201. Non-cyclic electron flow remains functional in a chloroplast, but cyclic flow around photosystem I is strongly inhibited. NADPH supply is adequate, while ATP becomes relatively deficient. Which Calvin-cycle consequence is most likely?
ⓐ. RuBisCO begins using NADPH as a substitute carbon-dioxide acceptor
ⓑ. ATP-dependent reduction and RuBP regeneration both become limiting
ⓒ. Carbon fixation accelerates as the ATP-to-NADPH requirement changes to \(2:3\)
ⓓ. PGA is converted into OAA to bypass every ATP-requiring reaction
Correct Answer: ATP-dependent reduction and RuBP regeneration both become limiting
Explanation: Cyclic electron flow provides additional ATP without adding NADPH. When this route is inhibited, non-cyclic flow may continue supplying both products, yet the chloroplast can develop a relative ATP shortage since the Calvin cycle demands ATP and NADPH in a \(3:2\) ratio. NADPH alone cannot maintain carbon assimilation. ATP is required during processing of PGA and during regeneration of RuBP. Reduced regeneration eventually lowers the available carbon-dioxide acceptor, while reduced energy supply also restricts formation of triose phosphate. RuBisCO cannot use NADPH as an acceptor, and the cycle’s fixed energy ratio does not reverse automatically. The changed light-reaction balance limits multiple ATP-dependent stromal steps even though reducing power remains available.
202. Four habitats are compared for the occurrence of a photosynthetic pathway.
| Habitat | Light | Day temperature | Seasonal water availability |
|---|
| P | Low | Cool | High |
| Q | High | High | Frequently limited |
| R | Low | High | Continuously high |
| S | Moderate | Cool | High |
In which habitat would plants with C4 characteristics be most commonly favoured?
ⓐ. Habitat P
ⓑ. Habitat Q
ⓒ. Habitat R
ⓓ. Habitat S
Correct Answer: Habitat Q
Explanation: C4 plants are commonly associated with dry tropical environments that combine intense illumination, relatively high temperature and periods of restricted water availability. Habitat Q contains this complete set of conditions. The C4 mechanism concentrates carbon dioxide around RuBisCO and limits photorespiration, which is especially advantageous when high temperature and strong light would otherwise increase the loss of fixed carbon in C3 plants. This ecological association does not mean that every plant in a warm or dry habitat must be C4, or that C4 photosynthesis occurs only under drought. It identifies the environment in which the pathway commonly provides a physiological advantage. The decision must use all three habitat variables rather than selecting the row with only high temperature or only reduced water availability.
203. Consider the following statements about the ecological association of C4 plants.
I. They are commonly found in dry tropical regions.
II. Strong light and relatively high temperature often favour their performance.
III. Every plant growing in a dry habitat necessarily uses the C4 pathway.
IV. Habitat alone is insufficient to prove the photosynthetic pathway of an unknown plant.
Which combination is valid?
ⓐ. Statements I and III are correct; statements II and IV are incorrect
ⓑ. Statements II and III are correct; statements I and IV are incorrect
ⓒ. Statements I, II and III are correct; statement IV is incorrect
ⓓ. Statements I, II and IV are correct; statement III is incorrect
Correct Answer: Statements I, II and IV are correct; statement III is incorrect
Explanation: C4 plants are commonly associated with dry tropical habitats, high irradiance and relatively high temperature, making statements I and II valid. Statement III converts a frequent ecological association into a universal rule. Dry habitats can contain C3 plants with other adaptations, and the C4 pathway may occur in plants growing under conditions that are not continuously dry. Statement IV provides the necessary evidential qualifier. Habitat can suggest a likely pathway, but reliable identification should use anatomical or biochemical evidence such as Kranz anatomy, initial fixation by PEP carboxylase, formation of OAA and compartmentation between mesophyll and bundle-sheath cells. The correct combination recognises the ecological pattern without treating climate as a complete diagnostic test.
