Diffusion and Osmosis


Plants and animals to stay alive, chemicals must be able to move easily:

  • From one part of a cell to another.
  • Into and out of the cell.
  • From one cell to another.


Movements of molecules from an area where they are more concentrated to area where they are less concentrated, down a concentration gradient until they are equally distributed.

Diffusion of one solute

Factors affecting the rate of diffusion

  • Thickness of the membrane– The thinner the membrane the faster the diffusion
  • Concentration gradient – The difference in concentration between the two regions, a region of high concentration next to the concentration of low concentration. The higher concentration gradient the faster the rate of diffusion is.
  • Surface areavolume ratio– The larger the surface area to volume ratio, the faster he rate of diffusion.
  • Size of molecules and ions– Larger molecules and ions diffuse slower than smaller ones
  • Temperature– As temperature increases rate of diffusion also increases. At lower temperature, molecules have less kinetic energy and diffusion rate is slow. At high temperature, molecules gain more kinetic energy and diffusion rate is faster.


Examples of diffusion in living organisms

In plants (gaseous exchange takes place by diffusion)

  • Movement CO2 needed for photosynthesis in to the leaf: The concentration of CO2 is high at the atmosphere and inside the air space of the leaf. As result of this concentration gradient CO2 diffuse into the leaf through the stoma (tiny opening of the leaf) from the atmosphere by the diffusion.
  • Movement of O2 produce in photosynthesis out of the leaf: In the leaf air space O2 concentration is high and in the atmosphere it is relatively low. Due to the concentration gradient, O2 moves out through the stoma by diffusion.

gas-exchange-through-stomata-of-a-leafgas exchange through stomata of a leaf2
Gas exchange through the stomata of a leaf

In animals

  • Movement of O2 from the alveoli of lung into the red blood cell: O2 concentration is high in the alveoli and low in the red blood cells. Due to the concentration difference, O2 diffuse into the red blood cells in solution as it has to pass a membrane
  • Carbondioxide in the blood diffuse in the alveoli: Due to the high concentration of CO2 in the blood and the low concentration of it in the alveoli, CO2 moves from the blood into the alveoli by diffusion. Hence gaseous exchange takes place in mammals by diffusion.

gaseous exchange in the lungs



Pure water or distilled water:  Solution which contains 100% free water molecules.

Dilute solutions:  Solutions which have relatively large number of free water molecules.

Concentrated solution: Solutions which have relatively fewer free water molecules.

Water potential:  The tendency of water molecules to move from one place to another.

Solutions having high water potential are dilute solution as they have more water molecules than solute molecules.
Solutions having low water potential are concentrated solution as they have less water molecules than solute molecules

Water always would move from a higher water potential to a lower water potential. Therefore it will always from a dilute solution to a concentrated solution.

Pure or distilled water has highest water potential.


Partially permeable membrane: Partially permeable membrane is permeable to small molecules like water, but impermeable to large water molecules like sucrose. Cell membrane of plant and animal cell is a natural partially permeable membrane. It is a living / biological membrane. Visking tube is an artificial partially permeable membrane.

Osmosis is movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

Osmosis is a special type of diffusion

Osmosis is said to be a special type of diffusion as water molecules move from where the potential of water is high to where the potential of water is low.

In osmosis, water diffusion through the partially permeable membrane, which has holes in it. Water molecules are very small compare to sugar molecules. Water molecules can pass through these holes but sugar molecules not.

Permeable membrane with tiny holes

In the above diagram the water in the beaker has higher water potential than the solution in the visking tubing (sugar solution).so water molecules moves into the visking tube by osmosis through the partially permeable membrane. After 30 minutes the level of liquid in tube x rise, as well as the level of liquid in the beaker (water level) decreases.


The effects of osmosis on animal cell

Animal cells in a solution having a higher water potential (dilute solution)

  • Water potential outside the cell would be higher than that of inside the cytoplasm.
  • Therefore water enters the cell by osmosis.
  • The cell stars expanding causing the cell membrane increase in size.
  • Since there is no cell wall, cell burst due to continuous entry of water.

The effects of osmosis on animal cell

Animals cells in a solution having a lower water potential “concentrated solution”

  • Water potential outside the cell would be lower than that of inside the cytoplasm
  • Therefore water leaves the cell by osmosis
  • The cell start shrinking causing the cell membrane to decrease in the size
  • Continuous loss of water causes the cells to become crenated.


The water potential inside most animal cells is often the same as the solution in which the cells are naturally bathed. There is little movement of water by osmosis into or out of the cell.

animal cells


The effect of osmosis on plant cells.

Plant cells in a solution having a higher water potential (dilute solution)

  • Water potential outside the cell would be higher than that of inside the cells sap.
  • Therefore water enters the cell by osmosis.
  • The vacuole increases in volume and pressing the cytoplasm against the flexible cellulose cell wall, exerting a pressure on the cell wall. This is turgor pressure.
  • The cell wall in turn exerts the pressure on the cell wall contents known as wall pressure
  • When the plant cell has absorbed all the water it possibly can, by osmosis, it is said to be fully turgid.


Plant cells in the solutions having lower potential (concentrated solution)

  • Water potential outside the cell wall would be lower than that of inside the cell sap.
  • Therefore water leaves the cell by osmosis.
  • The vacuole decreases in volume and causing the cell membrane detached and pull away from the cell wall.

Pressure is no longer exerted on the cell wall so the cell become flaccid and shrinks. Lots of water could cause the cell to be plasmolysed.



Importance of turgidity

Turgidity provides structural support o the plant tissues, keeping the stem upright. It keeps the leaves flat so that they can better absorb light. Turgor pressure is the main means of support herbaceous (soft stemmed non wood) plants like baisam of osmosis.

Active Transport

Active transport is the movement of substances from an area where they are low concentrated to an area where they are high concentrated, against the concentration gradient, through biological membrane.

A biological membrane is needed because, for substance to be transported against concentration gradient, energy is needed. The energy needed is released during respiration in living organisms.


Examples of active transport

uptake of mineral salts (ions) by plant.

Plants need o absorb minerals salts from the soil. However, the concentrations of minerals salts are low in the soil and concentrations of minerals salts are high in the root hair cell. Therefore, mineral salt are taken into the root hair cell by active transport using energy.

-absorption of digested food (e.g. glucose) from the ileum into the villi of small intestine.

The concentration of digestive food such as glucose is high in the cell of the villi, but low in the ileum. Therefore the digested food is absorbed into the cells of the villi by active transport using energy.

Difference between Diffusion, Osmosis and Active Transport

No membrane needed Partially permeable membrane Biological membrane
Any particle Only water molecules Any particle
Along the concentration gradient Along the water potential gradient Against the concentration gradient
High concentration to low concentration High water potential to low water potential Low concentration to high concentration