The Reactions (I)

 H 2 O is absorbed by the root epidermal cellsepidermal cells  Plants absorb water and carbon dioxide through stoma (a pore surrounded by guard cells) stoma  Light is absorbed by chlorophyll a (most important for green plants) and chlorophyll b  Chloroplasts contain chlorophyll (green) and carotenoids (red,orange and yellow) **Trees turn colours in fall because the green chlorophyll dies and the cartenoids take over photosynthesis.

 Overall reaction: 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 reactantsproducts  There are two major types of chemical reactions that occur: light reactions and dark reactions.

 When photons of light enter the chloroplast they excite electrons in the proteins of the chloroplast.  This excitement passes electrons along the membranes.  The passing of the electron stimulates the production of ATP (useable energy molecule)  Water molecules are split releasing O 2 and allowing H + to transfer to coenzymes for later use

Light enters the chloroplast in the form of photons A photon will excite a pigment molecule (chlorophyll a) called P680 That excitement energy will get passed on to the next pigment molecule and so on until it reaches the “reaction centre” The electron that has become excited in the reaction centre gets passed on to the primary acceptor in photosystem II

 An enzyme splits a water molecule into two electrons, two hydrogen ions, and an oxygen atom.  The electrons are supplied one by one to the P680 molecules, each replacing an electron lost to the primary electron acceptor

 Each photoexcited electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain.  The electron transport chain between PS II and PS I is made up of the electron carrier plastoquinone (Pq), cytochrome (b6f) complex and a protein called plastocyanin (Pc).  The cytochrome complex “actively” pumps hydrogen protons (H+) into the thylakoid space or lumen (center of thylakoid) where they add to a concentration gradient.

The ETC is passing electrons from photosystem II to photosystem I.

 The exergonic (energy releasing) “fall” of electrons to a lower energy level provides energy for the synthesis of ATP.  In a process called Chemiosmosis, the H+ protons in the thylakoid space diffuse through the thylakoid membrane via ATPase to the stroma (Proton Motive Force) resulting in the phosphorylation of ADP to ATP

 The light energy from the ECT is transferred to another antenna complex in photosystem I. The electron reaches the reaction centre and excites the P700 chlorophyll molecule.  The excited electron is then captured by PS I’s primary electron acceptor, creating an electron “hole” in the P700. The hole is filled by an electron that reaches the bottom of the electron transport chain from PS II.

 Photoexeited electrons are passed from PS I’s primary electron acceptor down a second electron transport chain through the protein ferredoxin (Fd).  The enzyme NADP+ reductase transfers electrons from Fd to NADP+.  Two electrons are required for its reduction to NADPH.