Osmosis: What Is It & How Does It Work?

What is Osmosis?

Osmosis is the movement of water particles from a region of high water concentration to a region of low water concentration through a partially permeable membrane.

A diagram showing osmosis and the partially permeable membrane

Did you know?

• Osmosis is a form of diffusion

• Osmosis is responsible for providing structure to plants so that they stand upright

• Osmosis occurs faster when the temperature is warmer

• Osmosis can lead to the bursting of animal cells but not plant cells

Ways to demonstrate osmosis

You can observe osmosis in very simple ways at home with solutions containing different quantities of dissolved salt or sugar. Kitchen items which could be used may include:

1. Naked eggs - eggs with the shells removed using vinegar

2. Expanding raisins

3. Gummy bears

Place the item in the solution and observe the changes.

History of osmosis

Observations were documented as far back as 1748 by French abbot Jean-Antoine Nollet. although the term ‘osmosis’ was coined in 1826 by French physician RJH Dutrochet.

The study of osmosis was advanced significantly after the development of the semi partially membrane by Moritz Traube in 1867.

Osmosis in Biological Systems

Water is considered a universal solvent, which means that it carries dissolved substances around within organisms. For example, glucose can be absorbed into the blood in the digestive system and urea can move into the blood from the liver, the blood is circulated around the body.

Substances dissolved in water can then be moved into and out of cells via the cell membrane, a selectively permeable membrane surrounding cells.  For example, glucose can move into respiring cells from the blood and urea can move into the nephron from the blood in the kidney.

Osmosis is responsible for  ensuring correct cellular shape which ensures efficiency of cell activity e.g. red blood cells have a biconcave shape which maximises surface area to volume ratio and therefore uptake of oxygen

Osmosis is required to provide water to photosynthesising plant cells. Water moves by osmosis into the root hair cells of a plant, driven by the high solute concentration of the cytoplasm within the root cells.

Osmosis in GCSE Biology

Osmosis is covered across many different parts of the GCSE and iGCSE specification, this varies between exam boards, but you will find links to osmosis in topics which cover:

1. Transport across membranes - Osmosis is one of the key processes by which substances more across the cell membrane along with active transport, facilitated diffusion and simple diffusion.

2. Absorption in the intestines - reabsorption of water in the intestine is done by the process of osmosis

3. Reabsorption in the kidney - The kidney is responsible for filtering the blood and removing waste as well as excess water and mineral ions.

4. Transpiration in plants - Not all of the transpiration stream uses osmosis, for example, water in the xylem does not move by osmosis as there is no partially permeable membrane for the water to move through. However, the the movement in the xylem is driven by evaporation of water from spongy mesphyll cells and subsequence osmosis of water through the cells in the leaves to replace the water lost.

5. Water and mineral uptake in root hair cells - mineral ions are actively transported into the root cells which results in osmosis of water down the concentration gradient into the cell.

6. Opening and closing of stomata -  osmosis of water into guard cells results in the opening of stomata. When the stomata are open, gas exchange occurs as well as transpiration

7. Practical investigations - You may study osmosis through observations made of plant tissue, such as potato, soaked in water of different concentrations. This investigation shows that water moves into the potato cells from solution when the solution has a higher water concentration than the potato cells and out of the potato cells when the solution  has a lower water concentration than the cells.

These concepts are covered in all Biology specifications, why not take a look at our course-specific revision notes:

• GCSE Biology Revision

• IGCSE Biology Revision

Osmosis in A Level Biology

In addition to the topics covered at GCSE, A Level courses also cover:

1. Translocation in plants - The movement of sugars around the plant is key to ensuring that all cells are provided with glucose for respiration. Osmosis of water from xylem to phloem aids the process of translocation of sucrose.

2. Tissue fluid formation - Tissue fluid is necessary to provide substances to respiring cells and for the removal of waste products. Water is initially forced from the capillaries due to high hydrostatic pressure, however, osmosis is responsible for the movement of water back into the capillaries containing waste products for removal from the body.

3. Urine production and control of blood water potential - water moves by osmosis to and from the filtrate in the kidney. This mechanism ensures that the water potential of the blood is kept within a range which is necessary for the human body cells to function efficiently. Urine volume and concentration is dependent on the water potential of the blood.

4. Practical investigations - similar to the investigations completed at GCSE, but also including the use of water potential calibration curves

These concepts are covered in all Biology specifications, why not take a look at our course-specific revision notes:

• A Level Biology Revision

Osmosis in Chemistry

Osmosis and GCSE Chemistry

Whilst osmosis is considered one of the fundamental components of GCSE and Alevel Biology courses, it also appears in Chemistry in a process called reverse osmosis.

What is reverse osmosis?

Reverse osmosis is a process used to produce drinking water from sea water. The process involves pushing salty water through a membrane to separate the salt molecules from the water. 

However, the process is called reverse osmosis, because it uses energy to make sure that the water is all extracted, this involves going against the water concentration gradient.

You can read more about reverse osmosis in our GCSE Chemistry Revision Notes.

Osmosis FAQs

Is osmosis active or passive?

Osmosis is a passive process because it requires no energy to move the water particles. This is because they move down a concentration gradient.

Does osmosis require energy?

No, no energy is required for osmosis as it is a passive process.

Where does osmosis occur in plants?

Osmosis occurs throughout all cells of a plant. From the movement of water into the root hair cells to the movement of water into the spongy mesophyl cells of the leaf where the water evaporates during transpiration. Osmosis is also responsible for the movement of sugars around the plant in the process of mass flow. Remember that all photosynthesising cells require water so water must move into these cells by osmosis.

What are the differences between osmosis and diffusion?

Osmosis and diffusion are similar in many ways, but the key differences are that osmosis applies to water particles only and requires a partially permeable membrane.

Is osmosis a type of active transport or diffusion?

Osmosis is a type of diffusion because it is the movement of particles down a concentration gradient without the need for energy. Whereas active transport is the movement of particles against the concentration gradient, this requires energy from ATP.

Glossary