Is Osmosis Active Or Passive Transport? Factors That Affect Osmosis & Its Significance

Cell membrane transport mechanisms can be active or passive, depending on whether they require the use of energy or not. Osmosis is a highly significant biological process in living organisms. A common question around osmosis is whether it is active or passive.

It is often mistaken for facilitated diffusion (which will be discussed later in the article), a passive form of cell membrane transport that has some similarities with active transport.

Understanding whether osmosis is a form of active or passive transport begins with grasping the concept of this biological process by itself. Also, in this article, you will understand the differences between active and passive transport, factors that affect osmosis, and many more.

Is osmosis active or passive transport?

Osmosis is a passive transport process. Osmosis is the net movement of water molecules across a semipermeable membrane, from a region of higher concentration to a region of lower concentration.

The solute concentration is lower in the region of higher water concentration and higher in the region of lower water concentration. This process occurs to create a balance in the concentration of substances in a solution.

Osmosis occurs down the concentration gradient without the cellular energy expenditure. This is one of the reasons why it is called a passive transport process. It occurs naturally through the potential of water molecules to move from regions of lower solute concentration to regions of higher concentration.

Furthermore, osmosis is dependent on the permeability of the membrane, not an active cellular machinery that runs active transport. The water molecules move across the lipid bilayer of the cell membrane and the integral membrane proteins.

Also, osmosis is not a specialized process peculiar to some living organisms. It is a simple process that occurs in all living cells.

What is the difference between active and passive transport?

The first difference worth noting is that active transport uses energy to move molecules from one region of the cell membrane to another, whereas passive transport does not energy. While they have their peculiarities, the major aim – transporting essential molecules – is achieved.

Active transport is a dynamic and rapid process that pumps molecules against the concentration gradient using ATP. On the other hand, passive transport is a more physical and slow process that moves molecules within and across the cell membrane without using ATP.

Also, the circulation in active transport is from a region of lower concentration to a region of higher concentration. In passive transport, circulation is from higher to lower concentration.

Both forms of transport aim to move essential molecules across the membrane but there are some differences. Passive transport targets transporting all soluble molecules such as water, carbon dioxide, oxygen, and lipids. Active transport moves proteins, complex sugars, ions, and large cells.

While active transport requires carrier proteins, passive transport does not. Because of this, active transport can be influenced by the presence of metabolic inhibitors.

Additionally, active transport is highly selective of solutes and can be affected by temperature. Passive transport is not fully selective and is not affected by metabolic inhibitors and temperature.

Exocytosis, endocytosis, and Na+/K+ -pump are some vital examples of active transport. Some examples of passive transport are osmosis, facilitated diffusion, and simple diffusion.

Is osmosis facilitated diffusion?

Osmosis is not facilitated diffusion. Facilitated diffusion is a type of passive transport but it operates quite differently from osmosis. This type of passive transport moves molecules or ions across the cell membrane via a specific transmembrane of integral proteins.

The use of a transmembrane of integral proteins is where this form of passive transport overlaps with active transport, however, it is not a form of active transport. A carrier molecule is used because of the large size and insolubility of the molecules that need to move across the membrane.

On the other hand, osmosis does not use any carrier molecules to move the molecules across or within the membrane. Both types of transport do not expend cellular or external energy.

Facilitated transport uses three types of transport proteins: channel proteins, carrier proteins, and aquaporins. These proteins behave differently across the membrane.

Channel proteins may be gated (ion channel proteins) or freely open constantly allowing molecules to go in and out of the cell, depending on the concentration gradient. They form hydrophobic passages across the membrane to permit specific hydrophobic molecules.

The carrier proteins bind to the target molecule and alter its shape before carrying it across the membrane. The aquaporins move water more swiftly across the membrane.

Types of osmotic solutions

Isotonic solution

When a cell is an isotonic solution, the concentration of the solute in the cell is the same as the concentration of the solute in the solution (the extracellular fluid). In this condition, there is no movement of water in and out of the cell, so the cell retains its original state.

Hypotonic solution

In a hypotonic solution, the concentration of solute in the extracellular fluid is lower than the concentration of the solute in the cells. Water from the solution moves into the cell, then, it swells and eventually ruptures.