204. A grass collected from a hot, open tropical site remains productive during a dry season. A researcher classifies it as C4 using only the collection site and seasonal performance. Which additional observation would provide the strongest direct support for the classification?
ⓐ. chloroplast-rich bundle-sheath cells ringed around vascular bundles
ⓑ. chloroplast-poor mesophyll cells forming a ring inside vascular bundles
ⓒ. chloroplast-rich epidermal cells replacing the bundle-sheath layer
ⓓ. vascular bundles lacking a distinct ring of bundle-sheath cells
Correct Answer: chloroplast-rich bundle-sheath cells ringed around vascular bundles
Explanation: The habitat and seasonal performance are consistent with a C4 plant but do not prove the pathway. Kranz anatomy provides direct structural evidence: each vascular bundle is surrounded by a conspicuous ring of large bundle-sheath cells containing numerous chloroplasts, with mesophyll cells arranged outside this ring. This organisation supports the biochemical separation of initial carbon fixation and the Calvin cycle. A waxy surface, extensive roots or reduced leaf area may assist survival under water stress, yet none specifically identifies C4 photosynthesis. The strongest confirmation is the anatomical feature linked directly with the pathway’s two-cell mechanism. A complete biochemical verification could additionally demonstrate PEP carboxylase activity in mesophyll cells and RuBisCO activity in bundle-sheath cells. Habitat and performance are suggestive but not diagnostic; classification requires direct anatomical or biochemical evidence such as Kranz organisation or the first stable product.
205. A transverse leaf section shows a vascular bundle surrounded by a continuous ring of large chloroplast-rich cells. Mesophyll cells form another layer outside this ring. This arrangement is identified as:
ⓐ. a granum formed by concentric thylakoids
ⓑ. an ordinary C3 mesophyll without cell specialisation
ⓒ. a C4 leaf showing specialised Kranz anatomy
ⓓ. a stomatal complex surrounding a vascular bundle
Correct Answer: a C4 leaf showing specialised Kranz anatomy
Explanation: Kranz anatomy is characterised by a wreath-like cellular arrangement around vascular bundles. Large bundle-sheath cells form the inner ring, and mesophyll cells occur outside them. Both cell types participate in photosynthesis but perform different biochemical roles. Mesophyll cells carry out initial inorganic-carbon fixation through PEP carboxylase, while bundle-sheath cells receive four-carbon acids and contain Calvin-cycle machinery centred on RuBisCO. The described structure is at the tissue level and must not be confused with a granum, which is a stack of thylakoids inside one chloroplast. An ordinary C3 leaf lacks this conspicuous two-cell concentric specialisation. The vascular bundle, chloroplast-rich bundle sheath and surrounding mesophyll together provide the decisive anatomical evidence. Their adjacency also supports short-distance transfer of four-carbon acids and helps retain released carbon dioxide near bundle-sheath RuBisCO. This compartmentation is the structural basis for the C4 carbon-concentrating mechanism.
206. Match each part of a C4 leaf with its spatial or functional description. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Vascular bundle | 1. Outer photosynthetic cells performing initial carbon fixation |
| Q. Bundle-sheath cells | 2. Central conducting tissue enclosed by the sheath |
| R. Mesophyll cells | 3. Wreath-like chloroplast-rich ring surrounding the bundle |
| S. Kranz arrangement | 4. Concentric organisation of sheath and mesophyll around vascular tissue |
ⓐ. P-3, Q-2, R-4, S-1
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-1, Q-4, R-2, S-3
ⓓ. P-2, Q-1, R-3, S-4
Correct Answer: P-2, Q-3, R-1, S-4
Explanation: The vascular bundle occupies the central conducting position and is enclosed by bundle-sheath cells. These sheath cells form the conspicuous chloroplast-rich ring that gives Kranz anatomy its wreath-like appearance. Mesophyll cells lie outside the bundle sheath and perform the initial fixation of inorganic carbon in the C4 pathway. The complete concentric organisation of vascular bundle, bundle sheath and surrounding mesophyll is termed the Kranz arrangement. The mapping distinguishes one tissue from the overall pattern it forms. Bundle-sheath cells are not merely an outer protective layer, and mesophyll cells do not occupy the central conducting position. Their spatial relationship is essential for transport of four-carbon acids inward and return of a three-carbon compound outward.
