What factors affect the rate of photosynthesis?

Toby Panton 11A

What is photosynthesis?

Photosynthesis is the process in green plants and certain other organisms by which carbohydrates are synthesised from carbon dioxide and water using light as an energy source. Most forms of photosynthesis release oxygen as a by-product. The Chemical equation for photosynthesis is:

6H2O + 6CO2 ----------> C6H12O6+ 6O2

Six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

Photosynthesis mainly takes place in a certain layer of a green leaf called the PALISADE cells. These are perpendicular to the cuticle (a waxy layer on both the upper and lower surfaces on the leaf to stop water loss) of the leaf and are packed with chloroplasts.

The structure of the chloroplast and photosynthetic membranes:

The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane.

Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. Whilst the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments.

Photosynthesis is a two-stage process. The first process is the Light Dependent Process (light reactions) and requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or dark reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term “Dark Reaction” is somewhat of a misnomer! The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

Light reaction: The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light.

Dark Reaction: The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the atmosphere with the use of ATP; can occur in the dark as long as ATP is present.

Grana: A series of stacked thykaloid disks containing chlorophyll; found in the inner membrane of chloroplasts.

Stroma: The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thykaloids, grana) inside the chloroplast.

Thylakoids: The specialised membrane structures in which photosynthesis takes place. They are the internal membranes in the chloroplast where the light reaction chemicals (prokaryotes and eukaryotes) are embedded. Collections of Thylakoids form the grana

What happens in the light reaction?

In the Light Dependent Processes (Light Reactions), light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions, the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions).

In the Light Independent Process, carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form Carbohydrates. The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilise light energy, but can convert it into C-C bond energy that can be released by glycolysis and other metabolic processes.

Overview of the two steps in the photosynthesis process. Image from Purves et al., Life: The Science of Biology

What Factors affect the rate of photosynthesis?

Light intensity: The first step in photosynthesis is the absorption of light by pigments. Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into chemical energy through a series of reactions. Therefore, with a higher light intensity photosynthesis rate will increase until a different factor comes into play and becomes the current ‘limiting factor’. The pigment, chlorophyll, in the leaf, absorbs light energy. Chlorophyll ...

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What Factors affect the rate of photosynthesis?

Light intensity: The first step in photosynthesis is the absorption of light by pigments. Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into chemical energy through a series of reactions. Therefore, with a higher light intensity photosynthesis rate will increase until a different factor comes into play and becomes the current ‘limiting factor’. The pigment, chlorophyll, in the leaf, absorbs light energy. Chlorophyll easily absorbs blue light, in the 400-450 N.M. range, and easily absorbs red light, in the 650-700 N.M. range. However, it does not easily absorb green or yellow light, rather it reflects them, decreasing the amount of light absorbed, and therefore the rate of photosynthesis. This can easily be controlled, simply by using the same lamp throughout the experiment.

Temperature: this affects the rate of photosynthesis because when the temperature is hot, the guard cells (stomata) close. Carbon dioxide used during photosynthesis first must pass through stomata into internal spaces within the leaf. It then diffuses into mesophyll cells where it becomes available for photosynthesis. When the stomata close gaseous exchange will no longer be occurring so therefore CO2 levels drop rapidly within the leaf, inhibiting the light-independent reactions. This then causes photosynthesis to stop.

Carbon dioxide supply: This affects the rate of photosynthesis because CO2 reacts with water to form O2 and glucose, so without enough CO2, the reaction will hit a ‘plane’ where the rate will no longer go any faster.

6CO2+ 6H20 + Light energy➔(chlorophyll)➔C6H12O6+ 6O2

I have chosen to study the factor Light Intensity: I will change the distance between a plant (I am using, elodea, also known as pondweed) and a low watt lamp. I will change the distance by moving the lamp along a ruler and will measure light intensity at designated points along the ruler. I will then integrate this evidence into a graph in order to show that light intensity has a direct effect upon rate of photosynthesis.

Fair testing: all other factors will remain the same through out the experiment in order to make sure that all results are uncorrupted and the most reliable they can be with our limited equipment.

Prediction: I predicted that as the intensity of light increased, so would the rate of photosynthesis. Furthermore, I hypothesise that if the light intensity increases, the rate of photosynthesis will increase (at a proportional rate until a certain level is reached). I suggest this because as stated earlier, the higher the light intensity, the higher amount of energy there is available to strike chlorophyll a in such a way as to excite electrons to a higher energy state. With this higher energy, the electrons are then, in a series of reactions, converted (along an electron transport process) into ATP and NADPH. This is used to make C-C bonds, which can be converted by glycolysis into energy. Eventually, a level will be reached where an increase in light intensity will have no further effect on the rate of photosynthesis, as there will be another limiting factor. In this case, I think that the limiting factor will be probably the number of chloroplasts available to photosynthesise at a moment in time. I say this because you can keep on increasing light, CO2, or temperature but you cant increase the amount of photosynthesising chloroplasts, unless you changed the type of plant used. Temperature can only be increased to a certain amount because otherwise it would denature the plant cells. Further more, when different light wavelengths are used, there will be different results predict this because as stated before, chlorophyll, in the leaf, easily absorbs blue light, in the 400-450 N.M. range, and easily absorbs red light, in the 650-700 N.M. range. However, it does not easily absorb green or yellow light, rather it reflects them, decreasing the amount of light absorbed, and therefore the rate of photosynthesis.

