The blade has several layers. The top layer is covered with a waterproof coat called the cuticle. Both the upper and lower layer contain pairs of guard cells. These guard cells protect the stomata, little openings in the leaf that allow the carbon dioxide to go in and oxygen to get out during photosynthesis. Water vapour is also emitted in a process called transpiration.
In between the top and bottom layers of the blade there are cells that are full of chlorophyll.These are essential for the plant to make its food during photosynthesis.
There are different types of leaves. They can change size, shape and colour. For this last change, they contain other substances called pigments that cause the beautiful autumn colours . These leaves are on deciduous trees, so they die and fall off and are replaced by new ones in spring. Conifers or trees that have cones can keep their needles for years and grow new ones as soon as they lose them.
Prediction
My prediction, is that the less light that gets to the plant, the slower the rate of photosynthesis . This is because light is the energy that activates the chloroplasts which cause the process called photosynthesis to happen. So I think that the less energy getting to the plant the fewer chloroplasts will be activated meaning that less photosynthesis takes place.
Apparatus
- Test Tube
- Beaker
- Canadian Pond Weed
- 40ml. Water for test tube
- 150ml. Water for beaker
- Bicarbonate
- 4 pieces white cardboard
- 40 watt light bulb
- Light bulb stand
10. Sellotape
11.Stop Watch
Presentation
Method
A Test tube was filled with 40ml. of H2O. A short stem of Canadian Pond weed was then put in and the test tube was placed in a beaker containing 150ml. of H2o. Next to this a 40 watt light bulb of in a light bulb holder was placed. To make it a fair test the whole setting was surrounded by white cardboard that stopped any rays of light reaching the plant and for extra accuracy all the lights in the lab were switched off. After that the light bulb was switched on and the experiment started.
At first there was a separation between plant and light-bulb of 5 cm. The number of H2O bubbles coming out of the stem were recorded. This process was repeated 6 times, each time adding an extra 5 cm to the original distance, finally ending up with a distance of 30 cm. between weed and bulb. Each reading, for which the light bulb was turned on for 2 minutes took 2 minutes,was repeated three times and at intervals of 5 minutes. The bulb was turned on for every 2 minute reading.
The measures taken to make it a fair test were the following ones:
-The amount of water in the test tube doesn´t change.
-The plant doesn´t change.
-The amount of water in the beaker doesn´t change.
-The class lights weren´t turned on.
-The setting was covered by the white card at all times.
- Three tries for each distance.
Table of Results
Graph of results
Conclusion
As we can see by the results, the rate of photosythesis is affected by the distance between weed and light. We have been able to see a decrease in the rate of bubbles each time the source of light moved further away. This was expected, because as I said earlier on in my prediction, the less energy reaching the chloroplasts the fewer are activated, which makes the food-making process, that involves letting out oxygen, slower.
Evaluation
On the whole I think that the experiment worked out quite succesfully, for the readings weren´t contradictory to my prediction. We were lucky that the pond weed didn´t die half way through, otherwise we wouldn´t have been able to take accurate readings.
If we had used a thermometer we would have able to make sure that the water temperatures didn´t rise.
