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DIFFUSION AND OSMOSIS

DIFFUSION

Definition:

Diffusion is the net movement of molecules (solid, liquid, or gas) from a region of higher concentration to a region of lower concentration, driven by random molecular motion, until dynamic equilibrium is achieved.

Examples:

  • When perfume is sprayed in a room, volatile molecules disperse from the high-concentration source to fill the entire space.
  • Oxygen diffuses from alveoli in lungs (high concentration) into capillaries (low concentration) during respiration.
Factors Affecting Diffusion Rate
  1. State of Matter: Gases diffuse fastest (molecules move freely), followed by liquids, then solids.
  2. Temperature: Increased kinetic energy at higher temperatures accelerates molecular movement (e.g., sugar dissolves faster in hot tea).
  3. Molecular Size: Smaller molecules (e.g., O₂) diffuse faster than larger ones (e.g., proteins).
  4. Concentration Gradient: Steeper gradients create stronger driving forces.
BIOLOGICAL SIGNIFICANCE OF DIFFUSION

In Plants:

  • Movement of manufactured food from the leaves to other parts of the plant via the phloem (This process is known as Translocation)
  • Stomatal gas exchange: CO₂ entry for photosynthesis and O₂ release.
  • Mineral absorption through root hairs (ion diffusion).

In Animals:

  • Gaseous exchange in paramecium, amoeba, gills of fishes etc.
  • Nutrient absorption in villi (small intestine) and nephrons (kidneys).
  • Exchange of nutrients between a foetus and the mother through the placenta.

OSMOSIS

Definition:

Osmosis is the movement of water molecules from a region of high water potential (low solute concentration) to a region of low water potential (high solute concentration), across a semi-permeable membrane until equilibrium is achieved.

Living Examples of Semi-Permeable Membranes
  • Sheep bladder
  • Pig bladder
  • Unripe paw-paw
  • Yam
  • Potato
Non-Living Examples of Semi-Permeable Membranes
  • Parchment membrane
  • Visking tube
  • Cellophane
  • Dialysis tube
Conditions Necessary for Osmosis to Occur
  1. There must be two solutions at different concentration levels
  2. The solutions must be separated by a semi-permeable membrane
Biological Significance of Osmosis

Water moves into and out of living cells by osmosis. When the solution inside the cell is more concentrated than the solution outside, the net flow of water is inward. When the solution outside is more concentrated than the solution inside, the net flow of water is outwards. The cell membrane acts as a semi-permeable membrane.

Processes Involving Osmosis in Plants
Processes Involving Osmosis in Animals

1. HYPERTONIC SOLUTION

A hypertonic solution is more concentrated than the cellular fluid of the cell. When a cell is placed in such solution, water molecules move from inside the cell to the surrounding solution (a process called exosmosis).

Effects on Plant Cells

The cell loses water to the surrounding solution. The cell volume decreases and the cell membrane pulls away from the cell wall. The plant cell becomes flaccid (weak or soft) and is said to be plasmolysed. This condition is called plasmolysis.

Effects on Animal Cells

The cell loses water to the surrounding solution. The cell volume decreases and the cell membrane becomes distorted, causing crenation (shrinkage).

2. HYPOTONIC SOLUTION

A hypotonic solution is less concentrated than the cellular fluid of the cell. Water moves from the solution into the cell (a process called endosmosis), increasing the cell volume.

Effects on Plant Cells

Water enters the cell by endosmosis, increasing cell volume. Turgor pressure develops within the cell and is resisted by the cell wall. When these opposing forces balance, the cell becomes turgid (fully charged with water), helping in plant support.

Effects on Animal Cells

Water enters the cell by endosmosis. Without a cell wall to contain the pressure, the cell may burst (a process called cytolysis).

3. ISOTONIC SOLUTION

An isotonic solution has the same concentration as the cellular fluid. The rates of exosmosis and endosmosis are equal, resulting in no net water movement. Both plant and animal cells remain normal in isotonic solutions.

Differences Between Osmosis and Diffusion

Diffusion Osmosis
Involves movement of molecules of gases, liquids and solids Involves movement of water only
No semipermeable membrane required Requires semipermeable membrane
Molecules diffuse into any available space Water moves between solutions of different osmotic pressure

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