The driving force behind water movement in plants is evaporation through the leaves, which acts like a magnet pulling water up the plant’s plumbing system. However, because water is evaporating from a living surface, it is called transpiration. The problem is that plants want to hold onto their water and not let it all out through transpiration. Therefore plants are constantly struggling to hang on to their water.
Before we get into the juicy details of water movement in plants, let’s back up and review how molecules move in general. Molecules are in constant motion, and through diffusion spread out evenly to take up whatever space is available to them. Of course water molecules diffuse too, but in living things water often diffuses across a membrane. This process is called osmosis. Water moves in and out of cells through osmosis, and always moves from regions of high concentration to regions of low concentration. Here’s the tricky part though: water concentration is different than solute concentration, which refers to the stuff dissolved in the water, and keeping those two concepts straight is necessary to figure out where water will go inside a plant.
Water is a polar molecule, and forms hydrogen bonds between the positively charged hydrogen atoms and the negatively charged oxygen atoms. Hydrogen bonds make water molecules stick together, a process known as cohesion. When water molecules form hydrogen bonds with other molecules, such as carbohydrates, it is called adhesion. The hydrogen bonds have tension between them, so water molecules stick together and move together. When water is pulled out through a leaf at the top of a plant via transpiration, the rest of the water molecules in the xylem are under tension and are pulled up the plant stem (or tree trunk), like water moving up a straw. The fancy name for this is the cohesion-tension theory. It describes the way water moves through the xylem using cohesion (the water molecules stick to each other) and tension (because transpiration is drawing water out of the leaves).
To discuss the way water moves in plants, we need a new term: water potential. Water potential is the property of water that determines which way it will flow, which depends on the concentration of solutes in the water and the pressure being exerted on the water. Another way to think about this is the water’s capacity to move or do work. Water potential is represented by the Greek letter psi, which looks like this: Ψ. It is pronounced like "sigh" and it looks like King Triton’s trident in The Little Mermaid. Perhaps King Triton was secretly a plant biologist and was measuring the water potential of seagrass.
We can actually measure the water potential of plants indirectly, by measuring the tension of water in a stem. We call this the pressure potential.
All water potentials are compared with the water potential of pure water, which is zero. Two things affect water potential: solute concentration and pressure. In symbols, that looks like this:
Ψ = Ψs + Ψp
In words, the above equation means the two components of water potential are the solute potential and the pressure potential. Solute potential reflects how much stuff is in the water. Any water with stuff dissolved in it (a solution) has a negative solute potential (Ψs), since solutes bind with water molecules and lessen their ability to move and do work.
Pressure potential is the amount of force being exerted on a solution. In living cells, this pressure comes from the contents of the cell pushing against the cell wall. This is positive Ψp. The cell wall pushes back, causing turgor pressure. Turgor pressure causes plant parts to be firm and erect.
When a plant cell loses turgor pressure, the solution the cell is in is hypertonic and the cell is plasmolyzed, which means water leaves the cell and the cell membrane shrinks. If water enters the cell at the same rate it leaves the cell, the cell is flaccid (and the solution is isotonic). When water enters the cell and pushes on the cell wall, the cell is turgid (and the solution is hypotonic). See figure below.
Individual cells can gain or lose water, but what does this look like at the whole plant level? When a plant loses water, turgor pressure decreases and the plant wilts. If pressure potential is negative, water is under tension; this is often the case for water in non-living cells like tracheids and vessel elements in the xylem.
Water travels along a gradient of high water potential to low water potential. Assume the water potential of the atmosphere is -20 and the water potential of a leaf is -2. If this is the case, where does water flow? Water will flow out of the leaf and into the atmosphere (thanks to evaporation). Remember that even though 20 is greater than 2, -20 is less than -2.
Stomata are essential to plants, since they take up gas that is used in photosynthesis. But since they are passageways into the plant’s insides, plants have to be able to control the opening and closing of the stomata. Plants only open their stomata when they need to, and politely close them when they don’t. They weren’t raised in a barn. Plants can lose a lot of water via evaporation through the stomata, and open stomata also provide pathogens with a means for entering the plant.
Two cells border each stoma (which is just a tiny hole in the leaf). These cells are called guard cells. Guard cells use turgor pressure to regulate the opening of stomata. When the plant wants to open its stomata, the guard cells take up ions (mostly K+ and Cl-) and sugars through ion channels and pumps. Because the solute concentration is now high inside the guard cells, water moves in and the cells expand. This expansion causes the guard cells to expand and puff out, opening the pore. To close a stoma, guard cells pump ions and sugars out of the cell, and water leaves too, resulting in a limp guard cell and a closed stoma.
