Teacher sheet · Ages 15–16 · about 45–50 minutes
Photosynthesis: the balanced equation and limiting factors
Lesson page: echoesof.science/school/plants/ages-15-16/ · Pupil worksheet: echoesof.science/school/plants/ages-15-16/print/
Learning aims
- Write and balance the photosynthesis equation.
- Describe transport in xylem and phloem.
- Investigate the effect of light on the rate of photosynthesis and evaluate the method.
- Recognise the large role of ocean phytoplankton in oxygen production.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: where do the atoms in a tree's wood come from? |
| 10 min | Read the text; balance the equation together. |
| 25 min | Pondweed practical in pairs. |
| 5 min | Plot the graph; discuss limiting factors. |
| 5 min | Quiz or sorter as homework. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Plants photosynthesise instead of respiring. | Plants respire all the time; photosynthesis happens alongside it in the light. |
| Rainforests produce most of the world's oxygen. | More than half is produced in the ocean, mostly by phytoplankton. |
| More light always means more photosynthesis. | Above a certain level another factor, such as CO₂ or temperature, limits the rate. |
Worksheet answers
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Reactants: carbon dioxide and water. Products: glucose and oxygen.
- Water evaporates from leaf cells and leaves through the stomata; this pulls a continuous column of water up the xylem from the roots.
- Bubbles per minute fall as distance increases; may level off at short distances.
- Bubbles differ in size; some gas is not oxygen; counting by eye is hard at high rates. Improvement: collect the gas in a measuring cylinder or syringe and measure its volume.
Quiz answers
- Which is the balanced equation for photosynthesis? 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Six of each reactant give one glucose and six oxygen.[1]
- Where in the leaf does most photosynthesis happen? In chloroplasts in the mesophyll. The mesophyll cells are packed with chloroplasts.[1]
- What drives water up the xylem? Transpiration from the leaves. Evaporation from the leaves creates the pull.[3]
- Phloem carries sugars from… sources such as leaves to sinks such as roots and fruits. Phloem moves sugar from where it is made to where it is used or stored.[3]
- Roughly how much of Earth's oxygen is produced in the ocean? More than half. WHOI: more than half, mostly from phytoplankton.[5]
- A shoot bending towards a window is an example of… phototropism. Phototropism is growth towards light.[4]
Going further
Repeat at one distance with different amounts of sodium hydrogencarbonate. Which factor limits the rate at 10 cm?
Sources used in this lesson
- [1] Biology 2e, 8.1 Overview of Photosynthesis (OpenStax, Rice University): reliability High
- [2] Concepts of Biology, 5.1 Overview of Photosynthesis (OpenStax, Rice University): reliability High
- [3] Biology 2e, 30.5 Transport of Water and Solutes in Plants (OpenStax, Rice University): reliability High
- [4] Biology 2e, 30.6 Plant Sensory Systems and Responses (OpenStax, Rice University): reliability High
- [5] Does the ocean produce oxygen? (Woods Hole Oceanographic Institution): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/plants/sources/
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