PHOTOSYNTHESIS The Details
Photosynthesis is divided into 2 sequential processes: 1. The Light Dependent Reactions (stages 1 & 2) 2. The Light Independent Reactions (stage 3) a.k.a. the Calvin Cycle
THE LIGHT REACTIONS The light reactions begin when a photon strikes a photosynthetic membrane. This process is divided into 3 parts: 1. Photoexcitation: absorption of a photon by an electron of chlorophyll 2. Electron Transport: transfer of electrons through a series of membrane bound electron carriers 3. Chemiosmosis: the movement of protons through ATP synthase to drive the phosphorylation of ATP
1. PHOTOEXCITATION Before a photon strikes an electron of chlorophyll, it is at the lowest possible potential energy level called ground state During its interaction, the electron gains energy called excitation As it returns, if it is not a part of a photosynthetic membrane, the loss in potential energy appears as heat and light! known as fluorescence
Most chlorophyll molecules do not fluoresce because the excited electron is captured by a special molecule called the primary electron acceptor (a compound embedded in the thylakoid membrane) Therefore, chlorophyll will become oxidized and the primary electron acceptor will be reduced http://www.buffalo.edu/news/releases/2016/03/072.html
PHOTOSYSTEMS Light is absorbed by chlorophyll and other accessory pigment molecules that are associated with proteins in clusters called photosystems A photosystem consists of Antenna Complex: composed of a number of pigment molecules including chlorophyll, set in a protein matrix in the thylakoid membrane. These pigments absorb photons and transfer the energy from pigment to pigment until it reaches a chlorophyll a molecule in the reaction center. Reaction Centre: an electron of the chlorophyll molecule absorbs the energy and is raised to a high energy level. A redox reaction transfers the electron to a primary electron acceptor.
PHOTOSYSTEMS ACT AS THE PRIMARY LIGHT HARVESTING UNITS OF CHLOROPLASTS!
PHOTOSYSTEMS There are 2 types of photosystems found in the thylakoid membranes: Photosystem I (PSI): chlorophyll a (a molecule in the reaction centre) is called P700 because its best absorption happens with wavelengths of 700nm Photosystem II (PSII): chlorophyll a (a molecule in the reaction centre) is called P680 because its best absorption happens with wavelengths of 680nm
2. ELECTRON TRANSPORT 3. CHEMIOSMOSIS Non cyclic pathway In this pathway, plants use PSI or PSII to produce ATP & NADPH
HOW DOES IT WORK? 1. A photon strikes PSII and eventually excites an electron of chlorophyll P680 2. The excited electron is captured by the primary electron acceptor 3. Through redox reactions an electron is transferred to plastoquinone (Pq), then to cytochrome B 6 -F complex to plastocyanin (Pc), and to PSI 4. As plastoquinone accepts electrons from PSII, it also gains protons from the stroma. These are released into the lumen (creating an electrochemical gradient) when the electrons are donated. 5. Water-splitting complex (associated with PSII) splits water in oxygen, hydrogen ions and electrons. The electrons are used to replace the missing electrons in P680
6. Oxygen leaves the cell as a waste product and the protons remain in the thylakoid lumen causing an electrochemical gradient 7. PSI gets hit with photons at the same time as PSII, exciting an electron of P700 8. Through redox reactions the electron is transferred to ferredoxin (Fd), then to NADP reductase 9. NADP reductase then reduces NADP + to NADPH 10. The electrochemical gradient in the lumen drives ADP to ATP using photophosphorylation
The ATP formed from this process will be used in the last stage (the Calvin Cycle) to synthesize glucose
HOMEWORK Section 5.2 p. 220-225 Know it 1-5