Cell Membrane Transport
Compare passive and active transport mechanisms across the selectively permeable cell membrane. Observe simple diffusion, facilitated diffusion, osmosis, and active transport (including sodium-potassium pumps and endocytosis). Adjust concentration gradients and ATP availability to see how cells maintain homeostasis.
ACROSS THE BORDER: MEMBRANE TRANSPORT
The plasma membrane is **selectively permeable**, meaning it controls what enters and exits the cell. This allows the cell to maintain internal homeostasis regardless of external conditions.
PASSIVE VS. ACTIVE TRANSPORT
1. **Passive Transport**: Movement from HIGH to LOW concentration. Requires NO energy. Includes **Simple Diffusion** (small non-polar molecules like ), **Facilitated Diffusion** (using protein channels for polar molecules), and **Osmosis** (water). 2. **Active Transport**: Movement from LOW to HIGH concentration (against the gradient). Requires **Energy (ATP)**. Includes **Ion Pumps** and **Bulk Transport** (Endocytosis/Exocytosis).
HOW TO USE THIS VISUALIZATION
1. **Observe Simple Diffusion**: Drop oxygen molecules and watch them pass through the phospholipids. 2. **Activate the Channel**: Watch glucose enter through a protein carrier. This is facilitated diffusion. 3. **Run the Pump**: Click the 'ATP' button to pump ions against their gradient. **Try This**: Increase the concentration of solutes outside. Watch the water molecules leave the cell (Osmosis). How does this affect the cell's turgor pressure?
AP EXAM CONNECTION
Unit: Unit 2: Cell Structure and Function (Topic 2.6)
Learning Objective: ENE-2.B
COMMON MISCONCEPTIONS
- Thinking facilitated diffusion requires ATP (it does not).
- Believing water 'wants' to move (it moves based on thermodynamic gradients).
- Confusing endocytosis with exocytosis.
KEY TAKEAWAYS
- Membranes are selectively permeable.
- Passive = No energy needed.
- Active = Needs ATP.
- Bulk transport uses vesicles.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): Which process is being used when a cell moves large amounts of waste out of the cell using a vesicle?
Show Answer & Explanation
Answer: Exocytosis (Active Transport).
Explanation: Bulk transport processes require energy to move vesicles and fuse them with the plasma membrane.
Q2 (CONCEPTUAL): A molecule is large and polar. Can it pass through the cell membrane via simple diffusion?
Show Answer & Explanation
Answer: No.
Explanation: The hydrophobic core of the phospholipid bilayer repels large, polar, or charged molecules. They require a transport protein to enter.
