Volume 10, 1997-1998
Number 2, November 1997
Biological Sciences Review Magazine Archive
Volume 10, 1997-1998
Number 2, November 1997
This graph compares two different stages / pathways within photosynthesis, known as C4 and C3. The graph shows that at a lower concentration of CO2 the C4 route of photosynthesis is operational. C3 , the metabolic pathway for most plants, only comes into effect at 100 CO2 concentration/ppm and increases at a generally steady rate. This article was published in 1997 so next century is taken to mean after the year 2000. We can see that at 380 CO2 concentration the C4 pathway begins to plateau. However in comparison the C3 metabolic pathway only begins to plateau at the end of the graph at around 950 CO2 concentration/ppm. These will be important when considering the effect of climate change and global warming. Climate change is largely caused by gases, known as greenhouse gases, which are being emitted through a variety of ways including the thawing of permafrost and industrial agriculture. This gas accumulates and gathers at the ozone layer. One of these greenhouse gases is carbon dioxide. The levels of carbon dioxide are constantly rising as is the world population. Carbon dioxide is an important molecule for all plants as it is used in photosynthesis. Theoretically if the limiting factor to photosynthesis rate in plants was the CO2 abundance in the vicinity of the plant. Therefore an increase in carbon dioxide through emissions would be good for plant growth and may potentially produce more fruit. This ‘model’ of carbon dioxide being the limiting factor is true for C4 up until 380 CO2 concentration/ppm and for C3 up until 950 CO2 concentration/ppm. After these points, when the lines plateau both plants have an abundant supply of CO2 and we can therefore conclude that the plants are limited by another reactant of photosynthesis such as water or light. With climate change the levels of CO2 will increase meaning C3 would produce more food which would be able to cope with the growing population however plants which use metabolic pathway C4 are already at an optimum level of efficiency of photosynthesis production. Therefore it is not possible for there to be more of it without devoting more agricultural land to growing it. C4 vital plants such as maize and sugar cane. The issue with more land being made into farming is that this land must come from somewhere which could be from clearing forests which would lead to more greenhouse gases being emitted (as trees absorb carbon dioxide). This article was written for educational purposes by Dr Amanda Bamford in the School of Biological Sciences at the University of Manchester with resources being used from the University of East Anglia. I believe that this article is valid as I do not feel these contributors would have an agenda to emphasise or downplay the results of the experiment in anyway.
The abiotic environment is dependant largely on ecological succession. The first way ecological succession shapes the abiotic environment is through edaphic factors. A sand dune for example begins with an inhospitable salty soil which contains very few mineral ions. However as these plants die their organic matter is left in situ. This decomposes and the organic matter contributes to the soil, this would include minerals such as nitrates and phosphates. This alters the pH making it less alkaline and the number of mineral ions steadily increases. Through the different stages of succession the soil becomes more hospitable allowing for larger plants which would then eventually decompose and keep contributing towards the edaphic factors allowing for larger plants to go. With more and more plants being able to survive in an area (due to an increase in the carrying capacity of the area) this means that water availability would decrease. Each plant sets roots into the soil which draw up water from the soil. At the start of succession, with the pioneer species, there may be a large amount of water available however when the climax community (consisting mainly of trees) is present they draw up a large volume of water leaving far less in the water. These roots, along with contributing to water availability, also make the soil more compact as the roots get into the soil and it becomes much more firm as soil and roots become intertwined. This means it is harder for future species to grow as it may be harder for future species to grow their roots to a sufficient length. The roots may also stop water run off which in heavy rain could cause flooding as the roots block and jam the water as it tries to run off. Light levels / availability for plants also changes throughout ecological succession. In the initial stage of succession there are very few pioneer species and the plants themselves are small. This means there is very little competition (either inter or intraspecific) between the plants. They can all get sufficient nutrients and light. However in stages similar to the climax community a taller plant may block the light from reaching small plant which does not grow upwards. The most obvious example being a large oak tree in the climax community shielding light from reaching a bush by the base of the tree.
I think I got 5 out of 6 for this answer. I shoul have expanded on my point about the root systems and should have mentioned their affect on soil erosion. I could have also discussed nitrogen fixing bacteria and their role in succession
Core practical 8: Investigate the effect of environmental conditions on water uptake in a plant shoot
A potometer has a predefined diameter of 1cm therefore we can work out the volume of water from the distance.
Volume of cylinder = ½ x π x r2 x h
If we take 3 squirts as an example, h is height which is the same as distance the bubble travelled as the water displaced would have been equal.
h = 0.8
r = 0.5
We can work out the rate of water uptake by dividing volume of water taken up over the course of the experiment divided by the time the experiment went on for
Analysis of results
We can see from these graphs that as the humidity of the air around the plant decreases the distance of the bubble travelled and therefore the rate of water uptake. This is because the transpiration stream works by having perpetual motion up the plant. To do this water needs to come in by the capillary tube and out the stoma of the leaves. The water exiting the stoma is done by a passive process known as diffusion. The rate of diffusion depends largely on the concentration difference between the leaf and the surroundings. By increasing the water content of the air around the plant we reduce the concentration difference. This causes the process to slow down which is reflected in the result. The rate of water uptake reduces from 0.5585 to 0.06108.
1.Why is it necessary to form an airtight seal?
It is necessary to form an airtight seal between the shoot and the capillary tubing. This is to make sure that there is no loss of pressure. The transpiration stream has a pull on the water from the potometer to draw it up into the xylem which requires closed conditions that means that you maintain suction. The purpose of this experiment is to simulate transpiration from the leaves to the roots in a natural plant. In real world conditions there would not be holes which would be detrimental to the turgor pressure of the stream.
2.The limitations of an investigation are factors that reduce accuracy and reliability of results. They may arise from variables that are difficult to control. What are the limitations of this procedure?
The first, and most impactful limitation was that to maintain accuracy we used the same plant for every experiment. We also used the same bag and because of the time constraints we only had a brief interval between humidity conditions. This meant that some residual increased humidity would affect the true value of humidity. So when I say 2 squirts it means 2 squirts and the remaining water in the air from the first experiment. The next limitation is that our bag was sealed by tying it at the base of the stem. Although this stops a majority of water escaping it doesn’t create a microclimate within the bag as there is still a gap in which water can escape. This happened for all experiments sop this limitation is consistent throughout all of the experiments but I cannot reliably estimate the volume of water retained in the bag.
3.Suggest ways of reducing the limitations to give more accurate and reliable data
I should make sure to do the experiments at different conditions at regular intervals of 24 hours. This would eliminate the effect of residual humidity. You would need to make sure that the conditions are similar on all occasions, this includes temperature, wind and light intensity. I would also use cable ties and make sure the base of the stem was covered off by the bag to make sure there was very little to no gap for water to escape through.