. Transpiration is driven by the water-vapour difference between the nearly saturated leaf air spaces and the air, against stomatal and boundary-layer resistances. The transpiration ratio (mol water lost per mol CO₂ fixed) is commonly taught as about 500 for C3, 250 for C4 and as low as 50 for CAM plants; water-use efficiency is its inverse.
For an ion the question is whether uptake is passive or active. The Nernst equation gives the potential at which an ion is at equilibrium: E = (2.303RT/zF) log(C_out/C_in), about 59 mV/z per tenfold gradient at 25 °C; an ion held beyond its Nernst distribution is actively transported. The plasma-membrane P-type H⁺-ATPase pumps protons out, making the cytosol inside-negative and the apoplast acidic, and the tonoplast V-ATPase and H⁺-pyrophosphatase acidify the vacuole. These gradients drive secondary transport: NO₃⁻, Cl⁻, phosphate and sucrose enter by H⁺ symport, Na⁺ leaves by Na⁺/H⁺ antiport, and K⁺ enters through inward-rectifying channels at millimolar external K⁺ but needs H⁺-coupled carriers at micromolar K⁺. Because an inside-negative membrane repels anions, anion uptake is always active.
Sugars move in the phloem by pressure flow (Münch): sucrose loaded into sieve elements at a source lowers their Ψs, water enters from the xylem, and turgor pushes sap to sinks, where unloading lowers it. Apoplastic loading uses H⁺-sucrose symporters (SUC/SUT); polymer-trap loading, as in cucurbits, converts sucrose arriving through plasmodesmata into raffinose and stachyose, too large to diffuse back.
3. Mechanism of stomatal movement
A stoma opens when its guard cells take up solutes, lower their Ψs, absorb water and swell. Cellulose microfibrils wrapped radially around kidney-shaped guard cells (dumb-bell-shaped in grasses) stop them widening, so they lengthen and bow apart. Blue light, sensed by phototropins, activates the plasma-membrane H⁺-ATPase by phosphorylating its C-terminus, which then binds a 14-3-3 protein. Proton extrusion hyperpolarises the membrane and opens inward-rectifying K⁺ channels; K⁺ enters with Cl⁻ and with malate²⁻ made from starch through PEP carboxylase, and sucrose adds to the osmotic load later in the day. The fungal toxin fusicoccin locks the H⁺-ATPase on and opens stomata wide.
Closure is the reverse, triggered by abscisic acid (ABA), high intercellular CO₂, darkness or dry air. ABA binds PYR/PYL/RCAR receptors, which inhibit PP2C phosphatases and so free the kinase OST1 (SnRK2.6). OST1 activates the SLAC1 slow anion channel and an NADPH oxidase that makes reactive oxygen species, and cytosolic Ca²⁺ rises. Anion efflux depolarises the membrane, which shuts the inward K⁺ channels and opens outward-rectifying K⁺ channels (GORK); K⁺, Cl⁻ and malate leave, water follows and the pore closes.
4. Nitrogen metabolism: nitrate assimilation and nitrogen fixation
Most crops take up nitrate by H⁺-coupled transporters (low-affinity NRT1/NPF and high-affinity NRT2) and reduce it to ammonium with eight electrons in two steps. Ammonium, whether from nitrate, from fixation or from photorespiration, is toxic and is assimilated at once by the GS–GOGAT cycle, with the net result 2-oxoglutarate + NH₄⁺ + ATP + 2e⁻ → glutamate. Aminotransferases pass the amino group on (glutamate + oxaloacetate → aspartate + 2-oxoglutarate), and asparagine synthetase makes asparagine, a nitrogen-rich transport form. In C3 leaves the NH₄⁺ released by photorespiration is refixed by chloroplast GS2 and Fd-GOGAT at rates that can exceed primary nitrate assimilation several-fold.
