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| Way | What moves, and how | Energy and direction |
|---|---|---|
| Simple diffusion | O2, CO2: through the lipid | no ATP; down the gradient |
| Facilitated diffusion | glucose, ions: through a channel or carrier protein | no ATP; down the gradient |
| Osmosis | water: through aquaporins (and a little through the lipid) | no ATP; toward more solute |
| Active transport | Na+, K+: by a pump protein | uses ATP; against the gradient |
Bulk transport. Large particles and big amounts of liquid cross in vesicles, and this uses ATP. In endocytosis the membrane folds in around material and pinches off a vesicle inside the cell: phagocytosis (“cell eating”, e.g. a white blood cell taking in a bacterium), pinocytosis (“cell drinking”) and receptor-mediated endocytosis (receptors in coated pits pick up a specific cargo, such as LDL cholesterol). In exocytosis a vesicle from the Golgi apparatus fuses with the membrane and releases its contents outside, e.g. insulin from pancreas cells or neurotransmitters at a synapse. Membrane fluidity lets the bilayer break and reseal during both.
Rate of diffusion (Fick’s law). Rate ∝ surface area × concentration difference / thickness of the membrane. Lungs, gut villi and root hairs are built for a large area and a thin barrier. Temperature raises the diffusion coefficient: in water, D ∝ T / viscosity (Stokes–Einstein), which is the rule the simulator uses.
Carriers saturate. Facilitated diffusion has a maximum rate (Vmax) when every carrier is busy, like enzyme kinetics; simple diffusion keeps rising in a straight line with concentration. Glucose transporters (GLUT) are carriers; aquaporins and ion channels are channel proteins. Channels can be gated by voltage or by a ligand.
Water potential (IB / A-level). Ψ = Ψs + Ψp. Solutes make Ψs negative; water moves from higher (less negative) to lower Ψ. In a turgid plant cell the wall pushes back (Ψp > 0) until there is no net flow. Red blood cells have no wall, so a hypotonic solution makes them burst (lysis); in a hypertonic solution they shrink and crenate.
Secondary active transport. The steep Na+ gradient made by the pump stores energy. The SGLT1 cotransporter in the small intestine lets Na+ flow back in and drags glucose in with it, against the glucose gradient. Sending out 3 positive charges for every 2 brought in also helps make the inside of the cell negative (the resting membrane potential used by nerve cells).
Model limits. The simulator uses a few dozen molecules and slowed-down time. Real membranes carry millions of transport events per second, and real carriers and pumps also change speed with temperature and can be damaged (denatured) above about 45 °C.
Name the parts of the fluid mosaic membrane. Drag each word onto its numbered box, or tap a word and then tap the box. Press Check to see what is right. Tap a filled box to take the word back.
Pick one answer for each question, then press Check my answers to see the explanations. New questions picks another set.
A 40–50 minute lesson. Start in Simple diffusion on a projector (Full screen button) and ask the class to predict where the O2 will end up. Then let students work through the six missions in pairs; each one asks for a prediction before the simulator shows the result, and the certificate prints when all six are done.
The simulator uses a few dozen molecules and model seconds, so runs differ a little: this is a good moment to talk about random motion and repeat trials. The temperature effect uses the real viscosity of water (Stokes–Einstein), the red blood cell uses the Boyle–van ’t Hoff relation (it bursts near 0.45 % NaCl, as in an osmotic fragility test), and the plant cell sap value (400 mOsm/L) is illustrative. The live graph and its data table (More options on phones) can be copied into a lab report.
Go deeper covers endocytosis, exocytosis, Fick’s law, carrier saturation, water potential and secondary active transport for AP Biology (Unit 2, topics 2.4–2.9) and IB Biology (B2.1).
Standards: NGSS MS-LS1-2, HS-LS1-2, HS-LS1-3 · AP Biology Unit 2 (2.4–2.9) · IB Biology B2.1
| Time | Step | What students do | Go to |
|---|---|---|---|
| 5 min | Hook | Predict where the oxygen goes; run Simple diffusion | Simulator |
| 15 min | Explore | Missions 1–3 (equilibrium, temperature, carriers) | Simulator |
| 10 min | Osmosis | Missions 4–5: red blood cell and plant cell | Simulator |
| 5 min | Energy | Mission 6: switch off the ATP | Simulator |
| 5 min | Label | Label the fluid mosaic membrane | Label Lab |
| 10 min | Check | Order the pump cycle, then the quiz | Order game |
Worksheet: a print-out for students and a separate answer key page for the teacher (opens the print dialog; choose "Save as PDF" to keep a file).
Checked against:
Key numbers: The membrane is a phospholipid bilayer about 7–8 nm thick; the sodium–potassium pump moves 3 Na+ out and 2 K+ in for each ATP.
What the model simplifies: Times are model seconds, slowed down so you can watch them, and the plant cell sap value is illustrative.
Standards: NGSS MS-LS1-2, HS-LS1-2, HS-LS1-3
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It is a phospholipid bilayer: two sheets of phospholipids with their water-loving phosphate heads facing out and their water-fearing fatty-acid tails facing in. Proteins, cholesterol and sugar chains are scattered through it, and the parts drift sideways, so it is called the fluid mosaic model.
Key takeaway: an oily, self-sealing bilayer with proteins in it.Passive transport (simple diffusion, facilitated diffusion, osmosis) moves substances down their concentration gradient and needs no energy from the cell. Active transport moves substances against their gradient and uses ATP, for example the Na+/K+ pump.
Key takeaway: down the gradient = free; up the gradient = costs ATP.Osmosis is the diffusion of water across a selectively permeable membrane toward the side with more solute. A hypotonic solution has less solute than the cell (water moves in), an isotonic one has the same (no net change) and a hypertonic one has more (water moves out).
Key takeaway: water follows the solute.Both take in water in a hypotonic solution. The red blood cell has only a membrane, so it swells until it bursts (lysis). A plant cell has a stiff cell wall that pushes back, so it becomes firm (turgid) instead. In salty water the plant cell loses water and the membrane pulls away from the wall (plasmolysis).
Key takeaway: the cell wall protects plant cells from bursting.Glucose is fairly large and polar, so it cannot dissolve in the hydrophobic middle of the bilayer. A carrier protein (such as GLUT) binds a glucose, changes shape and releases it on the other side. This is facilitated diffusion: it uses no ATP and only works down the gradient.
Key takeaway: big or polar molecules need a protein door.It uses one ATP to move 3 Na+ out of the cell and 2 K+ in, both against their gradients. The gradients it builds are used to send nerve impulses, to keep the cell’s volume steady and to pull glucose into gut cells (secondary active transport).
Key takeaway: 3 out, 2 in, 1 ATP.They move large particles or large amounts in vesicles and use energy. In endocytosis the membrane folds around material and pinches off a vesicle inside the cell (phagocytosis, pinocytosis, receptor-mediated endocytosis). In exocytosis a vesicle fuses with the membrane and releases its contents outside, for example hormones and neurotransmitters.
Key takeaway: vesicles carry big loads in and out.