the diagram compares diffusion of sugar molecules and osmosis. explain the difference between osmosis and diffusion in cells.

Osmosis and diffusion are both passive transport processes in cells, but they differ in what moves, how it moves, and whether a membrane is required.

Core difference

• Diffusion is the movement of any particles (like sugar, oxygen, carbon dioxide) from a region of higher concentration to a region of lower concentration, until they are evenly spread out.

• Osmosis is a special kind of diffusion: it is only the movement of water molecules across a partially permeable membrane, from where there is more water (lower solute concentration) to where there is less water (higher solute concentration).

In terms of what moves

• Diffusion can involve gases (like oxygen in the lungs), liquids, or dissolved solids (like sugar molecules in cytoplasm).

• Osmosis always involves water molecules as the moving particles, not the solute (like sugar or salt).

Role of membrane in cells

• Diffusion in cells can occur:
  • Directly through the cell membrane for small non‑polar molecules (like oxygen and carbon dioxide).

* Through channels or carrier proteins (facilitated diffusion) for ions or larger molecules.

• Osmosis in cells:
  • Requires a selectively (partially) permeable membrane, such as the cell membrane or tonoplast in plant cells.

* The membrane lets water pass but restricts solute molecules like sugar, which is why only water moves in osmosis diagrams.

Direction of movement

• In diffusion, particles move down their concentration gradient: from higher concentration to lower concentration of that same substance (for example, sugar molecules spreading out in water on both sides of the membrane, if they can cross).

• In osmosis, water moves:
  • From a region of lower solute concentration (more free water)
  • To a region of higher solute concentration (less free water), until water concentration becomes balanced (dynamic equilibrium).

How this relates to the sugar vs. osmosis diagram

• In the “diffusion of sugar molecules” part of the diagram, sugar moves from the side where it is more concentrated to the side where it is less concentrated, as long as the membrane allows sugar through.

• In the “osmosis” part, the membrane does not let sugar molecules cross, but it does allow water to move; water travels towards the sugar‑rich side, diluting the sugar and often making that side’s volume rise.

In living cells

• Diffusion in cells helps:
  • Oxygen enter cells and carbon dioxide leave
  • Small molecules like urea move out of cells.

• Osmosis in cells:
  • Maintains turgor pressure in plant cells (keeping them firm)
  • Helps animal cells balance water; if external conditions change too much, animal cells can swell or shrink due to osmosis.

TL;DR:

• Diffusion: any particles, no membrane needed, from high to low concentration.
• Osmosis: only water, across a partially permeable membrane, from dilute solution (more water) to concentrated solution (less water).