Movement of Substances

Movement of Substances

Diffusion

  • Definition: Net passive movement of molecules from high to low concentration.

  • Concentration Gradient: The difference in concentration; steeper gradient = faster diffusion.

Factors Affecting Diffusion Rate

  • Particle Size: Smaller particles diffuse faster.

  • Membrane Thickness: Thinner membranes promote faster diffusion.

  • Surface Area: Larger surface area increases diffusion rate.

  • Distance: Shorter distances facilitate faster diffusion.

  • Temperature: Higher temperatures increase particle kinetic energy, speeding up diffusion.

Partially-Permeable Membrane

  • Allows selective passage of molecules; typically permits small particles but not large ones.

Osmosis

  • Definition: Net passive movement of water from higher water potential to lower water potential across a partially-permeable membrane.

  • Water Potential: Higher water potential in dilute solutions compared to concentrated ones.

  • Osmosis in Cells:

  • Hypotonic: Cell gains water (swells).

  • Isotonic: No net water movement.

  • Hypertonic: Cell loses water (shrinks).

Tonicity

  • Isotonic: Equal solute concentration inside and outside the cell.

  • Hypertonic: Higher solute concentration outside the cell.

  • Hypotonic: Lower solute concentration outside the cell.

Active Transport

  • Definition: Movement of substances against a concentration gradient using ATP.

  • Involves specific carrier proteins; cells require energy, typically many mitochondria are present.

  • Examples: Transport of glucose and ions (Na+, K+) across membranes.

Facilitated Diffusion

  • Utilizes channel and carrier proteins for the passive movement of large or charged molecules down their concentration gradient.

  • Continues to be a passive process without the need for energy.

Endocytosis and Exocytosis

  • Endocytosis: Process of bringing materials into the cell via vesicles (phagocytosis for solids and pinocytosis for liquids).

  • Exocytosis: Releasing materials from the cell by vesicle fusion with the plasma membrane.

Surface Area to Volume Ratio

  • As organisms grow, their surface area to volume ratio decreases, affecting nutrient exchange.

  • Key principle: Larger surface area allows more efficient exchange; critical for cellular function.

Conclusion

  • Movement of substances is crucial for cellular function and is influenced by concentration gradients, membrane properties, and transport mechanisms. Efficient nutrient uptake and waste removal are essential for cell survival.