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Option 9.9 Biochemistry: 2. Determining a basic
equation for photosynthesis
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2. The notion that plants obtain nourishment from
water, light and air took nearly two centuries to evolve
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Students learn to:
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Students:
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Extract from Biology Stage 6 Syllabus (Amended October
2002) © Board of Studies, NSW.
[Edit: 18 June 09]
Prior learning: Stage 4, Structures and Systems 4.8.2 (c), 4.8.4(d) Interactions 4.10 (c); Stage 5, Interactions 5.11.2 (b), Structures and Systems 5.8.4(b).
Recall statements in Preliminary course: Preliminary module
8.2 (subsection 2) module 8.3 (subsection 4)
Background: The idea that plants obtain
nourishment from water, light and air took nearly two centuries
to evolve. Before this it was thought that plants took all
their nourishment from the soil.
outline the progress that occurred in the 17th and 18th centuries towards understanding plant growth by identifying:
- the observation of van Helmont that soil was not primarily responsible for a plants change in mass as it grew
- Stephen Hales’ proposal that plants extract some of their matter from air
- the work of Priestley in identifying that plants could ‘restore the air’ used by a candle and his subsequent discovery of oxygen
- Ingen-Housz’s demonstration of the importance of sunlight for oxygen production by plants
- Senebier’s demonstration of the use of carbon dioxide during photosynthesis
- Saussure’s conclusion that water was also necessary for photosynthesis
- In 1640 Van Helmont weighed 200 pounds of soil and planted a willow sapling weighing 5 pounds in a pot. He added only water to the pot. After five years he reweighed the sapling and found it weighed 169 pounds and the soil weighed 199 pounds. Van Helmont concluded that the tree had gained all its weight from the water he added.
- Stephen Hales grew a plant in a closed system above water. He measured a reduction in the volume of the air above the water of about 14%. This called attention to the fact that air was a participant in the process by which plants lived.
- In 1772 Priestley placed a mint plant in a container and connected it to a second container in which he had placed a candle that had been allowed to burn out (i.e. consume all the oxygen). After several days Priestley successfully relighted the candle. From this experiment he concluded that plants could “restore air which had been injured by the burning of candles”.
- In 1779 Igen-Housz showed that green plants “purified air” only when they were placed in light.
- In 1782 Senebier conducted three experiments. In the first experiment he placed leaves in two beakers: one beaker contained pure (decarbonised) water, the other beaker contained carbonised water from a well. Senebier collected the gas that was evolved and concluded that oxygen was produced by the leaves in the well water but not in the pure water. When the leaves in the beaker of well water stopped producing gas, Senebier replaced them with fresh leaves and found no gas was evolved. Senebier then placed the leaves he had removed from the well water into fresh well water. He found they again produced oxygen. From these experiments, Senebier concluded that it was “fixed air” (carbon dioxide) in the well water that was allowing the production of oxygen and that the leaves themselves were not producing any carbon dioxide for use in making oxygen.
- In 1804 Saussure grew plants in a closed system,
measuring the amount of water present initially, the mass
gained by the plant and how much water was present at the end
of the experiment. He concluded that the plant’s
increase in weight could not be solely due to uptake of
carbon and minerals but that water was necessary for the
photosynthetic production of organic materials.
Useful web sites:
History of Plant Physiology
by John Hanson, Biology Encyclopedia forum
History of photosynthesis
identify data
sources, plan and choose equipment or
resources to perform a
first-hand investigation to design experiments that could
test the observations of one of:
- van Helmont
- Hales
- Priestley
- Ingen-Housz
- Senebier
- Saussure
- Choose the experiment you wish to replicate and determine the type of data you need to collect. Plan your specific design for the experiment in light of the equipment and resources you have available, making modifications to the design of the investigation if necessary. Perform the experiment, recording the data appropriately. The experiment could be as simple as counting the bubbles of oxygen gas produced by an algal plant in water exposed to various light intensities to reproduce Ingen-Housz’ experiment.
