Cell Structures and Functions PDF

Cell Structures and Functions All living organisms are made up of one or more cells. The cells are the building block of life. Each contains materials that carry out basic life processes. These materials are called organelles. A useful technique for studying cell structure and function, which takes...

Cell Structures and Functions All living organisms are made up of one or more cells. The cells are the building block of life. Each contains materials that carry out basic life processes. These materials are called organelles. A useful technique for studying cell structure and function, which takes cell parts and separates major organelles and other subcellular structures from one another is called Cell Fractionation. Centrifuge speed for every cell structure: Nucleus - 3,000 rpm for 5 minutes Ribosomes - 28,800 rpm for 16 hours Mitochondria - 14,000 for 7 minutes Endoplasmic Reticulum - 35,000 for 70 minutes DNA (Deoxyribonucleic Acid) - 12,000 rpm for 2 minutes The Nucleus Typically, the nucleus is the most prominent organelle in a cell. Contains most of the cells’ DNA—its master plans—and controls the cells’ activities by directing protein synthesis. A prominent structure within the nondividing nucleus is the nucleolus, here a type of RNA is synthesized (rRNA or Ribosomal RNA). Also, in the nucleolus, the proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes. The Nuclear Envelope A double membrane structure that constitutes the outermost portion of the nucleus. The envelope is punctuated with pores—called nuclear pores—that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. Nucleoplasm - the semi-solid liquid inside the nucleus, where we find the chromatin and the nucleolus. Chromatin and Chromosomes Within the nucleus, DNA is organized into discrete unit called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with proteins—histone and nonhistone proteins. The complex of DNA and proteins making up chromosomes is called chromatin. Chromatin describes the material that makes up the chromosomes both when condensed and uncondensed. Ribosomes These are complexes made of ribosomal RNA and protein, are cellular components that carry out protein synthesis. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope. Two cytoplasmic locales: Free Ribosomes (cytosol) and Bound Ribosomes Free Ribosomes catalyze the first steps of sugar breakdown; The produced protein stays inside the cell. Bound Ribosomes are responsible for membrane insertion; Packaging within certain organelles and for secretion; The produced protein is transported to other cells. Cells with high rates of protein synthesis have particularly large numbers of ribosomes as well as prominent nucleoli, which makes sense, given the role of nucleoli in the ribosome assembly. The Endomembrane System This system carries out a variety of tasks in the cell, including synthesis of proteins, transport of proteins into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles. The Endomembrane System consists of the following: Nuclear Envelope Endoplasmic Reticulum Golgi Apparatus Lysosomes Various kinds of vesicles and vacuoles Plasma Membrane Endoplasmic Reticulum (ER) The Endoplasmic Reticulum is an extensive network of membranes that it accounts for more than half the total membrane in many eukaryotic cells. The Endoplasmic Reticulum consists of a network of membranous tubules and sacs called cisternae, a reservoir of liquid. The ER membrane separates the internal compartment of the ER, called the ER lumen (cavity) or cisternal space, from the cytosol. The ER membrane is a phospholipid bilayer  embedded with proteins, is continuous with the nuclear envelope. Smooth Endoplasmic Reticulum (SER) Functions in diverse metabolic processes, which vary with cell type. This processes include synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. Enzymes of the smooth ER are important in the synthesis of lipids, including oils, steroid, and new membrane phospholipids. Among the steroids produced by the smooth ER in animal cells are the sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands. Other enzymes of the smooth ER help detoxify drugs and poisons, especially in liver cells. Detoxification usually involves adding hydroxyl groups to drug molecules, making them more water-soluble and easier to flush from the body. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells. Rough Endoplasmic Reticulum (RER) So named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed though an electron microscope. Many cells secrete proteins that are produced by ribosomes attached to the rough ER. For instance, certain pancreatic cells synthesize the protein insulin in the ER and secrete this hormone into the bloodstream. Most secretory proteins are glycoproteins, proteins with carbohydrates covalently bond bonded to them. Secretory proteins depart from the ER wrapped in the membranes of vesicles that bud from like bubbles from a specialized region called transitional ER. Golgi Apparatus After leaving the ER, many transport vesicles travel to the Golgi apparatus. It serve as a warehouse for receiving, sorting, shipping, and even some manufacturing. In the Golgi apparatus, products of the ER, such as proteins, are modified and stored and then sent to other destinations. It is especially extensive in cells specialized for secretion.  The Golgi Apparatus also manufactures some macromolecules. Many polysaccharides secreted by cells are Golgi products. For example, pectin and certain other non-cellulose polysaccharides are made in the Golgi of plant cells and then incorporated along with cellulose into their cell walls. Lysosomes