Definition of osmosis, Practice Problems and FAQs

Do you know how contact lenses are stored after use? People who regularly use contact lenses always make sure to store their lenses in containers filled with a special solution. Any idea why?

This is because the lens solution has several ingredients each of which function separately to clean it, disinfect it and keep it moist. The solute concentration of the lens solution mimics that of the fluids in the eye which keeps the lenses soft, moist and comfortable when you keep your lenses inside the solution. Otherwise, the lenses may absorb moisture from the eye and make the eyes dry while wearing it. The process responsible for this is osmosis which causes water to be drawn into or out of a cell depending on whether it has less or more water content than its surroundings, respectively. Did you know that this process is of great significance for plants too?

Osmosis is the most important process of absorbing water from the soil and is also responsible for movement of water up through the stem. In fact, the swelling up of raisins soaked in water is also due to osmosis. In this article we will discuss osmosis in detail..

Table of contents:

• Definition of osmosis
• Experiments of osmosis
• Types of solutions
• Endosmosis and exosmosis
• Reverse osmosis
• Relation between osmotic pressure and water potential
• Osmosis in daily life
• Practice Problems
• FAQs

Definition of osmosis

The diffusion of water across a selectively permeable or differentially permeable membrane from its region of higher concentration to its region of lower concentration is called osmosis. Two factors are responsible for the direction and rate of osmosis. They are as follows:

• Concentration gradient
• Pressure gradient

Experiments of osmosis

To understand more about osmosis we can conduct three experiments. These are an U-tube experiment, potato osmometer experiment and thistle funnel experiment.

U-tube experiment

This experiment can show us the importance of concentration gradient in the process of osmosis.

Procedure of U tube experiment

To conduct the experiment we need a U tube with a selectively permeable membrane placed in the middle of the tube. Now we will add equal amounts of water to both the sides of the U tube. Next we will take some table sugar or sucrose and add it on the left side of the U tube and dissolve it. Sucrose acts as the solute in this experiment.

Observation of U tube experiment

The water level in the left side of the U tube will increase overtime and there will be a decrease in the water level in the right side. This will indicate that a movement of water has occurred from the right side to the left side. This movement occurs because the concentration of water molecules on the right side of the tube containing pure water is more compared to that on the left side which has a sucrose solution, that is, water mixed with a solute. Thus, water simply diffuses from its region of higher concentration (right) to its region of lower concentration (left). As the diffusion of water molecules occurs across a semipermeable membrane, this process is known as osmosis.

level on the left side

Water potential and osmosis

Water molecules in liquid and gaseous form exhibit random movement. This movement is rapid and occurs at all times. This random movement of water molecules is due to the kinetic energy of the water molecules and pure water has the highest kinetic energy, because it has more water molecules.

GIF: Movement of water molecules in the pure water

Now if we take an equal volume of a sucrose solution, the solution has both solute (sucrose) molecules and the solvent (water) molecules. Thus, in a solution the number of water molecules will be less compared to that of pure water. As a result, the kinetic energy of water molecules in the solution will also be less. So when pure water comes in contact with a solution, the water molecules will move from a region of higher kinetic energy to a region of lower kinetic energy.

This kinetic energy of water molecules can be expressed in another term called water potential and it is denoted by Greek symbol, psi or Ψ_w. It can be expressed in pressure units called Pascals.

So we can say that, during osmosis, the movement of water is from higher water potential to the lower water potential, until equilibrium is established.

Hence at equilibrium both sides of the U tube have nearly the same water potential and solvent concentration. Hence no more osmosis will take place.

Fig: Osmosis

Potato osmometer experiment

This experiment is also deprecating the concentration gradient in the osmosis.

Procedure of potato osmometer experiment

In this experiment, a large sized potato is peeled and its base is cut to make it flat. A hollow cavity is made in the centre, leaving a thin wall at the base. This cavity is half filled with a strong sugar or salt solution and the initial level of the solution is marked using a pin. This potato is then placed in a beaker containing pure water and observed after a few hours.

