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B4 D) Investigating Photosynthesis
B4 D) Investigating Photosynthesis
In the previous section, we learnt that the rate of photosynthesis is affected by light intensity, concentration of carbon dioxide and temperature. We can change one of these factors and see what effect it has on the rate of photosynthesis by undertaking an experiment. Before we look at the experiment, let’s remind ourselves of the equation for photosynthesis, which is shown below:
When a plant is photosynthesising it produces oxygen and water. We are going to be using the amount of oxygen produced to inform us as to what the rate of photosynthesis is. A higher quantity of oxygen produced means that the plant is carrying out more photosynthesis reactions, and a lower quantity of oxygen produced means that the plant is carrying out fewer photosynthesis reactions. The amount of oxygen produced is not a perfect way to measure the rate of photosynthesis and this is because some of the oxygen that the plant produces during photosynthesis will be used up in respiration reactions.
The experiment that we will use to measure the rate of photosynthesis is shown below.
The experiment that we will use to measure the rate of photosynthesis is shown below.
We place Canadian pondweed in water and leave it for a period of time. The pondweed will photosynthesise during the time that it is left. As it photosynthesises, it will produce oxygen and the oxygen will collect in the capillary tubing. When the period of time is up, we use the syringe to bring the oxygen bubble to the ruler part where we can measure the length of the oxygen bubble. The volume of oxygen produced will be in direct proportion to the quantity of photosynthesis reactions; a greater volume of oxygen means that there were more photosynthesis reactions meaning that the rate of photosynthesis was high, and a lower volume of oxygen produced means that there were fewer photosynthesis reactions and the rate of photosynthesis was low.
We will complete the experiment and change the factor that we are investigating (either light intensity, concentration of carbon dioxide or temperature). We would need to make sure that we only change the factor that we are investigating, and we keep all of the other factors constant. This is so that the changes in the volume of oxygen produced is solely down to the factor that we changed and not any other factors that we didn’t control/ keep the same. We then complete the experiment twice at each of the different conditions and find the mean of the results.
Sometimes in the exam you may be asked why you should use an experiment like this with capillary tubing, a ruler and a syringe to measure the rate of photosynthesis rather than counting the oxygen bubbles produced by the pondweed. The reason why you would use this experiment is because it will calculate the volume of oxygen produced more accurately. If you were counting bubbles, you may miss bubbles, the bubbles would be of different sizes etc. This will mean that counting bubbles is not an accurate way of measuring the volume of gas produced and therefore is not an accurate way to find the rate of photosynthesis.
I am now going to go through how we can modify this experiment to investigate the effect that light intensity, concentration of carbon dioxide and temperature have on the rate of photosynthesis.
We will complete the experiment and change the factor that we are investigating (either light intensity, concentration of carbon dioxide or temperature). We would need to make sure that we only change the factor that we are investigating, and we keep all of the other factors constant. This is so that the changes in the volume of oxygen produced is solely down to the factor that we changed and not any other factors that we didn’t control/ keep the same. We then complete the experiment twice at each of the different conditions and find the mean of the results.
Sometimes in the exam you may be asked why you should use an experiment like this with capillary tubing, a ruler and a syringe to measure the rate of photosynthesis rather than counting the oxygen bubbles produced by the pondweed. The reason why you would use this experiment is because it will calculate the volume of oxygen produced more accurately. If you were counting bubbles, you may miss bubbles, the bubbles would be of different sizes etc. This will mean that counting bubbles is not an accurate way of measuring the volume of gas produced and therefore is not an accurate way to find the rate of photosynthesis.
I am now going to go through how we can modify this experiment to investigate the effect that light intensity, concentration of carbon dioxide and temperature have on the rate of photosynthesis.
Concentration of CO2
We can investigate the effect that the concentration of carbon dioxide has on the rate of photosynthesis by adding sodium hydrogencarbonate into the water that the pondweed is in. The sodium hydrogencarbonate will dissolve into the water and produce carbon dioxide. The more sodium hydrogencarbonate that we add to the water, the greater the concentration of carbon dioxide will be in the water.
We then complete the experiment a few times with different amounts of sodium hydrogencarbonate dissolved in the water and measure the volume of oxygen produced. From our values, we can plot a graph with concentration of carbon dioxide/ amount of sodium hydrogencarbonate dissolved on the x axis, and the rate of photosynthesis/ volume of oxygen produced on the y axis. My graph is shown below.
We then complete the experiment a few times with different amounts of sodium hydrogencarbonate dissolved in the water and measure the volume of oxygen produced. From our values, we can plot a graph with concentration of carbon dioxide/ amount of sodium hydrogencarbonate dissolved on the x axis, and the rate of photosynthesis/ volume of oxygen produced on the y axis. My graph is shown below.
When we carry out the experiment, we will observe that a greater concentration of carbon dioxide (caused by a greater amount of sodium hydrogencarbonate being dissolved in the water) will lead to a greater volume of oxygen produced. This informs us that as the concentration of carbon dioxide increases, so too does the rate of photosynthesis. This increase in photosynthesis will only occur whilst the concentration of carbon dioxide is a limiting factor, which is at the start of the curve (the upwards sloping part). When another factor (light intensity or temperature) is a limiting factor, the rate of photosynthesis will not increase as the concentration of carbon dioxide increases (this is the flat part of the curve).