207. A mutation makes the walls of C4 bundle-sheath cells unusually permeable to gases while leaving PEP carboxylase and RuBisCO functional. The most likely consequence is:
ⓐ. greater carbon-dioxide leakage from bundle-sheath cells
ⓑ. complete prevention of initial carbon fixation in mesophyll cells
ⓒ. direct conversion of RuBisCO into PEP carboxylase
ⓓ. permanent accumulation of OAA inside vascular elements
Correct Answer: greater carbon-dioxide leakage from bundle-sheath cells
Explanation: Bundle-sheath cells in C4 leaves possess thick walls that are relatively resistant to gaseous exchange. This feature helps retain carbon dioxide released by decarboxylation of four-carbon acids near RuBisCO. If the walls become unusually permeable, more carbon dioxide can diffuse away from the bundle-sheath region, weakening the local concentration mechanism. Initial fixation by PEP carboxylase in mesophyll cells may still occur, since the mutation does not directly damage that enzyme. RuBisCO also remains chemically distinct from PEP carboxylase. The primary defect lies in retaining the carbon dioxide delivered by the C4 shuttle. Reduced retention would lower the advantage of concentrating carbon dioxide around RuBisCO and could allow greater competition from oxygen at its active site. Greater gas leakage lowers the carbon-dioxide concentration surrounding RuBisCO, increasing oxygen competition and weakening the principal advantage of the C4 mechanism.
208. Four leaf sections show the following features.
| Section | Bundle-sheath chloroplasts | Bundle-sheath walls | Intercellular spaces around sheath | Mesophyll arrangement |
|---|
| P | Few | Thin | Large | Irregular |
| Q | Absent | Thin | Large | Irregular |
| R | Moderate | Thin | Numerous | Scattered |
| S | Numerous | Thick | Little or none | Concentric |
Which section is most consistent with a C4 leaf?
ⓐ. Section P
ⓑ. Section Q
ⓒ. Section R
ⓓ. Section S
Correct Answer: Section S
Explanation: Section S combines the major structural features of a C4 bundle sheath. The sheath cells contain numerous chloroplasts, possess thick walls relatively impervious to gaseous exchange and have little or no intercellular space. Mesophyll cells are arranged concentrically outside the bundle sheath, completing the Kranz pattern. These features support both biochemical activity and retention of carbon dioxide released within bundle-sheath cells. The other sections lack the complete relationship: thin walls and large intercellular spaces would permit easier gas diffusion, while few or absent chloroplasts would not support the specialised photosynthetic role of the sheath. The correct identification requires synthesis of all four columns rather than recognition of one isolated trait such as chloroplast presence. The decisive row combines the relevant observation with the condition needed to interpret it biologically in the stated C4 pathway.
209. A leaf section from an unknown grass resembles sections prepared from maize and sorghum: vascular bundles are surrounded by conspicuous chloroplast-rich bundle-sheath cells, with mesophyll cells outside. The unknown grass is most appropriately classified as:
ⓐ. a non-photosynthetic plant identified only by its grass habit
ⓑ. a C3 plant in which the Calvin cycle occurs only in vascular elements
ⓒ. a C4 plant with chloroplast-rich bundle-sheath cells in Kranz anatomy
ⓓ. an aquatic plant lacking biochemical compartmentation
Correct Answer: a C4 plant with chloroplast-rich bundle-sheath cells in Kranz anatomy
Explanation: Maize and sorghum are standard examples of C4 plants, but the classification of the unknown grass does not rest only on resemblance of plant names. Its leaf section displays the diagnostic Kranz organisation: a prominent chloroplast-rich bundle sheath around each vascular bundle and surrounding mesophyll tissue. This anatomy supports division of photosynthetic labour between two cell types. Initial fixation occurs in mesophyll cells, while carbon dioxide released from transported four-carbon acids enters the Calvin cycle in bundle-sheath cells. The wording “likely C4” is appropriately evidence-based, since anatomy strongly supports the pathway and could be supplemented with enzyme-localisation data. The presence of vascular tissue does not make the vascular elements themselves the Calvin-cycle site.
