Light IntensityLight Intensity

D. Crowley, 2008

Light IntensityLight IntensityTo understand how light intensity affects photosynthesis

Wednesday, April 19, 2023

TalkingTalking Why can you argue that it is good for your plants to talk to

them?!

Remember, when plants grow they do so via photosynthesis – for this to occur plants need carbon dioxide, water and light (and when you speak, you exhale carbon dioxide)!

carbon dioxide + water glucose + oxygen

6CO2 + 6H2O C6H12O6 + 6O2

chlorophyll light energy

Rate Of PhotosynthesisRate Of Photosynthesis What can affect the rate of photosynthesis?

Photosynthesis needs carbon dioxide, water and light (as well as a variety of nutrients)

If any of these are taken away, they become a limiting factor of photosynthesis – reducing the amount of photosynthesis which can take place…

Light Intensity ExperimentLight Intensity Experiment How could you measure how light intensity affects the rate

of photosynthesis?

Hydrogen carbonate indicator reacts when carbon dioxide is present: -◦ In normal atmospheric conditions, the indicator is

orange/red◦ When there is lots of carbon dioxide the indicator is yellow◦ When there is not much carbon dioxide the indicator is

purple

So we can set up an experiment, changing the light intensity, and then measure the hydrogen carbonate indicator to see how the CO2 levels vary…

ExperimentExperiment1. Take 2 similar pieces of elodea2. Place 1 piece of elodea in a test tube and submerge

with hydrogen carbonate indicator3. Place 1 piece of black paper around the test tube,

ensuring the top is sealed (use the rubber band)

4. Repeat this experiment, this time placing 1 piece of tracing paper around the test tube

5. Finally set up a control – a test tube with only hydrogen carbonate indicator and no elodea in it or paper around it!

Leave test tubes in a rack, by the window for >20 mins…

PredictionPrediction Write down your prediction – remember the photosynthesis

equation and the colour changes of the hydrogen carbonate indicator…

carbon dioxide + water glucose + oxygen

Hydrogen carbonate indicator reacts when carbon dioxide is present: -◦ In normal atmospheric conditions, the indicator is

orange/red◦ When there is lots of carbon dioxide the indicator is yellow◦ When there is not much carbon dioxide the indicator is

purple

PredictionPrediction We should find the test tube with tracing paper (most light

availability) should have photosynthesised the most – used up most of the CO2 so the indicator should be purple

The test tube with black paper should have stopped most of the light getting through, so not much photosynthesis would have taken place (respiration would actually increase CO2 content) – used least CO2 so the indicator should be yellow

Our control should remain orange/red