Photosynthesis - Light Reaction Notes

Occur in the thylakoids of the chloroplast.
Convert solar energy to chemical energy in the form of ATP and NADPH.
ATP and NADPH are released into the stroma to play crucial roles in the Calvin cycle.
The process involves taking in ingredients and producing products.

Light travels in electromagnetic waves, which are disturbances of electric and magnetic fields.
Wavelength is the distance between the crests of electromagnetic waves.
The electromagnetic spectrum includes a range of radiation from gamma rays (less than a nanometer) to radio waves (more than a kilometer).
Visible light is the narrow band from 380 nm to 750 nm in wavelength, detectable by the human eye as various colors.
Light also behaves as discrete particles called photons.
Photons have a fixed quantity of energy; the amount of energy is inversely related to the wavelength of the light.
- $E \propto \frac{1}{\lambda}$
- Violet light photons have more energy than red light photons.

Pigments absorb light of different wavelengths; the wavelengths absorbed disappear.
The color we see is the color most reflected or transmitted by the pigment.
Chlorophyll absorbs violet-blue and red light and reflects or transmits green light, which explains why leaves appear green.
Spectrophotometer: An instrument that measures a pigment's light absorption versus wavelength, producing an absorption spectrum.
Absorption spectrum: A graph plotting a pigment's light absorption versus wavelength.
Chlorophyll a: Key light-capturing pigment that participates directly in the light reactions.
Chlorophyll b: An accessory pigment.
Carotenoids: Another group of accessory pigments.
The action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving photosynthesis.
Theodor W. Engelmann demonstrated the action spectrum for photosynthesis in 1883 using bacteria to measure rates of photosynthesis in filamentous algae.

Slight structural differences cause them to absorb at slightly different wavelengths in the red and blue parts of the spectrum.
Chlorophyll a appears blue-green, and chlorophyll b appears olive green under visible light.

Hydrocarbons that absorb violet and blue-green light, appearing yellow and orange.
Function in photoprotection by absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen.

When a molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital with more potential energy.
Ground state: The normal orbital of an electron in a pigment molecule.
Excited state: When a pigment molecule absorbs a photon and an electron is boosted to an orbital of higher energy.
Only photons with energy equal to the energy difference between the ground state and an excited state are absorbed.
Excited electrons drop back down to the ground-state orbital in a billionth of a second, releasing excess energy as heat.
Fluorescence: The afterglow when excited electrons fall back to the ground state and photons are given off.

Chlorophyll molecules, along with other small organic molecules and proteins, are organized into complexes called photosystems in the thylakoid membrane.
Photosystem: Composed of a reaction-center complex surrounded by several light-harvesting complexes.
Reaction-center complex: An organized association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor.
Light-harvesting complex: Consists of pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) bound to proteins.
Light-harvesting complexes act as an antenna for the reaction-center complex.

The thylakoid membrane is populated by two types of photosystems that cooperate in the light reactions: photosystem II (PS II) and photosystem I (PSI).
Photosystem II functions first in the light reactions.
P680: The reaction-center chlorophyll a of photosystem II, best at absorbing light having a wavelength of 680 nm.
P700: The chlorophyll a at the reaction-center complex of photosystem I, most effectively absorbs light of wavelength 700 nm.