Safety and evidence boundary
Use diagrams, paper models, or supplied simulation data. Do not use concentrated solutions, unknown chemicals, or biological tissues unless an instructor has explicitly approved a safe protocol.
Explicit science instruction
Diffusion is the net movement of particles from regions of higher concentration to lower concentration due to random molecular motion. Individual particles move in many directions; the net pattern emerges from unequal concentrations.
Facilitated diffusion also moves substances down an electrochemical or concentration gradient but uses membrane proteins. It does not directly require cellular energy input for the transport step.
Osmosis refers to net water movement across a selectively permeable membrane in response to differences in water potential or solute conditions. School models often say water moves toward the side with more solute, but the complete process depends on which solutes can cross and on pressure.
Active transport moves substances against a gradient or maintains unequal distributions by coupling transport to energy-releasing processes, often ATP hydrolysis or another gradient.
Vocabulary: diffusion; facilitated diffusion; osmosis; concentration gradient; electrochemical gradient; active transport; equilibrium; dynamic equilibrium.
Worked example with reasoning
A cell has 2 mM glucose outside and 8 mM glucose inside. If a glucose carrier allows passive transport only, the net tendency is outward, from higher to lower glucose concentration. If the cell must accumulate glucose to a level above the external concentration, passive transport alone cannot maintain that gradient; an energy-coupled process is needed.
Guided question
Compare simple diffusion and facilitated diffusion. What do they share, and what structural difference makes facilitated diffusion possible for substances that cannot cross the lipid region easily?
Independent work
- Classify four transport scenarios as simple diffusion, facilitated diffusion, osmosis, or active transport.
- For each, identify the direction relative to the gradient.
- Explain why equilibrium does not mean particles stop moving.
- Write one sentence correcting the phrase “osmosis is solute moving through water.”
Misconception check
Diffusion does not mean every particle moves from high to low concentration. Individual motion is random; the net movement reflects the gradient.
Formative understanding check
At equilibrium, has molecular movement stopped?
Show the worked response
No. Molecules continue moving randomly in both directions, but the opposing flows are balanced so there is no net concentration change.
Open-book formative practice only; not an official mastery attempt or grade.
Mentor evidence conference
Ask the mentor to test one transport classification and make you justify it using gradient direction, protein involvement, and energy—not vocabulary alone.
Open Archaemenes for this lessonTeacher guidance
Foundation: use arrow diagrams and high/low labels. Core: include protein-mediated passive transport and active transport. Extended: introduce electrochemical gradients and explain why osmotic models are simplified.
Preserve learning evidence
Preserve the transport classification table, one gradient explanation, and an equilibrium correction in Tuesday’s evidence field.
Return to tuesday Evidence Workspace