Problem 146 - Objective Biology for NEET

Question

The accepted mechanism for the translocation of sugar from source to sink is called (a) Pressure flow hypothesis (b) Mass flow hypothesis (c) Transpiration pull hypothesis (d) Both (a) and (b)

Step-by-Step Solution

Answer

The correct answer is (d) Both (a) and (b). The mechanisms for translocation of sugar from a source to a sink are described by both the Pressure Flow Hypothesis and Mass Flow Hypothesis.

Step 1: Recognizing the Type of Biological Hypotheses

Firstly, understand what each option represents. The pressure flow hypothesis, the mass flow hypothesis, and the transpiration pull hypothesis are all concepts related to the movement of substances in plants.

Step 2: Comparing Hypotheses with the Sugar Translocation Mechanism

Cross-examine each of these hypotheses with the mechanism of sugar translocation from source (where it's produced) to sink (where it's needed or stored) in plants.

Step 3: Analyzing the Pressure Flow Hypothesis

According to the pressure flow hypothesis, sugar is moved from a high pressure source, like a living cell, to a low pressure sink, along a gradient. This process is driven by turgor pressure and occurs within the phloem of plants.

Step 4: Analyzing the Mass Flow Hypothesis

The mass flow hypothesis, which is another name for the pressure flow hypothesis, suggests that the movement of sugars in the phloem occurs from areas of high sugar concentration (source) to areas of low sugar concentration (sink), facilitated by the flow of water.

Step 5: Analyzing the Transpiration Pull Hypothesis

The transpiration pull hypothesis, on the other hand, is related to the movement of water from the roots towards the leaves of a plant, but not the translocation of sugars.

Step 6: Selecting the Correct Hypothesis

From the comparison and analysis, It is clear that the pressure flow hypothesis and the mass flow hypothesis are the same, and both explain the process of sugar translocation in plants. The transpiration pull hypothesis isn’t related to the query. Thus, the correct answer is both (a) and (b).

Key Concepts

Mass Flow Hypothesis

The Mass Flow Hypothesis is a key concept in understanding how plants move nutrients, particularly sugars, from one part of the plant to another. This process is sometimes also known as the Pressure Flow Hypothesis. Essentially, it describes how substances such as sugars are transported in the phloem from 'source' regions such as leaves, where photosynthesis produces the sugars, to 'sink' regions like roots or fruits.

The process works like this:

• Sugars produced in the leaves are actively transported into the phloem sieve tubes, increasing solute concentration.
• This high solute concentration causes water to enter the tubes by osmosis, creating turgor pressure.
• The high turgor pressure causes the sap to flow towards areas of lower pressure – typically the sinks, where sugars are used or stored.

It's like a pipeline system where the sugars flow from high pressure to low pressure areas, driven largely by water movement.

Sugar Translocation

Sugar Translocation occurs within a plant's phloem tissue and is crucial for distributing the energy-rich products of photosynthesis throughout the plant. Plants often produce sugars in their leaves through photosynthesis and need an efficient system to move these sugars to different parts, from the tip of a growing root to seed development.

Translocation is facilitated by several key factors:

• Source-Sink Relationship: Sugars move from 'source' tissues, like leaves, to 'sink' tissues, including roots and developing fruits.
• Turgor Pressure: Created by osmosis as water moves into the phloem.
• Active Transport: Sugars are actively loaded and unloaded from the sieve elements of the phloem.

This complex mechanism enables plants to maintain energy balance and supports growth and development.

Phloem Transport

Phloem Transport is an integral plant function responsible for the movement of organic compounds, primarily sugars, throughout the plant. Unlike water transport that occurs in the xylem, phloem transport can occur in any direction required to feed plant parts thus meeting their metabolic needs.

Key features of phloem transport:

• Bidirectional Flow: Depending on the plant's needs, phloem can carry nutrients to and from leaves, roots, flowers, and fruits.
• Sieve Tubes: Specialized cells called sieve tube elements form a network through which the transport occurs.
• Companion Cells: These cells assist in the loading and unloading of materials into the sieve tubes.

The efficiency of phloem transport allows plants to quickly redirect nutrients to areas of growth or storage, ensuring their survival and reproduction.