Chemiosmotic phosphorylation
Chemiosmotic phosphorylation is the third pathway that produces ATP from inorganic phosphate and an ADP molecule. This process is part of oxidative phosphorylation.
The complete breakdown of glucose in the presence of oxygen is called cellular respiration. The last steps of this process occur in mitochondria. The reduced molecules NADH and FADH2 are generated by the Krebs cycle and glycolysis. These molecules pass electrons to an electron transport chain, which uses the energy released to create a proton gradient across the inner mitochondrial membrane. ATP synthase then uses the energy stored in this gradient to make ATP. This process is called oxidative phosphorylation because oxygen is the final electron acceptor and the energy released by reducing oxygen to water is used to phosphorylate ADP and generate ATP.
Non-Cyclic Photophosphorylation
Non-cyclic photo- phosphorylation,cytochrome b6f uses the energy of electrons from PSII to pump protons from the stoma to the lumen. The proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form AT
Non-cyclic photophosphorylation Process
- Non-cyclic photophosphorylation takes place inside a chloroplast, on or in a thylakoid membrane.
- A photon of light energy strikes the leaf and hits photosystem 2. The energy will pass from one antenna pigment molecule to another until it reaches a reaction center molecule (p680). The light energy will then energize 2 electrons.
- The energized electrons now pass to an electron acceptor this creates an electron "hole" within PS2.
- Water is split inside the thylakoid, providing the electrons to fill the "hole" for photosystem 2. The hydrogen ions, that for the moment are inside the thylakoid, and oxygen which will diffuse out of the chloroplast and the cell.
- From the electron acceptor the electrons pass to plastoquinone (PQ).
- From plastoquinone (PQ) the electrons pass on to a complex of cytochromes.
- As the electrons move from PQ to the cytochrome complex they release enough energy to power the active transport of hydrogen ions from the stroma into the thylakoid space. This generates a large hydrogen ion gradient.
- From the cytochrome complex the electrons pass on to Photosystem 1 to fill an electron "hole" in PS1.
- A photon of light energy strikes the leaf and hits photosystem 1. The energy will pass from one antenna pigment molecule to another until it reaches a reaction center molecule (p700). The light energy will then energize 2 electrons.
- The energized electrons now pass to an electron acceptor this creates the electron "hole" within PS1.
- From the electron acceptor the electrons pass to ferredoxin (Fd).
- From ferredoxin (Fd) the electrons and 2 hydrogen ions are used to reduce NADP+ to NADPH + H+. The NADPH + H+ is going to be utlized in the Calvin Cycle (dark reactions).
- The hydrogen ions that have been pumped into the thylakoid space pass down a concentration gradient through the ATP synthetase complexes. As the ions pass through the synthetase complex their chemiosmotic energy is released.
- The energy released by the hydrogen ions is used to help convert ADP and a phosphate group into ATP. Some of this ATP is going to be utlized in the Calvin Cycle (dark reactions).
Constitutive Secretion
In constitutive secretion Proteins are continuously secreted from the cell regardless of environmental factors. No external signals are needed to initiate this process. Proteins are packaged in vesicles in the Golgi apparatus and are secreted via exocytosis, all around the cell. Cells that secrete constitutively have many Golgi apparatus scattered throughout the cytoplasm. Fibroblasts, osteoblasts and chondrocytes are some of the many cells that perform constitutive secretion.
Disulfide Bonds
Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium. Since most cellular compartments are reducing environments, disulfide bonds are generally unstable in the cytosol with some exceptions as noted below.
Disulfide bonds in proteins are formed between the thiol groups of cysteine residues. The other sulfur-containing amino acid, methionine, cannot form disulfide bonds. A disulfide bond is typically denoted by hyphenating the abbreviations for cysteine, e.g., the "Cys26-Cys84 disulfide bond", or the "26-84 disulfide bond", or most simply as "C26-C84" where the disulfide bond is understood and does not need to be mentioned. The prototype of a protein disulfide bond is the two-amino-acid peptide, cystine, which is composed of two cysteine amino acids joined by a disulfide bond (shown in Figure 2 in its unionized form). The structure of a disulfide bond can be described by its χss dihedral angle between the Cβ − Sγ − Sγ − Cβ atoms, which is usually close to ±90°.
The disulfide bond stabilizes the folded form of a protein in several ways: 1) It holds two portions of the protein together, biasing the protein towards the folded topology. Stated differently, the disulfide bond destabilizes the unfolded form of the protein by lowering its entropy. 2) The disulfide bond may form the nucleus of a hydrophobic core of the folded protein, i.e., local hydrophobic residues may condense around the disulfide bond and onto each other through hydrophobic interactions. 3) Related to #1 and #2, the disulfide bond link two segments of the protein chain, the disulfide bond increases the effective local concentration of protein residues and lowers the effective local concentration of water molecules. Since water molecules attack amide-amide hydrogen bonds and break up secondary structure, a disulfide bond stabilizes secondary structure in its vicinity. For example, researchers have identified several pairs of peptides that are unstructured in isolation, but adopt stable secondary and tertiary structure upon forming a disulfide bond between them.
"Disulfide bond." Wikipedia, The Free Encyclopedia. 20 Jul 2009, 07:46 UTC. 20 Jul 2009 <http://en.wikipedia.org/w/index.php?title=Disulfide_bond&oldid=303092230>.
