Photosynthesis: the balanced equation and limiting factors
The balanced equation, where the oxygen comes from, transport in xylem and phloem, and investigating the rate of photosynthesis with pondweed.
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Photosynthesis is the process by which autotrophs turn light energy into chemical energy. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light absorbed by chlorophyll.[1]
In plants it takes place mainly in the leaves, in chloroplasts in the mesophyll layer. Gas exchange happens through stomata, which the plant can open and close.[1]
The glucose produced is the starting point for the plant's other molecules and is broken down in respiration to release energy. Heterotrophs, including humans, depend on it through food chains.[2]
Water and dissolved minerals move up the xylem, pulled by transpiration from the leaves; sugars move in the phloem from the leaves (sources) to growing tissues and stores (sinks).[3]
Photosynthesis on land is only part of the story. More than half of Earth's oxygen is produced in the sunlit top ~200 m of the ocean, mostly by phytoplankton. One single genus of tiny bacteria, Prochlorococcus, is estimated to produce 5–10%.[5]
Plants also respond to their environment: shoots grow towards light (phototropism) and roots grow downward with gravity (gravitropism), which puts leaves where light is and roots where water is.[4]
Try it: Pondweed bubbles and light
You need
- Pondweed (Elodea or Cabomba), a 5–8 cm fresh sprig
- Boiling tube in a rack, filled with water
- A pinch of sodium hydrogencarbonate (baking soda) to add CO₂
- LED lamp (an LED gives off little heat)
- Metre ruler
- Stopwatch
- Paper clip to hold the weed under water
Safety
Use an LED lamp and keep it and its cable away from the water. Wipe up spills. Wash hands after handling pondweed. Do not release pondweed into ponds or rivers; dispose of it in the school bin.
What to do
- Add a small pinch of sodium hydrogencarbonate to the water and stir. Put the pondweed in the tube, cut end up, held down by the paper clip.
- Place the lamp 10 cm from the tube. Wait 2 minutes for the plant to adjust.
- Count the bubbles leaving the cut end in 1 minute. Repeat twice and take the mean.
- Move the lamp to 20, 30, 40 and 50 cm and repeat. Keep the room lights off or dimmed.
- Plot the mean number of bubbles per minute against the distance. Describe the trend.
What happens, and why?
Light provides the energy for photosynthesis, so with more light the plant makes oxygen faster and gives off more bubbles. At short distances another factor, such as the carbon dioxide supply or temperature, may limit the rate, so the curve can level off.[1]
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Other ages: Ages 6–7 · Ages 10–11