This worksheet challenges students to identify different types of cellular transport, including simple diffusion, osmosis, and active transport using ATP.

Here's the complete solution to the worksheet “Passive and Active Transport,” with explanations for each question.

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Question 1: Lipid Movement

Diagram: Shows lipids (black circles) on both sides of a phospholipid bilayer. More lipids are on the bottom side.

Task:
- Draw an arrow showing direction lipids will move.
- Circle terms that describe this transport.

✔ Answer:
- Arrow direction: From bottom → top (from high concentration to low concentration).
- Circled terms: Passive, Diffusion

Explanation:
Lipids are nonpolar molecules that can dissolve in the lipid bilayer. They move by simple diffusion — from an area of higher concentration to lower concentration — without energy or a protein carrier. This is passive transport because no energy is used.

> ❗ Note: Lipids do NOT require facilitated diffusion here — they pass directly through the membrane via simple diffusion.

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Question 2: Water Movement

Diagram: Shows water molecules (blue circles) on both sides of a membrane with a channel protein. More water is on the left.

Task:
- Label sides as “HIGH” (high water concentration) and “LOW” (low water concentration).
- Draw arrow showing water flow.
- Circle correct terms.

✔ Answer:
- Left side = HIGH (more water molecules)
- Right side = LOW (fewer water molecules)
- Arrow direction: Left → Right
- Circled terms: Passive, Osmosis

Explanation:
Water moves across a semi-permeable membrane from an area of higher water concentration to lower water concentration — this is called osmosis. It’s a type of passive transport because it doesn’t require energy. The channel protein may be an aquaporin, which makes this *facilitated osmosis*, but since “Osmosis” is listed and is the most specific term, we circle Osmosis and Passive. Facilitated Diffusion could also apply if you’re being precise about the protein, but typically in introductory biology, osmosis is treated as its own category under passive transport.

> 📌 For this level, circling Passive and Osmosis is sufficient and expected.

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Question 3: Fill-in-the-blank

Text:
“The water molecules will continue to move with this (semi-permeable membrane / concentration gradient) until they reach (diffusion / equilibrium). The cell membrane helps maintain (homeostasis / diffusion).”

✔ Answer:
- “...with this concentration gradient...”
- “...until they reach equilibrium.”
- “...helps maintain homeostasis.”

Explanation:
- Water moves *down* its concentration gradient (from high to low water concentration).
- Movement stops when concentrations are equal on both sides — this is equilibrium.
- The cell membrane regulates what enters/exits to keep internal conditions stable — this is homeostasis.

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Question 4: Ion Movement Using Energy

Diagram: Shows ions (gray circles with minus signs) moving through a protein channel. A lightning bolt (⚡) indicates energy use. Ions are moving from right to left, even though there are more ions on the left already.

Task:
- Label each side as High or Low.
- Draw arrow showing ion movement.
- Circle correct terms.

✔ Answer:
- Left side = HIGH (more ions)
- Right side = LOW (fewer ions)
- Arrow direction: Right → Left (against the gradient!)
- Circled terms: Active

Explanation:
Ions are moving from low concentration (right) to high concentration (left) — against the concentration gradient. This requires energy (shown by the lightning bolt), so it’s active transport. No other terms apply here — it’s not passive, osmosis (water only), or simple/facilitated diffusion (which are passive).

> ✔ Only Active should be circled.

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## ✔ Final Answers Summary:

1. Lipid Movement

- Arrow: Bottom → Top
- Circled: Passive, Diffusion

2. Water Movement

- Left = HIGH, Right = LOW
- Arrow: Left → Right
- Circled: Passive, Osmosis

3. Fill-in-the-blanks

- concentration gradient
- equilibrium
- homeostasis

4. Ion Movement (Active Transport)

- Left = HIGH, Right = LOW
- Arrow: Right → Left
- Circled: Active

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