Open with the phenomenon question: 'Why does a freshwater fish placed in the ocean die within hours — and why does a saltwater fish placed in a freshwater lake die just as fast?' Show two labeled beaker photos on the slide (fish + tonicity arrows). Give students 2 minutes to write a prediction in their notebook, then 3 minutes to share with a partner. Cold-call two pairs. Expect answers about 'salt getting in' — that IS the misconception you'll fix today (osmosis moves WATER, not solute). Bridge: 'By the end of this block you'll be able to draw the water arrows, name the transport proteins involved, and explain why water — of all molecules — is the one doing the moving.'

1. 8m

   Homeostasis and the Membrane as a Selectively Permeable Bilayer

   Content

   Homeostasis is the cell's job of keeping its internal conditions — water, ions, pH, temperature — steady even when the outside changes. The gatekeeper is the cell membrane, which is NOT a solid wall. It is a phospholipid bilayer: two layers of phospholipids arranged tails-to-tails, with hydrophilic phosphate heads pointing outward toward water and hydrophobic fatty-acid tails hidden in the middle. This gives the membrane its selective permeability. Small nonpolar molecules like O₂ and CO₂ slip through the hydrophobic core easily. Water, though polar, is small enough to trickle through and also uses aquaporin channels. Ions like Na⁺, K⁺, Ca²⁺, and large polar molecules like glucose CANNOT cross the tails on their own — they need transport proteins.

   Delivery

   Emphasize the word 'selective' — the membrane picks. Contrast a brick wall (blocks everything) with a nightclub door with a bouncer (chooses). Pre-empt the misconception that the membrane is a solid barrier by naming the fluid mosaic: phospholipids drift, proteins float in the bilayer. Quick check: 'Which of these can cross the bilayer without help — O₂, Na⁺, glucose, CO₂?' (Answer: O₂ and CO₂.) Foreshadow: the reason ions need pumps is exactly why active transport exists.

2. 8m

   Passive Transport: Diffusion, Facilitated Diffusion, and Osmosis

   Content

   Passive transport moves substances DOWN their concentration gradient and costs the cell zero ATP — the energy comes from random thermal motion of particles. Three flavors: (1) Simple diffusion — O₂ and CO₂ move straight through the bilayer from high to low concentration. (2) Facilitated diffusion — ions and glucose ride through channel proteins (like a tunnel) or carrier proteins (which change shape); still down the gradient, still no ATP. (3) Osmosis — the diffusion of WATER across a selectively permeable membrane, from the side with LOWER solute concentration to the side with HIGHER solute concentration. In a hypotonic solution the cell gains water and swells (can lyse); in hypertonic it loses water and shrinks (plants plasmolyze); in isotonic there is no NET movement.

   Delivery

   Hammer the osmosis rule: water moves toward more solute. Students constantly say 'the salt goes in' — correct them out loud: 'Salt cannot cross. Water crosses. Water goes where the salt is.' Use the slide's three-panel red blood cell diagram (hypertonic shriveled, isotonic normal, hypotonic swollen) as your visual anchor. Do a fast turn-and-talk: 'A cell is placed in 10% salt solution. Which way does water flow?' (Out of the cell — the outside is hypertonic.) Confirm this is what they'll SEE in the onion lab in 20 minutes.

3. 8m

   Active Transport: Pumps, ATP, and the Na⁺/K⁺ Pump

   Content

   Active transport moves substances AGAINST their concentration gradient — from low to high — and this uphill work costs ATP. The classic example is the sodium-potassium pump in animal cells: with each ATP hydrolyzed, the pump ejects 3 Na⁺ out of the cell and brings 2 K⁺ in. Result: high Na⁺ outside, high K⁺ inside — a gradient the cell uses for nerve impulses and for co-transport of glucose. Other examples: proton pumps in the stomach (H⁺ out, giving pH ~2), Ca²⁺ pumps that keep intracellular calcium low. Endocytosis and exocytosis are bulk active transport for very large items (whole particles, vesicles of neurotransmitter).

