Chapter 3 · The Cell — Interactive Lab · Created by Dr. Meera Salama

Experiment Lab — where does life come from?

Four historical experiments, ready to run: Redi's jars (1668), Needham's broth (1745), Spallanzani's sealed flasks (1765) and Pasteur's swan-neck flask (1862). Together, they resolved the debate over spontaneous generation.

Day: 0 / 7 · maggots: 0

Select an experiment, then run it. In Redi’s jars only the open jar can acquire maggots. In Pasteur’s flask the bends trap airborne particles, and the broth remains sterile until the neck is broken.

Osmosis Lab — water movement across the membrane

Adjust the external salinity with the slider and observe the water. Blue dots represent water, orange dots external solutes, and purple dots the cell’s own solutes. Toggle the cell wall on and off to compare the two outcomes.

Outside salinity 35

Quick setups:

Outside solutes 35% Inside solutes 35%

The cell starts isotonic with its surroundings: water enters and exits at the same rate. Change the external salinity and observe the direction of net water movement: water moves toward the higher solute concentration.

Shape Studio — cell shapes and arrangements

Select a cell shape and an arrangement, then observe single cells divide into the formation. Pairs, chains, squares and clusters — the pattern is determined by the plane of cell division.

cells 5

Score: 0 / 0

A coccus is a spherical cell. When daughter cells remain attached: pairs = diplococcus, chains = streptococcus, grape-like clusters = staphylococcus, four in a square = tetrad. Review each formation, then take the quiz.

Gram Stain Lab — the four-step staining procedure

Two cells start colourless. Work through the four steps in order and observe where the dye is retained, and why. This procedure, developed by Hans Christian Gram in 1884, remains a standard method in diagnostic laboratories.

Each step has a specific function. Crystal violet stains all cells. Iodine forms a large complex with the dye inside the cell. Alcohol is the differentiator: only a thick peptidoglycan wall retains the purple. Safranin counterstains the decolourised cells pink. Begin with step 1.

Chemotaxis Runner — how bacteria locate nutrients

A bacterium has no eyes and no brain. It alternates straight runs with tumbles that reorient it at random. Observe how lengthening runs that improve the nutrient signal produces net movement up a gradient — then switch chemotaxis off to compare.

time 0.0 s distance 0 px nutrient signal 0% best run —

Press Start. The cell alternates runs (flagella bundled) with tumbles (flagella splayed, reorienting at random). With chemotaxis ON, runs that improve the nutrient signal are extended, producing net drift toward the source. With chemotaxis OFF, run lengths are random and progress is far slower.

Endospore Survivor — stress resistance of the endospore

Left: a normal vegetative cell. Right: an endospore — a dormant, highly resistant cell form. Subject both to boiling, radiation, chemicals and prolonged storage, then review the six-step sporulation process.

spore survived: 0 / 0 attacks · vegetative cells lost: 0

Run the attacks to compare survival. Vegetative cells are fragile, but the endospore is dehydrated, contains calcium–dipicolinate and is enclosed by a cortex and a protein coat. This is why sterilisation requires an autoclave (121 °C, 15 psi) rather than boiling water alone.

Transport Doors — mechanisms of membrane transport

The membrane is a phospholipid barrier that most solutes cannot cross freely. Four transport systems are available: simple diffusion, facilitated diffusion, an ATP-driven pump and the PTS group-translocation system. Assign CO₂, glucose and K⁺ to the correct mechanism and see why each system is needed.

ATP 4 molecules delivered: 0 / 4

Instructions: drag each molecule and drop it on a transport system. An incorrect match returns to the membrane with an explanation. Some systems are passive, one consumes ATP, and one chemically modifies the cargo during entry.

Cell Explorer — prokaryotic cell anatomy

An annotated prokaryotic cell. Click the dotted circles to inspect each structure: its name, pronunciation and function. Then use the Find it exercise to test your knowledge.

Explore the cell

Click any of the dotted circles to open information about the structure it marks.

Scale note: the real cell is only a few micrometres long; about 100 such cells placed side by side would span one millimetre.

Press Find it to begin: a structure will be named, and you select it on the diagram. score: 0 / 0

A prokaryote has no nucleus and no membrane-bound organelles. Its DNA lies in the nucleoid; it has 70S ribosomes, storage granules, plasmids and a cell envelope comprising wall, membrane and, in some species, a capsule. All structures shown are covered in Chapter 3.