1Measuring photosynthesis
Pondweed in water gives off bubbles of gas as it photosynthesises. Counting the bubbles in one minute shows the rate: the more bubbles, the faster photosynthesis is happening.
rate
Lesson 19.3 notes
Factors that affect photosynthesis
Measuring photosynthesis with pondweed, light, carbon dioxide and temperature, a fair test, limiting factors and greenhouses.
2Testing the gas
Collect the gas in an upside-down test tube of water. A glowing splint relights in it, which shows that the gas is mainly oxygen.
glowing splint
3Light intensity
Moving the lamp closer gives the pondweed more light, so it gives off more bubbles each minute. Plotted on a graph, the rate falls as the lamp moves away.
light intensity
4A fair test
The lamp distance is the independent variable: you change it. The bubbles a minute are the dependent variable: you measure them. Control variables, such as the temperature, stay the same.
independent variabledependent variablecontrol variables
5Carbon dioxide
Sodium hydrogencarbonate dissolved in the water releases carbon dioxide. With more carbon dioxide, the pondweed photosynthesises faster, until something else runs short.
carbon dioxide
6Temperature
Photosynthesis is controlled by enzymes. Warming a plant speeds it up to about 30 °C, but above about 40 °C the enzymes are damaged and the rate falls.
enzymes
7Limiting factors
Whichever factor is in shortest supply holds the rate back: the limiting factor. In greenhouses, growers add light, heat and carbon dioxide so their crops grow faster.
limiting factor
Explore: Pondweed lab
Move the lamp, add carbon dioxide or change the temperature, count the oxygen bubbles for a minute, and plot your results.
Common mistake: Too much heat slows photosynthesis down
Photosynthesis is controlled by enzymes. Warming a plant speeds it up to about 30 °C, but above about 40 °C the enzymes are damaged and the rate falls fast.
Remember
- Pondweed gives off bubbles that are mainly oxygen. Count the bubbles in a minute to measure the rate of photosynthesis.
- More light or more carbon dioxide speeds photosynthesis up, until something else runs short: the limiting factor.
- Warmth speeds it up to about 30 °C, but too much heat damages the enzymes. Greenhouses control light, warmth and carbon dioxide.
Going further at GCSE