| Enzyme | Site | Reaction | Notes |
|---|---|---|---|
| Nitrate reductase | Cytosol | NO₃⁻ + NAD(P)H + H⁺ → NO₂⁻ + NAD(P)⁺ + H₂O (2e⁻) | FAD, haem and molybdenum cofactor; induced by nitrate and light, inactivated in the dark by phosphorylation |
| Nitrite reductase | Plastid | NO₂⁻ + 6 Fd(red) + 8H⁺ → NH₄⁺ + 6 Fd(ox) + 2H₂O (6e⁻) | Siroheme and an Fe₄S₄ cluster |
| Glutamine synthetase (GS) | Cytosol (GS1), plastid (GS2) | Glutamate + NH₄⁺ + ATP → glutamine + ADP + Pi | High NH₄⁺ affinity; inhibited by glufosinate |
| GOGAT | Plastid | Glutamine + 2-oxoglutarate + 2e⁻ → 2 glutamate | Fd-GOGAT in leaves; NADH-GOGAT in roots and nodules |
| Nitrogenase | Bacteroid or free-living diazotroph | N₂ + 8e⁻ + 8H⁺ + 16 ATP → 2NH₃ + H₂ + 16 ADP + 16 Pi | Fe protein reduces the MoFe protein, which binds N₂; irreversibly inactivated by O₂ |
Nitrogen fixation in legume nodules starts with a chemical dialogue: root flavonoids induce the rhizobial nod genes through NodD, and the bacteria return Nod factors (lipo-chitooligosaccharides) that trigger root-hair curling, an infection thread and cortical cell division. Because O₂ destroys nitrogenase yet bacteroids need O₂ to make ATP, the nodule keeps free O₂ very low with a cortical diffusion barrier and leghaemoglobin, which binds O₂ and delivers it to respiration. Temperate legumes such as pea export fixed nitrogen as amides (glutamine, asparagine); tropical legumes such as soybean export ureides (allantoin, allantoic acid).
5. Photosynthesis: the light reactions
Two photosystems work in series in the thylakoid, the Z-scheme. Photosystem II (reaction centre P680), concentrated in the appressed grana, oxidises water at the oxygen-evolving complex, a Mn₄CaO₅ cluster: 2H₂O → O₂ + 4H⁺ (into the lumen) + 4e⁻. Electrons pass through pheophytin, Q_A and Q_B to the plastoquinone pool, then to the cytochrome b₆f complex, which pumps more protons by the Q-cycle, then via plastocyanin to photosystem I (P700, in the stroma lamellae with ATP synthase), and through ferredoxin and ferredoxin–NADP⁺ reductase to NADPH. Each electron is moved by two photons, one per photosystem, so one O₂ needs at least 8 photons. The proton gradient, mostly a ΔpH, drives the CF₀CF₁ ATP synthase.
Cyclic electron flow returns electrons from ferredoxin to the plastoquinone pool, pumping protons and making ATP without NADPH; it supplies the ATP that linear flow leaves short of the Calvin cycle's 3:2 ATP:NADPH demand. The case for two photosystems came from Emerson's red drop (quantum yield falls beyond about 680 nm) and the enhancement effect (adding shorter-wavelength light restores it). Two herbicides fix the sequence: DCMU (diuron) occupies the Q_B site and stops electron flow out of PSII, and paraquat takes electrons from PSI and passes them to O₂, generating superoxide.
6. The C3, C4 and CAM cycles
The Calvin–Benson (C3) cycle runs in the stroma in three phases. Carboxylation: Rubisco adds CO₂ to ribulose-1,5-bisphosphate (RuBP), giving two 3-phosphoglycerate (3-PGA). Reduction: 3-PGA is phosphorylated (ATP) and reduced (NADPH) to glyceraldehyde-3-phosphate. Regeneration: five triose phosphates are rearranged into three RuBP, the last step by phosphoribulokinase using ATP. The cost is 3 ATP and 2 NADPH per CO₂: 9 ATP and 6 NADPH per exported triose phosphate, 18 ATP and 12 NADPH per hexose. Light switches the cycle on through Rubisco activase, a rise in stromal pH and Mg²⁺, and thioredoxin reduction of FBPase, SBPase and phosphoribulokinase.
