A level Biology exam revision resources written by A level Examiners


Plants don't have a circulatory system like mammals. They actually have two transport systems, which move substances from one part of the plant to another. These two systems are named after the cells that they are made from. You should be familiar with them from GCSE. They are the xylem and the phloem transport pathways. At AS and A level it is important to know the mechanisms by which substances are transported by these systems. This article explores these mechanisms.   Exam Advice

mainly water and mineral salts
evaporation driven transpiration

mainly organic molecules e.g. sucrose and hormones
hydrostatic pressure

Root and Stem Structure

Figure 1 : Root cross section


The central area of the root is called the stele, both xylem and phloem tissues are located here. Sometimes we refer to clusters of xylem and phloem cells as vascular bundles. The majority of the root is the cortex - consisting of parenchyma cells, these tend to be larger than most other plant cells, and can be involved in storage. The outer layer of root cells makes up the epidermis - this is where root hair cells are found.

Figure 2 : Root hair cell

The advantage to the plant of having root hair cells in the epidermis is that they hugely increase the surface area of the root for the uptake of substances from the soil. They are responsible for the uptake of water (by osmosis) and mineral ions (mainly by active transport) from soil into the root.

Figure 3 : The Stele

Figure 3 shows the central portion of the root. The circular band of cells around the stele is the endodermal layer (endodermis). This is where cells with suberin in their cell walls form the Casparian strip. The Casparian strip is a waterproof layer which blocks the passage of substances travelling through the cell wall. The xylem vessels are found in a star shaped formation in the centre of the stele, with phloem vessels found in between the points of the stars.

Stem Structure

Figure 4 : Stem cross section


Figure 4 shows how the xylem and phloem are arranged in vascular bundles. These vascular bundles are located around the edge of the stem. The centre of the stem (or trunk) is called the pith. In the vascular bundles, the phloem tissue is closer to the outside of the stem, and this is why in ringing experiments where the bark is peeled off the movement of water through xylem is unaffected while the movement of sugars through phloem is stopped.

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