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Differences between diffusion and osmosis, Practice Problems and FAQs

Have you ever travelled in a heavily crowded train? Imagine being trapped in a train compartment thronged with people as shown in the gif below! Wouldn’t you just wish to get down from the train the first chance you get? This issue prevails for most public transports. However, if we have our personal means of transport, travel becomes more comfortable.

Please enter alt text
                 GIF: Crowded train                                     Fig: Empty train

Let us consider another scenario of us travelling in a heavily crowded compartment of a metro train. Suddenly if we figure out that the neighbouring compartment is comparatively less crowded, won’t we try to cross the vestibule and reach the less crowded space of the neighbouring compartment? Yes, we would, for a more comfortable journey. A similar logic drives the movement of molecules in and out of a cell. Every molecule prefers moving from a crowded place (high concentration of the molecule) to a place where it's not crowded or less crowded (less concentration of the molecule). The difference in concentration of substances across a membrane is called concentration gradient and molecules always prefer to move along their concentration gradient. As the concentration gradient drives the movement of the molecules in such cases, no extra energy is required and hence such movements are classified as passive movements.


                            Fig: Movement of molecules from high concentration to low concentration

Such movements are common in plant and animal cells. Diffusion and osmosis are two common types of passive movement of molecules. In spite of both being means of passive transport, diffusion and osmosis have a lot of dissimilarities too. In this article, we are going to find out the differences between diffusion and osmosis.

Table of contents:

  • Diffusion
  • Osmosis
  • Differences between diffusion and osmosis
  • Practice Problems
  • FAQs

Diffusion

The movement of particles from a region of higher concentration to a region of lower concentration until the equilibrium is achieved is called diffusion or simple diffusion.


                      Fig: Diffusion of molecules across a biomembrane

Diffusion of molecules occurs for shorter distances and it is a slow process. The movement of molecules via diffusion continues until the molecules are evenly distributed in a diffusing medium. This may happen from one part of the cell to the other, or from one cell to another, or over short distances like from the intercellular spaces of the leaf to the outside. Simple diffusion does not involve expenditure of energy, which means no ATP is involved. In plants, gas molecules like CO2 and O2 are exchanged with the atmosphere through simple diffusion across pores such as stomata and lenticels.

The process of diffusion does not depend on living systems and the direction of one substance during diffusion is independent of another one. Diffusion is common in fluids such as gases and liquids and it is uncommon in solid substances. This is because in fluids the molecules are less tightly packed and have room for movement whereas in solids the molecules are very tightly packed and there is no room for movement, except for vibrating in their positions.

The major factors that affect the rate of diffusion across the cell membrane are as follows:

  • Size of molecules - The cell membrane has small pores or channels and hence it is difficult for large molecules to diffuse across the cell membrane.
  • Temperature - increase in temperature leads to increase in the rate of diffusion as the kinetic energy of molecules increases
  • Concentration gradient - Greater the concentration gradient, faster is the rate of diffusion from the higher to the lower concentration
  • Diffusion pressure gradient- greater the difference in concentration of diffusing particles between two solutions, greater is the diffusion pressure deficit and faster is the rate of diffusion
  • Density - rate of diffusion is inversely proportional to the square root of density of the diffusing substances. Thus, denser the fluid, lesser is its diffusion rate
  • Surface area of the membrane - More the available surface area of the membrane faster is the rate of diffusion.

The molecules which are lipid soluble can easily pass through the lipid bilayer of the plasma membrane by simple diffusion. But, the molecules that are not soluble in the lipid bilayer adopt a specialised mechanism called facilitated diffusion.

Facilitated diffusion

The diffusion of the hydrophilic or charged polar molecules, which are insoluble in lipids, across the cell membrane is facilitated by specialised transport proteins (carrier proteins/permeases or channel proteins) present in the membrane itself. This movement of ions or polar, water soluble molecules through the cell membrane, from a higher to a lower concentration, with the help of transport proteins and without the use of energy is known as facilitated diffusion. The transport proteins used in facilitated diffusion are highly specific to the molecules that are being transported.


                                 GIF: Facilitated diffusion

Diffusion pressure

Diffusion pressure or DP is the pressure resulting from the diffused particles. DP is directly proportional to the number or the concentration of the diffusing particles. Hence the movement of particles is from higher DP to lower DP.

