Question Video: Describing the Function of Xylem Biology

Which substance is the xylem primarily responsible for transporting?

03:13

Video Transcript

Which substance is the xylem primarily responsible for transporting? (A) Glucose, (B) water, (C) carbon dioxide, (D) sucrose, or (E) oxygen.

Many plants have vascular systems composed of specific tissues that are used to transport different substances around. The word “vascular” originates from the Latin word “vascularis,” which refers to vessels or tubes. This is because the vascular systems of different organisms are generally made up of a complex system of tubelike vessels responsible for transporting various essential substances from one place to another.

In vascular plants, the two main vascular tissues are grouped together into regions called vascular bundles, as has been shown here in the plant stem. These vascular bundles extend throughout the plant from its roots, through the stem, to the leaves. These bundles consist of two tissues: the xylem and phloem. Let’s discuss what the roles of each of these tissues are.

Most plants are able to produce sugars in the form of glucose by reacting carbon dioxide with water in the presence of light energy through a process called photosynthesis. Glucose can react with oxygen in all the living cells of the plant through cellular respiration. This process produces carbon dioxide and water but, more importantly, releases energy for various cellular processes.

As glucose easily reacts with oxygen in cellular respiration, it is difficult to transport long distances. For this reason, glucose is converted into another sugar called sucrose to be transported to the various cells of the plant and then converted back into glucose when it is needed for cellular respiration. Using this information, we can eliminate answer choice (A), as we know that glucose is not transported around the plant.

Sucrose and other solutes, like amino acids, are transported in the phloem either up or down the plant stem to all the cells that might require them. As this question is asking for the substance that the xylem is primarily responsible for transporting and we know that the phloem, not the xylem, transports sucrose, we can eliminate answer choice (D) too.

While the sucrose and amino acids transported via the phloem are dissolved in water, it is not the main function of the phloem to transport water in bulk. That is the function of the xylem. The xylem transports water and dissolved mineral ions from the roots, which usually absorb them from soil, up the stem to all the various parts of the plant that require them. This suggests that the correct answer to this question is (B). But let’s double-check the last couple of answer choices to make sure.

While there will be some dissolved gases, like oxygen and carbon dioxide, present in both the phloem and the xylem, it is not the role of the vascular system to transport these substances. Instead, these gases are constantly exchanged between the plant and its external environment through pores in its leaves and stems. So we can eliminate answer choices (C) and (E).

With this information, we can confirm the correct answer to this question. The substance that the xylem is primarily responsible for transporting is (B), water.

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