Movement in Plants

Just like the circulatory system moves materials throughout your body, plants also rely on a system to transport materials. Remember that xylem and phloem are responsible for moving material within a plant, but without a main pump, how is it that plants can move materials against gravity to heights that surpass the tallest human?

Some of the tallest trees in the world are located in south western British Columbia. The giant Sequoia can grow between 50 - 85 metres high. That is the about the length of a football field. Nevertheless, water molecules travel from the roots to the tip of the tree every moment.

Xylem Xylem vessels are very long, narrow, and hollow tubes that do not contain any living material. They are connected end to end and stretch all the way from the roots to the leaves. Xylem cells only transport water and minerals that have been absorbed by the root hairs. Water and dissolved minerals enter the root hairs by osmosis. Recall that osmosis is the movement of water from an area of high concentration to an area of low concentration. The illustration shows the process of osmosis across a cell membrane.

For water to enter the root hairs by osmosis there must always be a lower concentration of water within the root of the plant. The only way that this can happen is if the water is continuously being pulled out of the root or if there is a higher concentration of ions with the root. Each scenario mentioned lowers the concentration of water within the root. The water enters the xylem through pits in the vessels called tracheids.

Root Pressure Plant roots can build up pressure and the pressure forces the water to move upwards. The increase in pressure can be caused by active transport of water into the xylem or active transport of ions to create a concentration gradient.

Capillary Action Capillary action is based on a specific property of water called adhesion. Water has the ability to cling to water molecules (cohesion) or to cling to other molecules (adhesion). Imagine you have a straw and you place it in a glass of water. The water molecules move up the straw because of the attraction between the water and the straw. 

However, water molecules are also attracted to each other, so as one water molecule moves closer to the straw molecule (adhesion), a second water molecule moves closer to the original water molecule (cohesion). The xylem cells are made up of tiny thin tubes. The smaller the diameter of the xylem cells, the higher the water will climb.

Cohesion Tension As was mentioned with capillary action, water molecules have an affinity to each other and that is what helps them move up through the xylem. Remember the specialized structure in the leaf that allows carbon dioxide gas to enter the plant? The stoma is also the spot where water molecules can escape. Therefore, as the stoma opens to allow carbon dioxide gas in, water molecules move out. As one molecule leaves, it pulls other water molecules up.

Water leaves a plant in a process called transpiration. Transpiration allows a plant to control its temperature through evaporative cooling. Excessive water loss through transpiration causes plants to wilt and may also lead to plants dying. An increase in transpiration increases the movement of water through the xylem.

Transpiration Animation http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841

Practice Questions How do the following weather conditions affect the rate of transpiration? Wind Light Temperature Humidity

Answers Wind: Windy conditions increase the rate of transpiration because the wind reduces the effects of a special layer on the leaf that slows the rate of transpiration. Light: The stoma open during the day when sunlight is available to drive the process of photosynthesis. Carbon dioxide enters through the stoma for photosynthesis; however, water can also leave through this opening if the temperature is hot. Temperature: Warmer air will increase transpiration and cooler air slows down the process of transpiration. Humidity: High humidity decreases the amount of water loss through transpiration. Dry air increases the rate of transpiration.

Practice Questions Explain what is happening in the diagram below.

Answers Water has two important properties: cohesion and adhesion. Cohesion is the attraction of water molecules to each other and adhesion is the attraction of water molecules to other molecules. This image shows the cohesive property of water. The water droplets stick together and form a droplet on the leaf because of its higher affinity to water droplets than to the surface of the leaf.

Homework Take another look at the animation on transpiration: http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841 Click on “transpiration” under where it says lesson media objects. You could also take a look at the associated readings in the lesson outline on the left!