Lecture Notes: Understanding Cell Membranes and Transport Mechanisms

Introduction

• Exploring the environment inside a cell:
  • Genetic material, cytoplasm, ribosomes.
  • Found in both prokaryotes and eukaryotes.
  • Eukaryotic cells have additional membrane-bound organelles.
• Importance of cell interaction with the environment to maintain homeostasis.
  • Cell membrane plays a critical role in regulating homeostasis.

Cell Membrane Structure

• Composed of a phospholipid bilayer:
  • Two layers of phospholipids.
  • Hydrophilic (polar) heads and hydrophobic (nonpolar) tails.
• Functions:
  • Regulates what enters and exits the cell, maintaining homeostasis.

Transport Mechanisms

Passive Transport

• Simple Diffusion:
  • Movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) across the membrane.
  • Moves with the concentration gradient (high to low concentration).
  • Does not require energy.
• Facilitated Diffusion:
  • Involves transport proteins:
    • Channel proteins or carrier proteins.
    • Assist molecules that are too large or polar to pass through the membrane on their own.
  • Examples include glucose transport and movement of ions through protein channels.
  • Osmosis: special case for water movement through aquaporins.
  • Still moves with the concentration gradient and requires no energy.

Active Transport

• Moves molecules against the concentration gradient (low to high concentration).
• Requires energy, generally in the form of ATP.
  • ATP releases energy when its phosphate bond is broken.
  • Energizes transport proteins, like the sodium-potassium pump.

Bulk Transport

Endocytosis

• Endocytosis:
  • Process of taking large molecules into the cell by engulfing them.
  • Forms vesicles for transport inside the cell.
  • Types include:
    • Phagocytosis: "cell eating"; amoebas use pseudopods to engulf particles.
    • Receptor-mediated endocytosis: specific, requires binding to receptors.
    • Pinocytosis: "cell drinking"; uptake of fluids.

Exocytosis

• Reverse process of endocytosis.
• Used for expelling waste and releasing cell-produced materials, such as polysaccharides for plant cell wall construction.

Conclusion

• Cell membranes are essential for maintaining the internal environment of a cell and managing transport processes.
• Encouragement to explore further and stay curious about cell biology.