12.5 Transport In Vascular Plants

Background Information

Figure 1 Overview of transport routes in plants
Figure 1 Overview of transport routes in plants


Figure 1 shows an overview of the process involved in the transportation of substances in a plant. Water in the environment is actually a solution containing dissolved nutrients which can be absorbed by the plant. Nutrients from the water are transported strictly in the xylem while sugars are transported only in the phloem.
  • Roots absorb water and minerals through their plasma membrane and transports them up as xylem sap (blue)
  • Leaves take in CO2 and expel O2 (red) and H20 through the stomata during photosynthesis (green)
  • Xylem sap is transported upward using a root pressure and capillary action
  • Sugars produced by photosynthesis are transported as phloem sap throughout the plant

Transport of Water and Nutrients

In plants moving water and nutrients from the roots to the leaves in a complex process. Water and nutrients are transported through 3 stages: (a) from the soil into the roots, (b) roots to the stem, and (c) from the stem to the leaves. Water used by the plants is mostly returned into the environment.

a) Transport into Roots

Water and nutrients are transported into the roots through two different processes. Water enters the root cells through osmosis while nutrients enter by active transport.

Figure 2 root pressure process in roots
Figure 2 root pressure process in roots


Figure 2 shows the root pressure process of absorbing water and nutrients:
  • The cytoplasm of plant cells have a lower concentration of water molecules then the soil water, and the cell membrane let the molecules cross freely
  • The concentration of nutrients in the cytoplasm of a plant cell is higher then that of soil water and therefore must enter through active transport
  • Once the nutrients/water are within the cytoplasm of the outer root cells, it proceeds through the cells of the cortex towards the endodermis (this is made easier due to the fact that adjacent plant cells have interconnected strands of cytoplasm)
  • Once nutrients/water enter the root cell they do not have to cross another cell membrane until they reach the vascular cylinder
  • The casparian strip is made of a wax-like substance which prevents all substances from passing through the spaces between the endodurmal cells and therefore must pass through the endodurmal cells.
  • The Casparian strip's key role is to prevent back flow back into the cortex
  • Once inside the vascular cylinder, the nutrients are pumped across cell membranes into the xylem




b) Transport into the Stem

Once water molecules and dissolved nutrients have crossed the Casparian Strip they form a liquid called xylem sap. Substances in the the xylem sap move up the roots towards the stem. As more nutrients are pumped into the xylem, their concentration increases. This creates root pressure in plants to help move the xylem sap upward. capillary action, which is the tendency of a liquid in a narrow tube to rise or fall, also helps the xylem sap move. Figure 3 shows an example of capillary action.
Figure 3 Capillary Action Example
Figure 3 Capillary Action Example


  • In capillary action, a column of liquid is held together by attractive forces between molecules and rises because of the attractive forces between the liquid and the sides of the tube.(figure 3)
  • The xylem tube is similar but the attraction forces occur between water molecules and molecules in the cell wall which results in the water molecules of the xylem sap sticking together and being drawn up the sides of the xylem tube


Xylem sap also moves from one xylem tube to another, through the pits in the cell walls of the xylem cell. It can also move outside the xylem tube through the pits into surrounding tissue. This way all cells in the plants body receive water and nutrients.

c) Transport into the Leaves

Capillary action and root pressure alone cannot pump the xylem sap all the way from the roots to the leaves. The main force that transports the xylem sap around a plant is transpiration.
  • The epidermis of leaves contain many stomata (pores formed by guard cells) which release water vapour during transpiration
  • due to the attractive force of water molecules, when a water molecule moves up the xylem column it pulls a neighbouring water molecule with it, then second molecule then pulls the one behind it and so forth
  • This pulling force continues down the xylem and if a plant does not transpire the water column wont move


If a plant does not have sufficient water, it may wilt. A plant cell stores water and dissolved nutrients are stored in its central vacuole. When it is full it exerts pressure against the cell wall, this is also known as turgor which helps support the plant.
The following video is an overview of plant structure and goes into other details pertaining to the transport of water and nutrients








Did You Know...?

  1. The xylem is made up of dead cells while the phloem is made up of living cells
  2. Vascular plants date back to 350 million years ago
  3. Lycopods dominated the landscape 400 million years ago but descendants can be found today

Glossary


Active Transport: substances that are transported across a membrane from an area of low concentration to an area of high concentration. (Always requires energy)
Capillary action: The tendency of a liquid to rise or fall because of the attractive forces between the liquid molecules
Casparian Strip: the wax-like strip that runs through the cell wall of an endodermal cell
Osmosis: the diffusion of water molecules across a selectively permeable membrane, from an area of higher concentration to an area of lower concentration
Root Pressure: The osmotic force pushing xylem sap upward in root vascular tissue
Transpiration: Evaporation of water through the stomata of plant leaves
Turgor: pressure caused by the fluid contents of the central vacuole,which pushes against the wall of a plant cell
Vascular Cylinder: The central portion of a root that contains the xylem and phloem


Mini-Quiz

  1. What does the xylem & phloem transport?
  2. What 3 things contribute to xylem sap getting to the top of tall trees?
  3. What methods are used when transporting nutrients into the roots?
  4. What is between the cortex and vascular cylinder of a plant and what does it do?


Mini-Quiz (Answers)
  1. Xylem transports nutrients while the phloem transports sugars.
  2. 3 contributing factors are: Root Pressure, Capillary Action, and Transpiration
  3. Water is absorbed through osmosis while nutrients enter using active transport
  4. The endodurmus and casparian strip lie between the vascular cylinder and cortex. key role of the casparian strip is to prevent back flow