Chris has a master's degree in history and teaches at the University of Northern Colorado.
Transporting Water in Plants
This is a plant. Now, I'm no botanist, but I'm pretty sure I know how to water it. You pour water in the pot. Ta-da! The goal is to get the plant to drink up the water through the roots, but the entire plant needs water right, not just the roots? So, how do we get water from the roots, to the rest of the plant?
The transport of water and solutes - the molecules dissolved within water, including minerals from the soil and sucrose made from photosynthesis - is an important part of how a plant survives.
While there are many processes at work here, the basic idea is osmosis, the diffusion of a solution across a semi-permeable membrane. That's how water gets from one cell into the next, and it happens because nature likes things to be balanced, to be equal. So, if the amount of solutes in the water on one side of the membrane is different than the amount on the other side, water will diffuse through the membrane from low concentration to high concentration until both sides are equal. That's a very basic look at what's going on, but let's get a bit closer and see what really happens when you water your plants.
So, let's start with how water gets from the ground into the rest of the plant. Water in the soil is absorbed by root cells through osmosis across the cell membrane. From there, the water enters into the xylem, a system of plant tissue cells designed to transport water upwards. When we say the tissues of plants, we mean the cells that give it structure, strength, and shape, like what our muscles do for us. But what does this look like? The most easily recognizable xylem is probably the wood within trees.
The xylem sap is a water-based solution with very few solutes, and it's one of the ways that water is transported through the plant. The low-concentration sap of the xylem diffuses into cells containing lots of solutes, like leaf cells. This is important, since leaves have to keep pores open to receive air and therefore lose most of their water to evaporation. The water in the xylem sap keeps these cells hydrated.
This is, however, a tricky process. After all, moving water through the xylem, up the trunk, and into leaves means fighting against gravity. How does the plant do this? It's a combination of two main factors.
First is tension created from evaporation of water in leaves which creates a drop in pressure. As we know, water of high pressure always moves naturally into areas of low pressure. The other factor is cohesion, or the natural tendency of water molecules to stick to each other and other things. The xylem structure encourages this and contains a matrix for water to cling to. The combination of tension and cohesion to move water against gravity through the xylem is called the cohesion-tension theory.
So the xylem is how water gets from the roots throughout the rest of the plant. But that water is low in solutes, which means it's low in nutrients.
The majority of the nutrients used by a plant are generated by photosynthesis at the exact opposite end of the plant from the roots. So at the same time that water is being transported up the xylem, we also have water distributing the nutrients from the leaves through the phloem. This is another set of transportation tissue cells, but instead of only moving water in one direction like the xylem does, the phloem moves water all over.
The phloem sap is also quite different than that of the xylem. It is full of solutes such as sucrose created by photosynthesis. Most sap only contains some sucrose, but if you've ever had maple syrup, then you know that some trees can go a little overboard on the amount of sugar they send through the phloem. The entire process of transporting organic molecules throughout the plant is called translocation.
So how does this work? Let's start at the sugar source, the location where sugar molecules are created by photosynthesis. Leaves are sugar sources. Special transportation molecules take these molecules and actively transport them into the phloem. Once in the phloem, they will pass through specialized phloem cells called sieve-tube elements. Sieve-tube elements have pores to transport solutions, but very little else. Most sieve-tube elements don't even have nuclei; they don't want anything to get in the way of transporting the sap. These sieve-tube elements transport the sap from sugar sources to sugar sinks, the areas of the plant that need those sugars, like roots or fruits.
Now, while xylem flow is driven by tension and cohesion, flow through the phloem is a result of hydrostatic pressure. Basically, as sugars are loaded into the sieve-tube elements, this creates an imbalance between sap in the xylem, which has few solutes, and the sap in the phloem, which has many solutes. Due to this imbalance, water from the xylem diffuses into the sieve-tube elements, resulting in an increase in pressure. So, now there is higher pressure at the top of the phloem than the bottom, and this pressure pushes the sap down the plant.
Once that sap reaches a sugar sink, molecules in those cells actively pull the sugars out of the phloem and use them to power the cell. So, sugar sources make sugars and then actively transport them into sieve-tube elements, creating a pressure increase that pushes sap down the plant towards the sinks where sugars are removed from the phloem. And you thought watering plants was as simple as pouring water in the pot.
Plants take in water and minerals at the roots, but photosynthesize organic molecules in the leaves, and all of this needs to be transported around. There are two main systems to do this.
The first is the xylem, a system of tissue cells that bring water from the roots upwards through the plant. The xylem is powered by the balance of tension - the pressure imbalance created by evaporation in the leaves - and cohesion - the tendency of water to cling to other molecules. Xylem sap tends to have few solutes.
The other transport pathway is through the phloem, a system of tissue cells that distribute solute-rich water throughout the plant. The process of transporting water and solutes through the phloem, called translocation, is driven by hydrostatic pressure. As solutes are moved into sieve-tube elements, the imbalance in water concentration makes water diffuse into the phloem. This creates a higher pressure at the top of the phloem than the bottom, and the sap is pushed down the plant.
There's a lot that goes into this, but thankfully the plant does it all on its own. All we have to do is provide it sunlight, air, and plenty of water.
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