Edexcel A Level (IAL) Biology -5.6 Absorption & Action Spectra- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -5.6 Absorption & Action Spectra- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -5.6 Absorption & Action Spectra- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 5.6 understand what is meant by the terms absorption spectrum and action spectrum
Absorption Spectrum vs Action Spectrum
🌱 Introduction
Photosynthesis depends on light energy absorbed by pigments like chlorophyll. Different wavelengths (colours) of light are absorbed unequally, and this affects the rate of photosynthesis. To study this, scientists use absorption spectra and action spectra.
🔹 Absorption Spectrum
Shows how much light energy each pigment absorbs at different wavelengths.
- Measured using a spectrophotometer.
- Each pigment (like chlorophyll a, chlorophyll b, carotenoids) absorbs light best at specific wavelengths.
| Pigment | Main Absorption Peaks (Approx.) |
|---|---|
| Chlorophyll a | Blue (~430 nm) and Red (~662 nm) |
| Chlorophyll b | Blue (~453 nm) and Orange-red (~642 nm) |
| Carotenoids | Blue-violet (~450 nm) |
Low absorption occurs in green light, which is why plants appear green (green light is reflected).
🔹 Action Spectrum
Shows the rate of photosynthesis (e.g., amount of O₂ released or CO₂ absorbed) at different wavelengths of light.
- Peaks in blue and red regions – where absorption by chlorophyll is greatest.
- It indicates which wavelengths are most effective for photosynthesis.
Observation: The action spectrum closely matches the combined absorption spectra of chlorophylls and carotenoids, proving these pigments capture light for photosynthesis.
🔹 Relationship Between the Two
| Feature | Absorption Spectrum | Action Spectrum |
|---|---|---|
| Definition | Light absorption by pigments at each wavelength | Photosynthetic rate at each wavelength |
| Measured by | Spectrophotometer | Measuring O₂ evolution or CO₂ uptake |
| Main pigments involved | Chlorophyll a, b, carotenoids | All pigments together (total activity) |
| Peak regions | Blue & Red | Blue & Red |
| Purpose | Identifies which light pigments absorb | Identifies which light drives photosynthesis most effectively |
⚡ Quick Recap
| Concept | Key Point |
|---|---|
| Absorption Spectrum | Measures light absorbed by pigments |
| Action Spectrum | Measures photosynthetic rate under each wavelength |
| Peak Effectiveness | Blue and Red light |
| Green light | Least effective (mostly reflected) |
| Relation | Action spectrum mirrors absorption spectrum |
| Inference | Confirms chlorophyll pigments power photosynthesis |