Fair testing: I will make sure that temperature remains the same by placing the test tube containing the strand of elodea in a beaker of water to make sure that heat from the close lamp will not affect the water temperature inside the test tube which would corrupt the evidence and make it an unfair test.

The plant will be placed in the same amount of water from the same source to make sure that each water sample has the same amount of CO2 gas diffused in the water.

No chemicals will be added to the reaction and the experiment will be done in cloudy conditions (to make sure no extra sunlight affects the reaction) in order to maintain fair testing.

Measuring the rate of photosynthesis: I will measure the effects of the change in light intensity by the amount of O2 gas that is produced by the elodea in the allotted time allowance of 1 minute. This will be an indicator of the rate of photosynthesis, because when photosynthesis is occurring O2 is produced. I will measure the amount of O2 gas produced with extreme precision with micro burettes. I will clear the micro burette of all gas to maintain a precise experiment. Then I will start the stopwatch immediately and wait for the 1-minute to elapse fully. At which time I will draw all the O2 gas produced into the micro burette where I will measure the volume of gas with great precision in order to gain results that are more reliable. The results will then be put into a table for easy and simple access.

Apparatus list:

Elodea plant: I am using this plant because the O2 gas created can be collected and measured easily.
Beaker: Used to hold the test tube and regulate a constant temperature of the elodea plant.

Micro burette: I am using this measuring equipment because it is extremely precise and gives reliable results that may be read easily. This piece of equipment will measure the volume of gas created by the elodea plant in photosynthesis.
Knife: used to cut the bottom of the elodea plant so that the O2 gas created can escape, used a knife because it gives a cleaner cut than tearing and is consistent and will give more reliable results.
Clamp: used to hold the Audus apparatus in place

Thermometer: Insert a thermometer into the beaker, and record the temperature at the beginning and end of each experiment, merely as a precaution against a significant rise in temperature, which is not expected.

Test-tube: will contain the water that holds the elodea.
Cold water: will hold the elodea in suspension (pond water would suit better.)
Stopwatch: used to time the 1 minute in which the gas is collected, much better than a clock, as it is far more precise and will give more reliable results.
Lux measurer: this piece will measure the amount of light energy reaching the plant, and will do so with precision, I will be referring to light energy later.
Ruler: With this, I will measure distances the lamp is from the elodea. I am using a ruler because they are accurate, efficient, and can be easily repeated.
Lamp: this will be the source of the light energy used to stimulate photosynthesis, I am using a lamp simply because it is the optimal choice for this experiment.

Preliminary work:

I did a preliminary investigation in which I recorded the volume of oxygen given off in a given time of 1 minute at various light intensities. To alter the light intensity, I placed a lamp at determined distances from the plant; I measured the distances precisely to get results that are more reliable. When the experiment was taking place the other factors, which could affect the rate of photosynthesis, were maintained. I also therefore needed a way of accurately measuring the light intensity, and I did this using a photometer. I recorded the Lux reading (unit of light intensity) at each distance. While preliminary work was taking place I monitored all inconsistencies that occurred and measured the effects of these mistakes, if I have time I will reperform the experiment with the calculated corrections In order to collect more reliable results. Whether I perform another Experiment will depend on the consistency of my results, if my results are precise I will not partake in an apparently needless repeat.

With a preliminary experiment completed, I can begin to justify the range of the measurements I will take. Through this experiment I have decided that the range of 10-60 cm will be sufficient, as with a distance greater than that (as shown by the preliminary work) the natural light begins to be the dominant energy source, not our lamp, which would make any further measurements irrelevant.

Step wise Method:

200cm³ of water is to be put into a glass beaker. Set-up in front of the beaker will be a meter ruler with range varying from 0cm at the beaker to 200cm furthest away from the beaker. This will also be accompanied by a light bulb being placed beside the meter rule, ready for the first reading. The light bulb will not be switched on at this point.
The microburette will be placed in the beaker, held up by a clamp. The syringe will be down and the screw at the top tightened. A fresh piece of elodea will be cut and placed in the beaker – the cut end being placed inside the collecting bulb of the microburette.
Readings would then be ready to be taken. The light bulb will be placed at 10cm and turned on. A reading from the Lux meter will then be taken and the stopwatch would be started. After two minutes, a reading from the microburette will be taken. The process would be repeated again for each distance away from the plant.
Step 3 will then be repeated with all distances again twice. This would mean there would be three sets of results. This is to further increase the reliability of the results and single out any anomalies if any occur.

