Osmosis and diffusion are both passive transport processes, but they describe movement of different substances and under different membrane conditions. Core distinction

• Diffusion: Random movement of solute particles from an area of higher concentration to an area of lower concentration. It can involve any type of molecule and does not require a membrane that is specific to the substance being transported. The driving force is the concentration gradient of the solute, and diffusion can occur across any permeable medium.
• Osmosis: A specific type of diffusion that involves only water moving across a selectively permeable (semipermeable) membrane. Water moves from a region of lower solute concentration (higher water potential) to a region of higher solute concentration (lower water potential) to equilibrate solute concentrations on both sides of the membrane.

Key features to remember

• Substance involved:
  • Diffusion: can be gases, liquids, or solutes (e.g., sugars, ions) moving through a medium or membrane.
  • Osmosis: only water (the solvent) moving across a semipermeable membrane.
• Membrane constraints:
  • Diffusion: movement depends on membrane permeability to the solute; some solutes may diffuse easily, others may require channels or transporters.
  • Osmosis: movement depends on the membrane being permeable to water but relatively impermeable to dissolved solutes; water moves to dilute the higher solute side.
• Energy requirement:
  • Both diffusion and osmosis are passive; they do not require metabolic energy (no ATP) when the gradient exists.
• Direction of flow:
  • Diffusion follows the solute concentration gradient from high to low.
  • Osmosis follows water potential differences, moving toward the side with higher solute concentration to balance solute levels and water potential.

Practical implications in cells

• If a cell’s membrane is permeable to a solute but not to water, diffusion can move that solute across the membrane, while osmosis governs water movement in response to solute distribution.
• When a cell is placed in a solution with higher solute concentration outside, water tends to move out of the cell by osmosis, causing shrinkage; if the outside solution is less concentrated, water enters by osmosis, potentially causing swelling.
• Diffusion explains the spread of small nonpolar molecules (like O2 and CO2) and some polar molecules through membranes, whereas osmosis specifically explains water movement across a membrane in response to solute gradients.

If you’d like, I can tailor this explanation to a specific diagram or example (e.g., xylem transport in plants or red blood cell behavior in different saline solutions) and annotate how each process would appear in that context.