116. Plants take up water from the soil predominantly by the apoplastic and symplastic modes of transport.
Which one of the following statements is true?
(1) Apoplastic transport is ATP-dependent, symplastic is ATP-independent.
(2) Symplastic transport is ATP- dependent, apoplastic is ATP-independent.
(3) Both apoplastic and symplastic transport are ATP-dependent.
(4) Both apoplastic and symplastic transport are ATP-independent.

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Detailed Explanation:

Question:
Plants take up water from the soil predominantly by the apoplastic and symplastic modes of transport. Which one of the following statements is true?

✅ Correct Answer: (2) Symplastic transport is ATP-dependent, apoplastic is ATP-independent.

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Water Uptake in Plants: Two Main Pathways

Plants primarily use two pathways for the movement of water and solutes from the soil to the xylem:

1. Apoplastic Pathway

• In this route, water moves through the cell walls and intercellular spaces without crossing any membranes.

• It is a passive process, and no ATP (energy) is required.

• Water moves freely until it encounters the Casparian strip in the endodermis, which acts as a barrier to passive flow.

2. Symplastic Pathway

• Water moves from cell to cell through plasmodesmata, which are cytoplasmic connections between cells.

• This requires active transport mechanisms at certain points, particularly for ion transport to maintain osmotic gradients.

• Hence, symplastic transport is ATP-dependent, especially when substances are actively moved across membranes to maintain cellular water potential.

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Why Option (2) is Correct:

• Symplastic transport is ATP-dependent due to the involvement of active transporters and maintenance of osmotic gradients.

• Apoplastic transport is ATP-independent, as water moves freely through the cell wall matrix by simple diffusion or capillary action, driven by transpiration.

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Why the Other Options Are Incorrect:

• (1) Incorrect – Apoplastic is not ATP-dependent.

• (3) Incorrect – Only symplastic, not both, may require ATP.

• (4) Incorrect – Symplastic transport often involves ATP-driven mechanisms.

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Conclusion:

The correct understanding of water uptake in plants shows that:

• Symplastic transport is ATP-dependent due to its reliance on active processes.

• Apoplastic transport is ATP-independent, relying on passive flow through cell walls.

✅ Correct Answer: (2) Symplastic transport is ATP-dependent, apoplastic is ATP-independent.