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Plant Water Relations: Plasmolysis and Imbibition, Practice problems and FAQs

Plant Water Relations: Plasmolysis and Imbibition, Practice problems and FAQs

You must be aware that some people maintain beautiful gardens at their homes and put in a lot of effort into it. Adding fertiliser or manure to the soil is an integral part of gardening and is known to promote the growth of plants. But did you know that adding excessive fertiliser to the soil can actually kill the plants? Even farmers are taught to limit the use of excessive fertilisers in their crop lands as this might adversely affect the crop yield. Why do you think this happens? Why is the excessive use of fertilisers in the soil, harmful for the plant? We will discuss the scientific reason behind this phenomenon in this article.

Now let us take another example of a wooden door. Have you noticed how wooden doors jam during rainy seasons? Do you know the scientific explanation for this phenomenon? Come let us have a look into it.

List of contents:

  • Types of solution
  • Plasmolysis
  • Imbibition
  • Practice problems
  • FAQs

Types of solutions

We know that water is essential for the movement of materials inside the plant body. Transport of water within the plant can be across short distances or across long distances. Short distance transport of water involves the exchange of water between the cell and the environment or the movement of water from one cell to the other. This is mainly achieved by the process of osmosis, diffusion and imbibition.

Diffusion is the movement of molecules from their region of higher concentration to their region of lower concentration unless an equilibrium is achieved between the two regions. When water diffuses from its region of higher concentration to its region of lower concentration, across a semipermeable membrane, the process is known as osmosis. Imbibition is a surface phenomenon in which hydrophilic cell walls of the plant cells adsorb the water molecules on their surface.

The water movement by osmosis in plant cells depends on the type of solution surrounding the cells. There are three types of solutions - isotonic solution, hypotonic solution and hypertonic solution. How are these solutions classified? Let's check it out.

Isotonic solution

Just like the name indicates, it is the solution having the same solute concentration as that of the cell sap. Because ‘Iso’ = same and ‘tonic’ = concentration of the solution. If the external solution balances the osmotic pressure of the cytoplasm, then it is said to be isotonic. Supposedly, if we keep a cell (or tissue) in an isotonic solution, then there will be no net flow of water towards the inside or outside. Thus, no net osmosis occurs when a cell is placed in an isotonic solution. When the amount of water going in and out of the cell is in equilibrium, the condition of the cell is said to be flaccid.

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Fig: No net movement of water occurs when cell is placed in isotonic solution

Hypotonic solution

Here ‘Hypo’ means less than and ‘tonic’ means the concentration of the solution. Hence a hypotonic solution has a relatively lower solute concentration than that of the cell sap. The water potential in the solution is higher when compared to the water potential inside the cell. So, the water will move from the outside to the inside of the cell, i.e., endosmosis will occur.

Endosmosis leads to gradual swelling of the cell and the pressure of the cell contents on the cell membrane (in animal cells) or cell wall (in plant cells) also increases. This pressure is said to be the turgor pressure or pressure potential and the condition of the cell, when it is fully swollen and can no longer take in water by endosmosis, is said to be turgid.

Hypotonic solution acts differently on animal cells and plant cells. Due to endosmosis, animal cells like RBC burst or get lysed due to the increased pressure on the cell membrane. But in a plant cell, the endosmosis will make the protoplast exert pressure on the cell wall, making the cell turgid or swollen.

Fig: Effect of hypotonic solution on animal cells and plant cells

Hypertonic solution

In the term hypertonic, ‘Hyper’ = more and ‘tonic’= concentration of the solution. A hypertonic solution has a relatively higher solute concentration than that of the cell sap. The water potential of the cell sap is higher than the water potential outside the cell. So, the water moves out of the cell by the process of exosmosis and the cell shrinks. Due to the exosmosis, animal cells shrink while in plant cells, a phenomenon named plasmolysis is observed.

