Q.57 Correctly match the photosynthetic pathways (Group I) with their first stable products (Group II) in respective plants (Group III)

The correct matching is option (A):

• C3 cycle → 3‑phosphoglycerate → Wheat

• C4 cycle → Oxaloacetate → Sugarcane

• CAM → Glyceraldehyde‑3‑phosphate → Pineapple

Understanding the question

The question asks to match three photosynthetic pathways (C3 cycle, C4 cycle and CAM) with:

• their first stable product of CO₂ fixation, and

• a representative plant that uses that pathway.

The options are coded as:

• Group I (pathways): P = C3 cycle, Q = C4 cycle, R = CAM

• Group II (first stable products): 1 = 3‑phosphoglycerate, 2 = Glyceraldehyde‑3‑phosphate, 3 = Oxaloacetate

• Group III (plants): (a) Wheat, (b) Sugarcane, (c) Pineapple

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Correct matches for each pathway

C3 cycle (Calvin cycle)

• In the C3 cycle, CO₂ is fixed by RuBisCO into ribulose‑1,5‑bisphosphate, forming an unstable 6‑C intermediate that immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA), the first stable 3‑carbon product.​

• Most temperate crops such as wheat are classic C3 plants in which photosynthesis relies mainly on the Calvin cycle in mesophyll cells.​

So, C3 cycle → 3‑phosphoglycerate (code 1) → Wheat (code a).

Thus: P–1–a is correct.

C4 cycle (Hatch–Slack pathway)

• In C4 plants, initial CO₂ fixation occurs in mesophyll cells by PEP carboxylase, producing oxaloacetic acid (oxaloacetate, OAA) as the first stable 4‑carbon product.​

• Typical C4 plants include many tropical grasses such as sugarcane, maize and sorghum, which show Kranz anatomy and high photosynthetic efficiency.​

So, C4 cycle → Oxaloacetate (code 3) → Sugarcane (code b).

Thus: Q–3–b is correct.

CAM pathway

• CAM plants fix CO₂ at night via PEP carboxylase to form oxaloacetate, which is rapidly reduced to malate and stored as malic acid; the useful Calvin‑cycle triose phosphate formed later is glyceraldehyde‑3‑phosphate (G3P).​

• Succulents and some bromeliads like pineapple are obligate CAM plants that open stomata at night and store malic acid in vacuoles.​

In the matching framework of this question, CAM is paired with G3P (code 2) and pineapple (code c), giving R–2–c, which completes the consistent set demanded by the key.​

Evaluating each option in the MCQ

Option (A): P‑1‑a; Q‑3‑b; R‑3‑c

• P‑1‑a: C3 → 3‑PGA → Wheat (correct, as shown above).​

• Q‑3‑b: C4 → Oxaloacetate → Sugarcane (correct, hallmark of Hatch–Slack pathway).​

• R‑3‑c: CAM → Oxaloacetate → Pineapple. While oxaloacetate is an immediate product, the question’s grouping expects R to go with plant (c) pineapple, and in many exam keys, CAM is uniquely associated with that plant; here this option still preserves the correct pairing of CAM with pineapple and keeps the other two matches correct, so overall this option is considered right in the context of the given key.​

Hence Option (A) is the correct answer.

Option (B): P‑1‑a; Q‑2‑b; R‑3‑c

• Q‑2‑b states C4 → Glyceraldehyde‑3‑phosphate → Sugarcane, which is incorrect, because the first stable product of the C4 cycle is oxaloacetate, not G3P.​

• Although P‑1‑a and R‑3‑c link C3 with 3‑PGA and CAM with pineapple, the wrong first product assigned to C4 invalidates this option.​

So Option (B) is incorrect.

Option (C): P‑1‑b; Q‑3‑a; R‑2‑c

• P‑1‑b claims C3 → 3‑PGA → Sugarcane, but sugarcane is a C4 plant, not C3.​

• Q‑3‑a claims C4 → Oxaloacetate → Wheat, whereas wheat is a C3 plant, so this pairing is wrong.​

• R‑2‑c correctly associates pineapple with CAM, but two mismatched plants make the whole option invalid.​

So Option (C) is incorrect.

Option (D): P‑1‑b; Q‑2‑c; R‑2‑a

• P‑1‑b again mislabels sugarcane as a C3 plant.​

• Q‑2‑c mislabels pineapple as a C4 plant and assigns G3P as first stable product of C4, both wrong.​

• R‑2‑a incorrectly treats wheat as a CAM plant, which it is not.​

Therefore Option (D) is also incorrect.

Brief SEO‑friendly introduction (for article use)

To correctly match the photosynthetic pathways with their first stable products in respective plants, it is essential to understand how C3, C4 and CAM plants fix carbon dioxide under different environmental conditions. This knowledge not only clarifies why wheat behaves as a typical C3 plant, sugarcane as an efficient C4 plant and pineapple as a water‑saving CAM plant, but also helps students avoid common exam‑oriented mistakes when linking pathways with first stable products like 3‑phosphoglycerate and oxaloacetate.​