2.8.A.1 Movement of Substances

The movement of substances between cells and into and out of a cell differs in the following respects:

• The movement of substances may occur across a selectively permeable membrane (allows only specific substances to pass) such as the plasma membrane
• The substance whose movement is being described may be solvent or solute
• Movement of substances may occur from higher to lower concentrations (down the concentration gradient) or the reverse (up or against the gradient)
• A solute may be:
  • Hypertonic (a higher concentration of solutes)
  • Hypotonic (a lower concentration of solutes)
  • Isotonic (an equal concentration of solutes) relative to another region
• The movement of substances may be passive or active (active movement requires the expenditure of energy and usually occurs up a gradient)

Bulk Flow

• Collective movement of substances in the same direction in response to a force or pressure
• Example: Blood moving through a blood vessel

Passive Transport Processes

• Movement of substances from regions of higher to lower concentrations (down a concentration gradient)
• Do not require (ATP) energy

1. Diffusion/Simple Diffusion
   • Movement from higher to lower concentration
   • Occurs as a result of random and constant motion of molecules
   • Eventually molecules gets uniformly distributed (Equilibrium)
2. Osmosis
   • Diffusion of water molecules across selectively permeable membrane
   • Hydrostatic pressure (osmotic pressure) build up inside the body
   • Example: Turgor pressure is developed when water enters into plants and microorganisms
3. Dialysis
   • The diffusion of solutes across a selectively permeable membrane
   • Different solutes are separated by selectively permeable membrane
4. Plasmolysis
   • Movement of water out of a cell (osmosis) causing collapse of cell mainly in plants
5. Facilitated Diffusion
   • Diffusion of solutes or water through channel proteins and even without the aid of specialized proteins
   • Aquaporins increase the rate of transfer
6. Countercurrent Exchange
   • The diffusion of substances between two regions in where substances move by bulk flow in opposite directions
   • Example: Fish gills – water flows opposite to direction of blood flow thus maximizing the oxygen diffusion

Active Transport

• Movement of solutes against a gradient and requires energy (usually ATP)
• Use transport proteins (Na+, K+, Cl–, H+, amino acids, and monosaccharides)

Vesicular Transport

• Use vesicles or other bodies in cytoplasm to move macromolecules or large particles

1. Exocytosis
   • Vesicles fuses with plasma membrane and releasing their contents out
2. Endocytosis
   • Plasma membrane engulfs substances into vesicle
   • Three types:
   1. Phagocytosis (cellular eating) – occurs when undissolved material enters cell (solid particles) – Example: white blood cells
   2. Pinocytosis (cellular drinking) – occurs when dissolved substances enter the cell (liquid particles)
   3. Receptor-mediated endocytosis – occurs when specific molecules (ligand) in fluid surrounding the cell bind to specialized receptors in coated pits
      • Example: Low-density lipoproteins or LDL cholesterol