Diffusion is the net movement of molecules from a region of its higher concentration to a region of its lower concentration. Molecules move down a concentration gradient, as a result of their random movement.

Dynamic Equilibrium:

In liquids and gases, molecules possess kinetic energy and constantly move about.

As this movement is random, an equilibrium is reached when the molecules are evenly spread out, where there is no more net change – dynamic equilibrium reached.

Passive Transport:

The steeper the gradient for a substance, the faster the rate of diffusion. No energy input is required for diffusion because the particles already are in motion. Hence, it is known as passive transport.

Facilitated diffusion:

For living cells, the principle of the movement down a concentration
gradient is the same, but there is one restriction.  The cell is surrounded by a cell membrane, which does not allow the free movement of the molecules because cell membrane is a selective permeable membrane. The composition of the membrane (lipid and protein) allows some molecules to cross with ease, but others with difficulty or not at all.

Proteins in the cell membrane  act like as channels to help the movement along. And of course there is an in-between transport process where very small molecules are able to cross a semi-permeable membrane.
Sometimes, proteins are used to help move molecules more quickly. It is a process called facilitated diffusion. The simplest sort of selection is based on the size of the molecules.

It could be as simple as bringing in a glucose molecule. Since the cell membrane will not allow glucose to cross by diffusion, helpers are needed. The cell might notice outside fluids rushing by with free glucose molecules. The membrane proteins then grab one molecule and shift their position to bring the molecule into the cell. That is an easy situation of passive transport because the glucose is moving from higher to lower concentration. It is moving down a concentration gradient. If you needed to remove glucose, the cell would require energy.

Concentration Gradient:

A concentration gradient occurs when the concentration of particles is higher in one area than another. In passive transport, particles will diffuse down a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced.

Why concentration gradient is important?

Organisms that need to move a substance in or out of their cells, for example, may use the movement of one substance down its concentration gradient to transport another substance.


Sometimes cells are in an area where there is a large concentration difference. For example, oxygen molecule concentrations could be very high outside of the cell and very low inside. Those oxygen molecules are so small that they are able to cross the lipid bilayer and enter the cell. There is no energy needed for this process. In this case, it’s good for the cell because cells need oxygen to survive.

Factors affecting diffusion:

Concentration gradient: The greater the concentration difference, the faster the rate of diffusion.

Temperature: The higher the temperature, the faster the rate of diffusion

 Pressure: increasing the pressure increases the kinetic energy of the particles, thus increasing the rate of diffusion.

Size of particles: Heavier particles will move more slowly and so will have a slower rate of diffusion. Smaller particles on the other hand will diffuse faster because they can move faster.

 Surface area to volume ratio:  When the cell increases in size, the volume increases faster than the surface area, because volume is cubed where surface area is squared. When there is more volume and less surface area, diffusion takes longer and is less effective.

Importance of the process of diffusion in living things:

Diffusion helps living organisms to:

  • obtain many of their requirements.
  • get rid of many of their waste products.
  • gas exchange for respiration.

Carbon dioxide uses by plants for photosynthesis is diffuses from the air into the leaves, through the stomata (pores at the surface of leaves). There is a lower concentration of carbon dioxide inside the leaf, as the cells are using it up. Oxygen (waste product of photosynthesis diffuses out in the same way).

Flowering plants use diffusion to attract pollinators like bees.

Some of the products of digestion are absorbed from the ileum of mammals by diffusion.


Respiration produces waste carbon dioxide, causing the amount of carbon dioxide to increase in the cell. Eventually, the carbon dioxide concentration in the cell is higher than that in the surrounding blood. The carbon dioxide then diffuses out through the cell membrane and into the blood. This exchange of gases take place in alveoli as well where carbon dioxide diffuses out and oxygen diffuses in to purify the blood.

Water diffuses into plants through their root hair cells. The water moves from an area of high concentration (in the soil) to an area of lower concentration (in the root hair cell). This is because root hair cells are partially permeable.



Osmos means water and osmosis means diffusion of water molecules from higher concentration, or it refers to the movement of water molecules from a region of higher water potential to a region of lower water potential across a semi or partially permeable membrane.

What is a semipermeable membrane?

The cells in our bodies are surrounded by a wall-like structure called a cell membrane. This membrane is special because only water and very small molecules can pass through it.

We use the word semipermeable to describe the ability to only let certain things pass through a membrane. It separate 2 solutions: cytoplasm and solution around the cell. If the solutions are of different concentrations, osmosis occurs.

Osmosis is a passive transport and no extra energy required and molecules move by their own energy.

Hypotonic, Isotonic, Hypertonic

A hypertonic solution contains a higher concentration of solutes compared to another solution.

A hypotonic solution is a solution that has a lower solute concentration compared to another solution.

A solution is called an isotonic when it has the same concentration of solutes as another solution across a semipermeable membrane.

A solution cannot be hypotonic, isotonic or hypertonic without a solution for comparison.


Effects of Osmosis on plant and animal cells:

What happens when you place an animal cell in a hypotonic solution?

Animal cell will swell and may bust in a hypotonic solution because it does not have a cell wall. Remember, water moves from a region of low osmolarity to a region of high osmolarity. In this case, since the extracellular fluid has low osmolarity, the water would rush into the cell. The cell would then expand and eventually lyse or burst.

What will happen if a plant cell is placed in hypotonic solution?

The water moves from a region of low osmolarity (extracellular fluid) to a region of high osmolarity (inside the cell). The cell would then expand. Unlike an animal cell, the plant cell does not burst. This is because plant cells have a rigid cell wall around the plasma membrane. Upon swelling with water they become turgid.

What will happen if an animal cell is placed in hypertonic solution?

In this case, water will leave the cell since the cell has a lower osmolarity than the extracellular fluid. As a result the cell would shrink that is called plasmolysis.

What will happen if an animal cell is placed in hypertonic solution?

What happens when you place an animal cell in an isotonic solution?

The osmolarity of both fluids is equal. As such, though water diffuses in and out, there is no net change in the volume of the cell.

What happens when you place a plant cell in an isotonic solution?

The osmolarity of both fluids is equal. Though water diffuses in and out there is no net change in the volume of the cell.

Turgor in plants:

  • Turgor is very important in maintaining the shape of soft tissues in plants
  • Young stems and most leaves, esp. those of herbaceous or non-woody plants, can remain firm and erect because of the turgor pressure within their cells.
  • When there is a high rate of evaporation of water from the cells, they lose their turgidity and the plant wilts
  • The movement of certain plants are due to changes in turgor. For example, opening and closing of the stomata, folding of leaflets of the Mimosa plant etc.


Active Transport:

Active transport is the process of transferring substances into, out of, and between cells, using energy. In some cases, the movement of substances can be accomplished by passive transport, which uses no energy. However, the cell often needs to transport materials against their concentration gradient. Active transport is required in these cases.


Some examples of active transport in plants include:

  • Ions moving from soil into plant roots.
  • Transportation of chloride and nitrate from the cytosol to the vacuole.
  • Sugars from photosynthesis moving from leaves to fruit.
  • Calcium using energy from ATP to move between cells.
  • Minerals traveling through a stem to various parts of the plant.
  • Water moving from plant roots to other plant cells through root pressure.

Here are some examples of active transport in animals and humans:

  • Sodium-potassium pump(exchange of sodium and potassium ions across cell walls)
  • Amino acids moving along the human intestinal tract.
  • Calcium ions moving from cardiac muscle cells.
  • Glucose moving in or out of a cell.



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