3.2 Osmosis

2026 Syllabus Objectives

  1. Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport
  2. State that water diffuses through partially permeable membranes by osmosis
  3. State that water moves into and out of cells by osmosis through the cell membrane
  4. Investigate osmosis using materials such as dialysis tubing
  5. Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations
  6. State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall
  7. Describe osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
  8. Explain the effects on plant cells of immersing them in solutions of different concentrations by using the terms: turgid, turgor pressure, plasmolysis, flaccid
  9. Explain the importance of water potential and osmosis in the uptake and loss of water by organisms

The Importance of Water in Living Organisms 💧

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Water is one of the most important compounds in living organisms, making up approximately 80% of some organisms' bodies. It has many critical functions, particularly as a solvent for numerous substances.

Water as a Solvent

A compound is a substance formed by the chemical combination of two or more elements in fixed proportions. Water acts as an essential solvent in organisms for several key processes:

In Digestion:

  • When we swallow food, some of it dissolves in water in the alimentary canal
  • Enzymes are also dissolved in this water and can act on the dissolved food

In Excretion:

  • The kidneys excrete urea dissolved in water to form urine
  • This allows waste products to be removed from the body in solution

In Transport:

  • Substances are transported around the body dissolved in the water in blood plasma
  • This enables nutrients, gases, and other materials to move efficiently throughout the organism

Understanding Osmosis 🔬

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Basic Concept of Osmosis

Every cell in an organism's body has water inside it and outside it. Many different substances are dissolved in this water, and their concentrations may be different inside and outside the cell. This creates concentration gradients down which water and solutes will diffuse, if they are able to pass through the membrane.

Consider a simple situation involving a concentrated sugar solution separated from a dilute sugar solution by a membrane. The membrane has holes or pores in it which are very small. An example of such a membrane is dialysis tubing.

Partially Permeable Membranes

A partially permeable membrane is a membrane (very thin layer) that lets some particles move through it, but prevents others from passing through.

Key characteristics:

  • Water molecules are very small (made of two hydrogen atoms and one oxygen atom)
  • Sugar molecules are many times larger than water molecules
  • In dialysis tubing, the holes are big enough to let water molecules through, but not sugar molecules

How Osmosis Works

When two solutions of different concentrations are separated by a partially permeable membrane:

Concentration gradients exist for both molecules:

  • There is a higher concentration of sugar molecules on one side
  • There is a higher concentration of water molecules on the other side

Movement of sugar molecules:

  • Sugar molecules cannot pass through the membrane because the holes are too small
  • They cannot diffuse to equalize their concentration

Movement of water molecules:

  • On the side with dilute solution, there is a high concentration of water molecules
  • On the side with concentrated solution, the concentration of water molecules is lower because a lot of space is taken up by sugar molecules
  • Because there are more water molecules on the dilute side, at any one moment more of them will "hit" a hole in the membrane and move through
  • Over time, there will be an overall, or net movement of water from the dilute side to the concentrated side

🔑 Key Definition: Osmosis is the diffusion of water molecules through a partially permeable membrane.

Important insight: Osmosis is really just a kind of diffusion. It is the diffusion of water molecules in a situation where the water molecules but not the solute molecules can pass through a membrane.

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