Ch 1 · Xylem and Phloem · O Level Biology 5090

Topic 1 of 4 in Transport in Plants — O Level Biology 5090 — 48 lessons.

Introduction

A plant has no heart, yet water absorbed by a root tip reaches the topmost leaf, and sugar made in that leaf feeds the growing root. Two transport tissues do this work: xylem and phloem, grouped together in vascular bundles. Examiners expect you to name each tissue's function precisely, to locate it in root, stem and leaf sections and to link the structure of xylem vessels to their function.

Key ideas — Functions of xylem and phloem

Xylem has two functions:

🧑‍🏫 Teacher's explanation

Think of a plant like a high-rise apartment building. The xylem is like the building's plumbing system: it consists of rigid, hollow pipes that carry water and minerals from the ground up to every floor. Because these pipes are reinforced with a tough material, they also act like the building's structural pillars, helping the plant stand upright.

📖 Definitions

🔍 Worked example — Easy

Identify the two main functions of the xylem in a dicotyledonous plant.

🔍 Worked example — Standard

A student observes a cross-section of a stem under a microscope. They note a tissue consisting of cells joined end-to-end with no end walls and thick, lignified walls. Identify this tissue and state why the lack of end walls is important for its function.

🔍 Worked example — Exam-style

Compare the substances transported by the xylem and the phloem.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Where the tissues lie in root, stem and leaf

A quick rule: xylem is nearer the centre of the stem and the top of the leaf; phloem is nearer the outside of the stem and the bottom of the leaf. In micrographs, xylem vessels are the larger, thick-walled, empty-looking cells; phloem cells are smaller and thin-walled.

🧑‍🏫 Teacher's explanation

Think of a plant like a high-rise building with a complex plumbing and elevator system. The xylem is like the heavy-duty water pipes running through the core of the building; it needs to be strong and central to support the structure and move water upwards against gravity. The phloem is like the delivery elevators, moving food (sucrose) to where it is needed most, often positioned closer to the outer walls to access the 'kitchens' (leaves) and 'storage' (roots).

📖 Definitions

🔍 Worked example — Easy

A student observes a transverse section of a dicotyledonous stem under a microscope. They identify a vascular bundle. Describe the relative positions of the xylem and phloem within this bundle.

🔍 Worked example — Standard

Compare the arrangement of xylem and phloem in a root versus a stem of a dicotyledonous plant.

🔍 Worked example — Exam-style

A micrograph shows a leaf cross-section. A large, thick-walled, empty-looking cell is identified. State the tissue type and its position relative to the phloem.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Describe the arrangement of xylem and phloem in the root. [2] (b) State the function of the xylem in the plant. [1] [3 marks]

Key ideas — Root hair cells

Just behind each root tip, single epidermal cells grow long, thin extensions called root hairs. A root hair cell is identified by its narrow, finger-like projection into the soil, its thin cell wall and the absence of chloroplasts (it is underground, so it does not photosynthesise).

🧑‍🏫 Teacher's explanation

Imagine you are trying to soak up water from a sponge. If you use a flat, smooth piece of plastic, you can only touch a tiny bit of water at a time. But if you use a sponge with millions of tiny, long fingers, you can grab much more water much faster. This is exactly what a root hair cell does for a plant.

📖 Definitions

🔍 Worked example — Easy

A root hair cell has a surface area of 200 mum^2. After growing a long extension, its surface area increases to 1200 mum^2. Calculate the percentage increase in surface area.

🔍 Worked example — Standard

Describe how the structure of a root hair cell is adapted to its function of absorbing water and minerals.

🔍 Worked example — Exam-style

A plant is placed in a soil solution with a higher water potential than the cell sap. Explain the movement of water into the root hair cell.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Structure of xylem vessels

how xylem structure fits its function. A xylem vessel is made of many cells stacked end to end. As each cell matures:

🧑‍🏫 Teacher's explanation

Imagine a building's plumbing system. If you wanted to move water from the basement to the roof efficiently, you wouldn't want a bunch of small, cluttered rooms with furniture and doors blocking the way. You would want one long, smooth, reinforced pipe.

📖 Definitions

🔍 Worked example — Easy

Identify the two main functions of the xylem in a plant.

🔍 Worked example — Standard

Explain how the absence of cytoplasm and organelles in a xylem vessel aids its function.

🔍 Worked example — Exam-style

A student observes a cross-section of a stem under a microscope. They note that the xylem vessels have very thick, dark-stained walls compared to the surrounding parenchyma cells. Explain how this structural feature relates to the function of the xylem.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Question. The diagram shows a transverse section through the stem of a young sunflower. (a) Name the tissue labelled X, on the inside of each vascular bundle, and state its two functions. [3] (b) Describe how the arrangement of xylem differs in the root. [2] (c) Explain how two features of a xylem vessel suit it to transporting water. [4]

Ch 1 · Xylem and Phloem · O Level Biology 5090

All chapters in Transport in Plants