ACT Science: Diffusion and Osmosis—How Substances Move Across Membranes

Diffusion: Molecules move from high concentration to low concentration (down the concentration gradient) without energy input. Example: Perfume diffuses from a bottle throughout a room. Facilitated diffusion: Molecules still move down the gradient, but need help from channel proteins. Example: Glucose enters cells via glucose transporter proteins. Osmosis: Water moves across a semipermeable membrane toward higher solute concentration (low water concentration). A cell in hypotonic solution (low solute outside) swells as water enters. A cell in hypertonic solution (high solute outside) shrinks as water leaves. Isotonic: solute concentration equal; no net water movement. All three are passive—no ATP required. On the ACT, you'll predict cell behavior given solution type, calculate water movement direction, or identify diffusion types. Understanding concentration gradients and membrane selectivity is key.

Key insight: Osmosis is water diffusion toward solutes. High salt outside = water leaves cell. The cell shrinks. High salt inside (rare) = water enters. The cell swells. This matters for cells in different environments.

Misconception 1: Diffusion requires energy. False. It's passive (down the gradient). Active transport requires energy (against the gradient). Misconception 2: Facilitated diffusion is active transport. False. Facilitated diffusion uses channel proteins but still moves down the gradient (passive). It's just... facilitated. Misconception 3: All water movement is osmosis. False. Osmosis specifically refers to water movement across a semipermeable membrane in response to solute concentration differences. Misconception 4: A cell in hypotonic solution will burst. Not always. Plant cells have cell walls that prevent bursting; they turgid (firm). Animal cells may lyse (burst). Remember: Hypotonic = cell swells/enters. Hypertonic = cell shrinks/exits. Isotonic = equilibrium, no net movement.

Checklist: (1) Identify if it's diffusion, facilitated diffusion, or osmosis. (2) Determine concentration gradient direction. (3) Predict substance/water movement. (4) Predict cell outcome (swell, shrink, equilibrium). (5) Note: plant cells behave differently due to cell wall.

Scenario 1: Animal cell in hypotonic solution (pure water). Prediction: Water enters by osmosis. Cell swells. May lyse if solution is very hypotonic. Scenario 2: Plant cell in hypertonic solution (5% salt). Prediction: Water leaves by osmosis. Cell shrinks (plasmolysis). Cell wall prevents bursting. Scenario 3: Red blood cell in isotonic solution (0.9% saline). Prediction: No net water movement. Cell maintains shape. Scenario 4: Glucose diffusing across a cell membrane via facilitated diffusion. Prediction: Glucose moves down gradient (high outside, low inside). Requires glucose transporter proteins. Scenario 5: Oxygen diffusing into muscle cells during exercise. Prediction: Oxygen diffuses down gradient (high in blood, low in cells). No protein channel needed. For each scenario, explain: Is it passive or active? What's the direction? What happens to the cell?

Daily drill: Sketch cell diagrams showing hypotonic, isotonic, hypertonic scenarios. Predict water and solute movement. Practice explaining plasmolysis vs. lysis.

About 1-2 cell biology questions per ACT Science section test diffusion, osmosis, or cell behavior in different solutions. These are conceptual and predictive: given a scenario, predict cell outcome. If you understand the driving force (concentration gradient), you can reason through any scenario, even unfamiliar ones. Students who grasp diffusion and osmosis fundamentally can answer even tricky questions by applying the core concept: substances move down concentration gradients; water follows solutes.

Spend 2 days on passive transport. Memorize solution types (hypotonic, isotonic, hypertonic) and their effects on animal and plant cells. Practice predicting cell behavior. By test day, these questions will feel manageable and you'll score points many students miss.