Problem 213 - Objective Biology for NEET

Question

Assertion: Function of sieve tubes is controlled by nucleus of companion cells. Reason: Companion cells help in maintaining the pressure gradient in sieve tubes.

Step-by-Step Solution

Answer

Both assertion and reason are correct, and they are closely connected as well. The function of sieve tubes is indeed controlled by the nuclei of companion cells, while companion cells help maintain the pressure gradient in sieve tubes.

Step 1: Understanding the roles of companion cells and sieve tubes

Companion cells and sieve tubes are both part of the phloem, a tissue in plants responsible for the transport of organic nutrients. The companion cells help in maintaining the pressure gradient in the sieve tubes, as they are metabolically active cells with many mitochondria. On the other hand, sieve tubes are meant for the conduction of food. They lack a nucleus, but their functioning is controlled by the companion cells next to them.

Step 2: Analyzing the assertion

The assertion is that the function of sieve tubes is controlled by the nucleus of companion cells. Given the general understanding from Step 1, it can be stated that this is correct. Even though sieve-tubes lack a nucleus, their functioning is indeed controlled by the companion cells adjacent to them.

Step 3: Analyzing the reason

The given reason is that the companion cells help in maintaining the pressure gradient in sieve tubes. This is also correct. The companion cells assist in loading and unloading sugars into the sieve-tubes, helping to maintain the pressure gradient that facilitates the flow of nutrients.

Key Concepts

Companion Cells

Companion cells serve as the lifelines to sieve tubes in the plant transport system - specifically within the phloem. In contrast to other cells, companion cells are packed with organelles like mitochondria, which power the active transport mechanisms necessary for moving substances into and out of the sieve tubes.

Acting akin to a command center, the nucleus within companion cells is responsible for the regulation of the phloem's function. Despite sieve tubes having no nucleus of their own, it’s the companion cells that take charge, ensuring the precise movement of nutrients through a process known as translocation. This synergetic relationship is vital for plant survival and growth, as it maintains the flow of energy-rich substances throughout the plant.

Sieve Tubes

Structure and Functionality

Sieve tubes are fascinating components of the plant's circulatory system, characterized by their elongated, tubular shape. As structural engineers of the plant world, these tube-like cells create a vast network for transporting sugars and nutrients. Despite seeming simplistic due to the absence of nuclei, ribosomes, and other typical cellular structures, sieve tubes are uniquely adapted to their role.

Each sieve tube is formed by a sequence of sieve tube elements, aligned end-to-end, which are interrupted by sieve plates - porous end walls that allow the flow of phloem sap. Nutrients, principally sugars made during photosynthesis, move from where they are produced or stored to where they are needed through these tubes, powered by the osmotic pressure gradients established by companion cells.

Plant Transport System

The plant transport system is a remarkable innovation of nature, consisting mainly of the xylem and phloem - each with their distinct functions. Phloem plays a crucial role in distributing sugars and amino acids, which are products of photosynthesis, from leaves to other parts of the plant - both upwards and downwards, depending on the plant's needs.

The phloem is more than a highway for food distribution; it's the plant’s equivalent of a trade route, delivering vital nutrients across different locales of the plant. This bidirectional flow is what distinguishes phloem from the predominantly unidirectional mechanism of xylem, which transports water and minerals upward from the roots. Efficient phloem function hinges on the well-coordinated activities within the sieve tubes and companion cells, illustrating the sophistication of plant internal transport mechanisms.