Biology (0610) | Xylem and phloem

Welcome to the Plant Plumbing System!

Have you ever wondered how a giant tree gets water all the way up to its highest leaves, or how the food made in those leaves reaches the roots underground? That’s the job of the plant’s amazing internal transport system!
In this chapter, we will explore the two types of transport vessels in plants: Xylem and Phloem. Think of them as the plant's essential highways and plumbing pipes. Understanding these structures is crucial for mastering Transport in Plants!

1. Introduction to Vascular Bundles (The Highways)

Plants rely on special tissues bundled together to move substances around. These bundles are called Vascular Bundles, and they contain both Xylem and Phloem vessels.

Core Concept: Functions of Xylem and Phloem (8.1 Core)

These two vessels have very distinct roles. It's vital to remember what each one carries:

• Xylem: Transports water and mineral ions (from roots upwards). It also provides support to the plant.
• Phloem: Transports sucrose (sugar, the main product of photosynthesis) and amino acids (the building blocks of protein).

Core Concept: Position of Xylem and Phloem (8.1 Core)

The position of the Xylem and Phloem within the vascular bundle changes depending on whether you look at a root, a stem, or a leaf (in non-woody dicotyledonous plants).

• In the Root: The Xylem is usually found in the centre, often shaped like a star, providing great support for the plant against pulling forces. The Phloem is located between the arms of the xylem star.
• In the Stem: The vascular bundles are typically arranged in a ring. The Xylem is found towards the inside of the stem, and the Phloem is found towards the outside.
• In the Leaf: Xylem and Phloem are together in the vascular bundles (the "veins" of the leaf). The Xylem is usually on the side facing the upper epidermis (top), and the Phloem is on the side facing the lower epidermis (bottom).

Key Takeaway: Xylem moves water up and Phloem moves food around. Their specific arrangement in the root and stem provides structure and efficiency.

2. Xylem Vessels: The Water Pipes

Xylem Function (Core)

The primary job of the Xylem is to move water and dissolved minerals from the roots, all the way up to the leaves, using a process called the transpiration pull. They also provide crucial physical support to keep the plant standing upright.

Structure of Xylem Vessels (8.1 Supplement/Extended)

The structure of the xylem vessel is perfectly adapted for its function as a water pipeline.

Imagine a very efficient, durable, continuous pipe system made for high-pressure movement.

• Thick Walls with Lignin: The walls of the xylem vessels are thickened by a tough, waterproof substance called Lignin. This prevents the vessels from collapsing inwards due to the pressure changes during water transport, and provides the necessary support for the plant structure. (You do not need to know the details of how lignification happens).
• No Cell Contents: Xylem vessels are actually dead cells. They have no cell contents (no cytoplasm, no nucleus) when mature.
  Why is this important? If there were cell contents, they would block the water flow! Being hollow makes water movement extremely easy.
• Long Continuous Tube: The cells are joined end to end, but the cross walls (the walls separating individual cells) have disappeared or been broken down.
  This forms a single, long, uninterrupted tube from the root to the leaf, allowing water to flow freely under tension.

Quick Review Box: Xylem Adaptation

• Hollow tube: Efficient flow.
• Lignified walls: Strength and support, prevents collapse.
• Continuous: No cross walls to impede flow.

3. Phloem Vessels: The Food Delivery Network

Phloem Function (Core)

The Phloem transports manufactured food, mainly in the form of sucrose (a type of sugar) and amino acids. These substances are transported both up and down the plant.

Process: Translocation (8.4 Supplement/Extended)

The movement of sucrose and amino acids within the phloem is called Translocation.

• Unlike water movement in the xylem (which is passive, driven largely by evaporation), translocation requires energy (active transport) and is usually driven by pressure differences.

Source and Sink (8.4 Supplement/Extended)

For translocation to happen, you need a starting point (Source) and an end point (Sink).

a) Sources

Sources are parts of the plant that produce or release sucrose or amino acids.

• The main sources are the leaves, where sugar is made during photosynthesis.

b) Sinks

Sinks are parts of the plant that use or store sucrose or amino acids.

• Examples of sinks include:
  • Growing points: Buds, flowers, fruits, or new leaves (for growth).
  • Storage organs: Roots, stems, or underground tubers (like potatoes, for storage).
  • Roots: Need energy for respiration and active transport of mineral ions.

The Changing Roles of Sources and Sinks (8.4 Supplement/Extended)

Don't worry if this seems tricky at first, but a key idea is that the role of a plant part can change with the seasons or the stage of the plant's life.

A plant part may act as a source and a sink at different times.

Example: A potato tuber.

• Summer (Growing Season): The mature leaves are the sources, making sugar. The potato tuber (underground) is the sink, storing starch.
• Spring (Early Growth): The stored starch in the tuber is converted back into sucrose and transported up to feed the new, developing shoots. In this case, the tuber is temporarily acting as the source until the new leaves are large enough to photosynthesise effectively.

Key Takeaway: Phloem transports food (sucrose and amino acids) via translocation from areas where they are made or stored (Sources) to areas where they are needed (Sinks).

4. Comparison: Xylem vs. Phloem Summary

Here is a quick comparison of the main features of these two vital transport tissues:

Comparison Table

Feature	Xylem	Phloem
Substances Transported	Water and mineral ions.	Sucrose and amino acids.
Direction of Flow	Upwards only (from roots).	Up and down (from source to sink).
Cell Status (Extended)	Dead cells.	Living cells (companion cells help them function).
Walls (Extended)	Thick, lignified walls.	Thin walls (not lignified).

Keep practicing identifying these structures in diagrams of roots, stems, and leaves! Good luck with your studies!