How a Plant Feeds Itself

An earlier tutorial in this subject establishes what photosynthesis needs and what it produces, at a level a younger reader can follow. This track asks the questions that one leaves open, starting by cutting a leaf across: light arrives from above so the chloroplast-packed cells are at the top under a transparent layer, carbon dioxide has to reach every cell so the tissue below them is mostly connected air, and because the surface would dry out it is sealed with wax and the openings are underneath. Those openings are not holes but gaps between two cells that bend apart when water enters them by osmosis, and the board makes unavoidable the fact that admitting carbon dioxide and losing water vapour are one event rather than two. From there the track derives the limiting factor rule from a single observation — more light helps until abruptly it does not — then turns to evidence, walking through the starch test in an order where the two steps that look skippable, destarching and the foil, are the two carrying the whole argument. It closes on what photosynthesis cannot do: glucose is carbon, hydrogen and oxygen, so proteins and chlorophyll need elements that only the soil supplies.