Photosynthesis

Activities to try

1. 1

   Feed the plant: vary each input and watch the response

   Why it works: Light energy is the driver of photosynthesis and each of CO₂, water and light is necessary; glucose is built by the plant from air and water, and plants respire (use oxygen) as well as photosynthesise.

   1. Work with a plant model (physical bench setup, a simulation, or a teacher-run demonstration) that has three controllable supplies: a light source (brightness), air/CO₂, and water from the soil.
   2. Starting with all three supplies on, raise each one in turn and observe and record what happens to the rate of oxygen bubbling (and, in a simulation, the displayed amount of glucose being built; glucose cannot be watched directly on a real bench).
   3. Set water and CO₂ to maximum, then switch the light OFF. Predict first, then observe: does the plant keep making food? Write down what this tells you about which input drives the process.
   4. In the simulation or model, switch each remaining input to zero one at a time and record whether photosynthesis stalls, concluding which inputs are necessary (in a real plant, cutting off water or CO₂ slows photosynthesis gradually rather than switching it off instantly).
   5. Leave the plant in the dark and reveal/consider the plant's own use of oxygen; state in a sentence why this shows plants also respire, not only photosynthesise.
2. 2

   van Helmont's willow: predict where the mass comes from

   Why it works: Almost all of a plant's extra mass comes from carbon dioxide in the air combined with water, using light energy; soil supplies water and small amounts of minerals, not the bulk of the mass.

   1. Read/consider the classic experiment: a small willow is planted in a weighed pot of soil and watered for about five years until it gains many kilograms, while the soil loses only a tiny fraction of that (in the usual account, about 2 ounces, roughly 57 g).
   2. Before the result is revealed, predict how much soil mass will be lost and, importantly, explain WHY.
   3. Compare your predicted reason with the result: the soil barely changed, so the tree's large mass gain cannot have come mainly from the soil.
   4. State in your own words where nearly all the extra mass (the wood) actually comes from. Explain that the experiment shows it did not come mainly from the soil (van Helmont himself concluded it came from water), and that later evidence showed the carbon comes from carbon dioxide in the air.
   5. Answer follow-up probes: What happens to a well-watered plant kept in total darkness and why? Which part of the carbon dioxide ends up in the wood? Does a plant need oxygen, and when?
3. 3

   Name and source the parts: leaf anatomy and input origins

   Why it works: The leaf is the 'factory' where carbon dioxide from the air and water from the soil are turned into glucose and oxygen using light energy captured by chlorophyll; reactants and products can be named and correctly attributed to their origin.

   1. On a whole-plant diagram, draw or place arrows showing CO₂ entering leaves, water and minerals rising from roots, and light striking the leaf canopy.
   2. Label a leaf cross-section with stomata (gas in/out), chloroplasts (where light is captured), and veins (bringing water).
   3. Sort a set of items — {carbon dioxide, water, minerals, glucose, oxygen} — into reactants of photosynthesis, products of photosynthesis, or neither (minerals are needed by the plant but are not reactants in the photosynthesis word equation). Then sort only the materials the plant takes in (carbon dioxide, water, minerals) into from-air vs from-soil, noting that glucose and oxygen are made by the plant.
   4. Explain, in a lift-the-flap or written response, the myth 'the plant eats the soil': state that the plant builds its food from carbon dioxide from the air and water, using light energy, and that the soil supplies water and small amounts of minerals, not the bulk of the food.