Grade 7

Photosynthesis

How a plant builds its own food out of light, air and water, and why everything that eats depends on it.

6 lessons

  1. 1Where does a plant's mass actually come from?Overwhelmingly from the air, not the soil. The plant takes in carbon dioxide through its leaves and builds sugar from it, which is why a tree can add hundreds of kilograms while the soil around it barely loses any mass at all. From the soil come water and a small quantity of minerals.30m
  2. 2Why are leaves green?Because chlorophyll absorbs red and blue light and reflects green. The colour we see is precisely the colour the plant does not use — it is bounced back to our eyes instead of being taken in. That is also a real limitation: some of the light energy reaching the leaf is simply never collected.30m
  3. 3How is a leaf built and what do the stomata do?A leaf is built like a flat, wide factory: a protective layer on top, packing tissue rich in chloroplasts in the middle, veins bringing water in and carrying sugar out, and stomata on the underside. A stoma is a pore through which carbon dioxide enters — and through which, unavoidably, water leaves.35m
  4. 4What is the equation of photosynthesis and where does the oxygen come from?Six molecules of carbon dioxide and six of water, driven by light energy, become one molecule of glucose and six of oxygen. The oxygen the plant releases comes from splitting the water, not from the carbon dioxide — and that is the point most people remember the wrong way round.30m
  5. 5How do you test a leaf for starch and what makes an experiment fair?Boil the leaf in alcohol to strip out the chlorophyll, then add iodine: wherever there is starch it turns blue-black. A fair experiment changes one factor and holds everything else constant, and a variegated leaf is the elegant case where the control sits inside the very same leaf.40m
  6. 6What is the difference between photosynthesis and cellular respiration?Photosynthesis builds sugar from carbon dioxide and water using light; cellular respiration breaks that sugar down using oxygen to release energy. They are opposites in their raw materials and products. A plant does both — photosynthesis only in light, respiration all the time.35m