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.
What does Osmosis in Plant Tissues Mean?
How does Osmosis Happen in Plants?
Examples of Osmosis in Plants
Importance of Osmosis in Plants
Investigating Osmosis in Plant Cells
What does Osmosis in Plant Tissues Mean?
As any cell with a semipermeable membrane, plant Cells can undergo osmosis, i.e. the exchange of water from a solution with low concentration to one with a high concentration of solutes.
Plant cells contain rigid cell walls made of cellulose. This structure helps with maintaining a characteristic called turgidity - this describes the cellular state of being firm and upright with water. This is highly important in stabilizing plant tissue and preventing wilting. Turgidity is only achieved when plant cells are placed in a hypotonic solution as water diffuses into the cell via osmosis. As Water Molecules enter the cell, they exert pressure against the cell membrane which is then forced to press against the cell wall. This is called turgor pressure or hydrostatic pressure. The plant cell will appear swollen and firm under these conditions.
When placed in a hypertonic solution, plant cells will undergo plasmolysis. This is the process by which water leaves the cell via osmosis, causing the cytoplasm to shrink away from the cell wall. The plant cell's appearance is said to be flaccid.
When placed in an isotonic solution, plant cells are also said to be flaccid. There is no net movement of Water Molecules, thus the cells are neither turgid nor have undergone plasmolysis.
As a result of osmosis, plant cells perform best in hypotonic environments due to their turgidity.
Fig. 1 - Structure of plant cells in different solution types
Hypotonic solution - a solution with low solute concentration.
Isotonic solution - a solution with the same solute concentration as that one of the solution at the other side of a membrane.
Hypertonic solution - a solution with a high solute concentration.
When talking about osmosis, we can use the definition above -the movement of water from an area of high concentration to an area of low concentration through a semi-permeable membrane- 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 the definitions of osmosis 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.
Fig. 2. Types of solutions according to tonicity and the direction of water flow through a semipermeable membrane.
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!
How does Osmosis Happen 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.
Plant Root hair cells
Plant Root hair cells are specialized cells found on the roots of plants. They are responsible for nutrient and water absorption. Plant root hair cells are adapted to maximise the absorption of water and ions. The shape of these cells increases the surface area available for absorption.
The uptake of water in plant root hair cells relies on osmosis. The cytoplasm and vacuole of plant root hair cells contain many dissolved solutes, meaning it has a lower water potential than the soil. Due to this water potential gradient, water molecules move into the plant root hair cell from the soil. This water potential gradient is maintained as the water moves into neighbouring cells, down a water potential gradient through each cell's semipermeable membrane.
Even though osmosis is a passive process, the root hair cells are 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, plant root hair cells 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.
Fig. 3. Active transport diagram. Note that the molecules being transported have a higher concentration on the side of the membrane to which they are being transported, hence the need for energy (ATP).
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.
Examples of Osmosis in Plants
Let's go into more detail regarding hyper-, iso- and hypotonic solutions. 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.
Fig. 4. The osmotic gradient around a plant cell can condition its shape and health, as water is essential to plants.
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.
Investigating Osmosis in Plant Cells
We have looked at how osmosis works so we can now perform an experiment to investigate the process. We are going to look at how we can work out the water potential inside potato cells using dilutions of sucrose solution.
For this experiment, you'll need to know what isotonic solutions are because this is what we are going to calculate! If you haven't already, read the section titled 'what is tonicity?' and then check back here.
A potato
A cork borer
Set of weighing scales
Distilled water
Sucrose solution
Boiling tubes
Water bath
Using the cork borer, cut 6 uniform potato pieces of similar sizes and surface area. Use a paper towel to dry each piece.
Weight each potato piece.
Make serial dilutions from a 1M sucrose solution using distilled water. Use concentrations of 1.0, 0.8, 0.6, 0.4, 0.2 and 0.0M.
Add 10cm³ of each sucrose concentration solution to a boiling tube and label each tube.
Add a potato piece to each boiling tube, making sure each piece is fully immersed in the sucrose solution.
Place the boiling tubes in a water bath and leave for 20 minutes.
After 20 minutes, extract the potato pieces from each boiling tube. Use paper towels to blot the excess solution from each potato piece.
Weight each potato piece.
Calculate the percentage change in mass for each potato piece and its corresponding sucrose solution.
The percentage change in mass will tell you which potato pieces have gained or lost water as a result of osmosis. From the data recorded, we can create a calibration curve to work out the water potential of the potato pieces. Calibration curves are used to determine an unknown concentration by comparing the unknown with known standard concentrations (standard curves).
To create your calibration curve, follow these steps:
The plot at which the line of best fit crosses your X-axis (X-intercept) indicates the water potential of your potato pieces. At this point, there is no change in mass because the sucrose concentration is considered isotonic to the water potential inside the potato pieces.
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.
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