Living organisms are made up of chemical building blocks
VERN BENDER
Our cells are bounded by a membrane that separates them from all else. The plasma membrane is the border between the interior and exterior of a cell. The cell membrane, also called the plasma membrane, is found in all cells and separates the cell’s interior from the outside environment. The cell membrane consists of a lipid bilayer that is semipermeable. The cell membrane regulates the transport of materials entering and exiting the cell.
It controls the passage of various molecules, including sugars, amino acids, ions, and water, into and out of the cell.
Life is built on polymers: Organic polymers play a crucial role in living things, providing basic structural materials, and participating in vital life processes. For example, the solid parts of all plants are made up of polymers. Starches, important sources of food energy derived from plants, are natural polymers composed of glucose.
polymer: A relatively large molecule consisting of a chain or network of many identical or similar monomers chemically bonded to each other.
monomer: A relatively small molecule that can form covalent bonds with other molecules of this type to form a polymer.
Living organisms are made up of chemical building blocks: All organisms are composed of a variety of these biological macromolecules.
Biological macromolecules are important cellular components and perform a wide array of functions necessary for the survival and growth of living organisms.
The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids.
Examples of membrane–bound organelles are the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, plastids, lysosomes, and vacuoles.
The ER captures selected proteins from the cytosol as they are being synthesized. These proteins are of two types: transmembrane proteins, which are only partly translocated across the ER membrane and become embedded in it, and water-soluble proteins, which are fully translocated across the ER membrane and are released into the ER lumen. Some of the transmembrane proteins function in the ER, but many are destined to reside in the plasma membrane or the membrane of another organelle. The water-soluble proteins are destined either for the lumen of an organelle or secretion. Regardless of their subsequent fate, all of these proteins are directed to the ER membrane by the same kind of signal sequence and are translocated across it by similar mechanisms.
According to the signal hypothesis, the secreted protein should be extruded into the lumen of the microsome during its synthesis in vitro. This can be demonstrated by treatment with a protease: a newly synthesized protein made in the absence of microsomes is degraded when the protease is added to the medium. The same protein made in the presence of microsomes remains intact microsomal membrane protects it. When proteins without ER signal sequences are similarly synthesized in vitro, they are not imported into microsomes and are therefore degraded by protease treatment.
The SRP receptor is located on the cytosolic side of the ER and binds to the SRP-ribosome complex, but not to free SRP as noted earlier. The SRP receptor plays an important role in the termination of the elongation arrest and in the translocation of polypeptides into the ER lumen
The ER signal sequence is guided to the ER membrane by at least two components: a signal-recognition particle (SRP), which cycles between the ER membrane and the cytosol and binds to the signal sequence, and an SRP receptor in the ER membrane. The SRP is a complex particle consisting of six different polypeptide chains attached to a single small RNAmolecule (Figure 12-41A). Homologs of the SRP and its receptor are found in all organisms studied, indicating that this protein-targeting mechanism arose early in evolution and has been conserved.
The Polypeptide Chain Passes Through an Aqueous Pore in the Translocator.
Combinations of Start-Transfer and Stop-Transfer Signals Determine the Topology of Multipass Transmembrane Proteins.
Translocated Polypeptide Chains Fold and Assemble in the Lumen of the Rough ER.
Most Proteins Synthesized in the Rough ER Are Glycosylated by the Addition of a Common N-linked Oligosaccharide.
Oligosaccharides Are Used as Tags to Mark the State of Protein Folding.
Most Membrane Lipid Bilayers Are Assembled in the ER.
Phospholipid Exchange Proteins Help to Transport Phospholipids from the ER to Mitochondria and Peroxisomes.
Gene negative feedback control: In the case of negative control, the genes in the operon are expressed unless a repressor protein switches them off. Thus the operon will be turned on constitutively (the genes will be expressed) when the repressor is inactivated. Repressor binding blocks RNA polymerase from binding with the promoter, thereby leading to repression of operon gene expression. A classic example of negative repressible regulation of gene expression involves the trp operon, which is regulated by a negative feedback loop. Bacteria have specific regulatory molecules that control whether a particular gene will be transcribed into mRNA. Often, these molecules act by binding to DNA near the gene and helping or blocking the transcription enzyme, RNA polymerase.
