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Osmosis in Plants

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Osmosis in Plants

There are many different ways that molecules can move from one place to another. One of these is osmosis. This is the movement of water from an area of high concentration to an area of low concentration through a semi-permeable membrane. Let's have a look at how and why osmosis occurs in plants.

Osmosis in Plants Definition

Osmosis is the movement of water from an area of high concentration to an area of low concentration. Concentration in this context refers to how much water is in each area; essentially, water is moving from a place where it is abundant to a place where there isn't much water. Osmosis happens through a semi-permeable membrane. If the membrane were not permeable, the water molecules would not be able to move from one area to another.

When talking about osmosis, we can use the definition above or another more complex one. Rather than discussing areas of high and low concentration, we can speak of water potential. Pure water has a water potential of 0, and every other solution has a negative water potential. The more negative the solution's water potential, the less water it has.

Osmosis is a type of diffusion. We use the term osmosis when talking specifically about water, whilst diffusion is used when talking about particles.

Let's look at pure water compared to a glass of lemonade. Lemonade has water in it but also has many different ions, flavourings and carbonic acid to make it fizzy. Pure water has a water potential of zero, so let's say that lemonade has a water potential of -100 (this is a theoretical number used to show us that it has more negative water potential than water). Water will move from the solution with a less negative water potential to a solution with a more negative water potential. In this case, water would move from the pure water (less negative, 0) to the lemonade (more negative, -100).

Both of these definitions are accurate; however, the definition which includes water potential is more accurate when discussing osmosis and the movement of water; and concentration is more accurately used when talking about active transport and diffusion.

Osmosis is a passive process, which means that it does not require energy. Osmosis does not require energy because we move molecules from an area of high concentration to an area of low concentration - we move with/down the concentration gradient. Energy is required when transporting molecules when we move against the concentration gradient.

The movement of water in plants from an area of less negative water potential to an area of more negative water potential is essential for plants. This helps the plants absorb moisture from the soil into the root and allows the transport of water up the plant via various pathways. Water is needed in the plant to support the stem and cool it down at the leaves, among other processes. None of these vital processes would be possible without osmosis!

Process of Osmosis in Plants

Let's talk through the process of osmosis in plants at the different areas of the plant where osmosis occurs. Osmosis occurs all over the plant, so let's start at the bottom.

At the root of the plant, water is absorbed from the soil into the plant. The soil has a less negative water potential than the plant, so water moves from an area of less negative water potential in the soil to an area of more negative water potential in the root of the plant cell.

Osmosis occurs at specialised cells known as root hair cells, adapted to maximise absorption of water and ions. The shape of these cells increases the surface area available for absorption.

Osmosis in plants, at the root hair cells and in other areas is closely associated with active transport.

Active transport is the movement of molecules from an area of low concentration to an area of high concentration. This movement is against the concentration gradient, so it requires energy.

When the water has moved from the soil into the plant's root, the concentration gradient flattens out at the root hair cell. With a flat concentration gradient, there is no difference in water potential on the inside of the plant compared to the surrounding soil. With this lack of a concentration gradient, water no longer moves in either direction. The plant would no longer have a supply of water.

In order to prevent this from happening, we need to pair up active transport with osmosis. Ions are constantly pumped from an area of low concentration in the soil to an area of high concentration in the root of the plant. Pumping these ions into the plant's root decreases the root's water potential. Eventually, the water potential of the plant becomes negative enough to create a concentration gradient which allows for osmosis to occur again.

Any time you see the phrase 'pumping' or 'active', assume the process being referred to requires energy and likely moves substances against the concentration gradient.

The xylem is the vessel that carries water up the plants. Getting water into the xylem is similar to the process described above at the root. Ions are first pumped into the xylem vessel to create a steep concentration gradient. Then, once a steep concentration gradient is established, water can move from the endodermis into the xylem.

The movement of water from one plant cell to another through different pathways also happens through osmosis. The xylem makes up part of the vascular bundle, with the rest of the vascular bundle composed of the phloem. The phloem transports sucrose and other solutes up and down the plant using a two-way movement and requires energy. The xylem transports water and ions up the plant passively.

Osmosis in Plants Examples

Let's cover another example of osmosis in plant cells. The movement of water in and out of plant cells when we place them in hypertonic or hypotonic solutions is an example of osmosis.

Hypertonic solutions are more concentrated than plant cells. This means that the surrounding solution has a more negative water potential than the plant.

Hypotonic solutions are less concentrated than plant cells. These solutions have less negative water potential than the plant.

Isotonic solutions are as concentrated as the plant cell. It has around the same water potential.

Let's start with placing a plant cell in a hypertonic solution. When this occurs, water will move from an area of less negative water potential in the plant cell to an area of more negative water potential in the surrounding cell by osmosis. Water leaving the plant causes a change in the shape of the plant, causing it to be flaccid. If the plant continues to lose water by osmosis, it will become plasmoylsed. This level of water loss is irreversible and will cause severe damage to the plant because, as we mentioned above, the plant requires water for support and multiple other processes. We will explore the importance of osmosis in plants in the section below!

