Diffusion and Osmosis Lecture Notes

1. Define Diffusion and Osmosis

• Diffusion: The net movement of molecules from a region of higher concentration to a lower concentration down a concentration gradient.

  • Occurs in gases and liquids.

  • Does not require energy (passive process).

  • Example: Oxygen diffusing from the lungs into the bloodstream.

• Osmosis: The movement of water molecules across a semi-permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).

  • Only water (solvent) molecules move, not solutes.

  • Also a passive process, requiring no energy.

2. Define and Distinguish Passive and Active Movement

• Passive Movement:

  • Movement of substances without energy.

  • Includes Diffusion, Facilitated Diffusion, and Osmosis.

  • Substances move down their concentration gradient.

• Active Movement:

  • Movement of substances against the concentration gradient, from low to high concentration.

  • Requires energy, usually in the form of ATP.

  • Includes Active Transport, Endocytosis, and Exocytosis.

  • ATP is broken into ADP, releasing energy for the process​.

3. Concentration Gradient and Its Role

• A concentration gradient is the difference in the concentration of a substance between two regions.

  • In passive transport, substances move down the gradient (high → low).

  • In active transport, substances move against the gradient (low → high), which requires energy input.

4. Effect of Temperature on Diffusion

• As temperature increases, molecules move faster because they have more kinetic energy.

  • This causes faster diffusion.

  • At lower temperatures, molecules move slower, so diffusion occurs more slowly.

5. Gases and Liquids Can Diffuse

• Gases (e.g., oxygen and carbon dioxide) and liquids (e.g., food dye in water) can undergo diffusion.

• In cells, gases diffuse through membranes—for example, oxygen enters cells and carbon dioxide exits by diffusion.

6. Only Solvent Molecules (Water) Involved in Osmosis

• In osmosis, only water molecules (solvent) pass through the semi-permeable membrane.

• Solute molecules (like sugar or salt) do not move through the membrane during osmosis.

• Osmosis is selective for water only.

7. Semi-Permeable Membrane in Osmosis

• A semi-permeable membrane allows only certain molecules (like water) to pass while blocking others (like solutes).

• It's crucial in osmosis to regulate water balance in cells.

• Example: Dialysis tubing experiment simulates this—only water moves through the tubing, not sugar or salt​.

8. Examples of Diffusion and Osmosis in Living Organisms

Diffusion Examples:

• Gas exchange in lungs: Oxygen diffuses into red blood cells; CO₂ diffuses out into alveoli.

• Nutrient absorption: Glucose diffuses from the small intestine into the bloodstream.

Osmosis Examples:

• Root hair cells absorb water from the soil by osmosis.

• Red blood cells swell or shrink in solutions due to osmosis.

• Experiment: Dialysis tubing filled with sugar solution placed in water gains mass because water enters by osmosis​