Overview
This lecture covers active transport in cells, focusing on its energy requirements, mechanisms, and key examples such as the sodium-potassium pump and vesicular transport.
Principles of Active Transport
- Active transport moves substances against their concentration gradient, from low to high concentration.
- This process always requires energy, typically from ATP (adenosine triphosphate).
- Transport occurs across cell membranes using protein pumps.
Types of Protein Pumps
- Pumps can move one or multiple substances at a time.
- Substances may move in the same (symport) or opposite (antiport) directions.
Sodium-Potassium Pump (Primary Active Transport)
- The sodium-potassium pump uses ATP directly to function (primary active transport).
- Pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell per ATP used.
- This creates high sodium concentration outside and high potassium concentration inside the cell.
- Pump sets up the negative resting membrane potential (more positive outside, more negative inside).
- Essential for electrophysiology and cell absorption processes.
Secondary Active Transport
- Uses ATP indirectly by relying on gradients established by primary active transport pumps.
- Example: Sodium gradient (created by the sodium-potassium pump) drives sodium back into the cell, simultaneously carrying glucose or amino acids against their gradient.
- One molecule moves down its gradient, while another is transported against its gradient.
Vesicular (Bulk) Transport
- Uses vesicles to move large particles or volumes across membranes; requires significant energy.
- Endocytosis brings substances into the cell via vesicle formation.
- Phagocytosis: "Cell eating" large particles.
- Pinocytosis: "Cell drinking" fluids and small solutes.
- Receptor-mediated endocytosis: Specific molecules bind to membrane receptors to initiate uptake.
- Exocytosis releases substances from the cell by vesicle fusion with the membrane.
- Transcytosis moves substances across the cell by combining endocytosis and exocytosis.
- Example: Release of digestive enzymes from pancreatic cells and acetylcholine at neuromuscular junctions.
Key Terms & Definitions
- Active Transport — movement of substances against their concentration gradient, requiring energy.
- ATP (Adenosine Triphosphate) — molecule that stores and supplies energy for cellular processes.
- Sodium-Potassium Pump — primary active transport protein moving sodium out and potassium into the cell.
- Primary Active Transport — uses ATP directly for moving substances.
- Secondary Active Transport — uses gradients established by primary pumps to move other substances.
- Symport — movement of substances in the same direction.
- Antiport — movement of substances in opposite directions.
- Endocytosis — process of bringing substances into the cell via vesicles.
- Phagocytosis — endocytosis of large particles ("cell eating").
- Pinocytosis — endocytosis of fluids ("cell drinking").
- Receptor-Mediated Endocytosis — uptake initiated by specific molecule-receptor binding.
- Exocytosis — process of expelling substances from the cell via vesicles.
- Transcytosis — moving substances across the entire cell using endocytosis and exocytosis.
Action Items / Next Steps
- Review the passive transport lecture if not already completed.
- Prepare for the next topic: cell cycle and cell division.
- Contact the instructor with any questions on active transport.