Aim: To investigate a factor that affects the rate of photosynthesis

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Aim: To investigate a factor that affects the rate of photosynthesis.

A piece of pondweed will be cut and placed into a beaker containing water. A lamp will be shined on to the pondweed and the amount of bubbles released from the plant will be counted. The lamp will be adjusted to different distances from the plant to try and obtain different results.

Photosynthesis Equation:

6CO2 + 6H2O light energy & chlorophyll C6H12O6 + 6O2

Variables:
Experimental Variable- Light intensity is to be the variable explored in this investigation. Increasing or decreasing the distance from the light source to the plant can vary light intensity.

Factors that influence the rate of photosynthesis

The rate of photosynthesis is dependent on the following environmental factors: light intensity, temperature, and the availability of carbon dioxide, the water supply, and the availability of certain minerals. A shortage of any one of these factors can limit the rate of photosynthesis, and an increase in the particular rate-limiting factor will, up to a point, speed up the process. The rate also varies with the plant species and its physiological state- for example its health, its maturity and whether or not it is in flower.

Fixed Variables-
Light Wavelength (color)- Light energy is absorbed by pigments in the leaf such as chlorophyll. Chlorophyll easily absorbs blue light, in the 400-450 nm range, and also easily absorbs red light in the 650-700 nm range. Chlorophyll does not absorb green light or yellow light effectively but tends to reflect them, decreasing the amount of light absorbed and decreasing the rate of photosynthesis. Why the rate of photosynthesis increases or decreased from the amount of light energy absorbed is what is being investigated in this experiment. The light color can be fixed by using the same lamp throughout the experiment.
Carbon Dioxide- CO2 concentration can affect the rate of photosynthesis since the more CO2 in the air, the more CO2 that can diffuse into the leaf. This variable can be fixed by adding a fixed amount of sodium hydrogen carbonate to the beaker and plant. The experiment should also be completed in one session and under two hours so the plant does not use up a significant percentage of the CO2.
Water- Water is required in the photosynthetic reaction. When plants lack water, their stomata close to prevent further water loss. At the same time, closing the stomata cells doesn't allow CO2 to diffuse into the leaf. Water is also therefore, linked to the carbon dioxide factor. Water can be kept a constant by keeping the same amount of water in the beaker.
Temperature- Enzymes are used in photosynthesis and the respiration of the plant. Therefore, increasing the temperature will increase enzyme reaction and the photosynthetic rate until a certain point is reached when the enzymes denature. The temperature can be kept somewhat a constant by performing the experiment in one session, when the air temperature shouldn't change enough to affect water temperature. A transparent glass block will also be placed in front of the lamp to retain some of the heat from the lamp.
Plant- Different species plants have different photosynthetic rates due to the different leaf structures of the plants. Even plants of the same species may have slightly different rates of photosynthesis since there may be more or less chlorophyll in the leaves to absorb light. The size of the plant is also important since this would affect the amount of surface area for gas exchange. The only solution to controlling this variable is by using the same plant throughout the experiment.
Limiting Factors- Light, carbon dioxide, temperature, and chlorophyll are all limiting factors, meaning that even when there is surplus of every other variable, the rate of photosynthesis will be limited by the limiting factor until there is an optimal amount of the limiting factor to increase the rate of photosynthesis further. Otherwise, the rate of photosynthesis can no longer increase.

Prediction: I predict that increasing the light intensity will increase the rate of photosynthesis at a proportional rate where LI is inversely proportional to 1/d2 when LI= light intensity and d= distance (from light source to plant). This is true to a certain point until another factor is limiting the rate of photosynthesis.

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Method:

1. Set up the apparatus as shown in the diagram above but leaving out the pond weed, funnel, test tube, water, and the sodium hydrogen carbonate.
2. Fill the beaker with 450 cm3 of water and 50 cm3 of NaHCO3.
3. Select 1 or 2 pieces of pond weed each roughly 5-10 cm long and cut off the stems.
4. Place the pond weed in the beaker and secure the funnel upside down over (on top of) the pond weed using the plasticine.
5. Place a water-filled test tube upside down and over the funnel (see diagram).
6. Place the ruler so that the ...

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