Observation of potato osmometer experiment

After a few hours, we will observe that the water level in the beaker outside the potato will decrease slightly and the solution level inside the potato will increase. This is because there is a higher solute concentration and a lower water potential inside the potato, while the outside has a zero solute concentration and hence a higher water potential. So, there is osmosis, i.e., movement of water from outside to inside.

Fig: Potato osmometer experiment

So now we can conclude that during osmosis water moves from higher water potential to lower water potential or from low solute concentration to high solute concentration.

Fig: Direction of movement of water during osmosis

Now we know the concentration gradient between two solutions is affecting the osmosis. Next we are going to discuss how the pressure gradient is affecting the osmosis. We can explain this through an experiment, which is the thistle funnel experiment.

Thistle funnel experiment

First let’s see what are the requirements for this experiment.

Requirements of thistle funnel experiment

Requirements for this experiment are as follows:

• A thistle funnel
• Rubber band
• Cellophane paper (semipermeable membrane)
• Sugar
• Water

Fig: Thistle funnel

Procedure of the experiment

The first step is to cover the mouth of the funnel using a cellophane paper and a rubber band. Next process is to fill the funnel with a sugar solution. Now this apparatus is placed inside a beaker containing pure water.

Fig: Initial stage of thistle funnel experiment

Observation of the experiment

After a while, the solution level in the funnel raises. This is due to osmosis. The water is moving from higher concentration, to lower concentration. So here it is moving from the beaker to the funnel.

We can also explain it on the basis of water potential. The movement of water is always from higher water potential to lower water potential. We know that the water potential of pure water is always higher. Hence, the water moves from the beaker to the funnel, across the semipermeable membrane, until equilibrium is reached.

Fig: Final stage of thistle funnel experiment

Here we were able to explain the concentration of water and how it is applied in the osmosis. Next we will check how pressure is affecting the process of osmosis.

As the water molecules enter the thistle funnel, the pressure inside the thistle funnel keeps increasing. There will come a time when the pressure inside the thistle funnel is high enough to prevent the entry of any further water molecules, even if the concentration of water on either side of the cellophane paper is not equalised. This can be demonstrated by applying pressure on the sugar solution in the thistle funnel using a piston while it is still inside the beaker with pure water. As we keep increasing the pressure, we reach a point where the pressure applied by the piston will prevent the entry of water molecules from the water in the beaker into the sugar solution in the thistle funnel. Thus no further osmosis can occur. This minimum amount of pressure needed to prevent osmosis is known as osmotic pressure.

Thus, when the pressure inside the solution reaches the osmotic pressure, there will be no movement of water, even though there is a difference in the water potential of the sugar solution in the funnel and the water in the beaker. When solute concentration is higher, more pressure needs to be applied to stop the process of osmosis. Hence osmotic pressure is directly proportional to solute concentration. Thus pure water has zero osmotic pressure and a dilute solution will have less osmotic pressure compared to a concentrated one. Hence, we can say that water moves by osmosis from a solution with less osmotic pressure to a solution with high osmotic pressure.

Reverse osmosis

The process of reverse osmosis can be explained through the thistle funnel experiment. We know that the osmotic pressure prevents the movement of water molecules from the beaker containing pure water into the thistle funnel carrying the sugar solution. What happens if we apply pressure that exceeds the osmotic pressure? Then the water will not move into the thistle funnel, rather it will move in the opposite direction. Which means, the water will flow from the lower concentration to the higher concentration ie; from the sugar solution in the thistle funnel to the pure water in the beaker. We can also explain this as the flow of water from the lower water potential to higher water potential.

Hence, reverse osmosis is the process by which the solvent moves in the opposite direction of the natural osmosis through a selectively permeable membrane. So the pressure applied in reverse osmosis is always higher than the osmotic pressure.

RO filtration system is an equipment used for the purification of water. This works on the principle of reverse osmosis. Through reverse osmosis, the large and unwanted molecules of chlorine, salt and dirt are removed from the water using a semi permeable membrane.