Temperature
We can modify this experiment to investigate the effect that temperature has on the rate of photosynthesis by placing the test tube with pondweed into a water bath. We then set the temperature of the water bath at a variety of different temperatures starting from a low temperature and measure the volume of oxygen produced. We start from a low temperature and end with a high temperature because if we go from a high temperature to a low temperature, we would denature the enzymes, which would mean that the enzymes would be unable to function at any temperature. If we didn’t have a water bath, we can place the test tube with pondweed into a beaker like what is shown below; we can add heat to the beaker by placing it on top of a Bunsen burner.
We can modify this experiment to investigate the effect that temperature has on the rate of photosynthesis by placing the test tube with pondweed into a water bath. We then set the temperature of the water bath at a variety of different temperatures starting from a low temperature and measure the volume of oxygen produced. We start from a low temperature and end with a high temperature because if we go from a high temperature to a low temperature, we would denature the enzymes, which would mean that the enzymes would be unable to function at any temperature. If we didn’t have a water bath, we can place the test tube with pondweed into a beaker like what is shown below; we can add heat to the beaker by placing it on top of a Bunsen burner.
We then plot the temperature on the x axis and the rate of photosynthesis/ volume of oxygen produced on the y axis. The graph will look like what is shown below.
What we should observe for the start of the curve is that as the temperature increases, so too does the rate of photosynthesis. This is because a greater temperature results in the reactants for photosynthesis (carbon dioxide and water) and the enzymes having more kinetic energy. This means that the reactants and enzymes move around faster and collide more frequently resulting in a greater rate of photosynthesis.
However, as the temperature increases too much, the rate of photosynthesis decreases. This is because the enzymes start to denature. Denaturing is where the excessive temperature causes the bonds that make up the enzyme to break. This causes the active site of the enzymes to change shape, which means that the reactants can no longer fit into the active site of the enzymes, thus resulting in the rate of photosynthesis decreasing. The denaturing temperature depends on the plant but it is usually around 35-45°C.
However, as the temperature increases too much, the rate of photosynthesis decreases. This is because the enzymes start to denature. Denaturing is where the excessive temperature causes the bonds that make up the enzyme to break. This causes the active site of the enzymes to change shape, which means that the reactants can no longer fit into the active site of the enzymes, thus resulting in the rate of photosynthesis decreasing. The denaturing temperature depends on the plant but it is usually around 35-45°C.
Light
We can investigate the effect that light intensity has on the rate of photosynthesis by having a lamp at different distances away from the test tube with the pondweed in. The light from the lamp needs to be a white light and a light that doesn’t get hot. The light should not get hot because temperature affects the rate of photosynthesis and if we had a hot light and moved it closer to the test tube, it would cause both light intensity and temperature to increase. If this was the case, we would be unable to say for sure whether the change in the rate of photosynthesis was down to light intensity changing or temperature changing. Therefore, we need to use a light that does not get hot. We set the apparatus up like what is shown below.
We can investigate the effect that light intensity has on the rate of photosynthesis by having a lamp at different distances away from the test tube with the pondweed in. The light from the lamp needs to be a white light and a light that doesn’t get hot. The light should not get hot because temperature affects the rate of photosynthesis and if we had a hot light and moved it closer to the test tube, it would cause both light intensity and temperature to increase. If this was the case, we would be unable to say for sure whether the change in the rate of photosynthesis was down to light intensity changing or temperature changing. Therefore, we need to use a light that does not get hot. We set the apparatus up like what is shown below.
We can plot our data from the experiment on a graph with the distance of the lamp away from the pondweed on the x axis, and the rate of photosynthesis/ volume of oxygen produced on the y axis.
From the above graph, we can see that an increase in the distance from the light source causes a decrease in the rate of photosynthesis. This is because a greater distance between the light source and pondweed causes the light intensity to decrease, which decreases the rate of photosynthesis. From looking at the graph, we can see that the data gives us a curve; this tells us that the relationship between distance and the rate of photosynthesis is not linear. The reasons for this non-linear relationship is because light intensity is inversely proportional to the square of the distance. The notation for this is shown below.
Inverse proportion means that the variables move in opposite directions; if one variable increases, the other variable decreases (and vice versa)
If we were to halve the distance of the lamp away from the pondweed (distance decreases), the light intensity would increase by 4 times (22 = 4). If we were to make the distance of the lamp away from the pondweed a third of what it was, the light intensity would be 9 times greater (32 = 9).
On the contrary, if we were to move the lamp twice as far away from the pondweed (distance increases), the light intensity would be 4 times less. Also, if we were to triple the distance of the lamp away from the pondweed, the light intensity would be 9 times less.
It may be the case that we are asked to work out the light intensity in arbitrary units for a particular distance. We do this by subbing the value for the distance into the above formula.
Example 1
Find the light intensity in arbitrary units when the distance is 5 cm.
We find the light intensity in arbitrary units by subbing d in as 5. The working is shown below:
This tells us that the light intensity in arbitrary units is 0.04.