Fig: Effect of hypertonic solution on animal cells and plant cells

Plasmolysis

Placing a plant cell in a hypertonic solution results in loss of water from the cell by exosmosis. This causes the protoplasm of the cell to shrink and draw away from the cell wall. This phenomenon is known as plasmolysis and the cell is said to be plasmolysed. The space between the cell wall and the shrunken protoplast is occupied by the hypertonic solution.

Fig: Plasmolysis

Fruits and vegetables are preserved by pickling them and adding excess salt in the medium which makes it hypertonic. The hypertonic solution helps in the plasmolysis of bacterial and fungal cells and prevents their growth, thereby preserving the food items.

Similarly, excessive addition of fertilisers to the soil can make the soil hypertonic and result in plasmolysis of the root hair cells. This will eventually lead to the death of the plant.

Fig: Pickles

De-plasmolysis

The process of plasmolysis is reversible. Reverse plasmolysis is called de-plasmolysis. If a plasmolysed plant cell is placed in a hypotonic solution which has higher concentration of water than the cell sap, the water from the solution flows into the cell by endosmosis and the protoplasm starts unshrinking. Gradually, due to entry of water, the protoplast starts exerting turgor pressure on the cell wall but the rigidity of the cell wall prevents the plant cells from rupturing.

Fig: De-plasmolysis

DPD and plasmolysis

Pure water has maximum diffusion pressure due to high kinetic energy. Addition of solute particles to water lowers its diffusion pressure. The lowering of the diffusion pressure of water in a solution over its pure state is known as diffusion pressure deficit (DPD). Basically, it is another term for water potential, but is always positive in magnitude.

When concentration of solutes increases in a cell, the water potential (measure of concentration and free energy of water in a system per unit volume) decreases, the cell tries to make up for it by taking in more water. So, Obviously OP (osmotic pressure) for the cell will increase. The cell will continue taking up more water until the cell is turgid again.

Hence it is clear that DPD is dependent on the osmotic pressure and turgor pressure. So if we want to calculate DPD, the equation is:

DPD = Osmotic pressure (OP) - Turgor pressure (TP)

In a turgid state,

OP = TP

DPD = OP - TP

DPD = 0

In a plasmolysed state,

TP = 0

DPD = OP - TP

DPD = OP - 0

DPD = OP

Imbibition

Imbibition is the absorption of water by hydrophilic colloids which causes them to increase in volume. It is a special kind of diffusion as water travels from higher to lower concentration. In the process of imbibition, the water potential gradient and the affinity between the adsorbent and the liquid are essential requirements. Imbibition of water by seeds helps them to germinate and emerge out of the soil. Dry wood also imbibes water and increases in volume.

Fig: Seeds germinating by imbibing water

Practice Problems

Q1. Suppose cell ‘A’ with OP = 10 atm and TP = 5 atm is in contact with the cell ‘B’ having OP = 15 atm and TP = 12 atm. Then how will the flow of water be?

A. from A to B
B. equal flow
C. from B to A
D. no flow

Solution: We know that, DPD = OP - TP. In cell A the DPD can be calculated as 10 - 5 = 5 atm. In cell B the DPD is 15 - 12 = 3 atm. The reduction in diffusion pressure (DP) of water in a system over its pure state is called DPD. The pressure exerted by the molecules that are undergoing diffusion are called diffusion pressure. The DP of pure solvent is maximum. Diffusion pressure deficit can determine the direction of movement of water. Water moves from an area of lower DPD to the area of higher DPD. Here cell B has a lower DPD than cell A. Then the flow of water will be from cell B to A.

Hence the correct option is c.

Q2. During the rainy season, a wooden peg inserted into a rock can cause it to break due to the development of

A. turgor pressure
B. osmotic pressure
C. imbibition pressure
D. plasmolysis

Solution: Imbibition is the absorption of water or any other solvent by solid colloidal particles that result in an increase in their volume but does not form a solution. The swollen solid substances develop a pressure called imbibition pressure. Wooden pegs inserted in rocks get wet during the rainy season and develop a huge imbibition pressure which causes the rocks to break into pieces.

