Question

In plants, active absorption usually does NOT occur when ____.

• A. stomata are open
• B. stomata are closed
• C. transpiration stops
• D. it is night time

Hint:

{Easy way to remember:}

• {Stomata open} → High transpiration → {Passive absorption dominates}
• {Stomata closed} → Low transpiration → {Active absorption occurs}

Think of transpiration pull as a {strong suction force}. When this suction is strong, plants do not need to actively pump water.

Answer 1

The Correct Option is A

Concept:
Plants absorb water mainly through their root hairs. The absorbed water is transported upward through the xylem to different parts of the plant. Water absorption occurs through two major mechanisms:

• Passive absorption
• Active absorption

Passive absorption is driven by the transpiration pull created by evaporation of water from the leaves. It accounts for nearly 95--96% of total water absorption in plants. Active absorption, on the other hand, occurs due to the metabolic activity of root cells. Energy in the form of ATP is used to actively absorb ions, which creates an osmotic gradient that helps in water uptake.
Step 1: Understanding passive absorption.
When stomata are open, transpiration occurs rapidly. This creates a strong transpiration pull in the xylem, which draws water upward from the roots to the leaves. During this condition:

• Water movement is mainly controlled by transpiration.
• Roots do not need to actively pump water.
• Thus, passive absorption dominates.

Step 2: Understanding active absorption.
Active absorption occurs when transpiration is low or absent. This typically happens under conditions such as:

• When stomata are closed
• During night time
• During conditions of high humidity or low transpiration

In such situations, root cells actively absorb mineral ions using ATP. This creates an osmotic gradient that pulls water into the root cells.
Step 3: Identifying when active absorption does not occur.
When stomata are open, transpiration is high and passive absorption supplies most of the water. Therefore, active absorption is usually not required under this condition. Final Answer:
Active absorption usually does not occur when stomata are open because passive absorption driven by transpiration pull becomes dominant.