MPSC 2025 Prelims Paper-I · Question 57 of 100

Correct — option (2), Potassium. Stomatal opening and closing is driven by changes in the turgor pressure of the two guard cells that flank each stomatal pore, and that turgor pressure is controlled chiefly by the movement of potassium ions (K+) into and out of the guard cells. When light, low internal carbon dioxide, or other favourable cues are sensed, proton (H+) pumps in the guard-cell membrane actively export hydrogen ions, and the resulting electrochemical gradient drives potassium ions into the guard cells through specific K+ channels. This potassium influx lowers the guard cells' internal water potential, so water follows osmotically by moving in as well, the guard cells swell and become turgid, and because their cell walls are unevenly thickened (thicker on the side facing the pore), the swelling bows the pair of guard cells apart and opens the stomatal pore. Reversing the process — potassium and water moving back out of the guard cells — deflates them and closes the pore, which is exactly what happens under water stress when the plant hormone abscisic acid (ABA) triggers potassium efflux to conserve water.

• (1)Magnesium — Magnesium's best-known physiological role in plants is as the central atom of the chlorophyll molecule, making it essential for photosynthesis and for the green colour of leaves, and its deficiency causes interveinal chlorosis. It has no established role as the ion whose accumulation directly drives guard-cell turgor and stomatal opening; that role belongs specifically to potassium.
• (3)Iron — Iron is required by plants mainly as a cofactor for enzymes involved in chlorophyll synthesis and electron-transport processes (photosynthesis and respiration), and its deficiency also produces chlorosis in young leaves. It is not the ion that guard cells accumulate to swell and open the stomatal pore.
• (4)Zinc — Zinc is a micronutrient required as a cofactor for several plant enzymes, including those involved in auxin (growth hormone) synthesis, and its deficiency causes symptoms like stunted growth and small, distorted leaves. It has no established role in driving the turgor-pressure changes that open and close stomata; that mechanism is specific to potassium ion movement.

Each stomatal pore on a leaf's epidermis is flanked by two guard cells whose relative turgidity determines whether the pore is open or closed. Guard cells are unusual among plant cells in having a cell wall that is thicker on the side facing the pore and thinner on the outer side; when the cells become turgid (swollen with water), this uneven thickening makes them bow outward and apart, opening the pore, while loss of turgor lets them relax back together, closing it. The osmotic driver of this turgor change is the active, regulated movement of potassium ions (accompanied by counter-ions such as chloride or malate, and by water following osmotically) into the guard cells to open the stomata, and back out again to close them, a process controlled by light, CO2 concentration, and hormonal signals such as abscisic acid under water stress.

MPSC's botany questions frequently test the ionic and hormonal mechanisms behind everyday plant physiology processes like stomatal movement, rather than just the anatomical fact that stomata open and close. The habit rewarded here is knowing potassium specifically (not any generic mineral nutrient) as the ion responsible for guard-cell turgor changes, since several mineral elements are tested together as distractors precisely because they have other, unrelated roles in plant nutrition.

• Stomatal opening is driven by potassium ion (K+) influx into guard cells, which lowers their water potential and draws in water osmotically, increasing turgor pressure.
• Guard cells have unevenly thickened walls (thicker toward the pore), so turgor-driven swelling bows them apart and opens the stomatal pore.
• Proton (H+) pumps in the guard-cell membrane create the electrochemical gradient that drives potassium influx during stomatal opening.
• The hormone abscisic acid (ABA) triggers potassium efflux from guard cells under water stress, closing stomata to reduce water loss through transpiration.

• Confusing potassium's role in guard-cell turgor with magnesium's role in chlorophyll structure or iron's role in photosynthetic electron transport — several minerals are tested together precisely because each has a distinct, specific physiological role
• Assuming stomatal opening is a purely passive/mechanical response to light, rather than an actively regulated ion-transport process
• Forgetting that stomatal closure (potassium efflux) is just as actively regulated as opening, notably through abscisic acid signalling under water stress

MPSC's biology section regularly tests which specific mineral ion or hormone drives a named plant physiological process, using a set of chemically or functionally similar options (here, several essential plant micronutrients) as distractors to check precise, not general, recall.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which plant hormone triggers potassium efflux from guard cells, closing stomata under water-stress conditions ?

  • (a)Auxin
  • (b)Gibberellin
  • (c)Abscisic acid
  • (d)Cytokinin

  Answer(c) Abscisic acid — it signals guard cells to release potassium ions (and water), deflating them and closing the stomatal pore to conserve water under drought stress.

• practice — not a real PYQ

  Guard cells are able to bow apart and open the stomatal pore when turgid mainly because of which structural feature ?

  • (a)Their cell walls are uniformly thin all around
  • (b)Their cell walls are thicker on the side facing the pore and thinner on the outer side
  • (c)They lack a cell wall entirely
  • (d)They contain no chloroplasts

  Answer(b) Their cell walls are thicker on the side facing the pore and thinner on the outer side — this uneven thickening makes a turgid guard cell bow outward, opening the pore, rather than simply expanding uniformly.