CIE AS/A Level Biology -13.2 Investigation of limiting factors- Study Notes- New Syllabus
CIE AS/A Level Biology -13.2 Investigation of limiting factors- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -13.2 Investigation of limiting factors- Study Notes- New Syllabus
Key Concepts:
- state that light intensity, carbon dioxide concentration and temperature are examples of limiting factors of photosynthesis
- explain the effects of changes in light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
- describe and carry out investigations using redox indicators, including DCPIP and methylene blue, and a suspension of chloroplasts to determine the effects of light intensity and light wavelength on the rate of photosynthesis
- describe and carry out investigations using whole plants, including aquatic plants, to determine the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis
Limiting Factors of Photosynthesis
📌 Key Points
- Photosynthesis rate depends on several environmental factors.
- When any factor is in short supply, it limits the overall rate, even if other factors are optimal.
🌱 Examples of Limiting Factors
| Factor | Effect on Photosynthesis |
|---|---|
| Light intensity | Low light → slower light-dependent reactions → less ATP and NADPH produced |
| Carbon dioxide concentration | Low CO₂ → slower Calvin cycle → less carbohydrate formation |
| Temperature | Too low → enzymes work slowly; too high → enzymes may denature |
🧠 Key Points
– A limiting factor is any condition that restricts the rate of photosynthesis.
– The most limiting factor at any moment is the one in shortest supply relative to plant needs.
– Understanding limiting factors helps in optimizing crop growth in agriculture.
Effects of Limiting Factors on Photosynthesis
📌 Overview
- The rate of photosynthesis depends on light intensity, CO₂ concentration, and temperature.
- Each factor can limit photosynthesis if it is too low or, in some cases, too high.
🌱 1. Light Intensity
- Low light: Rate of light-dependent reactions is slow → less ATP and NADPH → slower Calvin cycle.
- Increasing light intensity: Rate increases until another factor becomes limiting (e.g., CO₂ or temperature).
- Very high light: Can damage chlorophyll or photosystems → rate may plateau or decline.
🌱 2. Carbon Dioxide Concentration
- Low CO₂: Calvin cycle slows → less GP → less TP → reduced sugar synthesis.
- Increasing CO₂: Rate increases until another factor (light or temperature) becomes limiting.
- Very high CO₂: Can cause a slight inhibitory effect due to stomatal closure in some plants.
🌱 3. Temperature
- Low temperature: Enzymes in the Calvin cycle work slowly → reduced photosynthesis rate.
- Optimal temperature: Enzymes work efficiently → maximum rate of photosynthesis.
- High temperature: Enzymes may denature → photosynthesis rate drops sharply. Stomata may close → CO₂ uptake decreases.
🧠 Key Points
– Each factor has an optimum range for maximum photosynthesis.
– Limiting factor principle: The factor in shortest supply determines the rate.
– In nature, light, CO₂, and temperature interact to influence photosynthesis.
Investigating Photosynthesis Using Redox Indicators
📌 Overview
- Redox indicators (e.g., DCPIP, methylene blue) are used to monitor electron transfer in the light-dependent reactions of photosynthesis.
- Chloroplast suspensions provide the site for the reactions in vitro.
- Experiments can determine how light intensity and wavelength affect the rate of photosynthesis.
🌱 Principle
- DCPIP (2,6-dichlorophenol-indophenol) is blue when oxidized and colorless when reduced.
- Methylene blue is blue when oxidized and colorless when reduced.
- During light-dependent photosynthesis:
- Electrons from water reduce DCPIP or methylene blue, causing a color change.
- The rate of color change indicates the rate of electron transport → proxy for photosynthesis rate.
🌱 Method (Example Using DCPIP)
- Isolate a chloroplast suspension from fresh leaves.
- Mix chloroplasts with DCPIP solution in a test tube.
- Expose the mixture to light of different intensities or wavelengths.
- Include control tubes:
- Dark (no light) → ensures color change is light-dependent.
- Measure the time for DCPIP to decolorize or use a colorimeter to monitor absorbance.
- Compare rates under different light conditions.
🌱 Observations
| Condition | Result (DCPIP) | Interpretation |
|---|---|---|
| Light present | DCPIP turns colorless faster | Electron transport is active → high photosynthesis rate |
| Light absent (dark) | DCPIP remains blue | No electron transport → no photosynthesis |
| Different wavelengths | Blue and red light → faster decolorization; Green light → slower | Chlorophyll absorbs mainly blue and red light |
🧠 Key Points
– Redox indicators act as artificial electron acceptors, replacing NADP⁺.
– The rate of decolorization reflects the rate of light-dependent photosynthesis.
– Helps investigate: effect of light intensity (stronger light → faster reaction) and wavelength (blue/red → most effective).
Investigating Photosynthesis Using Whole Plants
📌 Overview
- Whole plants (terrestrial or aquatic) can be used to study how environmental factors affect photosynthesis.
- The rate of photosynthesis can be measured by observing O₂ production or CO₂ uptake.
🌱 Principle
- Photosynthesis produces oxygen as a by-product.
- Measuring bubbles of O₂ in aquatic plants (e.g., Elodea) or using gas sensors provides an estimate of the photosynthesis rate.
- Changes in light intensity, CO₂ concentration, or temperature affect the rate of photosynthesis.
🌱 Method (Example Using Aquatic Plant)
- Set up: Place an aquatic plant (e.g., Elodea) in a test tube or beaker with water. Invert a measuring cylinder or funnel to collect oxygen bubbles.
- Control variables: Same volume of water, same plant size, same initial conditions.
- Manipulate independent variable:
- Light intensity: Move lamp closer/further.
- CO₂ concentration: Add sodium bicarbonate to water.
- Temperature: Use water baths at different temperatures.
- Observation: Count the number of oxygen bubbles per minute. Repeat for accuracy.
- Record data: Plot rate of photosynthesis vs light intensity / CO₂ / temperature.
🌱 Expected Results
| Factor | Effect on Photosynthesis Rate |
|---|---|
| Light intensity | Rate increases with light until another factor becomes limiting |
| CO₂ concentration | Rate increases with CO₂ until light or temperature limits |
| Temperature | Rate increases with temperature up to optimum; high temperature → enzymes denature → rate drops |
🧠 Key Points
– O₂ production is a simple, measurable indicator of photosynthesis.
– Each factor has an optimum range; too low or too high can limit the rate.
– Helps understand plant adaptation and environmental responses.