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Question Video: Describing the Structure of the Xylem Biology

Which of the following best describes the structure of the xylem? [A] The xylem is a long, continuous tube of living cells. [B] The xylem is composed of many living cells with pores in the cell wall. [C] The xylem is a long, continuous tube of dead cells. [D] The xylem is composed of many living and dead cells that form a solid tube.

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Video Transcript

Which of the following best describes the structure of the xylem? (A) The xylem is a long, continuous tube of living cells. (B) The xylem is composed of many living cells with pores in the cell wall. (C) The xylem is a long, continuous tube of dead cells. Or (D) the xylem is composed of many living and dead cells that form a solid tube.

This question is asking about the specialized vascular cells found in plants called xylem. To answer it, let’s recall the key facts about vascular tissues in plants, looking at xylem in more detail.

Plants are autotrophs, making their own food by photosynthesis. Therefore, they start most food chains and provide us with the oxygen we need to stay alive. Vascular plants are complex, multicellular organisms, which means they are too large to rely on diffusion and osmosis alone to move substances between cells. They need a good transport system, which is made up of the vascular tissues xylem and phloem. Xylem transports water and mineral ions from the roots to the leaves. And phloem transports nutrients, most commonly sucrose and amino acids, to wherever they are needed in the plant. It is common to get the two types of tissues mixed up. So let’s look at the differences between them and how they are adapted for their function.

Xylem tissue is made up of two main types of cells: tracheids and vessel elements, or members. These cells grow on top of each other and end up forming long, hollow tubes like straws. But how does this happen? As the cells get older, they become lignified. This means lignin, a waterproof substance, is laid down in the cell walls. And this kills the cells, as nothing can get in or out of them. The end walls of the cells then break down, forming the long, hollow structure as shown in the diagram. The lignin provides support and allows the xylem to withstand pressure changes as the water moves up the plant. These structural adaptations allow the xylem to carry out its role of transporting water and mineral ions in one direction, from the roots to the leaves.

Phloem tissue, on the other hand, is mainly made of living cells, sieve tubes and companion cells. Sieve tubes form again by the cells growing on top of each other. This time, however, the end cell walls do not break down. Instead, they form pores, making sieve plates. These allow the movement of cytoplasm between the cells. The cells lose their nuclei, and there is little room for organelles. This is where the companion cells come in. They are connected to the sieve tubes through small pores. And as their name suggests, they assist the sieve tube cells. Companion cells provide energy and any substances required by the sieve tubes to function properly. Between the two, they carry out their role of moving the organic molecules up and down the plant to wherever they are needed.

Now that we have reviewed the structure of the vascular tissue in plants, we can return to the question, which asks “Which of the following best describes the structure of the xylem?” We have seen that phloem is mainly made of living cells with pores in their end walls, so we know the answer is not (B). Xylem is made only of dead cells. So we can discount any answers left with living cells mentioned. This means we can cross out (A) and (D). Therefore, the correct answer is (C). The xylem is a long, continuous tube of dead cells.

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