13.2 Investigation of Limiting Factors
- Syllabus
- 9700–2028–2029
- Topic
- 13.2
- Level
- A2
Learning objectives
13.2.1Limiting factors• Limiting factors: light intensity, CO2 concentration, temperature13.2.2Effects of limiting factors• Effects of changes in limiting factors on photosynthesis rate13.2.3Chloroplast suspension investigation• Investigate using redox indicators with chloroplast suspension: effects of light intensity and wavelength13.2.4Whole-plant photosynthesis investigation• Investigate using whole plants (including aquatic): effects of light intensity, CO2 concentration, temperature
A limiting factor is the variable currently holding a rate below its potential
A limiting factor is a variable that restricts the rate of photosynthesis under the stated conditions. It is conditional: light intensity, carbon-dioxide concentration or temperature can limit the rate when that factor is in short effective supply, even if the other requirements are adequate.
- Short supply → constrained rate: if a factor is below the level needed by the photosynthetic reactions, increasing it can increase the rate because the process can proceed closer to its potential.
- Another factor takes over: once the first factor is no longer restricting the rate, further increases have little effect if a different requirement—such as light, carbon dioxide or temperature—is now the limiting factor.
- The conclusion is conditional: the limiting factor is identified for a particular range of conditions, plant system and measured rate. It is not a permanent label attached to that factor.
- Fair evidence: change one independent factor while keeping the important alternatives and sample conditions constant, measure a rate proxy over a comparable time, repeat, and interpret the result only within the tested range.
- Rate boundary: oxygen, carbon-dioxide change, biomass or another supported signal can act as evidence for photosynthetic activity, but a proxy must not automatically be called direct carbohydrate production or the whole plant’s total photosynthesis.
Do not say that the factor with the lowest numerical value is always limiting, that a limiting factor stops photosynthesis completely, or that increasing any one factor must keep increasing the rate. Graph-reading details and specific DCPIP/aquatic-plant methods belong to later cards.
Each limiting factor changes a different photosynthetic requirement
| Factor increases | Why rate may rise | Why the curve plateaus or falls |
|---|---|---|
| Light intensity | More photons photoactivate chlorophyll, increasing electron flow and ATP/reduced NADP supply while light limits | Another factor such as CO2 or temperature limits; excessive light may damage systems but do not infer this without data |
| CO2 concentration | More substrate is available for rubisco carbon fixation, increasing Calvin-cycle throughput while CO2 limits | Light-dependent products, temperature/enzyme capacity or another factor limits |
| Temperature | Greater kinetic energy raises enzyme-substrate collision frequency and Calvin-cycle reaction rate up to an optimum range | Above the optimum, enzymes/proteins lose functional shape and stomatal/water effects may reduce rate |
Read axes first. A rising region shows the x-axis factor limits in that range; a plateau shows another factor now limits; a temperature peak followed by decline supports an optimum and adverse high-temperature effects. Compare curves only under matched conditions.
Increasing one factor cannot raise rate indefinitely. State mechanisms only for the tested range: a plateau does not mean photosynthesis has stopped, and one endpoint cannot identify the new limiting factor.
Redox indicators compare chloroplast electron transfer under different light
Oxidised DCPIP and methylene blue are coloured and become less coloured/colourless when reduced. In an illuminated chloroplast suspension, faster indicator reduction is a proxy for faster light-dependent electron transfer.
| Investigation | Vary | Keep constant and compare |
|---|---|---|
| Light intensity | Distance from a lamp or measured intensity | Wavelength/filter, chloroplast concentration/volume, indicator concentration/volume, temperature, pH, time and starting absorbance/colour |
| Light wavelength | Equal-intensity coloured filters or monochromatic light | Intensity at sample, chloroplast/indicator amounts, temperature, pH, time and starting absorbance/colour |
Prepare equal fresh chloroplast suspensions, add the same indicator and start timing consistently. Measure time to a defined endpoint or absorbance change per unit time; repeat each treatment and calculate a mean rate. Include a dark control and a no-chloroplast or heat-inactivated control. Faster reduction under a wavelength should be compared cautiously with pigment absorption/action evidence.
Indicator reduction is not direct oxygen, ATP, carbohydrate or whole-plant growth measurement. Distance changes can also heat the sample, so use a heat shield/water bath and monitor temperature; wavelength comparisons require matched intensity.
A whole-plant photosynthesis experiment needs biomass, gas or colour evidence and careful controls
A whole-plant or aquatic-plant investigation estimates photosynthetic activity from a measurable proxy such as oxygen produced over time, gas volume, bubble output or a change in biomass. The design must state what was measured and distinguish that proxy from direct carbon fixation or ATP production.
- Choose comparable material: use plants of matched species and similar mass, length or leaf area. Allow them to acclimatise, and keep the container, starting condition and observation period comparable.
- Vary one factor: for light, change distance or intensity while keeping temperature and carbon-dioxide availability controlled; for carbon dioxide, change the supported carbon source such as bicarbonate while matching light and temperature; for temperature, use controlled water baths while matching light, carbon dioxide and plant material.
- Measure a rate proxy: record oxygen gas volume with a suitable gas-collection method, or record a defined bubble/gas change over a fixed time. Use rate = measured change ÷ time, and state whether the result is net oxygen gain rather than total photosynthetic production.
- Use controls and repeats: include a dark or baseline treatment where appropriate, keep aeration, illumination, temperature, plant mass, solution and timing consistent, and repeat each condition so variation can be estimated.
- Conclude within the evidence: compare rates only when the proxy and controls are comparable. A change in gas output may reflect photosynthesis, respiration, gas exchange or stress; do not treat a proxy as direct evidence of ATP production or carbon fixation.
A fair comparison changes only the chosen factor and normalises the response to time and, where appropriate, plant mass or leaf area. Report the measurement method and uncertainty rather than treating bubble counts or one endpoint as an exact oxygen yield.
More bubbles or gas do not automatically mean more gross photosynthesis: plants respire, and gas can dissolve or escape. A whole-plant or aquatic-plant model gives conditional proxy evidence under the tested conditions, not a direct ATP, carbohydrate or carbon-fixation measurement.