   Delivery

   The single most common student error is 'both passive and active transport use ATP.' Say the correction explicitly and put it on the check-for-understanding: PASSIVE = FREE, ACTIVE = COSTS ATP. Use an escalator analogy — diffusion is like a ball rolling downhill, active transport is like pushing it uphill; the pusher needs energy. Reference the slide's Na⁺/K⁺ pump diagram and walk through the 3-out/2-in stoichiometry. Ask: 'Why would a cell BURN energy to move ions against a gradient?' (To set up gradients that do work later — nerve signaling, nutrient uptake.)

4. 8m

   Water's Properties That Make Homeostasis Possible

   Content

   Water is not a background prop — its molecular properties directly enable homeostasis. Because the O–H bond is polar, water molecules hydrogen-bond to each other. This produces (1) cohesion: water sticks to itself, giving surface tension and pulling continuous water columns up xylem in a 100 m redwood against gravity. (2) Solvent behavior: the polar water molecule surrounds ions like Na⁺ and Cl⁻ and dissolves them, which is why blood plasma can carry glucose, ions, and wastes, and why cytoplasm is a reaction medium. (3) High specific heat capacity (4.18 J/g·°C): water absorbs a lot of energy for a small temperature change, so a cell — which is ~70% water — resists temperature swings, and large bodies of water buffer coastal climates and aquatic habitats. (4) Less dense as a solid: ice floats, insulating lakes so fish survive winter.

   Delivery

   Directly attack the misconception that water's properties are 'just chemistry, not biology.' Every property maps to survival: cohesion → tall trees can exist; solvent → blood can transport; high heat capacity → your enzymes don't cook on a hot day; ice floats → aquatic ecosystems overwinter. Ask students to pair each property with an organism-level consequence. This is the framework they will use in the closing formative assessment.

1. 40m

   Onion Epidermis Plasmolysis Lab — Osmosis Under the MicroscopeLab

   Students prepare a wet mount of red onion epidermis, observe cells in distilled water, then flood with 10% NaCl and watch plasmolysis in real time, then reverse with distilled water. They record observations, sketch, and write an evidence-based claim. Setup (teacher, before class): Peel one thin red-purple sheet of epidermis from the concave side of each onion square; keep in water. Distribute 2 pre-cut squares per pair. Student lab handout: Onion Plasmolysis — Osmosis Investigation Question: How does the tonicity of the surrounding solution affect a plant cell? Hypothesis: Write an if/then prediction using the words water, hypertonic, and vacuole. My hypothesis: ______________________________________ Procedure — Part 1: Cell in distilled water 1. Using forceps, peel a single thin layer of red-purple epidermis from the onion square. 2. Place the epidermis flat on a slide. Add 1 drop of distilled water. Cover with a cover slip. 3. Observe at ×100, then ×400. Locate the cell wall, cell membrane, and the large central vacuole (purple). 4. Sketch 3–4 cells. Label cell wall, membrane, vacuole. Procedure — Part 2: Add 10% NaCl (hypertonic) 1. Do not remove the cover slip. Place 2 drops of 10% NaCl at ONE edge of the cover slip. 2. Touch a piece of paper towel to the OPPOSITE edge to wick the salt solution across the cells. 3. Wait 2–3 minutes. Re-focus at ×400. Watch the purple vacuole pull away from the cell wall. 4. Sketch 3–4 cells after salt. Label where the membrane has pulled away. Procedure — Part 3: Reverse with distilled water 1. Wick 10% NaCl out with paper towel while adding drops of distilled water to the opposite edge. 2. Wait 3 minutes. Sketch cells after rinse. Data table (fill in): - Part 1 (distilled water): vacuole appearance = ______, cell membrane position = ______ - Part 2 (10% NaCl): vacuole appearance = ______, cell membrane position = ______ - Part 3 (rinse): vacuole appearance = ______, cell membrane position = ______ Analysis questions: 1. In Part 2, which direction did WATER move — into or out of the cell? Justify with the solute concentrations. 2. Solute (NaCl) cannot cross the membrane freely. Then WHY does the vacuole shrink? (This is the point — answer in one sentence.) 3. Was Part 2 an example of passive or active transport? How do you know no ATP was involved? 4. Predict: if you had used 30% NaCl instead of 10%, would plasmolysis be faster, slower, or the same? Explain using the concentration gradient. 5. Claim–Evidence–Reasoning: Write a CER paragraph. Claim: what type of solution was the 10% NaCl relative to the onion cell? Evidence: cite your Part 2 sketch. Reasoning: connect osmosis to the phospholipid bilayer's selective permeability. Safety: Scalpels/razor blades are sharp — cut away from fingers. NaCl solution is not hazardous but wash hands after. Wipe up all spills; wet floors around microscopes are a slip hazard. Do NOT eat any lab material. Circulate and check: at Part 2, students should visibly see the purple vacuole detached from the cell wall — if not, wick more salt through. Common student error to correct on the spot: 'the salt went in.' Ask them: 'Point to the salt in your sketch.' They can't — because it stayed outside; only water moved.

