Osmosis/ Facilitated diffusion

Water can move across the (Semipermeable non polar lipid) membrane by simple diffusion (osmosis).

But polar molecules cannot pass through the non polar lipid bilayer, they require carrier proteins to facilitate their movement.

But water as well, is a polar molecule. How and why does it simply pass through the non polar lipid bilayer by simple diffusion and not by facilitated diffusion (i.e., through carrier proteins)?

Differences Between Osmosis And Facilitated Diffusion

In the body of an organism, molecules or ions move from one place to another by physiological processes. The main physiological processes are diffusion, osmosis and active transport. In the case of osmosis and facilitated diffusion they have some similarities as well as some differences. Osmosis is the spontaneous movement of water molecules across a semi permeable membrane from a region of high concentration of solution to a region of high solute concentration. Facilitated diffusion on the other hand is the process of spontaneous passive transport of molecules across a biological membrane via specific trans-membrane integral proteins. Some of the differences between osmosis and facilitated diffusion are:

Osmosis involves the movement of water molecules. Water molecules move from a region of high concentration to a region of low concentration. Facilitated diffusion on the other side involves insoluble compounds such as sugars, amino acids and ions which can pass through a partially permeable membrane.

Concentration gradient is the number of molecules within a particular area. It could also be taken as the gradual difference in solute concentration between two areas. The process of osmosis causes the molecules to move down a concentration gradient. This means that an osmotic pressure must be created so that solvent molecules move from a region of low solute concentration to a region of high solute concentration. Facilitated diffusion on the other hand causes molecules to go from a high concentration to a low concentration gradient. This brings in a difference between the two.

A membrane is a flexible enclosing or separating tissue forming a plane or film and separating two environments usually in living organisms. When it comes to osmosis a membrane is essential I order to create a concentration gradient between two different environments. Facilitated diffusion on the other case can take place either in the presence or absence of a membrane. This is because molecules can move freely from the area of high concentration to that of low molecule concentration.

Biologically a carrier is a protein in the membrane that aids the movement of molecules or ions from one region to another. Facilitated diffusion requires carriers in the movement of molecules. The molecules bind to membrane-spanning transport protein and together move form towards the region of low concentration. In the case of osmosis, a carrier is not required in the movement of the water molecules. This highlights a noticeable difference between osmosis and facilitated diffusion.

Osmosis involves movement of water molecules across a semi permeable membrane. This means that water is an essential for the process of osmosis to take place. On the other side facilitated diffusion does not require water molecules for other molecules to transfer. A major difference can be noted that osmosis requires water molecules but facilitated diffusion does not require any water molecules.

The process is how physiological means take place. Osmosis occurs when the medium surrounding the cell has a higher water concentration than the cell. The cell gains water molecules due to the osmotic pressure effect. Osmosis also occurs when water moves from one cell to another. Facilitated diffusion on the other hand occurs when the medium surrounding the cell is in high concentration of ions or molecules than the environment within the cell. The molecules move from the surrounding medium into the cell due to diffusion gradient.

From the differences highlighted above it is clear and evident that osmosis and facilitated diffusion differ in one way or another.


The passive movement of water across a selectively permeable membrane is called osmosis. Water molecules move across the plasma membrane from an area of greater water concentration (lower solute concentration) into an area of lower water concentration (greater solute concentration), either by crossing the plasma membrane straight or by moving through a channelprotein Osmosis plays an essential role in the functions of the cells and the entire body. Water molecules are the dominant components of cells and act as the solvent of the other chemicals. Likewise, the movement of water molecules into and from the cells has the capability to considerably impact the volume of cells and the concentration of the chemicals within them.

The beaker is divided into 2 compartments (A and B) by a selectively permeable membrane that permits water molecules however not sugar molecules to pass across it. Since the greater concentration of water remains in compartment A, water moves from compartment A into compartment B. Sugar molecules can not pass across the membrane, so water molecules from compartment A continue to move into compartment B, triggering the volume of the solution in compartment B to increase as the volume of water in compartment A reduces.

Like compartment B in figure, living cells likewise consist of numerous substances to which the plasma membrane is impenetrable. For that reason, any modification in the concentration of water across the plasma membrane will result in net gain or loss of water by the cell and a modification in cell volume and shape. The capability of a solution to impact the tone or shape of living cells by changing the cells’ water material is called tonicity.