This site describes several of the experiments. Photosynthesis , LMPC, DET, New South Wales
gather and process information from secondary sources to identify and describe relevant modern technologies not available to the above people that would have assisted them in their investigations
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Gather information from a wide variety of secondary sources on types of modern experimental equipment currently available , such as Geiger counters and data loggers.
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Process this information to identify how it could assist imporve the validity and reliability of classic experiments.
Sample information
Pure sources of gases
If pure sources of gas were available then as they are today, plants could be grown in pure atmospheres of CO2(g), O2(g), or N2(g) to determine which components of the atmosphere support photosynthesis. Plants grown in an atmosphere of pure oxygen or nitrogen would not survive while those in carbon dioxide would produce sufficient oxygen by photosynthesis to respire and survive, thus proving that carbon dioxide is the gas required for photosynthesis.
Geiger counters
Geiger counters or scintillation counters and a supply of radioactively labeled CO2(g), H2O(l) or O2(g) would have allowed the early experimenters to trace the movement of these substances in the plant. Using Geiger counters it is possible to follow, for example, the movement of water from the shoots to the leaves. Traces of radioactivity would be found in the leaves and in the atmosphere around the plant as some water would be transpired but other water molecules would be incorporated into glucose by photosynthesis. Using radioactive 14CO2(g) Calvin described the processes of the Calvin Cycle of photosynthesis in 1962.
Autoradiography
Autoradiography could have been used to determine the
location of the radioactively labeled substances. The cells
or tissues, washed clean of any radioactive material that was
not taken up, are fixed and preserved on glass slides. The
slides are covered with a layer of photographic emulsion and
kept in the dark. As the radioactive substance decays, it
exposes the photographic emulsion and when developed, the
sites of radioactivity can be examined under a microscope.
This technique would have revealed chloroplasts as the site
of photosynthesis.
Oxygen electrodes and dataloggers
These can be used to monitor long-term changes in gases
entering and leaving plants.
explain that, building on the evidence from earlier investigations, Mayer concluded that plants convert light energy to chemical energy
- Scientists before Mayer had shown that plants, in the presence of sunlight, used water and carbon dioxide to produce oxygen and increased their weight (grew) in the process. Mayer hypothesised that the sunlight provided the initial source of energy that was turned into chemical energy and used to make organic matter. Mayer used this information to formulate a crude equation for photosynthesis:
CO2 + H2O + sunlight → O2 + organic matter
gather and process information from secondary sources to identify observations and conclusions from the observations that led to the hypothesis of Blackman and Mathgel
- Gather data from the work of Blackman and Mathgel from a range of resources including scientific journals or other secondary sources.
- Process the information by using computer-assisted analysis to illustrate trends and patterns in the data.
- Describe the experiments of Blackman and Mathgel and identify their observations and conclusions. The website below is a starting point.
Photosynthesis
Hamburg University, Scroll down to about half way to find the reference to Blackman and Mathgel
identify that Blackman and Mathgel hypothesised that photosynthesis was a two-step process
Background information
In 1905, F.F. Blackman and G.L.C. Mathgel (also spelt Mathaei) cultivated plants under different carbon dioxide concentrations, light intensities and temperatures. They recorded the effects of these variables on the rate of photosynthesis. Blackman and Mathgel found that
- in conditions of excess carbon dioxide, the rate of photosynthesis increased with increasing light intensity. This was independent of temperature.
- in conditions of excess light, the rate of photosynthesis increased with increasing carbon dioxide concentration but that at high temperature the rate was greater than at low temperatures.
- Blackman and Mathgel concluded that photosynthesis was a two-stage process. The first stage was independent of carbon dioxide concentration and temperature and dependent only on light. The second stage was independent of light intensity and dependent on carbon dioxide concentration and temperature.
- In 1925, O. Warburg explained the results of Blackman as two classes of photosynthetic reaction: the light reaction and the dark reaction.
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