Study Guide Section 5-1- Passive Transport Answers
What Is Passive Transport?
Passive transport is the movement of molecules across a cell membrane without requiring any energy from the cell. Molecules move from areas of higher concentration to areas of lower concentration—this natural tendency is called diffusion.
Cells don't have to do anything for this to happen. The process relies entirely on the kinetic energy already present in the molecules. That's the whole point. No ATP, no cellular work, no effort.
The Three Main Types of Passive Transport
1. Simple Diffusion
Small, nonpolar molecules slip directly through the phospholipid bilayer. Oxygen, carbon dioxide, and nitrogen move this way.
Key characteristics:
- Works with tiny molecules only
- No membrane proteins involved
- Movement goes with the concentration gradient
2. Osmosis
Osmosis is just diffusion applied to water. Water molecules move across a selectively permeable membrane from an area of higher water potential to an area of lower water potential.
The direction of water movement depends on solute concentration:
- Hypotonic: Lower solute concentration outside the cell → water moves IN
- Hypertonic: Higher solute concentration outside the cell → water moves OUT
- Isotonic: Equal solute concentration → no net movement
Here's the thing students get wrong: water always moves toward the higher solute concentration. That's it. Remember that and osmosis makes sense.
3. Facilitated Diffusion
Larger or polar molecules can't squeeze through the membrane directly. They need channel proteins or carrier proteins to help them cross.
- Channel proteins form tunnels through the membrane
- Carrier proteins change shape to shuttle molecules across
- Both still move molecules from high to low concentration—no energy required
Passive Transport vs. Active Transport
Students constantly confuse these two. Here's the direct comparison:
| Feature | Passive Transport | Active Transport |
|---|---|---|
| Energy source | None (kinetic energy only) | ATP required |
| Direction | High to low concentration | Low to high concentration |
| Protein involvement | Only in facilitated diffusion | Always requires carrier proteins |
| Molecule size | Small molecules (or water) | Can move large molecules |
Common Section 5-1 Study Guide Questions and Answers
Q: What is the definition of passive transport?
A: The movement of molecules across a cell membrane without using cellular energy. Molecules move down their concentration gradient.
Q: Why does passive transport not require energy?
A: Because molecules naturally move from areas of higher concentration to lower concentration due to their own kinetic energy. No push from the cell is needed.
Q: What determines the direction of passive transport?
A: The concentration gradient. Molecules always move from where there's more of them to where there's less. Always.
Q: What is the difference between diffusion and osmosis?
A: Diffusion covers all molecules. Osmosis is specifically the diffusion of water across a selectively permeable membrane.
Q: What happens to an animal cell in a hypotonic solution?
A: Water moves into the cell. The cell swells and can burst (lyse) if too much water enters. Plant cells don't burst because they have a cell wall—that's why animal cells are more fragile.
Q: What happens to a plant cell in a hypertonic solution?
A: Water leaves the cell. The cytoplasm shrinks away from the cell wall, a process called plasmolysis. The cell becomes flaccid (limp).
Q: Why can't large polar molecules diffuse directly through the membrane?
A: The phospholipid bilayer has a hydrophobic core. Large or charged molecules can't pass through this oily interior. They need channel or carrier proteins to get across.
How to Actually Remember This Stuff
Most students try to memorize. Don't. Understand the logic instead.
- Think of crowds. Molecules naturally spread out from crowded areas to less crowded areas. That's diffusion. No one forces them—they just spread.
- Think of water and salt. Salt pulls water. In a salty environment, water leaves cells. In a freshwater environment, water floods in. The solute concentration controls water movement.
- Channel proteins are doorways. Facilitated diffusion just means molecules need a door to walk through. The door doesn't require a key (energy)—they just walk through when the door is open.
Quick Reference: Passive Transport Summary
- Simple diffusion: Small nonpolar molecules cross directly through the membrane
- Osmosis: Water moves across a selectively permeable membrane
- Facilitated diffusion: Large/polar molecules use proteins to cross
- All three: No energy required, move down concentration gradient
That's everything you need for Section 5-1. Know the three types, know how to distinguish hypotonic/hypertonic/isotonic, and know why passive transport needs no energy. The rest is just vocabulary.