Transport in Plants

Chapter Outline
Absorption of Water and Mineral from Roots
Transport in Xylem Sap

Objectives

Describe the role of root hairs and mycorrhizae in water and mineral absorption.
Describe how water and minerals are transported from the roots up to the leaves.

Key Words - You should be able to define and identify the definitions of the terms listed below.

root hairs
mycorrhizae
xylem
root pressure
transpiration-cohesion-tension mechanism

Notes

Absorption of Water and Mineral from Roots

Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and mineral absorption

Root hairs increase the surface area available for the absorption of water and minerals
Mycorrhizae are the symbiotic structures consisting of the plant's roots and fungal threads (hyphae). The hyphae absorb water and certain minerals which can be transported to the roots. Click here to see a picture of this.
After absorption of water and minerals into the root, they enter the xylem, the system of tubes in a vascular plant which transport the xylem sap up to the leaves.

Transport of Xylem Sap

There are two main mechanisms which xylem sap is transported up the xylem: (1) root pressure, and (2) the transpiration-cohesion-tension mechanism.

Root Pressure – Pushing Xylem Sap

Water entering the root cells through the process of osmosis contributes some pressure involved in transporting water and minerals up the xylem but in most plants this mechanism does not contribute very much to the overall transport up the xylem. In fact, in some trees, there is not root pressure at all.

The Transpiration-Cohesion-Tension Mechanism: Pulling Xylem Sap

Transpirational Pull

Before we can understand the process of transpiration, we must first learn about the structure of a leaf. Click here for a good explanation of the structure of leaves.
- The surface of a leaf is covered with small openings called stomata.
- Click here to see a picture of stomata.
- These openings are found mainly on the lower epidermis of a leaf.
- Traveling through the interior of a leaf are the veins, which are vascular bundles. You'll recall that one component of the vascular bundles is the xylem, the tissue that transports water and minerals up to the leaves.
- Between the lower and upper epidermis of a leaf are cells called mesophyll. There are two layers of mesophyll – one that is close to the upper surface of the leaf (palisade mesophyll) and a layer that is near the bottom of the leaf – the spongy mesophyll. As implied by the name, the cells of the spongy mesophyll are relatively far apart from each other; in other words, there is a lot of intercellular space. Click here to see this. This intercellular space contains air that is saturated with water. This water entered the space by exiting from the xylem of a leaf.
- Water enters the intercellular space of the spongy mesophyll and diffuses through the stomata and to the air outside of the leaf. The movement of water out of leaf is called transpiration. Refer to Figure 36.8, page 704.
- For an examination of the various factors that affect the rate of transpiration, click here.

Cohesion and Adhesion of Water

Cohesion refers to the attractive force that water molecules have for one another. This force is due to hydrogen bonding between the water molecules.

Summary

The transpiration-cohesion theory states that when water molecules evaporate from open stomata, the next water molecules in the column (xylem) tend to move up and replace them, and in turn, pull up the water molecules below them.



		Chapter 36 Transport in Plants Chapter Outline Absorption of Water and Mineral from Roots Transport in Xylem Sap Objectives Describe the role of root hairs and mycorrhizae in water and mineral absorption. Describe how water and minerals are transported from the roots up to the leaves. Key Words - You should be able to define and identify the definitions of the terms listed below. root hairs mycorrhizae xylem root pressure transpiration-cohesion-tension mechanism Notes Absorption of Water and Mineral from Roots Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and mineral absorption Root hairs increase the surface area available for the absorption of water and minerals Mycorrhizae are the symbiotic structures consisting of the plant's roots and fungal threads (hyphae). The hyphae absorb water and certain minerals which can be transported to the roots. Click here to see a picture of this. After absorption of water and minerals into the root, they enter the xylem, the system of tubes in a vascular plant which transport the xylem sap up to the leaves. Transport of Xylem Sap There are two main mechanisms which xylem sap is transported up the xylem: (1) root pressure, and (2) the transpiration-cohesion-tension mechanism. Root Pressure – Pushing Xylem Sap Water entering the root cells through the process of osmosis contributes some pressure involved in transporting water and minerals up the xylem but in most plants this mechanism does not contribute very much to the overall transport up the xylem. In fact, in some trees, there is not root pressure at all. The Transpiration-Cohesion-Tension Mechanism: Pulling Xylem Sap Transpirational Pull Before we can understand the process of transpiration, we must first learn about the structure of a leaf. Click here for a good explanation of the structure of leaves. The surface of a leaf is covered with small openings called stomata. Click here to see a picture of stomata. These openings are found mainly on the lower epidermis of a leaf. Traveling through the interior of a leaf are the veins, which are vascular bundles. You'll recall that one component of the vascular bundles is the xylem, the tissue that transports water and minerals up to the leaves. Between the lower and upper epidermis of a leaf are cells called mesophyll. There are two layers of mesophyll – one that is close to the upper surface of the leaf (palisade mesophyll) and a layer that is near the bottom of the leaf – the spongy mesophyll. As implied by the name, the cells of the spongy mesophyll are relatively far apart from each other; in other words, there is a lot of intercellular space. Click here to see this. This intercellular space contains air that is saturated with water. This water entered the space by exiting from the xylem of a leaf. Water enters the intercellular space of the spongy mesophyll and diffuses through the stomata and to the air outside of the leaf. The movement of water out of leaf is called transpiration. Refer to Figure 36.8, page 704. For an examination of the various factors that affect the rate of transpiration, click here. Cohesion and Adhesion of Water Cohesion refers to the attractive force that water molecules have for one another. This force is due to hydrogen bonding between the water molecules. Summary The transpiration-cohesion theory states that when water molecules evaporate from open stomata, the next water molecules in the column (xylem) tend to move up and replace them, and in turn, pull up the water molecules below them.