Together, the guard cells look like a pair of lips. When the guard cells have water and salts in them and the stoma is open, the guard cells are big and puffy like Angelina Jolie’s lips. When they are closed, the guard cells are much more similar to Jennifer Aniston’s lips.
We know guard cells regulate stomata by moving things in and out of their cells, but the exact conditions for when plants open and close their stomata are not well understood. In most plants, stomata are open during the day; they want to allow gas in and out while photosynthesis is going on.
As with any trend there are renegades who want to go out and do things differently. The plants that shake things up with regard to stomata and photosynthesis are called CAM plants, because they have Crassulacean Acid Metabolism. This is just a different way of doing photosynthesis. Most CAM plants are desert plants, and they open their stomata at nighttime, which is when they fix carbon dioxide. Don’t worry too much about these weirdos now—just know that they’re different and might be worth learning about another time.
Now we know how water moves up a plant—but how does water move between cells? Just as security officials monitor what goes through airports, cells restrict what goes in and out of their cells. Each cell has pores in its membrane, called plasmodesmata. The path through the plasmodesmata is called the symplast; if particles travel through the symplast they have gained access to the cell’s insides and the entire plant body.
In contrast to the symplast, some particles aren’t allowed into the inner membrane pathway and instead move through the apoplast, which is a path through cell walls and intracellular spaces. These two pathways regulate the substances that are allowed to travel through the plant. A layer of tissue called the endodermis surrounds the vascular tissue and doesn’t let things in. Because of the endodermis, only particles that move through the symplast get to travel through the plant because they have access via the plasmodesmata.
The pathways through the plant look like this:
The process of moving sugars through the phloem is called translocation. Phloem moves sugars from the places they are made (the leaves) to various non-photosynthetic parts of the plant. Since a leaf is the site of photosynthesis, it is called a sugar source. Storage organs such as roots can also be sugar sources if they are releasing sugars, such as after the winter. Phloem makes its deliveries to sugar sinks, which are places that don’t make sugar. Phloem moves in multiple directions; this is different than the direction of xylem movement, which moves water up the plant body.
The way sugar gets into the phloem and around the plant is similar to trucks delivering products from a factory. Sieve tube elements are the trucks that transport sugar, and they line up end to end like an everlasting traffic jam. The trucks (sieve elements) load sugars at the sugar factory’s loading dock (photosynthetic leaves) in a process called phloem loading. During phloem loading, solutes are actively transported into cells using a H+/ATP pump.
After sugars have been loaded, water moves into these cells through osmosis. The flow of water causes pressure to build up, forcing sieve elements to move. When the sugars arrive at their destination (a place where sugar concentration is low), the trucks unload their cargo. Phloem unloading occurs when water flows out of the sieve elements and carries the sugar with it.
Since sugars are being concentrated and making water flow through osmosis, there has to be fancy name for the movement of phloem. It is called the Pressure-flow Hypothesis. This just means that at the sugar sources (the leaves), sugar is found in high densities, which causes water to flow into cells through osmosis. Then the phloem moves to sugar sinks through turgor pressure. And there it is: pressure causes the phloem to flow, and we have the pressure-flow hypothesis.
The water makes its way back into the xylem and can be used again in the plant. The sugars are either used or stored for later; in the summer, sugars move from leaves to storage organs such as roots; in the winter, sugars move in the opposite direction as roots deplete their stored reserves to support new growth.
A plant can’t live on water and sugar alone. Plants also depend on nutrients that they can’t make themselves, so they have to get them from the soil. The main nutrients a plant needs are nitrogen, phosphorus and potassium. These are called macronutrients because plants need large quantities of them to be healthy. A few other macronutrients are calcium, magnesium and sulfur.
Some nutrients are essential to plant life, but plants don’t need very much of them. These are called micronutrients, because plants only need small quantities of them. Micronutrients include boron, copper, iron, chloride, manganese, molybdenum and zinc. Consult your chemistry textbook if you want to know more about these individual elements.
Nutrients have to be transported through the vascular tissue too. Roots take in nutrients from the soil and then inorganic molecules move up the plant through the xylem. Phloem takes care of the organic molecules. Nutrients are delivered to where they are needed in the plant, such as new leaves or branches.
Tiny insects called aphids, also known as plant lice, feed on phloem sap. They puncture a hole in the phloem and feed on sugars. Some ants "farm" aphids, collecting the secreted sugar (or "honeydew") from the aphid. Check out the BBC’s Wild Nature clip of ants farming aphids.