Diffusion and Osmosis: Biology

Diffusion

→Diffusion is the net movement of particles of a substance from a region of higher concentration to a region of lower concentration down a concentration gradient.
→The difference in concenrtrations of the substance between two regions is known as concentration gradient or diffusion gradient.
→Diffusion applies to all molecules, with the exception of H2O molecules.
→Diffusion can occur with or without a membrane. 
→Diffusion does not require energy.
→Diffusion will never stop, as substances will always be in constant motion.
→However, diffusion will finally result in an equal distance of substances, with the substances evenly spread out.

Partially-Permeable Membrane

→A partially-permeable membrane only allows some substances to pass through but not others.
→This restricts the movement of flow of molecules.

Examples of partially-permeable membranes:
→Visking tubing
→Cell membrane
→Cellophane bag

Note:Cell wall is a permeable membrane, not a partially-permeable membrane.

Osmosis
→Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration.
→Osmosis only occurs in H2O molecules.
→Osmosis requires a partially-permeable membrane.
→Osmosis does not require energy.

Water potential: measure of the tendency of water to move from area to another.

→Dilute solutions have more water(solvent) than dissolved substances(solutes) compared to another solution.
Example: 5% sugar solution:
               5% sugar molecules;
               95% water molecules

→Concentrated solutions have less water(solvent) than dissolved substances(solutes) compared to another solution.
Example: 10% sugar soluation:
               10% sugar molecules;
               90% water molecules


Active Transport*
→Active transport is the net movement of a substance against the concentration gradient(from low to high concentration).
→Active transport requires energy.

*more information required

Higher water potential                                                           Lower water potential

    Pure water-------------->Dilute sugar solution-------------->Concentrated sugar solution
    Pure water-------------->Dilute salt solution------------------>Concentrated salt solution

Lower concentration                                                           Higher concentrtion

    Pure water<--------------Dilute sugar solution<--------------Concentrated sugar solution
    Pure water<--------------Dilute salt solution<------------------Concentrated salt solution