Cell structure and functions-Complete insight (cytoplasm and endomembranes)

The Cytoplasm 
The cytoplasm is the substance of the cell outside the nu­cleus. A number of organelles, small bodies with specific structures and functions, are located in the cytoplasm. Organelles help the cell carry out its many activities. For example, ribosomes carry out protein synthesis, the endomembrane system packages the pro­teins for secretion or for use inside the cell, and mitochon­dria convert the energy of glucose into the energy of ATP, the molecule that supplies metabolic energy. In addition to these general duties, each cell also has specialized func­tions, which we will examine in later chapters.
Ribosomes and Protein Synthesis
In low-power electron micrographs, ribosomes appear to be small, dense granules, but at a higher resolution, two subunits can be seen. Each subunit contains proteins and ribosomal RNA (rRNA), one of the three types of RNA made in the nucleus, as was described earlier. Protein synthesis occurs at the ribosomes, and all three types of RNA play a role. Synthesis refers to the bonding of small organic molecules to make a larger one. In this case, amino acids are joined to make a polypeptide, a part of a protein.
Ribosomal RNA (rRNA) plays a structural role in pro­tein synthesis in that ribosomes are structures that contain rRNA. Messenger RNA (mRNA) carries a message; it has a copy of the genetic code passed to it by DNA. As discussed in chapter 2, one polypeptide differs from another by the sequence of its amino acids. The sequence of nitrogen bases in m RNA specifies the order of the amino acids for the polypeptide being made. Transfer RNA (tRNA) trans­fers the amino acids to the ribosome; there is a separate tRNA for each of the different types of amino acids found in polypeptides.
Ribosom occur within the cytoplasm but are often attached to endoplpsmic reticulum. Three types of RNA­ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA (tRNA)-play a role in protein synthesis at the ribos 
Endomembrane System
The endoplasmic reticulum, the Golgi apparatus, Iyso­somes, and peroxisomes are structurally and functionally related membranous structures that comprise the en­domembrane system. They work together to produce, transport, store, or secrete cellular products.
Endoplasmic Reticulum 
The endoplasmic reticulum (en-do-plaz'mic re-tik'u-Ium) (ER) forms a membranous system of tubular canals that begins at the nuclear envelope and branches throughout the cytoplasm. Ribosomes are attached to portions of the endoplasmic reticulum. If ribosomes are present, the retic­ulum is called rough ER; if ribosomes are not present, it is called smooth ER.
In certain cells, smooth ER contains enzymes that make lipids, such as steroid hormones. The administration of drugs causes an increase in the amount of smooth ER in the liver where smooth ER contains enzymes that detoxify drugs.
The ribosomes present on rough ER function in pro­tein synthesis. Proteins that are exported (secreted) from the cell are stored temporarily in the channels of the retic­ulum. Small portions of the ER then break away to form vesicles (small, membranous sacs) that migrate to the Golgi apparatus, where the product is received, modified, and repackaged before being secreted.
Peroxisomes 
Peroxisomes (pe-roks'f-somz) are membranous sacs that contain enzymes. Hydrogen peroxide, a normal product of metabolism, is harmful if allowed to accumulate. Peroxiso­mal enzymes break down hydrogen peroxide to water and molecular oxygen. In the liver and kidney, peroxisomes detoxify harmful substances, such as formaldehyde and al­cohol. Peroxisomes are believed to bud directly from the smooth ER.

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