C1.3.5 Absorption of specific wavelengths of light by photosynthetic pigments Notes

Photosynthetic Pigments Absorb Specific Wavelengths to Capture Light Energy

1. The primary pigments involved in photosynthesis are chlorophyll a, chlorophyll b, and carotenoids.
2. Each pigment absorbs light at specific wavelengths, which is crucial for capturing the energy needed to drive the photosynthetic process.
3. This light energy is crucial for the excitation of electrons within the pigment molecules, a step that ultimately converts light energy into chemical energy.

Photosystems Capture and Transfer Light Energy to Drive Electron Flow

1. Pigments are organized into photosystems, which are complexes of proteins and pigments that work together to capture and convert light energy.
2. Photosystems contain two main parts:
   1. Antenna Complex: A collection of pigments that absorb light and transfer energy to the reaction center.
   2. Reaction Center: A specialized pair of chlorophyll molecules that release excited electrons to an electron transport chain.

Light Absorption Excites Electrons, Driving ATP and NADPH Production

1. When light is absorbed by a pigment, the energy of the photons is transferred to the electrons within the pigment molecule. This process is called excitation.
2. The energy excites the electrons, causing them to move to a higher energy state.
3. These excited electrons are then transferred to an electron transport chain, which ultimately leads to the production of ATP and NADPH, two key molecules used in the Calvin Cycle (the dark reactions) of photosynthesis.

Conversion to Chemical Energy

1. The excited electrons from the reaction center are transferred to an electron transport chain.
2. As they move through the chain, their energy is used to:
   1. Pump protons (H⁺) into the thylakoid lumen, creating a proton gradient.
   2. Drive ATP synthesis through chemiosmosis.
   3. Reduce NADP⁺ to NADPH, a molecule that stores high-energy electrons.

Why Only Some Wavelengths Are Absorbed

1. Light is composed of photons, each with a specific wavelength and energy.
2. Shorter wavelengths (e.g., blue light) have higher energy, while longer wavelengths (e.g., red light) have lower energy.
3. Photosynthetic pigments can only absorb photons with energies that match the energy levels of their electrons.
4. This is why pigments absorb only certain wavelengths of light.
5. Chlorophyll a absorbs light most efficiently in the blue (around 430 nm) and red (around 680 nm) regions of the spectrum.
6. It reflects green light, which is why plants appear green.

Absorption Spectra

1. An absorption spectrum is a graph that shows the wavelengths of light absorbed by a pigment.
2. The x-axis represents the wavelength of light (in nanometers), often accompanied by the corresponding colors (e.g., blue, green, red). The y-axis shows the percentage of light absorbed.
3. Chlorophyll a shows peaks in the blue and red regions, while carotenoids absorb light in the blue-green region (around 400–500 nm) and reflect yellow, orange, or red light.
   1. Chlorophyll a: Absorbs strongly in the red (~680 nm) and blue-violet (~450 nm) ranges.
   2. Chlorophyll b: Absorbs light more efficiently in the blue (~455 nm) and red-orange (~675 nm) ranges.
   3. Carotenoids: Absorb in the blue-green range (400–500 nm).

Excitation of Electrons and Energy Transformation

1. When a pigment molecule absorbs a photon, an electron within the molecule becomes excited, jumping to a higher energy level.
2. This process is the first step in converting light energy into chemical energy.

Absorption Spectra and Photosynthesis

1. The absorption spectrum of a pigment correlates with its role in photosynthesis.
2. Wavelengths that are absorbed most efficiently (e.g., blue and red light for chlorophyll) are also the wavelengths that drive the highest rates of photosynthesis.
3. This relationship is illustrated by the action spectrum, which shows the rate of photosynthesis at each wavelength.
4. The action spectrum of photosynthesis closely matches the combined absorption spectra of chlorophylls and carotenoids, highlighting the importance of these pigments in capturing light energy.
5. If plants absorbed all wavelengths of light, they would appear black, as no light would be reflected.
6. However, absorbing all wavelengths would also generate excess heat, which could damage the plant.
7. By reflecting green light, plants balance energy absorption with protection against overheating.