Fig: RO water filtration system

Types of solutions

The absorption of water from the soil into the root occurs by the process of osmosis. The solute concentration of soil is relatively less than the solute concentration of the cell sap of the root hair cells. So the water potential of soil is higher than that of root cells. Hence the water moves from the soil into the root hair cells through their semipermeable cell membrane.

Water and solutes (minerals) are transported in the plants through the vascular tissue called xylem. Minerals are pumped by active transport from the soil into the root hair cells and are then transported passively through the cortex, pericycle and endodermis into the xylem tissues. Hence the solute concentration in the xylem is more compared to the peripheral tissues. So there will be a concentration gradient of solutes in the root cells. Since water potential increases as solute concentration decreases, there is an opposite gradient of water potential which decreases as we go towards the xylem. Because of this gradient, water is transported from cell to cell from higher water potential to lower water potential through osmosis until it reaches the xylem.

to xylem of the root cells

So the movement of solutes depends on the concentration of the solution inside and outside of the cell. According to the concentration of a solution there are three types of solutions as follows:

• Isotonic solution
• Hypertonic solution
• Hypotonic solution

Isotonic solution

If two solutions have the same osmotic pressure across the semipermeable membrane, there will not be any net change in the movement of solvent or water across the membrane. Which means the amount of water moving inside will be equal to the amount of water moving out. This kind of solution is called an isotonic solution. ‘Iso’ means ‘same’ and ‘tonic’ means ‘concentration of the solution’. Hence an isotonic solution has the same solute concentration and water potential compared to the solution separated from it by a semipermeable membrane.

When the cell (or tissue) is placed in an isotonic solution, there is no net flow of water towards the inside or outside. It is said to be isotonic if the external solution balances the osmotic pressure of the cytoplasm.

The cells are said to be flaccid, when the water moving into the cell and out of the cell are in equilibrium. Isotonic state is perfect for a cell. So that is why an isotonic solution is used to store the contact lens.

Hypertonic solution

If a solution has relatively more solute concentration compared to the cell sap, then it is called a hypertonic solution. Hyper means more and tonic means concentration.

If we place RBC and a plant cell in a hypertonic solution, water will move out of the cell, since the water potential is high inside the cell. As a result the RBC shrinks. The plant cell will also shrivel because of the same reason, because the solute concentration outside the cell is higher than the solute concentration inside the cell. This is known as plasmolysis.

Plasmolysis

Water moves out of the cell and the cell membrane of a plant cell shrinks away from its cell wall, when a cell is placed in a solution hypertonic to the protoplasm. This is known as plasmolysis. The space between the cell wall and the shrunken protoplast is occupied by the hypertonic solution.

Hypotonic solution

If a solution has relatively less solute concentration than the cell sap, then it is called a hypotonic solution. Hypo means less and tonic means concentration.

If we place RBC and a plant cell in a hypotonic solution, water will move into the cell, since the water potential is high outside the cell. As a result, the RBC will swell and eventually burst due to the internal pressure. The plant cell will also swell and become turgid because of the same reason. When water enters a plant cell, pressure is exerted by the protoplast and it starts to exert pressure on the cell wall of the plant cell. This is called turgor pressure or pressure potential.

Pressure Potential (or) Turgor Pressure

The pressure exerted by the protoplasts due to entry of water against the rigid cell wall is called pressure potential. It is also called turgor pressure and is represented by Ψ_p. Plant cells do not rupture like RBC due to the presence of cell walls. This turgor pressure is ultimately responsible for enlargement and extension or growth of cells. The pressure potential of a flaccid cell is low and the pressure potential of a turgid cell is high.

The equal and opposite pressure exerted by the cell wall on the protoplasmic contents of a turgid cell is known as wall pressure.

Endosmosis and exosmosis

The movement of water into and out of the cell can classify the process of osmosis into endosmosis and exosmosis. This depends on the concentration of the solution in which the cell is placed.

Endosmosis

A cell placed in a hypotonic solution will swell when water moves into the cell across the semipermeable membrane. This movement of water is known as endosmosis.