A solution with a lower concentration of solutes (greater concentration of water) than the cell is called a hypotonic solution A cell put in this solution will get water and boost in size, which might ultimately cause burst of the cell. A solution with a greater concentration of solutes (lower concentration of water) than the cell is referred to as a hypertonic solution A cell put in this solution will lose water and diminish, which might cause cell death A solution that has the exact same concentration of solutes (exact same concentration of water) as the cell is an is otonic solution When surrounded by this solution, a cell shows no net gain or loss of water and no modification in volume.

Segment 3: Connection to the Course

  • Passive transport and its 3 types are important for many reasons
  • Most significantly, bring in particles/molecules vital to the survival of the cell, such as water and sugars
  • Works in tandem with active transport. Cell cannot wastefully expend ATP on bringing in every type of molecule passive transport remedies this by allowing many important molecules to pass through naturally without extra energy used. This saved energy can be used to perform many other cell functions.

Passive transport also maintains homeostasis for the cell by adhering to the concentration gradient. When particles diffuse from high to low conc., they are helping to maintain balance so that the cell is not overwhelmed by too much or too little of something.

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Learn how plants use osmosis, facilitated diffusion, and active transport to ingest water and mineral salts

NARRATOR: In most land plants, oxygen and carbon dioxide enter through the leaves, while water and mineral salts enter through the root system. Substances can move into the root in several ways, with the principal method being osmosis.

During osmosis free-water molecules pass from the soil into the epidermal cells, using the root-hair membrane. Looking at this process at the molecular level, we see that the small water molecules pass easily through the selectively permeable membrane of the epidermal cells.

Diffusion of mineral salts occurs at the same time. Facilitated diffusion occurs when important molecules pass through the membranes via special holes called channels.

Active transport of other molecules may also take place in the root hair, depending on the needs of the plant. Active transport requires energy, as the required molecules must be pumped across the membrane against their concentration gradient.

Diffusion, osmosis, active transport diffusion facilitated diffusion

Diffusion is the net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. Some major examples of diffusion in biology:

Gas exchange at the alveoli oxygen from air to blood, carbon dioxide from blood to air.

Gas exchange for photosynthesis carbon dioxide from air to leaf, oxygen from leaf to air.

Gas exchange for respiration oxygen from blood to tissue cells, carbon dioxide in opposite direction.

Transfer of transmitter substance acetylcholine from presynaptic to postsynaptic membrane at a synapse.

Osmosis diffusion of water through a semipermeable membrane.

High Diffusion Rate: short distance, large surface area, big concentration difference (Ficks Law). High temperatures increase diffusion large molecules slow diffusion.

This is the movement of specific molecules down a concentration gradient, passing through the membrane via a specific carrier protein. Thus, rather like enzymes, each carrier has its own shape and only allows one molecule (or one group of closely related molecules) to pass through. Selection is by size shape charge. Common molecules entering/leaving cells this way include glucose and amino-acids. It is passive and requires no energy from the cell. If the molecule is changed on entering the cell (glucose + ATP glucose phosphate + ADP), then the concentration gradient of glucose will be kept high, and there will a steady one-way traffic.

Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution down the water potential gradient) Note: diffusion and osmosis are both passive, i.e. energy from ATP is not used. A partially permeable membrane is a barrier that permits the passage of some substances but not others it allows the passage of the solvent molecules but not some of the larger solute molecules. Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of particular solutes stimulates the membrane to open specific channels or trigger active transport mechanisms to allow the passage of those chemicals across the membrane. Some major examples of osmosis

Absorption of water by plant roots. Re-absorption of water by the proximal and distal convoluted tubules of the nephron. Re-absorption of tissue fluid into the venule ends of the blood capillaries. Absorption of water by the alimentary canal stomach, small intestine and the colon.

Osmoregulation Osmoregulation is keeping the concentration of cell cytoplasm or blood at a suitable concentration. (a) Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm (thus need more respiration/O2/ATP than isotonic (marine) Amoebae). (b) The kidneys maintain the blood (thus, whole body) at the correct concentration. Osmosis and Plant Cells (a) Plant Cells in a hypotonic (= weaker) solution cells have lower water potential

the plant cells gain water by osmosis. the vacuole and cytoplasm increase in volume. the cell membrane is pushed harder

against the cell wall causing it to stretch a little.

the plant tissue becomes stiffer (= turgid).