Osmosis

The diffusion of water across a selectively permeable or differentially permeable membrane from a higher to a lower concentration is called osmosis. This process also does not require any energy to take place. Osmosis always happens from higher water potential to lower water potential. Water potential is the difference in the free energy of water molecules in the solution and that of pure water at the same temperature and pressure. The water potential of pure water is the maximum and considered to be zero. As we add solutes to water, the water potential decreases and becomes negative. The direction and rate of osmosis is dependent on two factors:

  • Pressure gradient - water moves from higher osmotic pressure to lower osmotic pressure during osmosis
  • Concentration gradient - water moves from high concentration to low concentration during osmosis


                                  Fig: Osmosis

Osmotic pressure

It is the pressure that is required to prevent water from diffusing or undergoing osmosis in spite of a difference in water potentials. Osmotic pressure and solute concentration are directly proportional to each other.

Osmotic pressure can be also defined as the hydrostatic pressure developed in a solution in a rigid vessel, when it is separated by a semipermeable membrane from a pure solvent. Osmotic pressure is numerically equal to osmotic potential, but has a positive sign.

Reverse osmosis

The reversal of the process of osmosis by applying hydrostatic pressure on the solution is known as reverse osmosis. It can increase the escaping tendency of the solvent molecules.

Differences between diffusion and osmosis

The process of osmosis and diffusion are different from one another despite having some similarities. The similarities include the equalisation of concentration of two solutions, no need of extra energy and the movement of particles from high concentration to low concentration. So now let us check out the differences between the diffusion and osmosis.

Diffusion

Osmosis

Observed in both liquid and gaseous medium

Only observed in liquid medium

Molecules do not need a semipermeable membrane.

Molecules pass through a semipermeable membrane.

Water is not required for the movement of molecules.

Water is required for the movement in osmosis

Both solutes and solvents can diffuse from a higher to a lower concentration.

Only water can move through the semipermeable membrane from a higher to a lower concentration.

Flow of molecules is in every direction, for example, the molecules of a perfume can spread from one corner of the room to all across the room.

Flow of molecules is in only one direction, that is, from pure water or a dilute sugar concentration to a concentrated solution across a semipermeable membrane.

Diffusion stops when the concentration of the diffusing substance equalises to occupy all the available space.

Osmosis stops when the movement of water molecules generates a pressure equal to that of osmotic pressure in a direction opposite to the movement of water. This can happen even if the concentration of water on either sides of the semipermeable membrane is not equalised.

If pressure is applied the process cannot be stopped.

The process can be stopped by exerting pressure equal to that of the osmotic pressure of the solution, in a direction opposite to the direction of osmosis. If more pressure is exerted, the process of osmosis can also be reversed.

Molecules can move between similar and dissimilar types of solutions

Molecules move across similar types of solutions

Does not depend on pressure potential, solute potential and water potential

Depends on solute potential, water potential and pressure potential

Helps in the uptake of minerals and nutrients from soil by roots

Help in uptake of water from soil by roots

Practice Problems

1. Which of the following statements are exclusive for the process of diffusion but not for osmosis?

  1. The movement of particles from a region of higher concentration to a region of lower concentration through a semipermeable membrane.
  2. Both solute and solvent molecules can show the movement.
  3. Observed in both liquid and gaseous medium.
  4. Diffusion of molecules occurs for shorter distances and it is a slow process.
  5. Does not involve expenditure of energy.
  1. A, B, C, D, E
  2. B, C, D, E
  3. B, C, D
  4. A, B, D

Solution: The movement of particles from a region of higher concentration to a region of lower concentration until the equilibrium is achieved is called diffusion or also called simple diffusion. It does not require the presence of a semipermeable membrane. Diffusion of molecules occurs for shorter distances and it is a slow process. This may happen from one part of the cell to the other, or from cell to cell, or over short distances like from the intercellular spaces of the leaf to the outside.

Diffusion can be seen in both liquid and gaseous media as the liquid and gas molecules can move from one place to another due to intermolecular spaces between them. However, osmosis occurs in only liquid medium as it involves the movement of water molecules from a higher to a lower concentration across a semipermeable membrane.

In diffusion both solute and solvent molecules can diffuse from their region of higher concentration to lower concentration, but in osmosis only solvent molecules, that is, water shows movement.

