Lesson at a Glance | He Tirohanga Whakamua
Ngā Whāinga Ako | Learning Intentions
Students will know
- That passive transport moves solutes/water down a concentration gradient (from high to low concentration) without requiring cellular ATP energy.
- The difference between Simple Diffusion (direct lipid passage) and Facilitated Diffusion (using channel or carrier proteins for polar/charged particles).
- The definition of Osmosis: net movement of water molecules from a high water potential (low solute) to a low water potential (high solute) area across a semi-permeable membrane.
Students will demonstrate
- By solving 3 osmotic tonicity scenarios for plant and animal cells.
- By completing formative Section 3 in their class exercise book or revision folder.
Curriculum alignment
- NZC (2007) · Science · Level 7 · Living World: “Explore the diverse ways in which animals and plants carry out the life processes.”
Do Now | Tīmatanga Whakaaro (10 min)
Osmosis Scenario Prompt:
"If you place a limp celery stalk into a beaker of pure fresh water overnight, it becomes crisp and rigid again. Why does a crisp celery stalk placed in heavy saltwater go completely limp?"
Unpack: In pure water (hypotonic), water enters celery cells by osmosis down the water potential gradient, filling the vacuole and pressing against the cell wall to generate turgor pressure. In saltwater (hypertonic), water leaves the vacuole by osmosis, causing plasmolysis as the cell membrane pulls away from the cell wall.
Diffusion Spectrum (15 min)
1. Simple diffusion
Down the concentration gradient, through the bilayer itself. No protein, no ATP. This is how O₂ and CO₂ move, and why lipid-soluble drugs cross membranes so easily.
2. Facilitated diffusion
Still down the gradient, still no ATP — but the molecule cannot cross the fatty core alone, so it uses a channel or a carrier protein. Glucose and ions move this way.
3. What sets the rate
For both: how steep the gradient is, temperature, and how much membrane surface is available. For facilitated diffusion there is one more — how many transport proteins there are.
4. Two distinct passive mechanisms
Both mechanisms move substances down an electrochemical gradient without direct ATP use, but they are distinct. Simple diffusion passes through the lipid bilayer; facilitated diffusion requires a specific channel or carrier protein and can reach a transport maximum when those proteins are occupied.
Mahi | Do this: Plot rate of transport against concentration gradient for both types on the same axes. One curve keeps rising; the other flattens. Write one sentence explaining what has run out when it flattens — that plateau is the whole difference.
Tonicity & Cell Responses (15 min)
| Environment Solution | Water Potential (Ψ) | Animal Cell Response | Plant Cell Response |
|---|---|---|---|
| Hypotonic (Pure Water) | High water potential outside cell | Lysis (cell bursts - no wall) | Turgid (firm turgor pressure) |
| Isotonic (Equilibrium) | Equal water potential inside & out | Normal / Homeostatic | Flaccid |
| Hypertonic (Salt Water) | Low water potential outside cell | Crenation (shrivels) | Plasmolysis (membrane shrinks from wall) |
📁 Formative Cell Biology Mastery Portfolio — Section 3: Passive Transport & Osmosis
Complete this formative practice section in your exercise book. This osmosis work prepares you for the external standard; it is not a separate NZQA submission. Use the transport explanations and tonicity table on this page, so no separate workbook or case-study source is needed.
Section 3 Requirements:
1. Simple vs Facilitated Comparison Diagram: Draw molecular movement for simple diffusion (O₂) vs facilitated diffusion (Glucose via carrier protein).
2. Water Potential & Tonicity Diagram: Annotate Plant vs Animal cells in Hypotonic, Isotonic, and Hypertonic environments.
3. Scenario Analysis: A turgid plant cell is moved from pure water into concentrated salt solution. In one paragraph, trace the water-potential gradient, direction of net water movement, change in the vacuole, and the shift from turgor towards plasmolysis.
Exit Verification | Ka Mutu Hoki (5 min)
Exit Check:
"My Section 3 correctly defines osmosis in terms of water potential gradient and explains plant cell turgor pressure."
Teacher Planning & NCEA Alignment
NCEA Level 2 Biology Alignment (4 Credits External):
- Passive Transport: Demonstrate understanding of diffusion, facilitated diffusion, osmosis, and water potential.
- Cell Homeostasis: Explain plant turgor, plasmolysis, and animal cell volume regulation under varying osmotic conditions.
Vocabulary: Passive transport, simple diffusion, facilitated diffusion, osmosis, water potential gradient, hypertonic, hypotonic, isotonic, turgor pressure, plasmolysis, crenation, lysis.
Paired concept in Guided Media & Evidence: passive transport mechanisms →