Overview
This lecture covers the structure and function of the cell membrane, focusing on how cells control movement of substances to maintain homeostasis through passive and active transport.
Structure and Function of the Cell Membrane
- All cells have a cell membrane composed of a phospholipid bilayer.
- The bilayer has polar heads and nonpolar tails.
- The cell membrane maintains homeostasis by regulating entry and exit of substances.
Passive Transport
- Simple diffusion allows small, nonpolar molecules (e.g., oxygen, carbon dioxide) to pass directly through the membrane.
- Simple diffusion moves molecules from high to low concentration (with the gradient) and does not require energy.
- Facilitated diffusion uses membrane proteins to help large or polar molecules (e.g., glucose, ions, water) cross the membrane.
- Facilitated diffusion also moves with the concentration gradient and requires no energy (still passive transport).
- Aquaporins are protein channels specialized for rapid water movement via osmosis.
Active Transport
- Active transport moves molecules from low to high concentration (against the gradient), which requires energy (usually ATP).
- ATP (adenosine triphosphate) powers active transport, often by energizing transport proteins.
- The sodium-potassium pump is a classic example of active transport.
Bulk Transport: Endocytosis and Exocytosis
- Endocytosis brings large molecules into the cell by forming vesicles from the membrane.
- Types of endocytosis include phagocytosis (cell "eating"), receptor-mediated endocytosis (selective uptake), and pinocytosis (cell "drinking").
- Exocytosis is the process of expelling substances out of the cell, used for waste removal and secretion of large molecules like plant cell wall carbohydrates.
Key Terms & Definitions
- Cell Membrane — A phospholipid bilayer structure regulating what enters and exits the cell.
- Phospholipid Bilayer — Two layers of lipids with polar heads and nonpolar tails forming the cell membrane.
- Homeostasis — Maintenance of a stable internal environment in the cell.
- Diffusion — Movement of molecules from high to low concentration.
- Passive Transport — Transport across the membrane without energy input.
- Facilitated Diffusion — Passive transport using proteins to help molecules cross the membrane.
- Active Transport — Energy-requiring movement of substances against their concentration gradient.
- ATP (Adenosine Triphosphate) — Energy molecule used by cells for active transport.
- Endocytosis — Process of taking substances into the cell by engulfing them in a vesicle.
- Exocytosis — Process of expelling substances from the cell via vesicles.
Action Items / Next Steps
- Research different types of endocytosis (phagocytosis, receptor-mediated endocytosis, pinocytosis).
- Review the sodium-potassium pump as an example of active transport.
- Watch the biomolecule video for more details on carbohydrates.