Lesson overview
Limits on photosynthesis: What limits how fast a plant photosynthesises?
Count the bubbles from pondweed as you change the light, the carbon dioxide and the temperature, find what holds the rate back, compare gas exchange in light and dark with an indicator, and see how a leaf is built for photosynthesis.
Suggested sequence
- Lesson 1 · Measuring the rate of photosynthesis. Counting oxygen bubbles from pondweed as light, carbon dioxide and temperature change; fair tests and means; limiting factors (Supplement). Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Gas exchange in light and dark. Hydrogencarbonate indicator: respiration all the time, photosynthesis in the light, and the net gas exchange of a water plant. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · How a leaf is built for photosynthesis. A large, thin leaf; the structures of a leaf in section and how each one helps photosynthesis. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 10 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 9 questions, 15 min, lesson closed.
Learning goals
Part 1: Measuring the rate of photosynthesis
- Investigate and describe the effect of varying light intensity on the rate of photosynthesis (6.1.8)
- Investigate and describe the effect of varying carbon dioxide concentration on the rate of photosynthesis (6.1.8)
- Investigate and describe the effect of varying temperature on the rate of photosynthesis, linked to enzymes (6.1.8)
- Measure the rate of photosynthesis fairly: oxygen bubbles and the test for oxygen, variables, repeats and means, and the limits of counting bubbles (practical skill for 6.1.8)
- Supplement: identify and explain the limiting factors of photosynthesis in different environmental conditions (S6.1.11)
Part 2: Gas exchange in light and dark
- Describe gas exchange in light and dark: respiration all the time, photosynthesis only in the light, and the net exchange (6.1.9)
- State the colours of hydrogencarbonate indicator with more, the same and less carbon dioxide than in air (practical skill for 6.1.9)
- Investigate the effect of light and dark on gas exchange in an aquatic plant using hydrogencarbonate indicator, with a control (6.1.9)
Part 3: How a leaf is built for photosynthesis
- State that most leaves have a large surface area and are thin, and explain how these features are adaptations for photosynthesis (6.2.1)
- Identify in diagrams and images of a dicotyledonous leaf: chloroplasts, cuticle, guard cells and stomata, upper and lower epidermis, palisade mesophyll, spongy mesophyll, air spaces, vascular bundles, xylem and phloem (6.2.2)
- Explain how these structures adapt leaves for photosynthesis (6.2.3)
Misconceptions to listen for
- “Carbon dioxide: it turns limewater milky.” — In bright light photosynthesis takes in carbon dioxide faster than respiration gives it out, so the gas given out is mainly oxygen.
- “Oxygen: a lit splint goes out with a squeaky pop.” — The gas is oxygen, but the squeaky pop is the test for hydrogen. Oxygen relights a glowing splint.
- “Water vapour: it turns blue cobalt chloride paper pink.” — The bubbles rise through water, so they are not water vapour. They are the oxygen made by photosynthesis.
- “The bubbles are pure oxygen.” — They are mainly oxygen: the oxygen left over after respiration, with a little nitrogen and carbon dioxide from the water.
- “The lamp distance is the independent variable, because it was the one changed in the light test.” — In this investigation the lamp stays at one distance. Only the mass of sodium hydrogencarbonate is changed on purpose.
- “The bubbles are the independent variable, because they change.” — The bubbles change because of what she did: they are what she measures, the dependent variable.
- “Every count must go into the mean.” — An anomalous count does not fit the pattern, so it is left out. Including it would pull the mean up.
- “The mean is 43.” — That is the total of the three counts. Divide it by the number of counts, 3.
- “The rate rises by about the same amount each time the lamp comes 9 cm closer.” — The rises were 2, 3.5 and 4.5, but from 18 cm to 9 cm only 1.
- “The rate falls from 14 to 3 as the lamp comes closer.” — Read the table from the far lamp to the near lamp: 45 cm is the farthest, and the rate there is the lowest.
Scope
The effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis, investigated with pondweed (6.1.8); gas exchange of an aquatic plant in light and dark with hydrogencarbonate indicator (6.1.9); limiting factors in different conditions (Supplement 6.1.11); the large, thin leaf (6.2.1); the structures of a dicotyledonous leaf in diagrams and images (6.2.2) and how they adapt it for photosynthesis (6.2.3). Bubble counts and indicator colours in the experiment come from a teaching model.