Leaf structure and gas exchange sim
Cut across a model leaf with a blade, name each tissue of the cut face as it glows, and follow one CO₂ molecule through an open stoma and the air spaces into a chloroplast, then switch the lamp off to see the stomata close.
Leaf structure in transverse section (cuticle, upper and lower epidermis, palisade and spongy mesophyll, air spaces, vein with xylem and phloem, stomata and guard cells); how CO₂ reaches a mesophyll cell by diffusion; oxygen and water vapour out; stomata closed in the dark · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Leaf structure and gas exchange: a thickened model leaf on a cutting tile is cut across by a blade, and the cut face (a transverse section) is drawn large below. Top to bottom: the waxy cuticle, the upper epidermis (clear cells with no chloroplasts), the palisade mesophyll (tall cells packed with chloroplasts), the spongy mesophyll (rounded cells with air spaces), and the lower epidermis with stomata, each a pore between two guard cells, which have chloroplasts. In the vein the xylem is on the upper side and the phloem below it. In daylight CO₂ diffuses in through an open stoma, through the air spaces, dissolves in the film of water on a mesophyll cell wall and diffuses into the cell and a chloroplast, down a concentration gradient kept up by photosynthesis; oxygen and water vapour diffuse out the other way. In the dark the guard cells close the stomata (in most plants), and respiration goes on. Model leaf: 12 mm thick for a real leaf about 0.25 mm (250 µm) thick, so the model is 48 times too thick (12000 µm / 250 µm = 48). Simplified on purpose: not to scale: the cuticles are drawn thicker than they are (on the model leaf's cut face the epidermis too), and the model leaf is life-size in outline but 48 times too thick; a few cells stand for the many in a real leaf; one vein is shown, in one place, at the edge of the piece drawn; one CO₂ molecule's path is followed, through one stoma; in a real leaf CO₂ diffuses to every mesophyll cell; the CO₂ concentrations are illustrative (≈); once the CO₂ has dissolved (wet wall, cell, chloroplast) it is no longer in air, so those values are shown as the concentration in air that would match it; both stomata are drawn in the lower surface; some leaves also have a few in the upper surface, which are not drawn; cell contents other than chloroplasts (the nucleus, the vacuole) are not drawn; the vein's sheath is drawn as a ring of small cells, and the vein's other tissues are not named; stomata are drawn fully open in daylight and fully closed in the dark; real stomata open and close gradually; the section shows the cut face down to the depth cut so far; the cells are tinted so the layers can be told apart; in life the cell walls are clear and only the chloroplasts are green.
- Glow: the part to name now
- Close-up: the cut face, magnified (a transverse section, not to scale)
- Trail: the path of the gas followed
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