C4 photosynthesis is a CO₂ pump built on Kranz anatomy. In mesophyll cytosol, PEP carboxylase, which has no oxygenase activity and a high affinity for HCO₃⁻, fixes HCO₃⁻ onto phosphoenolpyruvate to give oxaloacetate, which becomes malate or aspartate. The C4 acid moves to the bundle sheath and is decarboxylated (by NADP-malic enzyme in maize, sugarcane and sorghum; by NAD-malic enzyme or PEP carboxykinase in other C4 types), so CO₂ builds up around Rubisco and photorespiration nearly vanishes. In the mesophyll pyruvate, phosphate dikinase regenerates PEP: pyruvate + ATP + Pi → PEP + AMP + PPi. Counting AMP as two ATP equivalents, the pump adds 2 ATP per CO₂, so the textbook cost for NADP-ME C4 is 5 ATP and 2 NADPH per CO₂. It buys a near-zero CO₂ compensation point (C3 leaves sit at several tens of µL L⁻¹) and better water- and nitrogen-use efficiency in hot, bright, dry habitats.
Crassulacean acid metabolism (CAM) separates the same two carboxylations in time rather than space. Stomata open at night; PEP carboxylase fixes CO₂ into oxaloacetate, which is reduced to malate and stored as malic acid in the vacuole, so titratable acidity rises overnight. By day the stomata close, malate is decarboxylated and Rubisco refixes the CO₂ behind closed stomata. Phosphorylation keeps the night-time PEP carboxylase active and less sensitive to malate. The ice plant Mesembryanthemum crystallinum switches from C3 to CAM under salt or drought stress (facultative CAM).
| Feature | C3 | C4 | CAM |
|---|---|---|---|
| First stable product of ¹⁴CO₂ | 3-PGA (3 C) | Oxaloacetate, then malate or aspartate (4 C) | Malate, at night |
| Separation of the two carboxylations | None | Spatial (mesophyll and bundle sheath) | Temporal (night and day) |
| Photorespiration | Significant, rises with temperature | Very low | Low while malate is decarboxylated |
| Examples | Wheat, rice, most trees | Maize, sugarcane, sorghum | Kalanchoe, pineapple, Opuntia, Agave |
7. Photorespiration: the C2 cycle
Rubisco also accepts O₂: RuBP + O₂ → 3-PGA + 2-phosphoglycolate. Phosphoglycolate inhibits chloroplast enzymes and is salvaged by the C2 cycle across three organelles. In the chloroplast, phosphoglycolate phosphatase gives glycolate. In the peroxisome, glycolate oxidase makes glyoxylate and H₂O₂ (removed by catalase), and transamination gives glycine. In the mitochondrion, the glycine decarboxylase complex with serine hydroxymethyltransferase turns two glycine into one serine, releasing CO₂ and NH₄⁺ and making NADH. Serine returns to the peroxisome to become hydroxypyruvate and glycerate, and glycerate is phosphorylated to 3-PGA in the chloroplast. Of the four carbons in two phosphoglycolates, three return to the Calvin cycle and one is lost as CO₂; the NH₄⁺ must be refixed by GS–GOGAT at a cost in ATP and reductant.
Oxygenation rises relative to carboxylation with temperature (CO₂ solubility falls faster than O₂ solubility and Rubisco's preference for CO₂ declines), with high O₂, and with low intercellular CO₂ when stomata close. That is why 21% O₂ inhibits C3 photosynthesis (the Warburg effect) and why C4 and CAM pay off in hot, dry places. Worked example: with one oxygenation per four carboxylations (vo/vc = 0.25), each carboxylation is accompanied by 0.25 × ½ = 0.125 CO₂ released, so 12.5% of the carbon fixed is lost again. Arabidopsis mutants in C2-cycle enzymes grow normally in high CO₂ but die in air, the classic proof that the cycle is essential.