Hence the correct option is c.

Q3. When a cell is placed in 0.5M concentrated sugar solution, there is no change in its volume. What happens if the same cell is placed in a 0.5M concentrated solution of sodium chloride?

A. Decrease in volume of the cell
B. Increase in volume of the cell
C. No change in the volume of the cell
D. Decrease in volume of the solution

Solution: A sugar solution is a non-electrolyte solution (no ions are produced) whereas a solution of sodium chloride is an electrolyte solution (ions are produced). The ionisation will add more solute particles to the solution as compared to a non-electrolyte of the same strength. Thus, the sodium chloride solution will have higher solute concentration than the cell sap and the cell placed in the sodium chloride solution will undergo exosmosis. This causes the cell to shrink and decrease in volume.

Hence the correct option is a.

Q4. During endosmosis, the volume of solvent in the cell will

A. decrease
B. increase
C. remain constant
D. Osmosis does not affect volume

Solution: Osmosis is the movement of water molecules from a solution having lower solute concentration to a solution having higher solute concentration when the two solutions are separated by a semipermeable membrane. The entry of water into the cell by osmosis is known as endosmosis and it results in an increase in the volume of solvent (water) within the cell.

Hence the correct option is b.

Q5. How does a hypotonic solution affect a plant cell and an animal cell?

Answer: A hypotonic solution has a lower solute concentration than that of the cell sap. So, the water will move from the solution into the cell placed in it, i.e., endosmosis occurs. Due to endosmosis, animal cells like RBCs burst or get lysed due to the increased pressure of the cell contents on the cell membrane. But in a plant cell, the endosmosis will make the protoplast exert pressure on the cell wall, making the cell turgid or swollen. The cell does not burst due to the presence of the rigid cell wall.

Q6. What is the difference between a hypotonic and a hypertonic solution?
Answer:
The differences between hypotonic and hypertonic solution are -

Hypotonic solution

Hypertonic solution

Relative concentration of solutes is less than that of cell sap.

Relative concentration of solutes is more than that of cell sap.

Cell placed in hypotonic solution undergoes endosmosis

Cell placed in hypertonic solution undergoes exosmosis

Cell placed in hypotonic solution becomes turgid

Cell placed in a hypertonic solution shrinks or becomes plasmolysed.

FAQs

Q1. What are the applications of plasmolysis in daily life?
Answer:
Plasmolysis offers a wide range of uses in our everyday lives. It is essential for the preservation of meat, jellies, and other goods, preventing bacteria and fungi from destroying them. The osmotic pressure of the preservation medium is raised when food items (to be preserved) are salted. Exosmosis causes bacteria and fungus to be plasmolyzed and destroyed. Plasmolysis is also involved in the chemical weedicides used to destroy weeds in lawns, orchards, and agricultural fields. It aids in the prevention of plant growth in the gaps of the walls. Salting such plants causes plasmolysis, which kills the undesirable plants.

Q2. Is there plasmolysis in animals?
Answer:
Plasmolysis occurs when a plant cell is exposed to a hypertonic environment, causing the cell membrane to shrink away from the cell wall. The protoplast shrinks away from the cell wall when water travels out of the cell. Plasmolysis does not occur in animal cells since they do not have cell walls.

Q3. What is incipient plasmolysis?
Answer:
Incipient plasmolysis is the first step of plasmolysis, when water begins to flow out of the cell, causing the cell to reduce in size and the cell wall to become visible. As a result, incipient plasmolysis is described as an osmotic situation in which 50% of cells are plasmolysed. The osmotic potential inside the cell at this point is equal to the osmotic potential of the medium.

Q4. Is there any heat emitted during the imbibition process?
Answer:
The water molecules get strongly adsorbed and immobilised during imbibition. During imbibition, the absorbed water molecules lose the majority of their kinetic energy in the form of heat, which is referred to as heat of wetting (or heat of hydration).

 

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