Diffusion and Osmosis - Difference and Comparison

Table of Contents

Osmosis is the result of diffusion across a semipermeable membrane. If two solutions of different concentration are separated by a semipermeable membrane, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis. At the cellular level, both processes are types of passive transport.

Semipermeable membranes are very thin layers of material that allow small molecules, like oxygen, water, carbon dioxide, ammonia, glucose, amino-acids, etc., to pass through. However, they do not allow larger molecules, like sucrose, protein, etc., to pass through.

Comparison chart

Diffusion versus Osmosis comparison chart

	Diffusion	Osmosis
What is it?	Diffusion is a spontaneous movement of particles from an area of high concentration to an area of low concentration. (ex. tea flavoring moving from an area of high to low concentration in hot water.)	Osmosis is the spontaneous net movement of water across a semipermeable membrane from a region of low solute concentration to a more concentrated solution, up a concentration gradient. This equalizes concentrations on both sides of the membrane.
Process	Diffusion results from the random motion of molecules by which there is a net flow of matter from a region of high concentration to a region of low concentration. This process can neither be stopped nor reversed.	It occurs when the medium surrounding the cell has a higher water concentration than the cell. The cell gains water along with important molecules and particles for growth. It also occurs when water and particles move from one cell to another.
Importance	To create energy; Helps in exchange of gases during respiration, photosynthesis, and transpiration.	In animals, osmosis influences the distribution of nutrients and the release of metabolic waste products. In plants, osmosis is partially responsible for the absorption of soil water and for the elevation of the liquid to the leaves of the plant.
Concentration Gradient	Particles move from a high concentration gradient to a low concentration gradient.	Water moves down the concentration gradient i.e., it moves from a diluted solution to a concentrated solution through a partially permeable membrane.
Water	Doesn’t need water for movement	Osmosis requires water for the movement of particles.
Examples	Perfume, air freshener, or the smell of rotten eggs, where gas molecules diffuse into the air spreading the aroma (or stench).	Movement of water into root hair cells.
Diffusion of molecules	Only molecules of the solvent undergo diffusion.	Molecules of both the solute and the solvent can diffuse in the process of osmosis.
Direction of particles	Particles only flow in one direction -- from high to low concentration areas.	Particles flow in all directions.
Occurs in	Diffusion occurs in gases (most frequently), liquids, and solids.	Osmosis occurs in liquids.

Process of Osmosis vs. Diffusion

Diffusion occurs when the spontaneous net movement of particles or molecules spreads them from an area of high concentration to an area of low concentration through a semipermeable membrane. It is simply the statistical outcome of random motion. As time progresses, the differential gradient of concentrations between high and low will drop (become increasingly shallow) until the concentrations are equalized.

The process of diffusion. Some particles (red) are dissolved in a glass of water. Initially, the particles are all near one corner of the glass. When the particles all randomly move around ("diffuse") in the water, they eventually become distributed randomly and uniformly.

Diffusion increases entropy (randomness), decreasing Gibbs free energy, and therefore is a clear example of thermodynamics. Diffusion operates within the boundaries of the Second Law of Thermodynamics because it demonstrates nature's tendency to "wind down", to seek a state of less concentrated energy, as evidenced by increasing entropy.

Osmosis is the process of diffusion of water across a semipermeable membrane. Water molecules are free to pass across the cell membrane in both directions, either in or out, and thus osmosis regulates hydration, the influx of nutrients and the outflow of wastes, among other processes.

Osmosis in a plant cell

For example, if the medium surrounding the plant or animal cell has a higher water concentration than the cell, then the cell will gain water by osmosis. The overall result is that water enters the cell and the cell is likely to hydrate and swell. If the medium has lower concentration of water than the cell, it will lose water by osmosis as this time more water leaves the cell than enters it. Therefore the cell will shrink. If the water concentration in the medium is exactly the same then the cell will stay the same size while that concentration balance remains. In every situation, the movement of solvent is from the less-concentrated (hypotonic) to the more-concentrated (hypertonic) solution, which tends to reduce the difference in concentration (equalization).

Differences in Function

While osmosis influences the distribution of nutrients and the release of metabolic waste products in animals; in plants, osmosis is partially responsible for the absorption of soil water and for the elevation of the liquid to the leaves of the plant.

Diffusion can occur through a cell membrane, and the membrane allows small molecules like water (H2O), oxygen (O2), carbon dioxide (CO2), and others to pass through easily. Hence while osmosis helps the plants in absorbing water and other liquids, diffusion helps other molecules to pass through and hence both facilitate the photosynthesis process. Both processes help plants to create energy and other important nutrients.

Different Types of Osmosis and Diffusion

The two types of Osmosis are:

Reverse Osmosis: The osmotic pressure defines at what point a differential gradient between high and low solute triggers osmosis. In reverse osmosis, increased volumetric or atmospheric pressure will "push" the higher solute particles past the membrane, overcoming the gap that may exist when the osmotic pressure won't allow diffusion through the membrane. This process is often used to filter water of impurities when their concentrations are too low for regular osmosis, but cleaner water is still needed, as in desalination and pharmaceutical operations.

Forward Osmosis: Unlike reverse osmosis, which goes from high-to-low concentrations, forward osmosis forces low solute particles to move to a higher solute — in essence, the opposite of the normal osmotic process. Whereas reverse osmosis "pushes" particles, forward osmosis "draws" them in, resulting in cleaner water.

Osmotic effect of different solutions on blood cells

The types of diffusion are:

Surface diffusion: Seen after dropping powdery substances on a liquid's surface.

Brownian motion: The random motion observed under a microscope as particles skip, slip, and dart within a liquid.

Collective diffusion: The diffusion of a large number of particles within a liquid that remain intact or interacting with other particles.

Osmosis: The diffusion of water through a cell membrane.

Effusion: Happens as a gas disperses through small holes.

Electron diffusion: The movement of electrons resulting in electric current.

Facilitated diffusion: Spontaneous passive transport of ions or molecules across a cell membrane (different because it happens outside the active phase of osmosis or intracellular diffusion).

Knudsen diffusion: A variable measure of particle interactivity within a membrane pore, related to the particle's size and the length and diameter of the pore.

Momentum diffusion: The spread of momentum between particles mainly in liquids, influenced by the liquid's viscosity (higher viscosity = higher momentum diffusion).

Photon Diffusion: Movement of photons within a material, then scattering as they bounce off of different densities within. Used in medical tests as diffuse optical imaging.

Reverse Diffusion: Similar to forward osmosis, with low concentration moving to high, but refers to a separation of particles, not just water.

Self-diffusion: A coefficient measuring how much diffusion a type of particle will have when the chemical gradient is zero (neutral or balanced).

Diffusion and Osmosis - Difference and Comparison

Comparison chart

Process of Osmosis vs. Diffusion

Differences in Function

Different Types of Osmosis and Diffusion

References