8. Respiration: glycolysis, TCA cycle and the plant electron transport chain
Plant glycolysis runs in both cytosol and plastids and has flexible steps: sucrose is cleaved by invertase or sucrose synthase; fructose-6-phosphate is phosphorylated by ATP-PFK or by pyrophosphate-dependent PFK (PFP), which saves ATP when phosphate is scarce; and PEP can bypass pyruvate kinase through PEP carboxylase and malate dehydrogenase, feeding malate to mitochondria. In the TCA cycle each pyruvate yields 3 CO₂, 4 NADH (counting pyruvate dehydrogenase), 1 FADH₂ and 1 ATP (plant succinyl-CoA synthetase makes ATP), and NAD-malic enzyme lets plant mitochondria oxidise malate straight to pyruvate.
| Component | Electron transfer | Pumps protons? | Inhibitor |
|---|---|---|---|
| Complex I | Matrix NADH → ubiquinone | Yes | Rotenone |
| Type II NAD(P)H dehydrogenases | Matrix or cytosolic NAD(P)H → ubiquinone | No | Insensitive to rotenone |
| Complex II | Succinate → ubiquinone | No | Malonate |
| Complex III | Ubiquinol → cytochrome c | Yes | Antimycin A |
| Complex IV | Cytochrome c → O₂ | Yes | Cyanide, azide, CO |
| Alternative oxidase (AOX) | Ubiquinol → O₂ directly | No | SHAM (salicylhydroxamic acid); cyanide-resistant |
Electrons that take the non-pumping routes conserve little energy, and the rest is released as heat. The thermogenic spadix of aroids such as the voodoo lily uses AOX this way, with salicylic acid triggering the heat burst; elsewhere AOX keeps the ubiquinone pool from over-reduction and limits reactive oxygen species under stress. The respiratory quotient (RQ = CO₂ released ÷ O₂ consumed) reveals the substrate: carbohydrate gives 1.0; fats, poor in oxygen, give about 0.7 (tripalmitin, C₅₁H₉₈O₆ + 72.5 O₂ → 51 CO₂ + 49 H₂O, RQ = 0.70), as in germinating oil seeds; organic acids give more than 1 (malic acid, C₄H₆O₅ + 3 O₂ → 4 CO₂ + 3 H₂O, RQ = 1.33).
Key takeaways
- Ψw = Ψs + Ψp + Ψg, with Ψs = −iCRT; water moves to lower Ψw. At full turgor Ψw = 0 and Ψp = −Ψs; at incipient plasmolysis Ψp = 0.
- The Casparian strip forces a membrane step into the stele; xylem water rises by cohesion–tension; transpiration ratios are about 500 (C3), 250 (C4) and 50 (CAM); the Nernst equation separates passive from active ion uptake; phloem moves by pressure flow.
- Blue light opens stomata through phototropin, H⁺-ATPase activation and K⁺ and malate uptake; ABA closes them through PYR/PYL, PP2C, OST1 and SLAC1, with K⁺ leaving through GORK.
- Nitrate reductase (cytosol, 2e⁻) and nitrite reductase (plastid, 6e⁻) make NH₄⁺ for GS–GOGAT; nitrogenase spends 16 ATP per N₂, always makes H₂ and is shielded from O₂ by leghaemoglobin.
- The Calvin cycle costs 3 ATP and 2 NADPH per CO₂; the C4 pump adds 2 ATP (5 per CO₂); CAM separates the carboxylations in time; photorespiration loses one CO₂ per two oxygenations.
- Plant respiration adds PFP, NAD-malic enzyme, rotenone-insensitive dehydrogenases and the cyanide-resistant, SHAM-sensitive alternative oxidase; RQ is 1 for carbohydrate, about 0.7 for fat and above 1 for organic acids.
Practice questions (17)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
A fully turgid parenchyma cell is at equilibrium with pure water at atmospheric pressure (ignore gravity). Which description of its water potential components is correct?
