IGCSE Biology: Movement Into and Out of Cells

Main Transport Mechanisms

1. Diffusion
2. Osmosis
3. Active Transport

Diffusion

• Definition: Net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient, due to random movement.
• Cellular Role: Moves essential molecules like glucose and proteins into cells and waste products like CO2 and lactic acid out of cells.
• Factors Influencing Diffusion:
  • Surface Area: Larger surface area increases diffusion rate.
  • Temperature: Higher temperature increases diffusion rate due to increased kinetic energy.
  • Concentration Gradient: Greater difference increases diffusion rate.
  • Distance: Shorter distance increases diffusion rate.

Osmosis

• Definition: Movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) through a partially permeable membrane.
• Importance of Water:
  • Solvent for transport and digestion.
  • Necessary for excretion (e.g., urea removal via urine).
• Example: Dialysis tubing experiment with sucrose solution demonstrates osmosis with a partially permeable membrane.
• Effects on Plant and Animal Cells:
  • Turgid: Plant cells become firm when water enters.
  • Flaccid: Cells shrink when water exits.
  • Plasmolysis: Excessive water loss causes cytoplasm to pull away from cell wall.

Role of Water Potential

• Plant Water Uptake: Water enters roots by osmosis due to water potential difference, maintaining turgidity for support.
• Wilting: Occurs when water loss exceeds uptake.

Active Transport

• Definition: Movement of particles from a lower to a higher concentration against the gradient using energy from respiration.
• Cellular Role: Allows absorption of nutrients against concentration gradients, important in root hair cells and epithelial cells of villi.
• Process:
  • Involves protein carriers in cell membranes.
  • Requires energy to change protein shape for transportation.

Comparison of Transport Processes

• Diffusion:
  • Movement of particles down a concentration gradient.
  • Energy from random kinetic movement.
• Osmosis:
  • Movement of water from high to low water potential.
  • Involves a partially permeable membrane.
• Active Transport:
  • Movement against a concentration gradient.
  • Requires energy from respiration.

Conclusion

• Understanding these processes is crucial for explaining how cells interact with their environment to maintain homeostasis.

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