Define Physiology: Cohesion & Adhesion Theory?
Transport in Plants : Transport of water, minerals and nutrients within vascular plants is dramatically different from animals such as humans. Whereas humans have a closed circulatory system powered by a pump - the heart- plants with open vessel systems must rely on a passive system to move materials around. This is a considerable task, given that some giant redwood trees like Sequoia sempirvirens must lift water to leaves in excess of 110 meters (360 feet) above the ground (See left. Note the size of the people standing next to the tree). While the transport of water and minerals can be explained, not all of the transport processes of photosynthetic food products are well understood.
Water and minerals are thought to be transported up to the leaves by two processes: root pressure and transpirational pull. Transpirational pull is covered in this lesson.
Transpirational Pull : is a term used to describe the net movement of water from the leaves into the atmosphere by evaporation as water vapor. It has been estimated that over 90% of the water entering the plant through the root system is given off through the process of transpiration. Water that is absorbed by the root hairs forms a continuous unbroken column that travels from the roots to the xylem in the stem, through the branches and twigs, into the leaves through veins and veinlets and into the mesophyll cells in the leaves. Upon reaching the mesophyll, water evaporates into the air chambers where the spongy parenchyma is loosely packed.
Recall that the epidermal layers of leaves have a waxy cuticle to minimize water loss from evaporation, requiring pores in the epidermal layer of leaves. These pores, called stomata, are controlled by guard cells, and it is through the stomata that water vapor enters the atmosphere.
Cohesion : Water being lost to the atmosphere creates a negative pull on the continuous column of water in the xylem because of cohesive forces between water molecules. Cohesion refers to the phenomenon of water molecules "sticking" to each other. Because water is a polar (See "Basics") molecule, having a positive pole and a negative pole, the oppositely charged ends of adjacent water molecules are attracted to each other, all of the water molecules can be thought of as bonded together as one gigantic continuous molecule of water.
Since the column of water extends all the way down the vascular system into the roots and root hairs, the water in the soil must continually be available for replenishment as transpiration occurs. If not, the plant will "wilt." Water is needed by plant cells to perform the following: to act as the medium in which chemical reactions occur, water supplies electrons needed in photosynthesis, water is a good solvent, water provides support in the central vacuole of plant cells, and water is used as the transportation medium for minerals and nutrients.
Surface tension : Another way of understanding the cohesive forces that exist between water molecules is to think about the many different examples that we can see around us. The droplets of water that hang off the end of a faucet or the cold mirror in a steamy bathroom; water strider insects that "walk" on water without falling through the pond's surface, or even the trick of floating a paper clip on the surface of a glass of water, and the way you can fill a glass with water slightly over the brim without water spilling over all demonstrate the cohesive property of water. In all of these examples, water molecules "cling" together without breaking the hydrogen bonds between them. This phenomenon of having a continuous unbroken liquid surface is referred to as "surface tension"
Adhesion : Besides cohesion, water has another property that enables it to be lifted by plants to such great heights off the ground. Water also has the property of adhesion. Adhesion refers to the ability of water molecules to adhere, or stick to another substance unlike itself (water to glass). This differs from cohesion, in which substances are attracted to a like substance (For example: water to water.) Water adheres to the surface of interior walls of xylem vessels much like water sticks to the inside surfaces of a drinking glass. You can observe adhesion of water to the inside of a glass of water or the meniscus (See image below) of a pipette or graduated cylinder. Look at where the edges of the water slope up the sides.
Therefore, if water sticks to the edge, it will pull other water molecules bonded to them as well, forming a continuous volume of liquid. Gravity acts against the water creeping further up the sides. However, water in tube with a very small diameter will creep up the inside of the tube by capillary action. Capillarity can be observed in tiny tubes such as those found in blood testing labs used to pick up blood samples from a drop. Xylem vessels and tracheids in plants are even smaller, resulting in the ability to lift water within to higher levels.
So to summarize, scientists believe that the combination of root pressure, adhesion, cohesion, and transpirational pull all work together to move water from the soil through the plant and into the atmosphere. The movement of water supplies the plant with water for chemical reactions, including photosynthesis and physical support and growth, and it also transports minerals and nutrients to the various plant organs.