Passive & Active Absorption of Water in Plants

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  • 0:04 Absorption in Plants
  • 0:32 Active Absorption
  • 2:25 Passive Absorption
  • 3:16 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

All living organisms need some amount of water. In this lesson, we'll look at both passive and active absorption of water in plants to see how they take place and how water moves once inside the cells.

Absorption in Plants

When your throat is very dry and you feel dehydrated, you'll most likely reach for a bottle or a glass of water. Just like you need water to feel and function at your best, plants need water to grow and thrive.

But unlike you, plants can't just grab a bottle of water, so what do they do? Plants absorb water from their environment, particularly the soil in which they're rooted. They accomplish this through two types of absorption: active and passive absorption.

Active Absorption

Any time you hear the word 'active' you know that it's something that requires energy and effort. The same holds true in the case of active absorption, which is the absorption of water through the activity of a plant's root hairs.

Root hairs are thin, hair-like structures that extend from the roots of the plant. The root hairs increase the surface area through which the plant can take up water. For example, they're like having many instead of a few straws.

In a high water concentration, the root hairs will absorb water through osmosis. Osmosis is the movement of water from an area of high concentration to an area of low concentration. When the concentration of water in the soil is higher than the concentration of water in the roots, the water will move into the root hairs. This action does not require a lot of energy.

On the other hand, there are times when the concentration of water in the soil is lower than the concentration of water in the roots. To overcome this obstacle, the root hairs use energy in the form of adenosine triphosphate, or ATP, to create a concentration gradient that will allow the water to move into the roots.

The root hairs create this gradient through endocytosis, the sodium/potassium pump, and exocytosis in order to move the molecules of solutes into the cells of the root hairs. This causes a shift in the concentration gradient to a point where the concentration of water is higher in the soil than it is in the plants, allowing the water to flow into the cells of the root hairs.

Once the root hairs have done their part, the water has to make its way into the plant. This happens through the symplast pathway, which is the movement of water from cell to cell through the plasmodesmata of the cell.

Recall that the plasmodesmata is the network of cytoplasm in plant cells. Water enters the cell sap and then flows into the cell, moving from one cell to the next until it reaches the root. From there, it can make its way to the rest of the plant.

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