   Materials

   • Compound light microscopes (1 per pair)
   • Microscope slides and cover slips
   • Fine forceps and scalpels or razor blades
   • Red onion (½ onion per class, pre-cut into 2 cm squares)
   • Distilled water in dropper bottles
   • 10% NaCl solution in dropper bottles
   • Distilled water again for reversal
   • Paper towels / bibulous paper
   • Student lab handout (below)

   Example outputs

   • Analysis Q2 example: 'The vacuole shrinks because water — not salt — moved. The 10% NaCl outside is hypertonic, so water diffused from the vacuole (low solute) across the membrane to the outside (high solute), leaving the vacuole and membrane pulled away from the cell wall.'
   • Analysis Q4 example: 'Plasmolysis would be FASTER with 30% NaCl because the concentration gradient between inside and outside would be steeper, so water would leave the vacuole at a higher rate.'
   • CER example: 'Claim: The 10% NaCl was hypertonic to the onion cell. Evidence: In my Part 2 sketch, the purple vacuole shrank and pulled away from the cell wall within 3 minutes. Reasoning: The phospholipid bilayer is selectively permeable — it blocks Na⁺ and Cl⁻ but allows water. Water moved by osmosis from the vacuole (lower solute) to the outside (higher solute), so the cell lost volume. When we rinsed with distilled water, water re-entered and the vacuole partially recovered.'
2. 25m

   PhET Diffusion Simulation — Rate vs. Temperature and Concentration Gradient

   Students use the PhET Diffusion simulation to quantify how temperature and initial concentration gradient affect diffusion rate, then graph and interpret. This directly tests the passive-transport claim: diffusion is driven by thermal motion and gradient — no ATP needed. Direct students to: https://phet.colorado.edu/en/simulations/diffusion Student data sheet: Diffusion Rate Investigation (PhET) Setup for every trial: Open the simulation. You will see a box divided by a removable barrier. You can add particles (A on the left, B on the right) and set the temperature. Part 1 — Effect of concentration gradient (hold temperature constant at 300 K) Run three trials. In each, add particles ONLY to the left side, then remove the barrier and start a stopwatch. Record the time (in seconds) until the particle count on each side is within 2 of each other (equilibrium). - Trial 1: 20 particles A on left, 0 on right → time to equilibrium = ______ s - Trial 2: 50 particles A on left, 0 on right → time to equilibrium = ______ s - Trial 3: 100 particles A on left, 0 on right → time to equilibrium = ______ s Part 2 — Effect of temperature (hold particles constant at 50 A on left, 0 on right) - Trial 4: 300 K → time to equilibrium = ______ s - Trial 5: 500 K → time to equilibrium = ______ s - Trial 6: 700 K → time to equilibrium = ______ s Graphing: - Graph A: x = initial number of particles on left, y = time to equilibrium (Part 1 data) - Graph B: x = temperature (K), y = time to equilibrium (Part 2 data) Analysis: 1. As the concentration gradient gets steeper, what happens to the rate of diffusion? Cite specific times from your data. 2. As temperature increases, what happens to the rate of diffusion? Explain in terms of particle kinetic energy. 3. At any point in your trials, did the simulation ask you to spend energy (ATP) to move particles? What does this tell you about diffusion? 4. Predict: in the onion lab, if we had done Part 2 at 40 °C instead of room temperature, would plasmolysis occur faster or slower? Why? Circulate: check that students are recording actual numbers, not eyeballing. If PhET is slow to load, have pairs share a screen. Data will show inverse relationship (higher gradient → shorter time; higher T → shorter time).