(b) Plant Cells in a hypertonic (=stronger) solution cells have higher water potential

the plant cells lose water by osmosis. the vacuole and cytoplasm decrease

in volume. the cell shrinks away from the cell

wall. shrinkage stops when the cell sap

is at the same concentration as the external solution.

the plant tissue becomes flaccid, it has shrunk slightly

may go on to become plasmolysed.

Turgor Turgor is the pressure of the swollen cell contents against the cell wall when the external solution more dilute than the cell sap of the vacuole. Role of Turgor in Plants

Mechanical support for soft non-woody tissue, e.g., leaves. Change in shape of guard cells forming the stomatal opening between them. Enlargement of young immature plant cells to mature size.

This is the tendency of water to move from one place to another. Values are always negative! Water always flows downhill i.e. towards the more negative number. Units are pressure (kPa) Calculations are not set, but this formula may be:

Water Potential () = Pressure Potential (p) + Solute Potential (s)

Pressure Potential = the force of the cell wall on the contents, so for animal cells, this is zero, thus, in animals:


Passive transport is a way that small molecules or ions move across the cell membrane without input of energy by the cell. The three main kinds of passive transport are diffusion (or simple diffusion), osmosis, and facilitated diffusion. Simple diffusion and osmosis do not involve transport proteins. Facilitated diffusion requires the assistance of proteins.

Diffusion is the movement of molecules from an area of high concentration of the molecules to an area with a lower concentration. For cell transport, diffusion is the movement of small molecules across the cell membrane. The difference in the concentrations of the molecules in the two areas is called the concentration gradient. The kinetic energy of the molecules results in random motion, causing diffusion. In simple diffusion, this process proceeds without the aid of a transport protein. It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration.

Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient". The end result is an equal concentration, or equilibrium, of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop. At equilibrium, there is equal movement of materials in both directions.

Not everything can make it into your cells. Your cells have a plasma membrane that helps to guard your cells from unwanted intruders.

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 &mdash in essence, the opposite of the normal osmotic process. Whereas reverse osmosis "pushes" particles, forward osmosis "draws" them in, resulting in cleaner water.

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).
  • Gaseous diffusion: Used mainly with uranium hexafluoride to produce enriched uranium for nuclear reactors and weapons.
  • 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).


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Osmosis, the spontaneous passage or diffusion of water or other solvents through a semipermeable membrane (one that blocks the passage of dissolved substances—i.e., solutes). The process, important in biology, was first thoroughly studied in 1877 by a German plant physiologist, Wilhelm Pfeffer. Earlier workers had made less accurate studies of leaky membranes (e.g., animal bladders) and the passage through them in opposite directions of water and escaping substances. The general term osmose (now osmosis) was introduced in 1854 by a British chemist, Thomas Graham.

If a solution is separated from the pure solvent by a membrane that is permeable to the solvent but not the solute, the solution will tend to become more dilute by absorbing solvent through the membrane. This process can be stopped by increasing the pressure on the solution by a specific amount, called the osmotic pressure. The Dutch-born chemist Jacobus Henricus van ’t Hoff showed in 1886 that if the solute is so dilute that its partial vapour pressure above the solution obeys Henry’s law (i.e., is proportional to its concentration in the solution), then osmotic pressure varies with concentration and temperature approximately as it would if the solute were a gas occupying the same volume. This relation led to equations for determining molecular weights of solutes in dilute solutions through effects on the freezing point, boiling point, or vapour pressure of the solvent.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Erik Gregersen, Senior Editor.

Osmosis Videos/Demonstrations

We want to use the Egg Osmosis demonstrations to explain the symptoms of the disease diabetes and why a women died from drinking too much water during a radio contest.

Two of the symptoms of diabetes are thirst and excessive urination. What happens in your body to cause this? What does is have to do with osmosis?

We will place a decalcified egg in pancake syrup to find out.

A women named Jennifer Strange wanted to win a Wii for her family back in 2007 when they were hard to get. (Remember that?) The contestants had to drink an excessive amount of water and the last person to urinate won the prize. Jennifer finished second but later passed away from a condition called hyponatremia. What happen inside Jennifer’s body and how does it have to do with osmosis?

Watch the video: Passive Transportsimple diffusion, facilitated diffusion u0026 osmosis (January 2022).