International A Level Biology / Plant structure and function

Xylem, phloem and sclerenchyma

Edexcel International A Level Biology (YBI11), chapter 10: Plant structure and function, specification point 4.5, 4.4. Free lesson.

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Xylem, phloem and sclerenchyma have different structures and functions, and their positions in a stem vascular bundle follow a set order. Knowing how their cell walls are built helps you explain their physical properties and distinguish these from their functions.

Where each tissue sits in a stem

A transverse section is a slice cut across a stem. Vascular bundles sit in a ring near the stem’s outside, which helps the stem resist bending. Within each bundle, the tissues have a reliable order from the outside towards the centre. From the outside in: sclerenchyma, phloem, then xylem. Sclerenchyma fibres form a cap on the bundle’s outer edge. Phloem lies just inside this cap, and xylem is on the inner side, facing the stem’s centre.

How the three tissues are built

A longitudinal section is cut along the stem, showing cells lengthways. Sclerenchyma fibres and xylem vessels are dead when mature and have lignified walls. Phloem sieve tube elements are living, with cellulose walls and no nucleus. Sclerenchyma fibres are long, narrow cells with tapering, overlapping ends and an empty space called a lumen. Xylem vessels form when cells join end to end and their end walls break down, making one continuous hollow tube. Lignin often forms rings or spirals in the walls. Unlignified gaps called pits let water move sideways between vessels. Phloem…

How their structures link to their functions

Comparing the tissues point by point helps keep their differences clear. Sclerenchyma provides support, while xylem and phloem transport different substances. Sclerenchyma fibres and xylem vessels are dead and lignified; sieve tube elements are living but have no nucleus and are not lignified. Sclerenchyma fibres have tapered, overlapping ends; xylem vessels have no end walls; sieve tubes have sieve plates. Pits occur in sclerenchyma and xylem, while plasmodesmata link sieve tube elements to companion cells. Sclerenchyma provides support. Xylem carries water and mineral ions and also…

Why the walls are strong, rigid and waterproof

The physical properties of xylem and sclerenchyma walls come from the arrangement of cellulose and lignin. These properties are not the same as the tissues’ functions. Cellulose chains are held together by hydrogen bonds in bundles called microfibrils. This gives the wall high tensile strength, meaning it is hard to pull apart. Microfibrils in each wall layer run at a different angle from those in the layer before. This makes the wall strong when pulled in different directions. Lignin is added as secondary thickening around the microfibrils. It makes the wall rigid, so it resists…

Use tissue position to identify what is stained

Water containing red dye travels through xylem. In a transverse section of the stem, the stained tissue is therefore found on the inner side of each vascular bundle.

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