Question

Phloem is composed of all except (a) Sieve tube elements (b) Companion cells (c) Phloem parenchyma and sclerenchyma (d) Vessels

Short answer

The correct answer is (d) Vessels. Vessels are not a component of the phloem, but belong to the xylem tissue in plants.

Step-by-step solution

Identify the components of Phloem

The phloem is a tissue in plants which transports food nutrients from leaves to the rest of the plant. Its main components are: - Sieve tube elements, which are the main cells in the phloem and are responsible for the function of nutrient transport - Companion cells, which assist and support the function of the sieve tube elements - Phloem parenchyma, which stores and packs nutrients -Sclerenchyma, which provides support to the phloem.

Compare the options with the established phloem components

Now compare the options given in the exercise to the components identified in step 1. (a) Sieve tube elements - As identified in step 1, these are a component of the phloem. (b) Companion cells - These are also a component of the phloem, providing support to the sieve tube elements. (c) Phloem parenchyma and sclerenchyma - These are both component parts of the phloem structure. (d) Vessels - This is not a component of the phloem. Vessels are a part of xylem tissue in plants, which opposes phloem in functionality (it transports water and mineral nutrients from roots to other parts of the plant).

Key concepts

Sieve Tube Elements

Sieve tube elements are crucial when exploring the moving parts within the plant's circulatory system. Think of them as the highways for the transport of food substances. These specialized cells interconnect to form continuous channels for the flow of nutrients, mainly the products of photosynthesis, throughout the plant.

Sieve elements, however, cannot maintain themselves due to a lack of essential organelles, such as a nucleus, at maturity. This dependency is where another crucial component comes into play – the companion cells, which closely associate with sieve tube elements to keep them functioning.

Unique Structure Tailored for Efficient Transport

Each sieve tube element is ended with a sieve plate, dotted with pores, facilitating the movement between connected elements. Their design is essential for the dynamic flow of sap—a sugary solution produced in the leaves—and this streamlined system is what keeps plants nourished from leaf to root.

Companion Cells

Companion cells serve as the loyal sidekicks to sieve tube elements—they share a common origin and are almost inseparable in function. These cells are packed with the life-supporting machinery that sieve tube elements lack.

They provide metabolic support, facilitating the loading and unloading of sugars into the sieve tubes. This partnership is vital for the survival of sieve tube elements, and ultimately, for the transport efficiency within the plant.

The Role of Companion Cells

Companion cells orchestrate the regulation of pressure in the phloem, which drives the long-distance movement of sap. Their densely packed organelles are a stark contrast to the vacant interior of sieve tube elements, highlighting their role as the control center in the phloem's nutrient distribution network.

Phloem Parenchyma

While sieve tube elements and companion cells are busy with transport, the phloem parenchyma has another key role—storage. These cells are like the warehouse managers of the plant's transport system, holding reserves of vital nutrients and sometimes aiding in transporting soluble organic material.

Unlike sieve tube elements, phloem parenchyma cells are alive and retain all their organelles. They serve different functions depending on the plant species, location within the plant, and time of year, contributing to the adaptability and resilience of plant transport systems.

Functions Beyond Transportation

Phloem parenchyma cells can store starch and fats and play a role in the plant's defense mechanisms. Their versatility and storage capacity provide an essential buffer to the sieve tube elements and companion cells, ensuring that the plant's nutritional needs are met even when photosynthesis is not occurring, such as during the night or seasonal changes.

Plant Tissue Transport

Plant tissue transport is an umbrella term encompassing the systematic movement of water, nutrients, and other vital substances within a plant. It is a complex network mirroring our own vascular system. The phloem that we've been discussing is but one half of this transport equation, paired opposite the xylem, which manages the upward movement of water and minerals from the roots.

The phloem's role in nutrient transport, specifically the translocation of sugars from leaves to growth sites and storage organs, is indispensable for plant life. This process is dynamic and actively regulated by the plant's metabolic needs.

Circulation is Key

The transport within the phloem is bidirectional, facilitating not only growth but also repair and response to injury. This bidirectional flow maintains a balance between different parts of the plant, harmonizing growth, and ensuring survival across various environmental conditions.