210. Initial fixation in a C4 mesophyll cell combines one molecule of carbon dioxide with a \(3\)-carbon acceptor and produces a stable \(4\)-carbon compound. The acceptor-product pair is:
ⓐ. PEP and OAA
ⓑ. RuBP and PGA
ⓒ. OAA and PEP
ⓓ. PGA and RuBP
Correct Answer: PEP and OAA
Explanation: The primary carbon-dioxide acceptor in C4 mesophyll cells is phosphoenolpyruvate, or PEP, which contains \(3\) carbon atoms. Addition of inorganic carbon produces the first stable \(4\)-carbon compound, oxaloacetic acid, or OAA. The carbon relation can be represented conceptually as \(3+1=4\), linking the acceptor, incoming carbon and product. RuBP and PGA belong to Calvin-cycle carboxylation: \(5\)-carbon RuBP receives carbon dioxide and gives rise to \(3\)-carbon PGA molecules. OAA is a product rather than the initial acceptor, while PGA is not the C4 mesophyll acceptor. The pathway name C4 refers to the carbon number of OAA, the first stable product, and not to the carbon number of PEP.
211. Assertion: OAA is the primary carbon-dioxide acceptor in the C4 pathway.
Reason: Initial fixation in mesophyll cells uses \(3\)-carbon PEP and produces \(4\)-carbon OAA.
ⓐ. Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion
ⓑ. Both the Assertion and the Reason are true, but the Reason does not explain the Assertion
ⓒ. The Assertion is true, whereas the Reason is false and cannot explain the Assertion
ⓓ. The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Correct Answer: The Assertion is false, whereas the Reason is true and cannot explain the Assertion
Explanation: The Assertion is false since OAA is the first stable product of initial C4 fixation, not the molecule that initially accepts carbon dioxide. The Reason states the correct relation. PEP contains \(3\) carbon atoms and acts as the primary acceptor in mesophyll cells. Fixation of inorganic carbon through PEP carboxylase produces the \(4\)-carbon compound OAA. Confusing acceptor and product also leads to a mistaken interpretation of the C4 name. The label refers to the first stable \(4\)-carbon product, while the acceptor itself has \(3\) carbon atoms. The Reason supplies the evidence that disproves the Assertion by clearly assigning PEP and OAA to their proper positions in the fixation event.
212. Following a very brief pulse of labelled carbon dioxide in a C4 leaf, the earliest substantial label appears in OAA within mesophyll cells, while bundle-sheath Calvin-cycle products become labelled later. The strongest inference is:
ⓐ. RuBisCO performs the initial fixation inside mesophyll cells
ⓑ. mesophyll fixation precedes carbon delivery to bundle-sheath cells
ⓒ. OAA is synthesised only after the Calvin cycle is completed
ⓓ. bundle-sheath RuBisCO fixes carbon dioxide before mesophyll OAA forms
Correct Answer: mesophyll fixation precedes carbon delivery to bundle-sheath cells
Explanation: A brief tracer pulse reveals early pathway events before labelled carbon spreads through downstream products. The first substantial label in mesophyll OAA identifies this cell type and compound as the initial fixation site and early stable product. Later appearance of label in bundle-sheath Calvin-cycle compounds establishes movement of fixed carbon from mesophyll to bundle sheath. The observation does not place RuBisCO in mesophyll cells; initial fixation there is catalysed by PEP carboxylase. OAA must arise before transport and decarboxylation, not after completion of the Calvin cycle. The time sequence provides direct evidence for spatially organised carbon flow: external inorganic carbon is initially captured in mesophyll cells and subsequently delivered in a four-carbon form to bundle-sheath cells. The delay between labelled OAA and labelled bundle-sheath products establishes pathway direction rather than merely showing that both cell types eventually contain radioactive carbon.