Both diffusion and osmosis are means of passive transport and hence do not require any energy to take place. Hence the correct option is c as statements B, C and D are exclusive for diffusion but not for osmosis..

2. Which of the following is true for osmosis?

  1. Suspended particles diffuse from higher concentration to lower concentration
  2. Suspended particles diffuse from lower concentration to higher concentration
  3. Water diffuses from lower concentration to higher concentration
  4. Water diffuses from high low to low concentration across a semipermeable membrane

Solution: The diffusion of water across a selectively permeable or differentially permeable membrane from a higher to a lower concentration is called osmosis. This process also does not require any energy to take place. Osmosis always happens from higher water potential to lower water potential. The direction and rate of osmosis is dependent on two factors, that are pressure gradient and concentration gradient. Hence the correct option is d.

3. Which of the following means of passive transports is adapted by the lipid insoluble molecules to get in or out of the cell?

  1. Simple diffusion
  2. Facilitated diffusion
  3. Osmosis
  4. All the above

Solution: The molecules which are lipid soluble can easily pass through the lipid bilayer of the plasma membrane by simple diffusion. But, the molecules that are not soluble in the lipid bilayer adopt a specialised mechanism called facilitated diffusion. The diffusion of the hydrophilic or charged polar molecules, which are insoluble in lipids, across the cell membrane is facilitated by specialised transport proteins (carrier proteins/permeases or channel proteins) present in the membrane itself. This movement of ions or polar, water soluble molecules through the cell membrane, from a higher to a lower concentration, with the help of transport proteins and without the use of energy and this is known as facilitated diffusion. The transport proteins used in facilitated diffusion are highly specific to the molecules that are being transported. Hence the correct option is b.

4. Assertion: Osmosis always happens from higher water potential to lower water potential.

Reason: Water potential of pure water is maximum and is considered to be zero.

Which of the following statements are correct according to the given assertion and reason?

  1. Both assertion and reason are true and reason is the correct explanation of assertion.
  2. Both assertion and reason are true and reason is not the correct explanation of assertion.
  3. Assertion is true but the reason is false.
  4. Both assertion and reason are false.

Solution: Osmosis is the process of movement of water molecules across a semipermeable membrane, from their region of higher concentration to their region of lower concentration. Water molecules will move from pure water to a solution during osmosis. This is because osmosis always happens from higher water potential to lower water potential. Water potential is the difference in the free energy of water molecules in the solution and that of pure water at the same temperature and pressure. The water potential of pure water is the maximum and considered to be zero. As we add solutes to water, the water potential decreases and becomes negative. Thus, water moves from higher to lower water potential. Hence the correct option is a.

FAQs

1. What is the importance of osmosis in plants?
Answer:
The diffusion of water across a selectively permeable or differentially permeable membrane from a higher to a lower concentration is called osmosis. It helps to maintain the water content inside the cell. Softer cells of a plant body become turgid due to osmosis. The absorption of water through root hairs is controlled by osmosis. The conduction of water from xylem elements to adjacent cells is also supported by osmosis. Higher osmotic pressure protects the plants from drought injury.

2. What are some of the daily life scenarios of diffusion?
Answer:
The movement of particles from a region of higher concentration to a region of lower concentration until the equilibrium is achieved is called diffusion or also called simple diffusion. There are many examples of diffusion that happen in our daily life. Some of them are as follows:

  • The smell of perfume in the air.
  • Making tea using tea bags.
  • The diffusion of CO2 into air while opening a cool drink.
  • Gas exchange in lungs.
  • The removal of toxic substances from our body by excretory system.
  • Sugar dissolves in water and makes the water evenly sweet.

3. What is diffusion pressure deficit?
Answer:
Diffusion pressure deficit or DPD is the difference between the diffusion pressure of pure solvent and solution.

Due to the presence of solutes in the solution and the forces opposing the diffusion, there is a reduction in the diffusion pressure of solvent in the solution over its pure state, it is known as the diffusion pressure deficit. DPD is also called suction pressure.

4. Can foods be preserved using the theory of osmosis?
Answer:
The preservation of jams, jellies and pickles is done using the principle of osmosis. In this process the food items are preserved in a hypertonic solution. Hypertonic solutions have more solute concentration. Hence the water will move from the higher concentration, which is the body of microbes, to the outside solution which has low concentration of water. This will make the microbes dehydrated and will kill them. This will prevent the food items from spoiling.

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