Plant Physiology — Transport — Practice

Question 1

What is the difference between the apoplast and symplast pathways of water movement?

Accepted Answer

In the apoplast pathway water moves through cell walls and intercellular spaces (non-living, no membranes), so it is fast and unregulated. In the symplast pathway water moves through the cytoplasm via plasmodesmata (living), which is slower but allows control. The Casparian strip forces water from the apoplast into the symplast at the endodermis.

Question 2

Explain the cohesion-tension theory of water transport.

Accepted Answer

Water evaporating from leaves (transpiration) creates a negative pressure (tension) in the xylem. Water molecules cohere through hydrogen bonds and adhere to xylem walls, forming a continuous unbroken column. This column is pulled upward from the roots to the leaves like a rope, drawing water in from the soil.

Question 3

What is guttation and what causes it?

Accepted Answer

Guttation is the exudation of water droplets from the margins of leaves through hydathodes. It is caused by root pressure, usually at night when transpiration is low.

Question 4

How do guard cells regulate stomatal opening?

Accepted Answer

Guard cells take up K+ ions, so water enters by osmosis and they become turgid, opening the pore. When K+ leaves, water exits, the guard cells become flaccid, and the pore closes.

Question 5

What is the role of the Casparian strip?

Accepted Answer

It is a water-impermeable band in the root endodermis that forces water and dissolved minerals from the apoplast into the symplast pathway, acting as a checkpoint for selective ion uptake into the xylem.

Practice — Plant Physiology — Transport

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