‹ Biology simulationsBiology · AS Level 9700 · 4 Cell membranes and transportEight simulations covering Topic 4: the fluid mosaic membrane (4.1) and movement into and out of cells (4.2). Spin a 3D membrane, follow a hormone signal into a cell, watch molecules diffuse and water move by osmosis, put cells into solutions, run the potato practical and power a sodium-potassium pump with ATP.
A cell surface membrane is a phospholipid bilayer about 7 nm thick with proteins floating in it like tiles in a mosaic. The phospholipids and many proteins move sideways all the time, so the membrane is fluid. Pick a component to pick it out of the crowd and read what it does.
A cell releases a signalling molecule, the ligand. It travels, for example in the blood, to every cell, but only cells with a receptor of complementary shape respond. Binding changes the receptor's shape and starts a chain of events inside the cell, often through a second messenger such as cyclic AMP, which amplifies one signal into a large response.
Molecules move at random, so more cross from where they are concentrated to where they are not: net movement down the concentration gradient, with no energy from the cell. Small non-polar molecules slip straight through the bilayer. Ions and polar molecules such as glucose need channel or carrier proteins, and the number of proteins limits the rate.
Blocks of agar made pink with an alkali and indicator are dropped into acid. The acid diffuses in from every face and the pink fades from the outside inward at the same speed in every block. Small blocks clear first because they have more surface for their volume. Cut the blocks in half to see how far the acid has reached.
Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Pure water has the highest water potential, 0 kPa. Dissolving solute lowers it, so every solution has a negative water potential. Water keeps moving until the two sides are equal.
Put a red blood cell and a plant cell into solutions of different water potential. With no cell wall, the red blood cell swells and bursts in water and shrinks and crenates in a concentrated solution. The plant cell's wall stops it bursting: it becomes turgid, and in a concentrated solution the protoplast pulls away from the wall, which is plasmolysis.
The classic practical. Weigh cylinders of potato, leave them in a range of sucrose solutions, blot and reweigh. Tissue gains mass where the solution has a higher water potential than the cells and loses mass where it is lower. Plot percentage change in mass against concentration: where the line crosses zero, the solution matches the water potential of the tissue.
Active transport moves ions against their concentration gradient using carrier proteins and energy from ATP made in respiration. The sodium-potassium pump pushes 3 Na+ out and brings 2 K+ in for every ATP. Large particles and liquids cross in bulk instead, wrapped in membrane: endocytosis brings them in, exocytosis lets them out, and both use ATP too.
Eight interactive 3D and 2D simulations for Cambridge AS Level Biology 9700 topic 4 Cell membranes and transport: the fluid mosaic model in 3D with phospholipids, cholesterol, channel and carrier proteins, glycoproteins, glycolipids and receptors, cell signalling with a second messenger, simple and facilitated diffusion, surface area to volume ratio with agar blocks, osmosis and water potential, red blood cells and plant cells in solutions, the potato tissue water potential practical, and active transport with the sodium-potassium pump, endocytosis and exocytosis.
Eight simulations covering Topic 4: the fluid mosaic membrane (4.1) and movement into and out of cells (4.2). Spin a 3D membrane, follow a hormone signal into a cell, watch molecules diffuse and water move by osmosis, put cells into solutions, run the potato practical and power a sodium-potassium pump with ATP.
Cell membranes and transport is a free, browser-based simulation for Cambridge International AS & A Level Biology 9700 (topic 4 Cell membranes and transport). The course is assessed in Paper 1 (Multiple choice), Paper 2 (AS structured), Paper 3 (Advanced practical), Paper 4 (A Level structured), Paper 5 (Planning, analysis, evaluation). Syllabus for examination in 2025-2027. There are 8 tabs, each a self-contained experiment with its own controls and live readings, and nothing to install: it runs on a laptop, tablet or classroom projector.
Full syllabus map: AS Level Biology 9700 topic 4.
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