Topic : 2 Molecular Biology
2.9 Photosynthesis
Production of carbon compounds
- Living organisms require complex carbon compounds to carry out life processes and build the structures in their cells
- Photosynthesis involves the conversion of light energy into chemical energy (carbohydrates, lipids, protein and nucleic acids).
- Chloroplasts absorb light energy from the sun and convert this energy into chemical energy (glucose) to be used by the organisms for energy
Light frequency
- Light from the sun is composed of a range of wavelengths.
- The visible spectrum is the portion of the electromagnetic spectrum that is visible to or can be detected by the human eye.
- Electromagnetic radiation in this range of wavelengths (400 to 700 nm) is called visible light.
- All these wavelengths together form white light, with violet/blue colours having shorter wavelengths (more energy) and red colours having longer wavelengths (less energy).
- The two main colors of light that are absorbed by chlorophyll are blue and red light.
- The main color that is reflected is green light, which is why most leaves look green
Action spectrum
- The electromagnetic spectrum consists of the entire range of electromagnetic radiation.
- The part of the spectrum that is involved in photosynthesis is called the visible light spectrum.
- An action spectrum is the rate of a photosynthesis plotted against wavelength of light. It shows which wavelength of light is most effectively used during photosynthesis.
- The highest rates of photosynthesis occur at red and blue wavelengths.
- The absorption spectrum shows the % of light absorbed by the photosynthetic pigments in chloroplasts at each different wavelength.
- The graphs are very similar because photosynthesis occurs when light is absorbed by the chlorophyll pigments; therefore the wavelengths that have greatest rates of absorption will also have high rates of photosynthesis.
- Green wavelength of light is reflected and therefore has a very low % absorption level on the absorption spectra (this is why most leaves are green).
- Peak approximately at 450nm
- Peak approximately at 670nm
- First peak is higher than second pea
Limiting factor or photosynthesis
- Light intensity, \(CO_2\) concentration and temperature can all be limiting factors for the rate of photosynthesis
- If any of these factors is below their optimal level, they can be limiting; however, only one of these factors can be limiting at one time
- This is usually the factor that is the furthest away from its optimal level
- This is the only factor that can increase the rate of photosynthesis
- As this factor gets closer to its optimal level, the limiting factor can change to one of the other factors