Water moves across membranes because of differences in water potential. Water will move from areas of higher water potential to areas of lower water potential. Water potential is a measure of potential energy between a given sample of water and pure water. There are four factors that impact water potential: solute potential; pressure potential (turgor in plants); gravity potential; and matric potential. Each of these factors either increases or decreases the potential energy of a sample of water in comparison to another sample of water.
You can think of water molecules as having a given amount of energy. Considering a sample of water with a solute dissolved within, the water molecules of that sample have less potential energy overall because they are interacting with the solute. This is the basics of the effect of solute concentration on water potential. Given two samples of water, one with a solute and the other pure water, the pure water will have more potential energy. This is assuming all other factors are the same. Now assume that you have these two samples separated by a barrier that is permeable to water and not the solute. There will be a net movement of water molecules from the side with pure water because of its relative greater potential energy. You can do a simple demonstration in the class room with two sticks of celery. Place one in a cup with pure water and the second in a cup with salt water. After some time the stalk in pure water will be rigid and the one in salt water will be flaccid. This happens because of water potential. The stick in fresh water was immersed in an environment with a higher water potential than its cells. The result is that cells take in water to their capacity, now full there is pressure exerted on their cell walls and the entire structure is rigid. In contrast the stalk in salt water was immersed in an environment with lower water potential than its cells. In this case water moved out of the cells and into the environment leaving the structure as a whole more flexible.
Pressure as it pertains to water potential is a more straight forward concept to understand. If two identical samples of water are separated by a semipermeable the sample with greater pressure will have more potential and there will be a net movement of water towards the side with less pressure. In plants the pressure of interest is turgor pressure. Turgor is developed within a plant cell through the control of ion concentrations and the central vacuole. This is unique to plant cells as they have rigid cell walls that which can withstand pressure. For example, the central vacuole can be expanded with fluid thus taking up more space in the space within the cell. Since the cell wall is ridged, space is finite, thus pressure can generated that impedes the flow of water into the cell. Imagine you and some friends are trying to get into a room that is 10’ x 10’. There is a balloon with a 10’ diameter in the center of the room. If that balloon is fully inflated it will severely limit how many people fit in the room and if it is deflated it will not have such a big impact.
Gravity is a factor when it comes to water potential but is often of no consequence as the interface between two fluid containing bodies is just that, an interface, thus they are exposed to the same gravitational forces by default. Of note though, is the fact that trees that are transporting water hundreds of feet from the ground and therefore the effect of gravity cannot be overlooked. That being said, we must leave it as just a thing of note when considering that the water potential within the root system must be less than that of the surrounding soil. A tree of any significant height must have a strategy to compensate for the increase of water potential caused by the elevation of the water within the vessels and trachea of the xylem. The gravitational pull on this continuous column of water from root to tree top will increase the energy potential.
Matric potential is similar to solute potential, but in this case the water molecules are interacting with surfaces. This interaction is adhesion. Plant cells have walls made of cellulose, a material that is highly hydrophilic, as in water has and affinity for it. Cellulose fibers are arranged in a matrix, thus the designation of a matric potential. The impact of this is important because it results in a lower water potential within the plant when compared to the environment, assuming other factors are equivalent to each other. When a plant is fully saturated its matric potential is zero. This is because all cellulose is associated with water. In contrast if the plant is not saturated, the matric potential creates significant negative water potential for the plant, encouraging water to be drawn into the root fibers. Matric potential is an important factor for water uptake by roots from soil. Soils can have a high matric potential as well because of the composition and shapes of soil particles. Particles of soil are irregular and full of tiny crevices that increase their surface area. Water acquisition by plants must overcome this attractive force between water and soils. This will be discussed in further along in this unit as surface tension in the xylem of the plant stem is addressed.
Roots: Water Absorption
The roots of a plant terminate in thin tubular extensions of root epidermal cells. These are referred to as root hairs. These hair-like extensions are the water collecting structures of the plant. They increase the surface area of the roots enormously, allowing for efficient water collection from the soil. In addition to these plant structures most plants have a mutualistic association with fungi that aid in water absorption. These fungal associations are called mycorrhizae and they further increase the surface area available for water collection with their fibrous hair like extensions. Ultimately water moves from soil to root because of water potential. A successful plant maintains water potential lower than that of the soil it is anchored in, thus facilitating the flow of water from soil to plant via osmosis. This is driven by evaporative loss of water at the leaves, the terminus of the plant’s water column.
