C1.3.6 Similarities and differences of absorption and action spectra Notes

Similarities and Differences of Absorption and Action Spectra

1. Photosynthesis depends on light energy, but not all wavelengths are equally effective.
2. To understand this, scientists use two types of graphs: the absorption spectrum and the action spectrum.
3. Absorption Spectrum and Action Spectrum are two important graphs used to analyze the process of photosynthesis.
4. While they are related, they convey different types of information.

Similarities Between Absorption and Action Spectra

1. Both Involve Wavelengths of Light: Both spectra plot data against the wavelengths of light, typically ranging from 400 nm (violet) to 700 nm (red).
2. Peaks in Blue and Red Regions: Both graphs show high activity in the blue (around 450 nm) and red (around 680 nm) regions of the spectrum.
3. Low Activity in Green Region: Both spectra show a dip in the green region (around 500–550 nm), explaining why plants appear green, they reflect rather than absorb this light.

Differences Between Absorption and Action Spectra

1. What They Measure: The absorption spectrum measures light absorption by pigments.
   1. The action spectrum measures the rate of photosynthesis (e.g., oxygen production or carbon dioxide uptake).
2. Data Source: The absorption spectrum is derived from individual pigments like chlorophyll a, chlorophyll b, and carotenoids.
   1. The action spectrum reflects the combined effect of all pigments and the photosynthetic machinery.
3. Broader Peaks in Action Spectrum: The action spectrum is broader because it includes the contribution of accessory pigments like carotenoids, which absorb light in regions where chlorophyll is less effective.

Photosynthesis Can be Measured By Tracking Oxygen Production and Carbon Dioxide Consumption

1. Measuring Oxygen Production

1. Aquatic plants like Elodea release oxygen bubbles during photosynthesis.
2. By counting these bubbles or using an oxygen sensor, you can measure the rate of photosynthesis.

2. Measuring Carbon Dioxide Consumption

1. Carbon dioxide uptake can be measured by observing changes in pH.
2. As $CO_2$ is consumed, the solution becomes less acidic, indicating photosynthesis is occurring.

Plotting an Action Spectrum

1. The action spectrum can be plotted by taking measurements of oxygen production or carbon dioxide consumption across various wavelengths.
2. This provides a visual representation of how effective photosynthesis is at different wavelengths.

Steps to Plotting an Action Spectrum

1. Collect Data: Measure the rate of photosynthesis (e.g., oxygen production) at different wavelengths.
2. Plot the Graph:
   1. X-axis: Wavelength of light (400–700 nm).
   2. Y-axis: Rate of photosynthesis (e.g., bubbles per minute or oxygen concentration).
3. Analyze the Peaks: Identify which wavelengths are most effective for photosynthesis.

Why Do These Spectra Matter?

1. Understanding these spectra helps us optimize conditions for photosynthesis.
2. Agriculture: Growers use red and blue LED lights to maximize plant growth in greenhouses.
3. Ecology: Knowing which wavelengths are most effective helps explain why plants dominate ecosystems with high red and blue light availability.