Suppose we place a plant cell in a hypotonic solution. In that case, water will move from an area of less negative water potential in the surrounding solution to an area of more negative water potential in the plant cell. As water moves into the plant cell via osmosis, the plant cell becomes turgid. This provides the plant cell with support. However, if too much water were to enter, the plant cell could burst - this is known as cell lysis. The presence of a cell wall in plant cells helps to prevent this from happening.

Animal cells are much better at controlling the amount of water that enters and leaves the cell and does not require a cell wall in the same way that plant cells do.

If we place the plant cell in an isotonic solution, there will be no net movement of water in or out of the plant cell. The solution surrounding the plant cell will have a similar water potential, so there will be no concentration gradient for water to move down via osmosis.

Importance of Osmosis in Plants

Osmosis happens in many different areas of the plant. Water is essential for plants, and osmosis is how water is transported around plants. Let's recap the important uses of water in the plant;

  • Turgor is the process of keeping the plant supported. Plant cells become turgid when they are filled with water. This may happen when we place plants into pure water, as the pure water will move from the solution into the plant cell by osmosis. If we put plant cells into a very concentrated solution (with a very negative water potential), water would move out of the plant and into the surrounding solution. This would lead to the plant becoming plasmolysed.

  • Photosynthesis - can you remember its equation? Water is a reactant in the process of photosynthesis, meaning that it is used up in the reaction. Without water, photosynthesis could not happen, and the plant would eventually die. Osmosis helps transport these water molecules from one area to another, allowing photosynthesis at the leaf's upper surface even though water enters from the root of the plant.

    Check out our articles on photosynthesis if you need to recap the process.

  • Water is a transport medium - lots of ions and minerals such as magnesium and calcium are transported in water through the plant. Without osmosis moving water from one area of the plant to another, these nutrients would not be available throughout the plant, and the plant would eventually die. Water also acts as a transport medium in humans, as our blood is made mostly of water!

    You can look at our articles on nutrient deficiencies in plants for more information on how these affect the plant!

  • Transpiration helps to keep the plant cool. Transpiration is the evaporation of water from the surface of leaf cells and the subsequent diffusion out of the plant. This evaporation of water helps to keep plants cool. This is very similar to the process of sweating in humans. Sweating is the loss of water through the skin and the evaporation of this water that then cools our body temperature.

Osmosis in Plant Tissues - Key takeaways

  • Osmosis is the movement of water from an area of less negative water potential to an area of more negative water potential through a semi-permeable membrane.
  • Osmosis is paired alongside active transport all over the plant to help transport water up the plant.
  • Water is needed in the plant for support, photosynthesis and as a transport medium.

Frequently Asked Questions about Osmosis in Plants

Movement of water from the soil into the root hair cells is an example of osmosis in plants.

Osmosis in plants is the movement of water from an area of the plant that has a less negative water potential to an area of the plant that has a more negative water potential.

Plants use osmosis in many different areas of the plant, in order to ensure water and ions are transported around the plant.

Osmosis occurs all over the plant, two specific areas are between the soil and root hair cell, and between the endodermis and vascular bundle.

Osmosis can affect plant cells by making them turgid, plasmolysed or flaccid depending on whether water enters or leaves the plant.

Final Osmosis in Plants Quiz

Question

What is osmosis?

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Answer

Osmosis is the movement of water from an area of high concentration to an area of low concentration through a semi-permeable membrane.

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What is the water potential of pure water?

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Zero

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What is diffusion?


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Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

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What is the definition of osmosis related to water potential?

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Osmosis is the movement of water from an area of less negative water potential to an area of more negative water potential through a semi-permeable membrane.

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​Is osmosis active or passive?


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Answer

Osmosis is a passive process.

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Does osmosis move with or against the concentration gradient?

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Osmosis moves with the concentration gradient.

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​Where in the plant is water absorbed?


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Answer

Water is absorbed by the plant at the root.

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Which cells are present in the root which help with absorption?

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Answer

Root hair cells.

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What are the uses of water in the plant?


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Photosynthesis, support and cooling the plant as well as transport of ions are all uses.

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What is transpiration?

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Answer

Transpiration is the evaporation of water from the surface of leaf cells and the subsequent diffusion out of the plant.

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What happens if you put a plant cell in a concentrated solution?


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Answer

Water leaves the plant cell via osmosis, causing the plant cell to become flaccid. 

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What happens if a plant cell gains too much water?

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If a plant cell gains too much water, it becomes turgid and could then eventually burst - cell lysis.

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What is a hypertonic solution?


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Answer

A hypertonic solution is a very concentrated solution.

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What is a hypotonic solution?

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Answer

A hypotonic solution is a less concentrated solution.

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Question

Which process is companied alongside osmosis at the root hair cells?


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Answer

Active transport.

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