Plan for an experiment - How does temperature affect the rate of photosynthesis?
An investigation into the effects Temperature has on the Rate of Photosynthesis
Aim: The aim of this experiment is to investigate the effect of various temperatures on the rate of photosynthesis by measuring the volume of oxygen produced by elodea.
Introduction: Temperature is one of the main factors that can speed up as well as limit the rate of photosynthesis in plants because it has a direct effect on plant enzymes, which are essential to photosynthesis. Enzymes are globular proteins which act like ‘biological catalysts’, as they’re able to increase the rate of reactions within living organisms by lowering the activation energy needed for a reaction without actually being used up themselves. Enzymes have an active site, which is a region on the surface of the enzyme that substrates bind to and catalyses a chemical reaction. The active sites are specific to certain substrates, as only a few substrates can fit the active site, this is called enzyme-substrate specificity. Enzymes have a small range of temperatures within which they’re able to work. When there’s an increase in temperature, enzyme activity generally tends to double with every increase of 10 degrees, as both the enzymes and substrates have more kinetic energy and are therefore moving faster and having more collisions, which would speed up the rate of photosynthesis. All enzymes have an ‘optimum temperature’, where they’re able to work at their best by having more kinetic energy and collisions, however after the optimum temperature, the temperature tends to be too high for the enzymes to function properly, as their active sites began to denature. For most plant enzymes, the optimum temperature is around 25 degrees C . When temperature denatures enzymes, the causes vibrations within the enzyme that ends up destroying the shape of the active site by breaking the hydrogen bonds in the enzyme, which would stop it from being able to function properly as it can no longer fit the specific substrate required. This would slow down the rate of photosynthesis. On the other end of the spectrum, if the temperature is too low, enzyme activity would decrease as there’s a decrease in collisions between enzymes and substrates because they have little to no kinetic energy, which would also slow down the rate of photosynthesis.
In photosynthesis, enzymes are prominent in both the light dependent and independent reaction. Ribulose-1,5-bisphosphate carboxylase oxygenase, also known as RuBisCO is an enzyme that’s particularly important in the Calvin Cycle to bond carbon molecules, from carbon dioxide that’s diffused into the stroma, to ribulose molecoles. This is where oxygen is evolved as a by-product of photosynthesis, thus the volume of oxygen that’s produced by the plant can be taken and measured using setting up a photosynthometer to give an accurate indication of the rate of photosynthesis.
My hypothesis for this experiment is therefore that as temperature increases, the volume of oxygen that’s produced by the plant will also increase, as more successful collisions are taking place, with temperatures between 20 - 30
This is a preview of the whole essay
C producing the largest amounts of oxygen. Below 10°c and above 35°c, I expect the volume of oxygen to decrease, as there isn’t enough energy for the enzymes to collide below 10°c and the enzymes active sites would start to denature around and above 35°c.
Independent Variable: Temperature between 5 – 40 degrees in intervals of 5 (°c)
Dependent Variable: Volume of oxygen(mm3) evolved, calculated from the length of oxygen bubbles in apparatus
For this experiment, I will be measuring the amount of oxygen that’s produced whilst the elodea is submerged in water of different temperatures, therefore I will be setting up a photosynthometer, which is shown in figure 1.
(figure 1) Diagram of a photosynthometer 
- If applicable, make sure the windows and any other light sources that aren’t from the lamp are sufficiently covered up with the black paper/sheets.
- Set up the 10 water baths at 5°C intervals each, ranging from 5°C to 40°C
- Whilst waiting for the water to heat up, set up the equipment as shown in fig 1.
- Measure out your elodea so that it’s about half an inch shorter than the test tube it will go into. Make sure to cut the elodea at an angle/slant so that no oxygen bubbles will be trapped and therefore affect your results. Keep a record of how long your elodea plant is.
- Place the elodea upside down in the test tube and submerge it in either the elodea water or the sodium hydrocarbonate solution.
- Set up the lamp so that it’s 30cm away from the equipment and shining above the water bath. Place a beaker of water in between the lamp and the equipment so that the water can absorb and disperse the heat energy of the lamp so that it can’t affect your results.
- Once everything has been set up, record the light intensity from beside the equipment directly facing the lamp with the light intensity meter and make a note of it. Do this for each condition of the experiment to make sure the light intensity stays consistent.
- Set the test tube with the elodea inside the first water bath. Start the stop watch and allow the elodea to sit in the water for one minute. This is to allow the elodea to acclimatise to the water.
- After one minute, restart the stop watch.
- Start the stop watch again and wait for 3 minutes. If you have more time to do the experiment, you may choose to wait for 5 minutes instead.
- As soon as the time is up, SLOWLY pull the oxygen bubbles that have been produced through the capillary tube with the syringe. Pull them along until they line up with the scale (see fig 1).
- Record the length of the oxygen bubbles and use this to work out the VOLUME of oxygen produced. You can work out the volume with this formula, where ‘r’ is half the diameter of the capillary tube and ‘h’ is the length of the oxygen bubbles. V = πr2h
- Repeat steps 7 – 12 for each temperature condition at least five times. This is to make sure your results are more reliable. Always make sure the light intensity and distance of the lamp from the equipment is consistent to keep the experiment fair.
As with all lab experiments, a lab coat and goggles should be worn at all times, particularly when handling NaHCO3 as this can irritate the eyes.