In the previous post on photosynthesis, you revised how there were four environmental factors that can affect rates of photosynthesis in a plant:
- light intensity
- light wavelength
- carbon dioxide concentration
This post will explain the results from experiments with Elodea in which one factor is altered (the independent variable) and the other three are kept exactly the same (control variables)
The independent variable (light intensity) is on the x axis and the dependent variable (number of bubbles per minute) is on the y axis.
How do we explain the pattern in this graph?
As the light intensity increases the rate of photosynthesis increases. This is because a higher light intensity gives more energy to the chloroplasts and so more reactions can happen per second and the rate goes up. But beyond the orange dot on the graph, the increases in rate slows down until at around 12 units of light, adding more light has no effect on the rate. At these high light intensities some other factor is now the limiting factor as opposed to light intensity. The limiting factor remember is the factor in the shortest supply. So perhaps above 12 units of light photosynthesis is limited by the concentration of carbon dioxide. The only way to find the limiting factor is to repeat the experiment with more carbon dioxide and see whether the rate is higher above 12 units.
Although this graph is not perfect, it does show how the rate of photosynthesis varies at different light wavelength.
Rates of photosynthesis peak in the blue-violet and red parts of the visible spectrum with a much lower rate in green light. The reason for this is that chlorophyll pigments do not absorb green light well.
Carbon Dioxide concentration
The pattern is similar to the light intensity relationship. When carbon dioxide concentrations are low, it is the limiting factor for photosynthesis and so increasing the concentration will increase the rate. As the graph levels off, some other factor is now the limiting factor – perhaps light intensity or temperature.
Temperature is a factor that affects photosynthesis because of enzymes. Many reactions in photosynthesis are catalysed by enzymes and enzymes all have an optimum temperature.
This pattern is not explained by limiting factors. At low temperatures the rate is low because the enzymes and the substrate molecules are moving really slowly. This means there are few collisions between the substrate and the active site of the enzyme. As temperature increases, the rate increases as there are more collisions and more enzyme-substrate complexes are formed per second. But high temperatures denature enzymes: the bonds that hold the enzyme in its precious 3-D shape are broken and the enzyme molecule unravels. So the active site may either change shape or may be lost as a catalyst. This slows the rate down to an extremely low rate.
Photosynthesis is the process occurring in plants in which sunlight is trapped by chlorophyll pigments and used to power the chemical reactions involved in making food molecules such as carbohydrates from carbon dioxide and water. Oxygen is released as a waste product of these reactions.
(I can’t write a chemical equation as I can’t find a way of writing subscript in WordPress….. Can anyone help?)
In the equation above, the carbohydrate produced is glucose, a six carbon sugar.
The reactions of photosynthesis happen in specialised mesophyll tissue in the leaf of the plant (see previous post) Inside the palisade and spongy mesophyll cells there are thousands of tiny organelles called chloroplasts in which the reactions of photosynthesis occur.
So what environmental factors could be altered to vary the rate of photosynthesis in a plant?
Light Intensity – light provides the energy for photosynthesis and so the higher the intensity of light, the more energy the chloroplasts receive to make carbohydrates.
Light wavelength – chlorophyll pigments absorb the blue-violet and red parts of the spectrum well but cannot absorb green light.
Carbon Dioxide concentration – this is a reactant for photosynthesis so increasing the concentration makes a collision between the reactant molecule and the enzyme inside the chloroplast that bind it more likely, so the rate will go up.
Temperature – many reactions in photosynthesis are catalysed by enzymes and enzymes are very affected by temperature: too low temperatures and the enzymes and substrate molecules move very slowly and so there are few collisions, too high temperatures and the enzymes change shape (denature) so the substrate molecules cannot fit into the active site.
NB – water availability is never a factor that can alter rates of photosynthesis even though it is a reactant molecule. This might seem unusual until one remembers that plants that are dehydrating will close the stomata in their leaves to minimise transpiration. Closed stomata mean that carbon dioxide cannot get into the air spaces in the leaf so this is ultimately what limits photosynthesis in a dehydrated plant.
The experimental set up above is the best way to measure rates of photosynthesis and so investigate the effect of any of the four factors listed above. Light intensity can be varied either with a dimmer switch as above or by altering the distance between the lamp and the plant. The heat shield is transparent to let light through but will absorb the heat from the bulb ensuring the temperature of the water stays constant. Carbon dioxide concentration can be altered by dissolving different masses of sodium hydrogen carbonate in the water. The wavelength of light will stay constant so long as the build remains the same.
How to measure rates of photosynthesis in this set up?
Well you could collect the gas produced over a long period of time and measure its volume with a gas syringe. This might sound more accurate than counting bubbles but in fact it is a less reliable way as you would have to leave the set up for a long time and variables might change. So it is fine to assume that the bubbles produced are oxygen and that every bubble is the same volume: if you do this, the rate of production of bubbles is directly proportional to the rate of photosynthesis in the Elodea plant.