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CIE A Level Biology Your notes
13.2 Investigation of Limiting Factors
Contents
Limiting Factors of Photosynthesis
Investigating the Rate of Photosynthesis
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Limiting Factors of Photosynthesis
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Limiting Factors of Photosynthesis
Plants need several factors for photosynthesis to occur:
The presence of photosynthetic pigments
A supply of carbon dioxide
A supply of water
Light energy
A suitable temperature
If there is a shortage of any of these factors, photosynthesis cannot occur at its maximum possible
rate
The main external factors that affect the rate of photosynthesis are:
light intensity
carbon dioxide concentration
temperature
These are known as the limiting factors of photosynthesis
If any one of these factors is below the optimum level for the plant, its rate of photosynthesis will be
reduced, even if the other two factors are at the optimum level
Exam Tip
Light intensity, CO2 concentration and temperature are the three limiting factors of photosynthesis
that you need to learn. Although a lack of water can reduce the rate of photosynthesis, water
shortages usually affect other processes in the plant before affecting photosynthesis.
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Limiting Factors of Photosynthesis: Effects
Light intensity Your notes
When temperature and carbon dioxide concentration remain constant, changes in light intensity affect
the rate of photosynthesis
The rate of photosynthesis increases as light intensity increases:
The greater the light intensity, the more energy is supplied to the plant and therefore the faster the
light-dependent stage of photosynthesis can occur
This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which
can then also occur at a greater rate
During this stage of the graph below, light intensity is said to be a limiting factor of photosynthesis
At some point, if light intensity continues to increase, the relationship above will no longer apply and
the rate of photosynthesis will reach a plateau
At this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting
the rate of photosynthesis
The factors which could be limiting the rate when the line on the graph is horizontal include
temperature being too low or too high, or not enough carbon dioxide
The Effect of Light Intensity on the Rate of Photosynthesis Graph
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The effect of light intensity on the rate of photosynthesis
Carbon dioxide concentration Your notes
The rate of photosynthesis increases as carbon dioxide concentration increases:
Carbon dioxide is one of the raw materials required for photosynthesis
It is required for the light-independent stage of photosynthesis when CO2 is combined with the
five-carbon compound ribulose bisphosphate (RuBP)
This means the more carbon dioxide that is present, the faster this step of the Calvin cycle can
occur and the faster the overall rate of photosynthesis
This trend will continue until some other factor required for photosynthesis prevents the rate from
increasing further because it is in short supply
The factors which could be limiting the rate when the line on the graph is horizontal include
temperature being too low or too high, or not enough light
The Effect of CO2 Concentration on the Rate of Photosynthesis Graph
The effect of carbon dioxide concentration on the rate of photosynthesis
Temperature
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As temperature increases the rate of photosynthesis increases as the reaction is controlled by
enzymes
However, as the reaction is controlled by enzymes, this trend only continues up to a certain Your notes
temperature beyond which the enzymes begin to denature and the rate of reaction decreases
The Effect of Temperature on the Rate of Photosynthesis Graph
The effect of temperature on the rate of photosynthesis
For most metabolic reactions, temperature has a large effect on the reaction rate
For photosynthesis, temperature has little significant effect on the light-dependent reactions, as
these are driven by energy from light rather than the kinetic energy of the reacting molecules
However, the Calvin cycle is affected by temperature, as the light-independent reactions are
enzyme-controlled reactions (eg. rubisco catalyses the reaction between CO2 and the five-carbon
compound ribulose bisphosphate)
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Exam Tip
Your notes
Interpreting graphs of limiting factors can be confusing for many students, so here's a quick tip.
In the section of the graph where the rate is increasing (the line is going up), the limiting factor is
whatever the label on the x-axis (the bottom axis) of the graph is. In the section of the graph where the
rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on
the x-axis – choose from temperature, light intensity or carbon dioxide concentration.
