3. Movement into and out of cells (Cambridge IGCSE Biology 0610 for exams in 2023,2024 and 2025)

Overview of Movement Into and Out of Cells

• aThree main processes:

  • Diffusion

  • Osmosis

  • Active Transport

Diffusion

• Definition: Movement of molecules in and out of cells via the cell membrane, from higher to lower concentration.

• Example: Nutrient (glucose, proteins) absorption into the cell and waste (carbon dioxide, lactic acid) removal.

• Key points:

  • Molecules move until concentration is balanced.

  • Driven by constant random movement and kinetic energy.

• Influencing factors:

  • Surface Area: Larger area increases diffusion rate (more molecules can move at once).

  • Temperature: Higher temperature increases molecular speed and diffusion rate.

  • Concentration Gradient: Greater difference in concentrations accelerates diffusion rate.

  • Distance: Shorter distances increase diffusion efficiency.

Osmosis

• Importance of water: Acts as a solvent, aiding transport and excretion in organisms.

• Definition: Net movement of water molecules from a region of higher water potential (dilute solution) to lower water potential (concentrated solution), across a partially permeable membrane.

• Terminology:

  • Water potential: High in dilute solutions; low in concentrated solutions.

• Example using dialysis tubing:

  • Tubing filled with concentrated sucrose solution in distilled water.

  • Water moves across the tubing into the concentrated solution, decreasing the water level outside.

• Effects on plant tissues:

  • Cell in pure water: Water enters, causing turgidity (swelling).

  • Cell in concentrated solution: Water leaves, causing flaccidity (shrinking) or plasmolysis (cytoplasm detaching from cell wall).

• Significance of osmosis:

  • Critical for plant water uptake, nutrient transport, and maintaining turgidity.

Active Transport

• Definition: Movement of particles through the membrane from lower to higher concentration (against a concentration gradient), requiring energy from respiration.

• Application: Essential when cells need additional nutrients despite higher internal concentrations.

• Mechanism:

  • Protein carriers in the cell membrane capture and transport molecules.

  • Energy from respiration changes protein shape to facilitate transport.

Comparison of Processes

• Diffusion: Movement of particles down a concentration gradient, driven by kinetic energy.

• Osmosis: Movement of water across a partially permeable membrane; relies on water potential.

• Active Transport: Movement of particles against a concentration gradient using energy from respiration.