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.
The leaf is the organ in a plant specially adapted for photosynthesis. You need to understand the structure of the tissues in a leaf together with their functions.
Upper Epidermis: this is the tissue on the upper surface of the leaf. It produces a waxy layer, called the cuticle, which is not made of cells but is a waterproof barrier to prevent excessive evaporation through the hot upper surface of the leaf. The upper epidermis cells have no chloroplasts so light passes through them easily.
Palisade Mesophyll: this tissue is where 80% of the photosynthesis takes place in the leaf. The palisade cells have many chloroplasts in their cytoplasm and the box-like shape and arrangement of these cells ensures they are packed tightly together.
Spongy Mesophyll: this tissue contains large air spaces which are linked to the atmosphere outside the leaf through microscopic pores called stomata on the lower surface. Spongy mesophyll cells also contain chloroplasts and photosynthesis occurs here too. The air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells and the fact that these cells have fairly thin cell walls which are coated with a film of water together means that gas exchange between air space and mesophyll is speeded up.
Lower Epidermis is the most dull tissue in the leaf. The only interesting thing about it is that it contains specialised cells called guard cells which enclose a pore called a stoma. Carbon dioxide can diffuse into the leaf through the stomata when they are open (usually at day time) and water evaporates out of the stomata in a process called transpiration.
Adaptations of a Leaf for Photosynthesis
- Large Surface Area – to maximise light harvesting
- Thin – to reduce distance for carbon dioxide to diffuse through the leaf and to ensure light penetrates into the middle of the leaf
- Air Spaces – to reduce distance for carbon dioxide to diffuse and to increase the surface area of the gas exchange surface inside the leaf
- Stomata – pores to allow carbon dioxide to diffuse into the leaf and water to evaporate out (transpiration)
- Presence of Veins – veins contain xylem tissue (carries water and minerals to the leaf from the roots) and phloem (transports sugars and amino acids away from the leaf)
- Chloroplasts – mesophyll cells and guard cells contain many chloroplasts. These organelles contain the light harvesting pigment chlorophyll and are where all the reactions of photosynthesis occur
The topic of gas exchange in plants is often tested in exams because it can be a good discriminator between A grade and A* grade candidates. If you can master the understanding needed for these questions, important marks can be gained towards your top grade.
Firstly you must completely remove from your answers any indication that you think that plants photosynthesise in the day and respire at night. Even typing this makes me feel nauseous…. Yuk? Respiration as you all know happens in all living cells all the time and so while the first half of the statement is true (photosynthesis only happens in daytime), respiration happens at a steady rate throughout the 24 hour period.
Although the equations above make it look like these two processes are mirror images of each other, this is far from the truth.
How can gas exchange in plants be measured?
The standard set up involves using hydrogen carbonate indicator to measure changes in pH in a sealed tube. In this experiment an aquatic plant like Elodea is put into a boiling tube containing hydrogen carbonate indicator. The indicator changes colour depending on the pH as shown below:
- acidic pH: indicator goes yellow
- neutral pH: indicator is orange
- alkaline pH: indicator goes purple
a) If the tube with the plant is kept in the dark (perhaps by wrapping silver foil round the boiling tube), what colour do you think the indicator will turn? Explain why you think this.
b) If the tube with the plant is kept in bright light, what colour do you think the indicator will turn and why?
c) If a control tube is set up with no plant in at all but left for two days and no colour change is observed, what does this show?
In order to score all the marks on these kind of questions, there are two pieces of information/knowledge you need to demonstrate. You need to show the examiner that you understand that carbon dioxide is an acidic gas (it reacts with water to form carbonic acid) and so the more carbon dioxide there is in a tube, the more acidic will be the pH. As oxygen concentrations change in a solution, there will be no change to the indicator as oxygen does not alter the pH of a solution.
Secondly you need to show that you understand it is the balance between the rates of photosynthesis and respiration that alters the carbon dioxide concentration. If rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide so the pH will fall (become more acidic). If the rate of photosynthesis in the tube is greater than the rate of respiration, there will be a net uptake of carbon dioxide (more will be used in photosynthesis than is produced in respiration) and so the solution will become more alkaline.
So to answer the three questions above I would write:
a) The indicator will turn yellow in these conditions. This is because there is no light so the plant cannot photosynthesise but it continues to respire. Respiration releases carbon dioxide as a waste product so because the rate of respiration is greater than the rate of photosynthesis, there will be a net release of carbon dioxide from the plant. Carbon dioxide is an acidic gas so the pH in the solution will fall, hence the yellow colour of the solution.
b) The indicator will turn purple in these conditions. This is because the bright light means the plant photosynthesises at a fast rate. Photosynthesis uses up carbon dioxide from the water. The plant continues to respire as well and respiration releases carbon dioxide as a waste product. As the rate of photosynthesis is greater than the rate of respiration in these conditions there will be a net uptake of carbon dioxide. Carbon dioxide is an acidic gas so if more is taken from the solution than released into it, the pH in the solution will rise as it becomes more alkaline, hence the purple colour of the solution.
c) This shows that without a living plant in the tube there is nothing else that can alter the pH of the solution. It provides evidence that my explanations above about the cause of the colour change is correct.