213. Use the cellular arrangement described below. Cell P lies outside the bundle sheath and contains PEP, while Cell Q forms the chloroplast-rich ring around the vascular bundle and contains RuBisCO. The enzyme catalysing the first fixation of inorganic carbon is located in:
ⓐ. Cell Q and is RuBisCO
ⓑ. Cell Q and is PEP carboxylase
ⓒ. Cell P and is PEP carboxylase
ⓓ. Cell P and is RuBisCO
Correct Answer: Cell P and is PEP carboxylase
Explanation: Cell P is a mesophyll cell, identified by its position outside the bundle sheath and the presence of PEP. Initial fixation of inorganic carbon in the C4 pathway occurs there through PEP carboxylase, producing OAA as the first stable four-carbon product. OAA may be converted into another four-carbon acid before transport inward. Cell Q is a bundle-sheath cell containing RuBisCO, which fixes carbon dioxide released after decarboxylation of the transported acid. The two enzymes act at different stages and occupy different cell types in the normal C4 arrangement. Interpreting the description requires combining cell position, acceptor identity and enzyme function rather than selecting an enzyme from its name alone. This spatial separation allows mesophyll cells to capture incoming inorganic carbon efficiently and bundle-sheath cells to expose RuBisCO to a concentrated carbon-dioxide supply.
214. A selective inhibitor enters only the mesophyll cells of a C4 leaf and blocks PEP carboxylase. Light reactions and bundle-sheath RuBisCO remain functional. The earliest major effect is:
ⓐ. less OAA formation and four-carbon-acid delivery to the bundle sheath
ⓑ. increased OAA production through bundle-sheath RuBisCO
ⓒ. direct replacement of mesophyll PEP carboxylase by RuBisCO
ⓓ. unchanged bundle-sheath carbon concentration despite reduced acid delivery
Correct Answer: less OAA formation and four-carbon-acid delivery to the bundle sheath
Explanation: PEP carboxylase catalyses the initial fixation of inorganic carbon in C4 mesophyll cells. Blocking it prevents normal conversion of PEP into OAA and sharply reduces the pool of four-carbon acids available for transport to bundle-sheath cells. RuBisCO may remain structurally functional in the bundle sheath, but it receives less concentrated carbon dioxide from decarboxylation of transported acids. The carbon-concentrating mechanism weakens after the initial fixation defect. RuBisCO does not replace PEP carboxylase by producing OAA, and relocation of RuBisCO cannot restore the missing initial fixation step. The earliest effect appears upstream in mesophyll carbon capture; later effects include reduced bundle-sheath carbon dioxide supply and diminished Calvin-cycle activity. Transport and decarboxylation of four-carbon acids raise carbon dioxide around bundle-sheath RuBisCO. The loss first appears before transport and decarboxylation, so later bundle-sheath effects are consequences of reduced mesophyll capture rather than primary RuBisCO damage.
215. Examine the following cell-specific proposals.
| Proposal | Cell type | Enzyme | Immediate product or role |
|---|
| P | Mesophyll | PEP carboxylase | Formation of OAA |
| Q | Mesophyll | RuBisCO | Primary formation of OAA |
| R | Bundle sheath | PEP carboxylase | Initial capture of atmospheric carbon dioxide |
| S | Bundle sheath | ATP synthase | Carboxylation of PEP |
Which proposal is internally consistent?