Stems: Water Transport
The stems of a plant have tubular cells referred to as xylem. Xylem is responsible for the vertical movement of water up a plant. There are many features of xylem that allow it to defy gravity and many other physical properties. The method employed is to utilize the surface tension of water to maintain a negative pressure environment. Recall that water will flow from areas of high pressure to areas of low pressure due to water potential. How this is achieved within the plant stem I cannot fully explain but I will share the main ideas as well as adaptations that plants have evolved to combat forces that would compromise the goal of moving water.
Tension is the force that moves water up through a plant to the leaves. This tension is the result of cohesive and adhesive forces of water. The end product is a pressure differential between the column of water in the xylem of a plant and the surrounding environment. The water is essentially in a negative pressure state. When a stomate is opened the water escapes as vapor and replacement water is pulled up from the root system at the base of the plant because the water column within the plant is in low pressure, contrast to the higher pressure at the root system. The issue is that gas bubbles, or embolisms, can easily form when water is in low pressure. Particularly when gas has dissolved in water of higher pressure than it is later subjected to, think of a can of soda being opened. The column of water within the xylem may be at a pressure equivalent to -20 atmospheres.
Gas bubbles, or embolisms, do not regularly form though in the column of water, gases stay in solution because of the surface tension of water and the lack of any sights for nucleation. The low pressure of the water column cannot support a gas bubble because the surface tension of water is a greater force. The walls of the xylem vessels are sufficiently smooth as to not support sites of nucleation or cavitation events. There are no areas for a bubble to be shielded from the surface tension forces of the fluid. A nucleation site can also occur at any microscopic area that is hydrophobic, giving a gas a sort of starting point. With the lack of any area to start a bubble, bubbles do not regularly form in the column of water within a plant. In fact bubbles do not normally form without a nucleation site. Consider that water can be super-heated and then explosively boil, well after it has reached the boiling temperature. This can occur in the home with a brand new pot or piece of glassware. The new material has the potential to be so smooth that there are no nucleation sites for a bubble to form. Often the mistake is that once a utensil is immersed in the super-heated water it instantly boils as the utensil has sites for nucleation.
The instance of embolism formation under regular circumstances is limited by the negative pressure and lack of nucleation sites, but it can happen. There are also structures that prevent a gas bubble from expanding and migrating should one form in the first place. There are perforation plates that separate xylem vessels vertically. Between a two sections of xylem vessels there are these barriers that all fluid must pass through to reach the next higher section of the column. These perforation plates limit the growth and propagation of embolisms by forcing the fluid through small holes in the perforation plate. These serve to decrease the size of any possible embolism and as the size of the bubble is decreased its ability to expand decreases as the effect of surface tension on the bubble is compounded. The smaller the bubble the more surface tension force there is on it because of its decreased surface area to volume ratio, thus the less likely it is to expand and grow.
There are also passageways between adjacent xylem vessels, these allow for the flow of water to continue upwards should a vessel be interrupted by damage or embolism. These are again minute passage ways that limit the ability for an embolism to pass through thus isolating the embolism in one vessel or vessel section.
Lastly, the interface between the column of water and the outside environment, at the terminus of the column in the leaf, must be addressed. Surely this could be an area with rapid evaporation as the water in the column is at such low pressure that it is essentially ready to boil out as vapor. The fact is though that the end of the column is not an opening equal in diameter to the tube itself. The end is layered with a matrix of cell wall material, creating an almost felt-like structure. Within this structure there are countless interfaces between water in the column and air. Each opening is so small though that the surface tension is strong enough to hold the water molecules in the liquid state, to a point. Picture how water forms a meniscus in a glass graduated cylinder, this curvature is formed by the attraction of water to the class and the surface tension. You can imagine that these tiny curves exist at each tiny interface as water has a high affinity for the cellulose in the cell wall. There exists a massive pressure difference between the water at these interfaces, because of small size, and the atmosphere. The difference is so great that it causes the water to curve in significantly. Water is of course lost or passed on to the environment from these points, but at a somewhat controlled rate. When the stomates of a plant open, the air from inside the leaf, containing water vapor is passed out as CO
is passed in.
This brings us to the end of the transport of water up through a tree, drawn in through the roots, transported vertically through xylem, and released through the stomatal openings. These openings are controlled by guard cells. The stomates are often large enough to be visible on the leaf. They are typically located on the underside of the leaf so as not to be subjected to direct sunlight when opening. They are moveable structure that are controlled hormonally, by environmental stimuli (light), atmospheric CO
concentration, humidity, and abiotic and biotic factors. The mechanisms are not fully understood, but the action is clear, opening allows for the influx of CO
to drive primary production and results in a loss of water as transpiration.
To sum things up, water movement up and through a tree has not been mimicked by any man-made invention. Trees achieve this feet without any direct energy input, relying solely on the properties of water. There are variations among plants, with some moving more water faster than others, as plants have evolved, filling ecological niches.