BTEC Applied Science Unit 5: Cell Transport Mechanisms

Overview

• Cell Transport: Mechanism by which substances enter and exit cells via the plasma membrane.
• Plasma Membrane: Made up of a semi-permeable lipid bilayer.
  • Fluid Mosaic Model: Describes the plasma membrane's structure as flexible and embedded with proteins.

Phospholipid Bilayer

• Structure:
  • Hydrophilic Head: Attracted to water.
  • Hydrophobic Tail: Repels water.
  • Forms a continuous layer around the cell.
• Cholesterol Molecules:
  • Make the layer less fluid and more rigid.
  • Reduce permeability to small molecules.
  • Prevent crystallization of phospholipid tails.

Types of Transport

Passive Transport

• Doesn't require energy.
• Types of Passive Transport:
  • Simple Diffusion:
    • Movement of small non-polar substances (e.g., O2, CO2).
    • Moves from high to low concentration.
  • Osmosis:
    • Diffusion of water through the membrane or aquaporins.
    • Moves towards higher solute concentration.
  • Facilitated Diffusion:
    • Involves protein channels or carrier proteins.
    • Protein Channels:
      • Allow ions and polar molecules to pass through.
      • Can be leak channels (always open) or gated channels (open in response to stimuli).
    • Carrier Proteins:
      • Change shape to transport substances across the membrane.

Active Transport

• Requires energy (ATP).
• Types of Active Transport:
  • Pumps:
    • Move ions against the concentration gradient.
    • Specific to certain ions (e.g., sodium-potassium pump).
    • Utilize ATP, converting it to ADP.
  • Vesicles:
    • Exocytosis: Transport substances out of the cell.
    • Endocytosis: Transport substances into the cell.
    • Phagocytosis: Engulfing pathogens by phagocytes.

Surface Area to Volume Ratio

• Importance:
  • Small cells have a larger surface area to volume ratio, facilitating diffusion.
  • Ensures efficient nutrient uptake and waste removal.
• Mathematics of SA:V Ratio:
  • Calculate surface area and volume.
  • Compare ratios for understanding cell size efficiency.

Conclusion

• Understanding of passive vs. active transport mechanisms is crucial.
• The fluid mosaic model and various transport methods maintain cellular function and homeostasis.