ⓐ. Proposal Q
ⓑ. Proposal P
ⓒ. Proposal S
ⓓ. Proposal R
Correct Answer: Proposal P
Explanation: Proposal P correctly combines the mesophyll location, PEP carboxylase and formation of OAA. PEP carboxylase adds inorganic carbon to PEP, creating the first stable \(4\)-carbon product of the pathway. In the standard C4 arrangement, RuBisCO is localised to C4 bundle-sheath cells rather than mesophyll cells, and it operates in the Calvin cycle after carbon dioxide has been released from transported four-carbon acids. PEP carboxylase is not assigned to bundle-sheath initial fixation. ATP synthase forms ATP through chemiosmosis and does not carboxylate PEP. The valid row must align all three dimensions: cell type, enzyme identity and biochemical outcome. A correct enzyme paired with the wrong cellular compartment would not describe the organised C4 mechanism. The correct row must align mesophyll location, PEP carboxylase and OAA formation within the same initial fixation step.
216. Before transfer from mesophyll to bundle-sheath cells, OAA may be converted into another \(4\)-carbon acid such as ______.
ⓐ. RuBP or PGA
ⓑ. PEP or RuBP
ⓒ. PGA or phosphoglycolate
ⓓ. Malate or aspartate
Correct Answer: Malate or aspartate
Explanation: OAA is the first stable \(4\)-carbon product formed after PEP carboxylation in mesophyll cells. It can be converted into other four-carbon compounds, including malate or aspartate, before transport to bundle-sheath cells. The carbon number remains \(4\), allowing these acids to function as carriers of initially fixed carbon. Inside the bundle sheath, decarboxylation releases carbon dioxide and leaves a three-carbon compound. RuBP and PGA are Calvin-cycle compounds with \(5\) and \(3\) carbon atoms, respectively, while PEP is the \(3\)-carbon acceptor regenerated in mesophyll cells. Phosphoglycolate is associated with RuBisCO oxygenation rather than the normal C4 transport step. The missing terms must fit both the carbon count and the intercellular shuttle function.
217. Arrange the following events in the early C4 pathway.
P. A four-carbon acid moves to a bundle-sheath cell.
Q. PEP carboxylase fixes inorganic carbon in a mesophyll cell.
R. OAA is formed.
S. OAA is converted into a transportable four-carbon acid.
ⓐ. R → Q → P → S
ⓑ. Q → S → R → P
ⓒ. Q → R → S → P
ⓓ. S → P → Q → R
Correct Answer: Q → R → S → P
Explanation: The pathway begins in a mesophyll cell when PEP carboxylase fixes inorganic carbon using PEP as the acceptor. This reaction forms OAA, the first stable \(4\)-carbon product. OAA may then be converted into a four-carbon acid such as malate or aspartate. The acid moves from the mesophyll cell to a bundle-sheath cell, carrying the fixed carbon inward. The sequence follows both biochemical dependency and spatial direction. OAA cannot be converted before it has formed, and transport cannot precede production of the molecule being transported. Later events include decarboxylation in the bundle sheath, entry of released carbon dioxide into the Calvin cycle and return of a three-carbon compound to the mesophyll. Each event creates the condition required for the next, making the order causal rather than arbitrary in the stated C4 pathway.
218. When a mutation blocks movement of all four-carbon acids into bundle-sheath cells, a C4 leaf still forms OAA normally in mesophyll cells. Which pattern is most likely?