                                           Fig: Endosmosis

Exosmosis

A cell placed in a hypertonic solution will shrink and become flaccid, because water moves out of the cell across the semipermeable cell membrane. This movement of water is known as exosmosis.

                                                 Fig: Exosmosis

Relation between osmotic pressure and water potential

Osmotic pressure is referred to as OP. This abbreviation is not for osmotic potential, which is equal to solute potential. If we add more solute molecules in a solution, then the solution solute potential of the solution increases. Such a solution will have higher osmotic potential and will tend to draw in more water by endosmosis. Hence more pressure is needed to stop the osmosis of diffusion. As a result, the osmotic pressure or OP is the negative version of osmotic potential.

We know that water potential(𝚿_W) is the sum of pressure potential (𝚿_P) and solute potential (𝚿_S). And we know that solute potential is equal to osmotic potential and osmotic pressure is the negative version of osmotic potential.

Hence we can replace the solute potential with negative osmotic pressure. So the equation of the water potential will be as follows:

Water potential (𝚿_W) = Pressure potential (𝚿_P) - Osmotic pressure (OP)

Fig: Relationship between water potential and osmotic pressure

Osmosis in daily life

Osmosis is the one process which helps in the absorption and transportation of water and minerals in the plant body. But it can also be observed in our daily life. Let’s check some of those.

• Gargling with salt water can provide relief from sore throat

Inflammation of the throat due to some infection causes the throat cells to swell up due to water retention which results in a sore throat. If you gargle with salt water, the water molecules move from the higher concentration (throat) to the lower concentration (outside environment filled with salt water). As a result the throat cells will return to normal conditions and it will relieve us from the pain. Salt water also helps to kill the bacterial cells causing the infection because water from these cells is drawn out and they become shrivelled and die.

• Raisins soaked in water

Raisins will swell when soaked in water. Water inside the raisins is less as they are produced by sun-drying grapes. Hence the water will move from outside to inside the raisins through their membrane, until it reaches equilibrium.

• Food preservation

While making jams and pickles, high amounts of sugar and salts are added for preservation. This high concentration of solute in the pickle makes it a hypertonic solution. As a result microorganisms cannot grow in it. The water content inside the microbes will leak out and lead them to death due to dehydration.

• Small and large intestine absorbs digested food

Chyme is the form of food that reaches the intestine after food is partially digested and mixed with the gastric juice in the stomach. It has a higher concentration of water than the intestinal epithelial cells. Hence the water along with some minerals will get absorbed through osmosis into the epithelial cells. Villi are present on the surface of the epithelial cells which helps in the absorption process by increasing the surface area.

• Purification of water

Reverse osmosis is the method used in the water purifiers. The solvents or impurities are passed through the semipermeable membrane in the opposite direction of natural osmosis. This happens when hydrostatic pressure greater than the osmotic pressure is applied.

• Salt is used to avoid slugs

Slugs and snails are killed by using the common salt. While putting salt over the slugs, the semipermeable membrane of their skin will draw water out of the cells through osmosis. This will make them shrivel up and die.

Practice Problems

1. Which of the following experiments can describe the process of osmosis?

1. U-tube experiment
2. Potato osmometer experiment
3. Thistle funnel experiment
4. All of the above

Solution: The diffusion of water across a selectively permeable or differentially permeable membrane from its region of higher concentration to its region of lower concentration is called osmosis. Experiments like the U-tube experiment, potato osmometer experiment and thistle funnel experiment can explain the process of osmosis. The U tube experiment explains the concept of water potential and it also demonstrates the process of osmosis in the terms of water potential. According to the U tube experiment, during osmosis, the movement of water is from higher water potential to the lower water potential, until equilibrium is established. Potato osmometer experiments conclude that during osmosis water moves from higher water potential to lower water potential or from low solute concentration to high solute concentration. In both the U-tube and potato osmometer experiments the concept of concentration gradient is described. The pressure gradient is explained through the thistle funnel experiment, which can show the concept of the osmotic potential. The thistle funnel experiment describes that water moves from a region of low osmotic pressure to a region of high osmotic pressure. Hence the correct option is d.