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Answer: A — Ψw is zero, and Ψp equals Ψs in magnitude but is opposite in sign
At equilibrium the cell's Ψw equals that of pure water, zero. With Ψg ignored, Ψs + Ψp = 0, so Ψp = −Ψs: the wall pushes back exactly as hard as the solutes pull water in. Ψp = 0 with Ψw = Ψs is the tempting answer but describes incipient plasmolysis, the opposite extreme. A cell in equilibrium with pure water cannot have Ψw above zero, and the vacuole still holds solutes, so Ψs stays negative.A parenchyma cell has a solute potential of −0.90 MPa. It is placed in a large volume of 0.15 M sucrose at 300 K and allowed to reach equilibrium. Assume that the cell's solute potential does not change, that sucrose does not dissociate (i = 1), that gravity can be ignored and that R = 0.008314 L MPa mol⁻¹ K⁻¹. The pressure potential of the cell at equilibrium is ____ MPa (to two decimal places).
Numerical answer — type the value.
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Answer: 0.53
The open solution has Ψp = 0, so its Ψw = Ψs = −iCRT = −(1)(0.15)(0.008314)(300) = −0.374 MPa. At equilibrium the cell's Ψw is also −0.374 MPa, so Ψp = Ψw − Ψs = −0.374 − (−0.90) = +0.526, which is 0.53 MPa. Subtracting the wrong way gives −0.53, and adding the magnitudes gives 1.27; both are slips.In a root mutant the endodermal Casparian strip fails to form, while aquaporins and plasmodesmata are normal. Which consequence is most expected?
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Answer: B — Solutes can reach the stele through cell walls without crossing a membrane, losing ion selectivity
The Casparian strip seals the apoplast at the endodermis, forcing water and solutes to cross a plasma membrane, where transporters select them. Without it, solutes can leak into the stele through the walls, so selectivity is lost. Root pressure is the tempting distractor, but it would fall, not rise, because the same gap lets ions accumulated in the stele leak back. The transmembrane and symplastic routes do not depend on the strip.Root cortical cells contain 100 mM K⁺ while the external solution contains 1 mM K⁺. The measured membrane potential is −90 mV (inside relative to outside). Take the Nernst potential of K⁺ as E = 59 log₁₀(C_out/C_in) mV at 25 °C. The value of (membrane potential − Nernst potential of K⁺) is ____ mV.
Numerical answer — type the value.
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Answer: 28
E_K = 59 × log₁₀(1/100) = 59 × (−2) = −118 mV. Membrane potential − E_K = −90 − (−118) = +28 mV. A positive driving force on a cation means its passive tendency is outward: the cells hold K⁺ above its equilibrium level, so uptake against this gradient must be active (H⁺-coupled). Writing log(C_in/C_out) gives +118 mV for E_K and a wrong sign throughout.Which of the following contribute to blue-light-induced stomatal opening?
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Answer: A — Phosphorylation of the plasma-membrane H⁺-ATPase and binding of a 14-3-3 protein to it; B — Influx of K⁺ through inward-rectifying channels after the membrane hyperpolarises; D — Synthesis of malate in the guard cells from starch through PEP carboxylase
Blue light, through phototropins, phosphorylates the H⁺-ATPase, which then binds 14-3-3 and pumps protons out; the hyperpolarisation opens inward K⁺ channels; and malate made from starch through PEP carboxylase supplies a counter-anion and osmoticum. SLAC1 activation by OST1 is part of ABA-induced closure: anion efflux depolarises the membrane and drives K⁺ out, the opposite of opening.Plants of an Arabidopsis mutant lacking the protein kinase OST1 (SnRK2.6) are sprayed with abscisic acid and then exposed to dry air. Compared with the wild type, the mutant leaves would most likely
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Answer: C — keep their stomata open, lose water quickly and look cooler under thermal imaging
OST1 is the SnRK2 kinase released when ABA-bound PYR/PYL receptors inhibit PP2C; it activates SLAC1 and the NADPH oxidase that close the pore. Without OST1, ABA cannot close stomata, so the leaves keep transpiring, wilt and stay cooler. Spraying ABA cannot help, which rules out the biosynthesis option: the block lies downstream of the hormone. OST1 is not needed for blue-light opening, and PP2C acts upstream of it.Which statement correctly contrasts nitrate reductase and nitrite reductase in a leaf cell?