   Materials

   • 1 laptop or Chromebook per pair
   • Student data sheet (below)
   • URL: https://phet.colorado.edu/en/simulations/diffusion

   Example outputs

   • Part 1 sample data: Trial 1 (20 particles) ≈ 42 s; Trial 2 (50) ≈ 28 s; Trial 3 (100) ≈ 18 s. Graph A slopes downward — steeper gradient → faster equilibrium.
   • Analysis Q2 example: 'Higher temperature means particles have more kinetic energy and move faster in random directions, so they cross the box and reach equilibrium in less time. At 700 K equilibrium took ~12 s vs. ~30 s at 300 K.'
   • Analysis Q3 example: 'The sim never asked for ATP. Diffusion is passive — it runs on the random thermal motion of the particles themselves, not on cellular energy.'

   No-equipment fallback

   If computers are unavailable, provide printed screenshots of the PhET simulation at t = 0, 5, 10, 20, 40 seconds for each of the six trials (data pre-collected by the teacher) and have students extract times and complete graphs and analysis from the printouts.

1. A plant root hair cell has an internal solute concentration of 0.4 M. It is placed in a soil solution measuring 0.9 M solutes. Predict the direction of net water movement and explain what happens to the cell using the terms hypertonic, osmosis, and selective permeability.

   short answerThe soil solution (0.9 M) is hypertonic to the cell (0.4 M). By osmosis, water moves across the selectively permeable membrane from the cell (lower solute) to the soil (higher solute). The cell loses water and plasmolyzes (shrinks; membrane pulls from cell wall). Solutes do not cross — only water moves.

2. Which of the following requires ATP? A) O₂ diffusing from an alveolus into a red blood cell B) Water entering a red blood cell through aquaporins in hypotonic plasma C) The Na⁺/K⁺ pump moving 3 Na⁺ out and 2 K⁺ in against their gradients D) Glucose moving through a GLUT carrier protein down its gradient

   multiple choiceC. The Na⁺/K⁺ pump moves ions AGAINST their concentration gradients, which is active transport and requires ATP hydrolysis. A, B, and D are all passive transport (down the gradient, no ATP).

3. From the PhET data, a group recorded times to equilibrium of 45 s, 28 s, and 15 s for initial concentrations of 20, 50, and 100 particles on one side. Construct an evidence-based argument for how concentration gradient affects diffusion rate.

   short answerClaim: A steeper concentration gradient increases the rate of diffusion. Evidence: With only 20 particles on one side the box took 45 s to equilibrate, but with 100 particles it took only 15 s — three times faster. Reasoning: A larger difference in concentration means more particles are randomly moving from the crowded side to the empty side per unit time, so equilibrium is reached faster. No ATP was required — thermal motion drives it.

4. Explain how TWO different properties of water support homeostasis in living organisms. Give one specific biological example for each property.

   short answerAccept any two of: (1) High specific heat capacity (4.18 J/g·°C) — a cell that is ~70% water resists rapid temperature changes, so enzymes remain functional on a hot day; large lakes and oceans buffer aquatic habitat temperatures. (2) Solvent behavior — water's polarity dissolves ions and glucose, allowing blood plasma to transport nutrients, ions, and wastes to and from cells. (3) Cohesion — hydrogen bonds pull continuous columns of water up xylem in plants, supplying water to leaves 100 m above the ground. (4) Ice floats — a floating ice layer insulates the water below, letting fish and aquatic organisms survive winter.

5. Calculate: A Na⁺/K⁺ pump hydrolyzes 1 ATP to move 3 Na⁺ out and 2 K⁺ in. If a neuron uses 6 × 10⁸ ATP molecules per second on this pump alone, how many Na⁺ ions are ejected per second?

   calculation3 Na⁺ per ATP × 6 × 10⁸ ATP/s = 1.8 × 10⁹ Na⁺ ions ejected per second.

• ☐Compound light microscopes (1 per pair)
• ☐Microscope slides and cover slips
• ☐Fine forceps
• ☐Scalpels or single-edge razor blades
• ☐Red onion (½ per class, pre-cut into 2 cm squares)
• ☐Distilled water in labeled dropper bottles
• ☐10% NaCl solution in labeled dropper bottles
• ☐Paper towels / bibulous paper
• ☐Laptops or Chromebooks (1 per pair) with internet
• ☐Onion plasmolysis lab handout (printed, 1 per student)
• ☐PhET diffusion data sheet (printed, 1 per student)
• ☐Formative assessment printed or in LMS