A2 Biology- Effects of temperature and carbon dioxide on photosynthetic rate in Elodea. Aim: to investigate the effects of temperature and carbon dioxide on the photosynthetic rate of Elodea. Background knowledge: Photosynthesis is the use of light energy from the sun to fix carbon dioxide i. e. converted to sugars. These sugars can then be converted into other essential substances- fats and proteins etc. – that plants need to live and grow. Photosynthesis can be represented using the following equation: 6CO2 + 6H2O ? C6H12O6 + 6O2 The light independent stage occurs in the stroma.
Firstly CO2 combines with a 5C compound called ribulose bisphosphate. This reaction is catalysed by the enzyme RuBPC. The 6C compound formed immediately splits into two molecules of glycerate-3-phosphate (GP). The GP molecules are converted into molecules of triose phosphate (TP) using energy from ATP and the hydrogen atom from NADPH. Some of the TP is used to regenerate RuBP. Finally the rest of the TP is used to produce other essential substances that the plant needs- fats, proteins etc. As light intensity is increased, photosynthesis begins, and some carbon dioxide from respiration is utilised in photosynthesis and so less is evolved.
With a continuing increase in light intensity a point is reached where carbon dioxide is neither evolved nor absorbed. At this point the carbon dioxide produced in respiration exactly balances that being used in photosynthesis. ‘This is called the compensation point’1. ‘Further increases in light intensity result in a proportional increase in the rate of photosynthesis until light saturation is reached’2. Beyond this point further increases in light intensity have no effect on the rate of photosynthesis.
If, however, more carbon dioxide is made available to the plant, further increases in light intensity do increase the rate of photosynthesis until light saturation is again reached, only this time at a higher light intensity. At this point the carbon dioxide concentration, or possibly some new factor such as temperature, limits the process. The effect of temperature on the rate of photosynthesis. ‘The photochemical reaction or light stage of photosynthesis is unaffected by temperature, but that the light independent stage (Calvin cycle) is temperature dependent’3 (this stage is described above).
Provided the light intensity and concentration of carbon dioxide are not limiting, the rate of photosynthesis is found to increase proportionately with an increase in temperature. The minimum temperature at which photosynthesis can take place is 0? c for most plants, although some arctic and alpine varieties continue to photosynthesise below this level. The rate of photosynthesis at these temperatures is very low. ‘The rate approximately doubles for each rise of 10? c up to an optimum temperature, which varies from species to species’4.
Above the optimum temperature, the rate of increase is reduced until a point is reached above which there is no increase in photosynthesis. The optimum photosynthetic rate for most plants is around 25? c. Above these levels further temperature increases lead to a levelling off and then a fall in the rate of photosynthesis. The fall occurs at temperatures too low for it to be entirely accounted for the denaturation of enzymes. The effect of carbon dioxide on the rate of photosynthesis. In the light-limiting region the rate of photosynthesis is not affected by lowering the CO2 concentration.
‘Thus it can be inferred that CO2 does not participate directly in the photochemical reaction’5. But at light intensities above the light-limiting region, increasing the CO2 concentration enhances photosynthesis. In short term studies it was found the rate of photosynthesis increased linearly with increased CO2 concentration up to about 0. 5% though continued exposure to this high concentration injured the leaves. ‘Very good rates of photosynthesis can be obtained with a CO2 content of about 0. 1%’6. The average CO2 content of the atmosphere is about 0. 035%.
Therefore plants in their normal environment do not have enough CO2 to make maximum use of sunlight falling on them. The effect of inorganic ions on the rate of photosynthesis. In the absence of certain inorganic ions, such as iron, chlorophyll cannot be synthesised. Other ions, like nitrogen and magnesium, are an integral part of the chlorophyll molecule and their absence likewise prevents its formation. ‘Where plants are grown on soils deficient in any one of these minerals, the chlorophyll concentration is reduced and the leaves become yellow, a condition called chlorosis’7.
Under these circumstances the rate of photosynthesis is substantially reduced. Other factors also affect the rate of photosynthesis, such as water and specific chemical compounds. A deficiency in water will clearly reduce the rate of photosynthesis. Water has so many functions in a plant that it is impossible to
In order to prevent this interfering with the reliability of the results this variable must remain constant throughout the experiment. i?? Distance of lamp- artificial light produces heat that will increase the temperature so it must remain at such a distance that will not affect the temperature of the environment. i?? Time spent counting- whilst the number of bubbles are being recorded the plant is still photosynthesising, so these new bubbles will not be recorded, therefore to eliminate this error one person will have to record the results whilst the other person observes the number of bubbles being released.
Method: The effect of carbon dioxide on the rate of photosynthesis. 1) Fill the beaker with 800cm3 of water. 2) Select 1 or 2 pieces of pondweed each roughly 10 cm long and cut off the stems. 3) Place the pondweed in the glass beaker and secure the funnel upside down over (on top of) the pondweed using the plasticine. 4) Place a water-filled test tube upside down and over the funnel. 5) Record the number of bubbles emitted after a 3 minute duration. 6) Repeat the experiment but add 600cm3 of water and 200cm3 of the 0. 05M solution of NaHCO3 instead of 800cm3 water.