2. Assertion: Reverse osmosis is the process by which the solvent moves in the opposite direction of the natural osmosis through a selectively permeable membrane.

Reason: RO filtration system is an equipment used for the purification of water.

1. The assertion and the reason are correct and reason is the correct explanation to the assertion.
2. The assertion and the reason are correct but reason is not the correct explanation of the assertion.
3. The assertion is correct, but the reason is wrong.
4. Both assertion and reason are wrong.

Solution: If we apply more pressure than the osmotic pressure in a system, then the water will move in the opposite direction. Which means, the water will flow from the lower concentration to the higher concentration. Reverse osmosis is the process by which the solvent moves in the opposite direction of the natural osmosis through a selectively permeable membrane. So the pressure applied in reverse osmosis is always higher than the osmotic pressure. RO filtration system is an equipment used for the purification of water. This works on the principle of reverse osmosis. Through reverse osmosis, the large and unwanted molecules of chlorine, salt and dirt are removed from the water using a semi permeable membrane. So here the assertion and reason are correct. But the reason is not explaining the assertion. Hence the correct option is b.

3. Gargling with salt water can provide relief from sore throat. Which of the following processes is the exact reason for this?

1. Endosmosis
2. Exosmosis
3. Diffusion
4. Active transport

Solution: The diffusion of water across a selectively permeable or differentially permeable membrane from its region of higher concentration to its region of lower concentration is called osmosis. The movement of water into or out of the cell can divide the process of osmosis into endosmosis and exosmosis. This depends on the basis of the concentration of the solution in which the cell is placed. A cell placed in a hypotonic solution will swell when water moves into the cell across the semipermeable membrane. This movement of water is known as endosmosis. A cell placed in a hypertonic solution will shrink and flaccid, because water moves out of the cell across the semipermeable cell membrane. This movement of water is known as exosmosis.

Sore throat is caused due to infections in the throat which causes inflammation in the area and water retention in the cells. If you gargle with salt water, the water molecules move from the higher concentration (throat) to the lower concentration (outside environment with salt water). As a result the throat cells will return to normal conditions and it will reduce the pain. Salt water also helps to draw out water from the bacterial cells infecting the throat and kill them.

So exosmosis is the process happening in this scenario. Hence the correct option is b.

4. What are the differences between endosmosis and exosmosis?

Answer: The movement of water into and out of the cell can be described as endosmosis and exosmosis, respectively. This depends on the basis of the concentration of the solution where the cell is placed. The differences between the endosmosis and exosmosis are as follows:

FAQs

1. What is osmoregulation?

Answer: The active regulation of the osmotic pressure of the body fluids of an organism is known as osmoregulation. It is detected by osmoreceptors. Osmoregulation can maintain the homeostasis of the organism, which means it maintains the fluid balance and concentration of electrolytes in the body. This will keep the body fluids in perfect concentration.

2. What is the difference between osmosis and chemiosmosis?

Answer: The movement of water molecules from higher to lower concentration through a semipermeable membrane is called osmosis. Chemiosmosis is the movement of ions from higher to lower concentration across the semipermeable membrane. Both the processes require a concentration gradient to work. Osmosis is usually seen in the transport of water molecules in the plants. Chemiosmosis is seen in cellular respiration or photosynthesis. No energy molecules are formed during osmosis. But chemiosmosis utilises the proton gradient to synthesise ATP molecules.

3. What is osmotic shock?

Answer: The sudden change in the solute concentration around the cell can cause physiologic dysfunction and it is called osmotic shock. It is also known as osmotic stress. This causes a rapid change in the movement of water across the cell membrane.

4. What is osmotic power?

Answer: The energy available from the difference in the osmotic pressure between sea water and river water is called osmotic power. It is also known as salinity gradient power or blue energy and is used to generate electricity in a renewable fashion. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two practical methods for harvesting osmotic power. Both procedures rely on membrane-mediated osmosis. Brackish water is a significant waste product in generating osmotic power. This byproduct is a result of the use of natural forces, specifically the flow of fresh water into saltwater of the seas.

Related Topics