Photosynthesis Photosynthesis is the process of converting light energy to chemical energy stored in carbon compounds. – Plants, algae, cyanobacteria, and some protists produce organic compounds from inorganic materials and light energy and are classified as photoautotrophs. – Chloroplasts are the site of photosynthesis. Photosynthesis like cellular respiration is performed through a series of redox reactions with glucose being reduced (gaining charge) through the oxidation of water.

Chemical Equation 6CO H 2 O + light energy  C 6 H 12 O O 2 + 6H 2 O 6CO 2 + 6H 2 O + light energy  C 6 H 12 O O 2 (simplified formula) – CO 2 + H 2 O  [CH 2 O] n + O 2 (reduced formula) the reduced formula is necessary to visualize that the sugar molecule is built one carbon at a time – Production of O 2 is from the splitting of water (not CO 2 ) confirmed by C.B. van Neil using the isotope O-18 – Carbon in the glucose comes from CO 2 – Hydrogen in the new water and in the water come from reactant water – Oxygen in the glucose and the new water come from the CO 2 in the reactants

Structures Chloroplasts - site of photosynthesis – primary pigment is chlorophyll (green) found in the mesophyll - tissue of the leaf's interior – contains chloroplasts – stroma - fluid filled compartment surrounded be 2 membranes – thalakoids - stack of membraneous sacs that separate the stroma from other compartments (place where chlorophyll resides) – grana - stack of thalakoids CO 2 enters through holes in the leaf's surface called stomata veins carry water to the leaves and export sugar to various parts

The Light Reaction The light reaction of photosynthesis takes advantage of the photoelectric effect (ability of photons to exert inertia on electrons) and the photosynthetic pigment chlorophyll. There are 3 main pigments (2 types of chlorophyll) used in plants. The action spectrum describes which wavelength of light works best for each (studied by looking at CO 2 uptake and O 2 production). – Chlorophyll a blue-green in color absorbs violet and red best reflects blue & green – Chlorophyll b yellow-green in color absorbs blue and red best reflects yellow & green – Carotenoids various shades of red, orange, and yellow called accessory pigments absorbs violet & blue-green

Photosystems Photosystems – light harvesting complex containing the pigments where light energy is harvested – When a photon is absorbed by a pigment it causes an e - to move from its ground state to an excited state. The electron (because it is unstable in its new configuration) immediately returns to its ground state releasing the stored energy. – photon energy is transferred from pigment to pigment until it reaches the reaction center – The energy is then harvested where it is transferred to an electron acceptor. NADP + the reaction center is composed of a chlorophyll a molecule & a primary electron acceptor – chlorophyll a boosts its e - to an excited state where it is oxidized as the primary electron acceptor removes it – this is the 1st step in changing light energy to chemical energy (glucose)

The thylakoid membranes are populated by two types of photosystems (PS I & PS II). The two work together to produce ATP & NADPH (8 steps in the non-cyclic flow of electrons) A photon strikes a pigment in PS II & is transferred until in reaches P680 in the reaction center. The excited electron is captured by an electron acceptor. Water is split – 2 H+ are produced (reduced) – 2 e- are given back to the P680 – O2 is produced from the oxidized O The excited e- is passed through an electron transport chain (ETC) to PS I – plastoquinine (Pq)  cytochrome complex  plastocyanin (Pc) The exergonic "fall" of electrons provides energy for the synthesis of ATP (via proton-motive force & ATP synthase) PS I absorbs a photon and donates an e- from P700 to its electron acceptor – 2 electrons from PS II fill in the space (electron hole) Electrons captured by P700 are transfered down a 2nd ETC through the protein ferredoxin (Fd) Enzyme NADP+ reductase transfers the 2 e- to NADP+ to NADPH (requires 2 e-) – NADPH is then transferred to the Calvin Cycle Occasionally the system may skip PSII and operate under cyclic flow. In cyclic flow, ATP is generated but no NADPH is produced for transfer into the Calvin Cycle