ⓐ. Carbon dioxide concentration rises normally around bundle-sheath RuBisCO
ⓑ. RuBisCO relocates into mesophyll cells and converts OAA directly into glucose
ⓒ. Four-carbon acids accumulate in mesophyll as bundle-sheath delivery falls
ⓓ. PEP regeneration becomes unnecessary since transport has stopped
Correct Answer: Four-carbon acids accumulate in mesophyll as bundle-sheath delivery falls
Explanation: Initial fixation and OAA production remain functional, so mesophyll cells continue generating four-carbon compounds. The transport block prevents these acids from reaching bundle-sheath cells, producing an upstream accumulation in mesophyll tissue. Decarboxylation in the bundle sheath receives less substrate, so less carbon dioxide is released near RuBisCO and the carbon-concentrating effect declines. The pathway does not compensate by relocating RuBisCO or converting OAA directly into glucose. PEP regeneration remains part of sustained cycling, although the transport defect disrupts completion of the shuttle. The predicted pattern follows the location of the block: material builds up before the blocked intercellular step, while downstream substrate and carbon delivery become deficient. Transport and decarboxylation of four-carbon acids raise carbon dioxide around bundle-sheath RuBisCO. OAA can accumulate in mesophyll cells, but bundle-sheath carbon concentration and Calvin-cycle fixation decline because the transport link between compartments is broken.
219. A transported C4 acid enters a bundle-sheath cell, but its decarboxylation enzyme is inactive. Initial mesophyll fixation and acid transport continue briefly. The most immediate downstream consequence is:
ⓐ. less carbon dioxide and three-carbon compound are released
ⓑ. increased release of carbon dioxide without formation of a three-carbon compound
ⓒ. direct fixation of the intact four-carbon acid by RuBisCO
ⓓ. immediate conversion of bundle-sheath RuBP into PEP
Correct Answer: less carbon dioxide and three-carbon compound are released
Explanation: Decarboxylation normally removes one carbon from the transported four-carbon acid as carbon dioxide and leaves a three-carbon compound. Blocking the responsible step prevents both linked products from forming normally. Carbon dioxide is not released efficiently near RuBisCO, so the local supply available for Calvin-cycle carboxylation falls. The three-carbon return compound also becomes deficient, disrupting its movement back to mesophyll cells and later regeneration of PEP. RuBisCO does not fix the intact four-carbon acid; it uses carbon dioxide released from that acid. Nor does bundle-sheath RuBP become PEP directly. The consequences extend in two directions from the blocked step: carbon delivery to the Calvin cycle declines, and completion of the intercellular shuttle is interrupted.
220. Four molecules of a \(4\)-carbon acid are completely decarboxylated in bundle-sheath cells. How many carbon-dioxide molecules and \(3\)-carbon compounds are produced, and how many carbon atoms are represented altogether after decarboxylation?
ⓐ. \(4\) carbon-dioxide molecules, \(2\) three-carbon compounds and \(10\) carbon atoms
ⓑ. \(2\) carbon-dioxide molecules, \(4\) three-carbon compounds and \(14\) carbon atoms
ⓒ. \(4\) carbon-dioxide molecules, \(4\) three-carbon compounds and \(12\) carbon atoms
ⓓ. \(4\) carbon-dioxide molecules, \(4\) three-carbon compounds and \(16\) carbon atoms
Correct Answer: \(4\) carbon-dioxide molecules, \(4\) three-carbon compounds and \(16\) carbon atoms
Explanation: Each \(4\)-carbon acid undergoes one decarboxylation event that releases one carbon-dioxide molecule and leaves one \(3\)-carbon compound. Four acid molecules produce \(4\) carbon-dioxide molecules and \(4\) three-carbon compounds. Carbon accounting gives
\[
4\times4=16
\]
carbon atoms before decarboxylation. Afterward, the carbon dioxide contains \(4\times1=4\) carbon atoms, while the three-carbon compounds contain \(4\times3=12\). The total remains
\[
4+12=16
\]
The calculation verifies conservation of carbon and clarifies the dual outcome of decarboxylation. Released carbon dioxide can enter the Calvin cycle, while the remaining three-carbon compounds participate in the return phase of the C4 shuttle. Each decarboxylation separates one carbon from a four-carbon acid, producing one carbon-dioxide molecule and one three-carbon compound. Four events give \(4\) of each, and the carbon check is \(4\times1+4\times3=16\), matching the original four acids. The products retain all original carbon atoms even though the carbon is now divided between two molecular classes.
