Light Dependent Reactions

Light Dependent Reactions: 2 Main components for recipe: Cyclic and Noncyclic

Light Dependent Reactions are reactions that take place on the thylakoid membranes inside the chloroplast. Inside of thylakoids is called the lumen, and outside the thylakoid membrane is called the stroma, all where light dependent reactions take place. Within the thylakoid membranes are integral membrane protein complexes which are responsible for the catalyzation of the light reactions that play a significant role in photosynthesis. Vital to the processes of photosynthesis major protein complexes which their are 4 of: Photosystem I, Photosystem II, Cytochrome B6F Complex, and lastly ATP synthase. All four of these greatly important integral membrane protein complexes work together to allow for photosynthesis to occur allowing for the products of ATP or energy and NADPH to be produced. During both photosystems I and II, highly energized light radiation is absorbed through photosynthetic pigments, primarily the photosynthetic pigment chlorophyll, which is why most plants are green because chlorophyll functions best at the same wavelength as green. All light dependent reactions start with photosystem II within the reaction center where a chlorophyll a molecule absorbs a photon which causes an electron to gain a higher energy level in the molecule. This electron now at a higher energy level is very unstable which causes it to be passed along and transferred from one molecule to the next forming a chain of redox reactions known as an electron transport chain or ETC. This electron transport chain creates an electron flow from photosystem II to cytochrome B6F to then photosystem I. In photosystem I, the electron gains energy from yet another photon leading it to the last electron acceptor of NADP. In both photosystems I and II, since they are both protein complexes, they absorb photons in the same way and use the energy created by the electron transport chain. Both of these process use light harvesting complexes in their recipes, in order to properly absorb the photons most efficiently. Furthermore reactions known as photoinduced charge separations are used to kickstart the electron flow and is very special because it is rare since it turns light energy into chemical energy. Photoinduced separations are when a specific pigment molecule on a photosynthetic reaction center absorbs a photon, then an electron in the pigment reaches an excited state causing it to be transferred to another molecule creating the aforementioned electron transport chain. There are 2 types of photosynthesis, oxygenic or whether there is oxygen present, or anoxygenic photosynthesis where there is not oxygen. The difference between these two types of photosynthesis is that in oxygenic photosynthesis water or H20 is always the first to donate an electron causing oxygen to be a waste product, whereas in anoxygenic photosynthesis there are a variety of potential of initial electron donors. Through a process known as photophosphorylation, both Cytochrome B6F and ATP Synthase are able to work together in order to create ATP as a product. The Recipe for Light Dependent Reactions can be carried out either with cyclic electron flow in photophosphorylation or noncyclic electron flow in photophosphorylation. In noncyclic electron flow photophosphorylation, cytochrome B6F uses the energy from electrons gain from photosystem II in order to pump proteins transporting them from the stroma to the lumen inside the Thylakoid Membrane. The proton gradient established along the thylakoid membrane causes the formulation of a proton force, where ATP synthase uses this force to form ATP. In cyclic photophosphorylation, Cytochrome B6F takes the energy from electrons gained from photosystems II and I in order to create more ATP, as well as to stop the further creation of NADPH. Cyclic phosphorylation is greatly important because it creates an abundance of ATP, but more important maintains the correct amount of NADPH by stopping the production of it once there is enough for the light dependent reactions to properly occur. An overall breakdown is that through potosystems I  and II through the absorption of light, and the formation of a concentration gradient caused by the flow of electrons allows for NADP+ to convert to NADPH, as well as for ATP synthase to use the concentration gradient to phosphorylate ADP, which adds a phosphate making ATP.

Cyclic electron flow:
  • In cyclic electron flow only Photosystem 1 is used in order to produce ATP as a product, but does not create any NADPH or 02 as a byproduct

  • Only occurs when plant cells are in need of ATP OR when NADPH is in need of being reduced, but there is no NADP+ present to do so

  • During photosystem I, electron gradients present in the thylakoid membrane power the pumping and transport of H ions into the thylakoid and their conversion of ADP+P into ATP. This is an example of phosphorylation because a phosphate is being added by the electron gradients to the ADP creating ATP or energy, all making this process more efficient.