Analysing evidence:

As I predicted, my results showed that when you increase the light intensity, the rate of photosynthesis increases.

From these results, I am able to say that an increase in light intensity does certainly increase the rate of photosynthesis. The gradual slowing in the rate that photosynthesis increases can be attributed to the other factors limiting the rate of photosynthesis. As light intensity increases, the photosynthetic rate is being limited by certain factors, such as carbon dioxide and temperature. These factors do not immediately limit the rate of photosynthesis, but rather gradually. As light intensity increases further, so the rate of photosynthesis is being limited by other factors more and more, until the rate of photosynthesis is constant, and so is almost certainly limited in full by another factor.

Overall, both my results support my predictions fully. My idea that the rate of photosynthesis would increase with light intensity was comprehensively backed up by my results. This is because a higher light intensity involves a greater level of light energy, which can then be transferred to a special protein environment designed to convert the energy. Here, the energy of a photon is used to transfer electrons from one chlorophyll pigment to the next. When enough energy has been gathered at a reaction centre, ATP can be synthesised from ADP. The oxygen collected in the experiment is in fact the by-product of this reaction, and so it is clear to see that the more light energy, the more ADP is being converted into ATP and more oxygen is produced as a result.

Through looking at the positive correlation apparent in my graph it shows that the greater the light intensity the greater the volume of O2 is produced. This is because with the higher intensity there is more light for photosynthesis (which creates the bubbles), these results match my prediction and also support my thoughts that light intensity was a major factor in photosynthesis. This experiment follows an obvious trend, it is plainly pointed out by the graph line of best fit that O2 bubbles produced is directly related to the light intensity.

pick out anomalies ... say 'greater/lower this : greater/lower that' ... say whatever you find ... then say if it matched your prediction ... use scientific knowledge etc

Evaluation:

I think that I had a very successful experiment, my readings were accurate and I recorded no major anomalies, I know this because all results were acceptably close together. I needed not to repeat my experiment, firstly, there were no anomalies and secondly because the results were already extremely accurate, mainly due to the micro burettes being so precise.

I do think however that there were minor errors in the main experiment:

Firstly, the distance between the light sources and the Canadian Pondweed were not measured to a very high degree of accuracy, especially when you note the fact that the distance should have been measured exactly from the filament of the light bulb to the centre of the plant, and it is possible here to find a percentage error. I estimate that the error could have been up to 0.5cm.

Percentage error = possible inaccuracy

% Error |distance

10 |5cm

1 |50cm

It is clear to see that the percentage error is much less for the larger distances. This is irrelevant to this experiment however as we measured light intensity with great accuracy.

Another error would have been due to background light in the vicinity. We tried to reduce this error by closing all blinds in the laboratory and making sure that It was cloudy weather outside, but due to practical reasons, we could not all perform the experiment in a separate room. Because of this, we therefore experienced light pollution from other student’s experiments. This would have had a very marginal effect on my results as a whole, but to eliminate this problem, it would have been necessary to perform the experiment in a totally dark room.

A further inaccuracy was in the heat generated by the lamp. As I have earlier described, temperature has a very noticeable effect on the rate of photosynthesis, and so any increase in the temperature of the water would have had serious effects on the accuracy of my results. To ensure this did not happen we placed the elodea in a separate beaker filled with cold water, which I hoped would prevent the elodea temperature from rising. It turned out not to be a problem, as over the short period of time taken to gather experimental readings, the temperature did not rise at all. However, if I were to extend the time of my experiment to 5 minutes for each reading for example, which would have the effect of reducing other percentage errors, I would have to find some way of keeping the temperature constant.

As I mentioned in my planning, carbon dioxide concentration could have been an error in the experiment, however, I feel that due to the short period of time taken, there is very little chance that the concentration would ever have been so low as to have become the limiting factor. Again, if I were to carry out the experiment over a longer time period in order to decrease percentage error, it would have been necessary to add sodium hydrogen carbonate to the water at regulated time intervals to increase the carbon dioxide concentrations.

The last inaccuracy, though a small one, was in the time keeping. The main problem here was in when to begin the minute. If for one reading, the minute was started just after one bubble had been produced, and in another reading it was just before, this could have had a negative effect on the accuracy of my results. I therefore ensured that in each case I started the stopwatch just after a bubble had been produced, thus heightening the accuracy and making sure the experiment was as fair as possible.

It would maybe have been productive to test the different wavelengths of light to test whether they affected the light as much as CO2 concentrations or light intensity. Alternatively, even to test with completely different lights, such as a florescent light source or maybe using something like a halogen or black light?