Transporting Solutes & Water in Plants

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  • 0:00 Transporting Water in Plants
  • 1:18 Xylem Transportation
  • 3:10 Phloem
  • 6:05 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How do plants move water and nutrients where they need to go? That's the question we'll be exploring in this lesson as we look at both xylem and phloem transportation systems and see how plants stay hydrated and energized.

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.

Xylem Transportation

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.

Phloem

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.

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