Overview
This lecture explains the structure and function of the cell membrane, focusing on how cells control transport to maintain homeostasis through passive and active transport mechanisms.
Cell Membrane Structure and Function
- All cells have a cell membrane made of a phospholipid bilayer (two layers of lipids).
- The membrane’s heads are polar (hydrophilic), and tails are nonpolar (hydrophobic).
- The cell membrane controls what enters and exits the cell to maintain homeostasis.
Passive Transport
- Simple diffusion is the movement of small, non-polar molecules (e.g., oxygen, carbon dioxide) directly through the membrane.
- Passive transport does not require energy; molecules move from high to low concentration (with the gradient).
- Facilitated diffusion uses transport proteins to help large or polar molecules cross the membrane.
- Facilitated diffusion is still passive; it moves with the gradient and requires no energy.
- Examples include ion channels for charged ions, glucose transporters, and aquaporins for water in osmosis.
Active Transport
- Active transport moves molecules from low to high concentration (against the gradient) and requires energy (ATP).
- ATP provides energy by breaking its third phosphate bond.
- Example: sodium-potassium pump moves ions against their gradients using ATP.
Bulk Transport: Endocytosis and Exocytosis
- Endocytosis brings large molecules into the cell by forming vesicles from the membrane.
- Types of endocytosis include phagocytosis (engulfing solids), pinocytosis (taking in fluids), and receptor-mediated endocytosis (specific uptake).
- Exocytosis expels substances from the cell by fusing vesicles with the membrane.
- Exocytosis is used to remove waste and export materials like carbohydrates for plant cell wall formation.
Key Terms & Definitions
- Phospholipid bilayer — Two-layered structure forming the cell membrane with polar heads and nonpolar tails.
- Homeostasis — Stable internal environment within the cell.
- Simple diffusion — Passive movement of molecules from high to low concentration without energy.
- Facilitated diffusion — Passive movement of molecules via transport proteins.
- Passive transport — Movement of substances across the membrane without energy input, from high to low concentration.
- Active transport — Movement of substances with energy input (typically ATP), from low to high concentration.
- ATP (Adenosine triphosphate) — Energy-carrying molecule used by cells.
- Endocytosis — Process of taking large substances into the cell by engulfing them in vesicles.
- Exocytosis — Process of expelling substances from the cell via vesicles.
Action Items / Next Steps
- Research more about the sodium-potassium pump and different types of endocytosis (phagocytosis, pinocytosis, receptor-mediated).
- Review biomolecule functions (especially carbohydrates) in cell structure.