Question

Active absorption is governed by _________ theory.

• A. Diffusion
• B. Mass flow
• C. Lecithin
• D. Donnan equilibrium

Hint:

Active absorption theories are divided into carrier-mediated transport theories (like the Lecithin theory) and electrochemical pump theories (such as Lundegardh's Cytochrome Pump theory).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plant roots absorb mineral nutrients from the soil solution through passive absorption (which does not require metabolic energy) or active absorption (which requires metabolic energy, ATP, to move ions against a concentration gradient).

Step 3: Detailed Explanation:
Let us analyze the mechanisms and theories in the options:
1. Lecithin Carrier Theory (C): Proposed by Bennet-Clark in 1956, this is a prominent metabolic carrier theory of active ion absorption.
It suggests that lecithin (a phospholipid) acts as an amphoteric carrier molecule within the cell membrane.
Lecithin binds both anions and cations at specific binding sites, forming a lecithin-ion complex.
The enzyme lecithinase then hydrolyzes this complex on the inner side of the membrane, releasing the ions into the cytoplasm.
The regeneration of lecithin from its byproducts requires ATP, making this an active, energy-dependent absorption process.
2. Diffusion (A) and Mass Flow (B): These are physical, passive mechanisms where ions move along concentration gradients or with the bulk flow of water, without requiring metabolic energy.
3. Donnan Equilibrium (D): This is a passive, electrochemical mechanism where ions accumulate within cells to balance non-diffusible, fixed ions, without directly consuming ATP.

Step 4: Final Answer:
Active absorption is explained by the Lecithin carrier theory, corresponding to option (C).