  • This cyclic form of phosphorylation only happens exclusively on the thylakoid membrane

  • In the cyclic electron flow, the electron starts off in a pigment complex known as photosystem I, then it moves from the primary acceptor to ferredoxin, then to cytochrome B6F, a similar complex found in the mitochondria of cells.

  • One in the cytochrome B6F, the electron goes to the plastocyanin before it returns back to chlorophyll

  • This passing of the electron creates a transport chains known as a proton-motive force, which pumps H+ ions across the membrane

  • This proton-motive force is important because it creates a concentration gradient that is used to power ATP synthase during chemiosmosis

  • This process creates an overall pathway which is cyclic photophosphorylation which is a process response for the production of ATP still, but does so with producing O2 or NADPH which noncyclic photophosphorylation does

  • Cyclic electron flow is like when you bake a cake, but instead of  making the cake and the frosting, only the cake is made since you don't have the ingredients for the frosting, but you have them for the cake, which still tastes good and provides energy. This relates to cyclic electron flow because only photosystem I is being used in order to make the "cake" or in this case energy, but no "frosting" is made without photosystem II, meaning that since there is no ingredients available or no NADP+ only the cake can be made, meaning only energy is produced and not energy and NADPH.  

Non cyclic electron flow:

  • 2 step process involving both of the 2 different chlorophyll photosystems

  • occurs in the frets or stma lamellae in the plant cell

  • Water molecule is broken down into through a process called photolysis which literally means the splitting of water

  • 2 electrons stay in photosystem II

  • Photons is absorbed by chlorophyll

  • Light taken from the sun is used to excite the electrons of each pigment

  • This excitement though energy causes a chain reaction that leads to the transfer of energy to the core of photosystem II

  • This causes the excitement of 2 electrons that are then transferred to the primary acceptor in Photosystem II, which is pheophytin

  • The electrons lost in the transfer are made up fro by then the capturing of more electrons form yet another water molecule

  • The electrons in the pheophytin are transferred to the plastoquinone, where the 2 electrons and 2 H+ atoms from the stroma to form PQH2, which is then quickly broken down into PQ causing the 2 electrons to be released

  • These 2 electrons are released to Cytochrome B6F complex, while the 2 H+ ions are released in the thylakoid lumen

  • The electrons then move through the Cyt b6 and Cyt f all leading to the plastocyanin which provides enough energy needed for hydrogen ions to be pumped into the thylakoid

  • This pumping of electrons, once again, creates a gradient much similar to the one established in cyclic photophosphorylation electron flow

  • This concentration gradient causes H+ ions to flow back into the stroma of the chloroplast, which in turn proves more energy for the creating of ATP or energy

  • The photosystem II complex replaces its lost electrons from other various sources, but they do not go back to photosystem II, like in cyclic electron flow, instead because they are excited they get transferred to a photosystem I complex, which in turn causes them to gain a higher energy level with a second solar photon

  • These newly excited electrons are then transferred to an acceptor molecule, and are passed on to the enzyme, Ferredoxin-NADP+ reductase, which uses the electrons to catalyze the reaction NADP+ + 2H+ + 2e → NADPH + H+

  • This reaction uses the H+ ions which were produced by the splitting of water leading to the formulation of 1/2O2, ATP, and NADPH+H+ , which uses solar photons and water

  • NADPH in chloroplasts assists in the regulation of electron pathways used in light reactions

  • During the Calvin cycle when the chloroplast runs low on ATP, NADPH is piled in large quantities causing the plant to shift from noncyclic to cyclic electron flow

  • Non-cyclic electron flow or photophosphorylation is like having the ingredients for both the frosting and the cake when you make a cake. The same goes for non-cyclic since both photosystem I and II are used. The products of this are a dynamic duo if NADPH and Energy just like Cake with Frosting, each one of them is good by themselves, but when you put them together they become amazing.