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Answer: C — Nitrate reductase works in the cytosol with NAD(P)H; nitrite reductase works in plastids with six ferredoxin electrons
Nitrate reductase is cytosolic, uses NADH (or NADPH) and carries FAD, haem and the molybdenum cofactor; it moves two electrons (nitrate to nitrite). Nitrite reductase is plastidic, uses reduced ferredoxin and moves six electrons (nitrite to ammonium), with siroheme and an Fe₄S₄ cluster. The molybdenum option is tempting, but the molybdenum enzyme is nitrate reductase, so molybdenum deficiency causes nitrate, not nitrite, to accumulate.Which of the following statements about nitrogen fixation and ammonium assimilation are correct?
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Answer: A — Nitrogenase reduces protons to H₂ as an obligatory part of reducing N₂; B — Leghaemoglobin keeps free O₂ low in infected cells while still supplying O₂ for bacteroid respiration
N₂ + 8e⁻ + 8H⁺ + 16 ATP → 2NH₃ + H₂: hydrogen evolution is built into the mechanism. Leghaemoglobin binds O₂, keeping free O₂ low enough for nitrogenase while delivering it to the bacteroid respiratory chain. GOGAT transfers glutamine's amide nitrogen to 2-oxoglutarate, giving two glutamate; aspartate is made by an aminotransferase. N₂ binds the MoFe protein (its FeMo cofactor); the Fe protein is the ATP-dependent electron donor.A C3 plant and a C4 plant each fix 1.76 g of CO₂. Their transpiration ratios are 500 and 250 mol H₂O transpired per mol CO₂ fixed, respectively. Taking molar masses CO₂ = 44 g mol⁻¹ and H₂O = 18 g mol⁻¹, how many more grams of water does the C3 plant transpire than the C4 plant?
Numerical answer — type the value.
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Answer: 180
Moles of CO₂ = 1.76/44 = 0.04 mol. Water lost: C3 = 500 × 0.04 = 20 mol; C4 = 250 × 0.04 = 10 mol. The difference, 10 mol, is 10 × 18 = 180 g. Forgetting to convert moles of water to grams gives 10, and using the CO₂ mass directly gives 440.Assume that the Calvin–Benson cycle needs 3 ATP and 2 NADPH per CO₂ fixed, and that the C4 pump of an NADP-malic enzyme species costs an extra 2 ATP per CO₂ (pyruvate, phosphate dikinase converts ATP to AMP, counted as two ATP). Ignoring photorespiration, the total number of ATP needed to fix enough CO₂ to make one hexose in this C4 plant is ____.
Numerical answer — type the value.
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Answer: 30
Per CO₂ the C4 plant spends 3 (Calvin cycle) + 2 (PEP regeneration) = 5 ATP. A hexose needs 6 CO₂, so 6 × 5 = 30 ATP (with 12 NADPH, the same as C3). A C3 plant would need 18 ATP; counting the PPDK step as only one ATP gives 24.Leaves of a succulent are sampled every few hours. Titratable acidity is highest at dawn and lowest at dusk, and ¹⁴CO₂ supplied at night first labels malate. Which interpretation fits these data?
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Answer: B — PEP carboxylase fixes CO₂ into malate at night; by day malate releases CO₂ that Rubisco refixes
Nocturnal labelling of malate and acid accumulation overnight are the signature of CAM: PEP carboxylase fixes CO₂ (as HCO₃⁻) into oxaloacetate, reduced to malate and stored as malic acid; by day the stomata close and decarboxylation releases CO₂ for Rubisco, so acidity falls. Rubisco does not make malic acid from 3-PGA. The C4-style option reverses the timing and invokes Kranz anatomy, and photorespiration needs light-driven RuBP oxygenation.Leaves of maize and of wheat are given a 5-second pulse of ¹⁴CO₂ in the light. In which compounds does most of the label appear first in each?
