Transport in plants (pt 2)
Monacia Williams, Contributor

Well, how are you this week? I hope that you are good and that you have been keeping up with the lessons as we go along. Last week, we began our study on transport in plants as we looked at the structure of the tubes that are responsible for the movement of substances. This week, we continue our study as we look at the arrangement of the tubes in the different areas of the plant, such as the root, the stem and the leaf.

You will need to remember that the tubes for conducting water and minerals are the xylem and those that transport organic materials are the phloem. These tubes are found together in tissues called vascular bundles. The arrangement of the bundles in the root, the stem and the leaves are shown below. You will need to know the arrangement in all three areas because, in the examinations, you might be asked to identify and label diagrams of each type. You will also need to be able to identify xylem and phloem tissue within a vascular bundle.

In the root, the vascular tissue is found at the centre; in the stem, the vascular tissue is arranged in a ring nearer to the outside and in the leaf, they form the mid-rib and the lateral veins.

How does water move through the plant?

Water moves through the plant by going through the following stages:

• Entering into the roots
• Moving across the cortex of the root
• Entering into the xylem
• Moving up through the xylem
• Moving into the intercellular air spaces
• Evaporating from the leaves

Let us now look at the stages.

Entry into the root and movement across the xylem

Many of the root cells have root hairs on them. The root hairs increase the surface area of the root, making them very suitable for absorption. They extend in-between the soil particles which are covered by a film of water, the soil water. Can you make a guess as to how water enters the root? Did you guess by osmosis? If you did, you are correct! Can you now say what governs this movement by osmosis? Did you say concentration/osmotic gradient? You did? Good for you! You are again correct! Water moves by osmosis wherever a concentration/osmotic gradient exists. This means that the concentration of the water molecules in the soil is greater than that in the root epidermal cell. As this water enters the cell, its water concentration now becomes greater than that of the cell next to it, so another gradient is created. This continues until the water reaches the xylem.

Movement of water up the xylem

The movement of water up the xylem depends on two properties of water: that of adhesion and cohesion. Adhesion is the attraction of water for the walls of the vessel while cohesion is the attraction of water molecules for each other.

Water moves up the xylem by three means:

• Capillarity

This depends on the two properties of water as well as the narrowness of the xylem tube. However, the height to which capillarity moves the water is not high enough to satisfy the requirements of tall trees.

• Root pressure

Water that is absorbed by the roots creates a pressure in the root cells. The pressure in the leaves is less than in the roots, so a pressure gradient is created. This causes water to move away from the roots in order to create equilibrium.

• Transpiration pull

As the water molecules evaporate, their cohesive properties cause them to pull other water molecules from the leaves. This creates a continuous stream of water from the root to the leaves.

Movement into the intercellular spaces and evaporation from the leaves

As water moves from the spongy mesophyll cell into the intercellular air spaces, other concentration osmotic gradients are created between the mesophyll cells. The creation of the gradients continues to the xylem, causing water to move from the xylem into the mesophyll cells. The intercellular spaces become filled with water vapour and create gradients between the spaces and the stomata. Water vapour will now move from these spaces through the stomata by evaporation. This evaporation of water is known as transpiration. The diagram below shows the movement of water from the leaves to the external environment.

Internal structure of a leaf

Next week, we will look at transpiration and the factors affecting this process in the plant. Have a good week and remember to study these diagrams.

Monacia Williams teaches at Glenmuir High School. Send questions and comments to kerry-ann.hepburn@gleanerjm.com