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Investigating the Rate of Photosynthesis
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Investigating the Rate of Photosynthesis: Redox Indicators
The light-dependent reactions of photosynthesis take place in the thylakoid membrane and involve
the release of high-energy electrons from chlorophyll a molecules
These electrons are picked up by electron acceptors and then passed down the electron transport
chain
However, if a redox indicator (such as DCPIP or methylene blue) is present, the indicator takes up the
electrons instead
This causes the indicator to change colour
DCPIP: oxidised (blue) → accepts electrons → reduced (colourless)
Methylene blue: oxidised (blue) → accepts electrons → reduced (colourless)
The colour of the reduced solution may appear green because the chlorophyll has a green colour
The rate at which the redox indicator changes colour from its oxidised state to its reduced state can be
used as a measure of the rate of photosynthesis
When light is at a higher intensity, or at more preferable light wavelengths, the rate of
photoactivation of electrons is faster, therefore the rate of reduction of the indicator is faster
Method
1. Leaves are crushed in a liquid known as an isolation medium
This produces a concentrated leaf extract that contains a suspension of intact and functional
chloroplasts
The medium must have the same water potential as the leaf cells (so the chloroplasts don’t shrivel
or burst) and contain a buffer (to keep the pH constant)
It should also be ice cold (to avoid damaging the chloroplasts and to maintain membrane
structure)
2. Small tubes are set up with different intensities, or different colours (wavelengths) of light shining on
them
If different intensities of light are used, they must all be of the same wavelength (same colour of
light)
If different wavelengths of light are used, they must all be of the same light intensity
3. DCPIP of methylene blue indicator is added to each tube, as well as a small volume of the leaf extract
4. The time taken for the redox indicator to go colourless is recorded
This is a measure of the rate of photosynthesis
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Exam Tip
Your notes
In chemistry, the acronym ‘OIL RIG’ is used to remember if something is being oxidised or reduced.
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
Therefore DCPIP in its oxidised state has not accepted electrons and in its reduced state has
accepted electrons.
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Investigating the Rate of Photosynthesis: Aquatic Plants
Investigations to determine the effects of light intensity, carbon dioxide concentration and Your notes
temperature on the rate of photosynthesis can be carried out using aquatic plants, such as Elodea or
Cabomba (types of pondweed)
The effect of these limiting factors on the rate of photosynthesis can be investigated in the following
ways:
Light intensity – change the distance (d) of a light source from the plant (light intensity is
proportional to 1/d2)
Carbon dioxide concentration – add different quantities of sodium hydrogencarbonate
(NaHCO3) to the water surrounding the plant, this dissolves to produce CO2
Temperature (of the solution surrounding the plant) – place the boiling tube containing the
submerged plant in water baths of different temperatures
Whilst changing one of these factors during the investigation (as described below), ensure the other
two remain constant
For example, when investigating the effect of light intensity on the rate of photosynthesis, a glass
tank should be placed in between the lamp and the boiling tube containing the pondweed to
absorb heat from the lamp – this prevents the solution surrounding the plant from changing
temperature
Method
1. Ensure that the water is well aerated before use by bubbling air through it
This will ensure that oxygen gas given off by the plant during the investigation forms bubbles and
does not dissolve in the water
2. Ensure that the plant has been well illuminated before starting the experiment
This will ensure that the plant contains all the enzymes required for photosynthesis and that any
changes in rate are due to the independent variable rather than an increase in enzyme activity
3. Cut the stem of the pondweed cleanly just before placing it into the boiling tube
Cutting the stem at an angle provides a larger surface area from which bubbles can form
4. Set up the apparatus (as shown below) in a darkened room
This ensures that the lamp is the only light source and so allows light intensity to be controlled
Ensure that the pondweed is fully submerged in sodium hydrogencarbonate solution (1%); this
ensures that the pondweed has a controlled supply of carbon dioxide for photosynthesis
5. Measure the volume of gas collected in the gas syringe in a set period of time, e.g. 5 minutes
6. Repeat step 5 at least twice more
7. Change the independent variable and repeat step 5 again
The independent variable could be the light intensity, carbon dioxide concentration or
temperature depending on which limiting factor you are investigating
8. Record the results in a table and plot a graph of the volume of oxygen produced per minute against the
independent variable
Investigating photosynthesis in an aquatic plant diagram
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Your notes
Aquatic plants can be used to measure the effect of limiting factors on the rate of photosynthesis; this is
because it is easy to collect the oxygen that they produce, providing a measure of the reaction rate
Exam Tip
Learn the 3 limiting factors and how each one can be altered in a laboratory environment:
Light intensity – the distance of the light source from the plant (intensity ∝ 1/d2)
Temperature - changing the temperature of the water bath the test tube sits in
Carbon dioxide - the concentration of NaHCO3 dissolved in the water the pondweed is in
Also, remember that the variables not being tested (the control variables) must be kept constant.
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