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Answer: D — Maize: oxaloacetate and malate; wheat: 3-phosphoglycerate
Maize is a C4 grass: PEP carboxylase in the mesophyll fixes carbon first into the C4 acids oxaloacetate and malate. Wheat is C3, so Rubisco makes 3-PGA first. The reversed pairing is the classic slip. Rubisco is not the only carboxylase (C4 leaves rely on PEP carboxylase for initial fixation), and PEP carboxylase is not the primary carboxylase in C3 leaves.In a C3 leaf at high temperature, Rubisco catalyses 3 oxygenations for every 10 carboxylations. Assume that the C2 cycle releases one CO₂ for every two RuBP oxygenations and that there is no other CO₂ loss. The CO₂ released by photorespiration, as a percentage of the CO₂ fixed by carboxylation, is ____.
Numerical answer — type the value.
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Answer: 15
Per 10 carboxylations (10 CO₂ fixed) there are 3 oxygenations, giving 3 × ½ = 1.5 CO₂ released in the mitochondrion by glycine decarboxylase. Loss = 1.5/10 × 100 = 15%. Taking one CO₂ per oxygenation gives 30%, the usual slip.In the photorespiratory C2 cycle, where and in which step are CO₂ and NH₄⁺ released?
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Answer: C — In the mitochondrion, as glycine decarboxylase turns two glycine into serine
The glycine decarboxylase complex, working with serine hydroxymethyltransferase in the mitochondrial matrix, converts two glycine into one serine and releases CO₂ and NH₄⁺ (and makes NADH). Glycolate oxidase in the peroxisome is the tempting answer because it is an oxidation, but it produces glyoxylate and H₂O₂, not CO₂. The serine-to-glycerate steps are a transamination and a reduction, releasing neither.Mitochondria isolated from a thermogenic aroid spadix keep consuming O₂ in the presence of cyanide, but O₂ uptake stops when salicylhydroxamic acid (SHAM) is added as well. The best explanation is that
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Answer: D — an alternative oxidase carries electrons from ubiquinol to O₂, bypassing complex IV, and SHAM inhibits it
Plant mitochondria have an alternative oxidase that takes electrons from ubiquinol straight to O₂, bypassing complexes III and IV and pumping no protons, so the energy appears as heat. It is cyanide-resistant and SHAM-sensitive, which is exactly the pattern observed. Complex IV is cyanide-sensitive in all plants; uncoupling protein carries protons, not electrons; and type II dehydrogenases reduce ubiquinone, not O₂.Which of the following statements about the light reactions of photosynthesis are correct?
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Answer: A — DCMU blocks electron transfer from Q_A to Q_B, so O₂ evolution stops although light is still absorbed; C — Oxidising two water molecules releases one O₂ and four protons into the thylakoid lumen
DCMU binds the Q_B site of PSII, so electrons cannot leave PSII and water is no longer oxidised. The oxygen-evolving complex oxidises 2H₂O to O₂, releasing 4H⁺ into the lumen and 4e⁻. Cyclic flow returns electrons from ferredoxin to plastoquinone, so it makes ATP but no NADPH. The distribution is the reverse: PSII is enriched in appressed grana, while PSI and ATP synthase sit in the stroma lamellae and grana margins.Germinating seeds of an oil-rich species show a respiratory quotient well below 1. Why?
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Answer: A — Fats are poorer in oxygen than carbohydrates, so more O₂ is consumed per CO₂ released
RQ = CO₂ released ÷ O₂ consumed. Fats are highly reduced, so their oxidation needs extra O₂ for the hydrogen: tripalmitin gives 51 CO₂ for 72.5 O₂, an RQ of about 0.7. Organic acids and fermentation both raise RQ above 1, the opposite of what is observed, and the alternative oxidase reduces O₂ to water without making CO₂.