Hypertonic solution

When a cell is a hypertonic solution, the concentration of solute in the extracellular fluid is higher than the concentration of solute in the cell. As a result, water flows from the cell into the surrounding solution and the cell shrinks.

Factors that affect osmosis

Concentration gradient

The difference in solute concentration on both sides of the membrane is known as the concentration gradient. In osmosis, water moves from a region of higher concentration to a region of lower concentration.

The concentration gradient determines the direction and rate of water movement across the semi-permeable membrane.


Osmosis works with temperature. The higher the temperature, the faster the rate at which molecules move across the semi-permeable membrane. Conversely, the lower the temperature, the slower the osmotic movement.


Osmosis is also pressure-dependent. However, the trend is quite different from the trend with temperature. Osmotic movement slows down with increasing pressure and increases with decreasing pressure on the side of higher solute concentration.

Time of reaction

Osmosis depends on time. Osmotic movement changes as the reaction prolongs and the concentration gradient across the membrane equalizes.

Surface area

The larger the surface area of the membrane, the faster the osmotic movement.

Molecular size and membrane permeability

Smaller molecules move faster than larger molecules, thereby causing faster osmotic rates. Similarly, a more permeable membrane increases the rate of osmosis. The permeability of a membrane depends on the type, thickness, and composition of the membrane.

Also, the more soluble the solute, the faster it moves through the membrane and the faster the rate of osmosis.

Presence of other solutes

A more soluble and permeable solute increases the rate of osmosis. However, the presence of non-permeable and insoluble solutes on one or both sides will reduce osmotic movement across the membrane.

Significance of osmosis to cell transport

Osmosis is a crucial process for living cells. It plays key roles in cell metabolism, maintenance, and survival. It is established that this biological process influences the transport of nutrients within and across cells. Osmosis also facilitates the release of metabolic waste products.

By moving water and solute molecules within and across cells, osmosis helps to maintain a balance between water and intercellular fluid levels. This also stabilizes the internal environment of the living organism through cell-to-cell diffusion.

Osmosis enhances the release of toxic metabolic wastes like urea through the semi-permeable membrane that surrounds the cell. It also aids the absorption of water from the intestines into the blood. This same process regulates the movement of dissolved solids, liquids, and gases across living cells.

Furthermore, through the process of osmosis, plants absorb water from the soil and conduct it to the upper parts of the plant through the xylem. Plants can, therefore, maintain their water content despite the effect of transpiration.

This supports plants’ adaptability and tolerance for dry weather conditions. The higher the osmotic pressure, the higher the plant’s resistance to drought injury. In addition, osmosis helps to maintain cell turgidity and support the movement of plant parts.  


Are there conditions where osmosis becomes active transport?

Osmosis naturally doesn’t require energy to move water from a region of high concentration to a region of low concentration. This process can always occur without the aid of specialized carrier proteins.

However, there could be exceptions when some cells regulate their water balance using ion pumps to create concentration gradients.

Is reverse osmosis active or passive?

Reverse osmosis is passive transport because it does not require energy expenditure. Reverse osmosis is the movement of water molecules through a semi-permeable membrane, like regular osmosis, but across the concentration gradient.

What is the difference between osmotic pressure and osmotic potential?

Osmotic pressure is the hydrostatic pressure required to balance and prevent the osmotic inflow of water into a concentrated solution, while the osmotic potential is the potential of a solution to cause water molecules to move from a hypotonic solution to a hypertonic solution.

Is facilitated diffusion active transport?

Facilitated diffusion is a type of passive membrane transport. It is often mistaken for active transport because it involves carrier proteins. However, it remains passive transport because it is the movement of solute molecules down the concentration gradient.


Osmosis is a biological process that moves water from a higher concentration (where solute molecules have a lower concentration) to regions of lower concentration (where solute molecules have a higher concentration). It is called passive transport because this happens without energy input.

Osmosis is a simple process of diffusion that occurs through channels. This process is crucial to the maintenance of water and solute balance in cells. While osmosis is always passive, there could be exceptions when the process is influenced by active transport mechanisms.

Learn about the similarities and differences between simple diffusion and facilitated diffusion.

Thanks for reading.