4 mistakes learners make with photosynthesis, each one named and explained.
Light is the energy source for photosynthesis, not one of the ingredients.
A rate graph levels off and you say the plant has as much light as it can use, when light is on the axis.
The graph only shows one factor, so that is the only one available to explain it.
Question. Rate against light intensity levels off at high light. Why?
A common answer. the plant cannot absorb any more light
The answer. some other factor, such as carbon dioxide or temperature, has become limiting
Why. A flat line means something not shown on the graph is now in shortest supply.
When a graph plateaus, name a factor that is not on the axes.
You halve the distance to the lamp and say the light intensity doubles.
Halving one thing doubling another is the pattern most relationships in science follow.
Question. A lamp is moved from 20 cm to 10 cm from a plant. What happens to the light intensity?
A common answer. it doubles
The answer. it becomes four times as great
Why. Intensity depends on one over the distance squared, so halving the distance multiplies it by four.
Square the change in distance, then turn it upside down.
You write light into the equation beside carbon dioxide and water, as though it is turned into glucose.
Light is written above the arrow, which puts it in the same picture as the reactants.
Question. Which of these becomes part of the glucose a plant makes?
A common answer. light
The answer. carbon dioxide
Why. The carbon in glucose comes from carbon dioxide. Light supplies the energy, not the atoms.
Ask where the atoms come from. Energy is not an atom.
You name temperature as the limiting factor and stop, as though the others no longer matter.
The phrase the limiting factor sounds like there is exactly one, and questions often ask for just one.
Question. A plant is in bright light at 10 degrees C with plenty of carbon dioxide. What limits the rate?
A common answer. nothing, because light and carbon dioxide are plentiful
The answer. temperature, and it would still be limited by whichever factor ran short next
Why. Whichever factor is in shortest supply sets the rate, and that can change as conditions change.
Raise the limiting factor and something else becomes limiting. There is always one.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.