Cell Structure Reference Materials

The Cell Cell is the basic functional unit of complex organisms The living substances of the cell is known as Protoplasm Protoplasm has two compartments: Cytoplasm and Karyoplasm Cytoplasm extending from the plasma membrane to the nuclear envelope Karyoplasm forming the contents of the nucleus Cytoplasm The bulk of cytoplasm is composed of water Organic and inorganic chemicals are dissolved or suspended in it. This fluid suspension is called Cytosol. Cytosol contains Organelles, Cytoskeleton and Inclusions Eukaryotic cell structure Cell Membrane •between The cell membrane forms a selectively permeable barrier the cytoplasm and the external milieu. Molecular Composition *phospholipids, Cell membrane is based on a lipid bilayer contains glycolipids and cholesterol *It is composed of a phospholipid bilayer and associated integral and peripheral proteins *Membrane proteins carry out most of the specialized functions of cell membrane by participating in membrane transport mechanisms, by forming attachments to cytoskeletal and extracellular proteins and by constituting the receptors of the plasma membrane *Glycoclayx composed usually of carbohydrate chains, coats the cell surface which are not in contact with cytosol Ribosomes Organelles Ribosomes are Small particles composed of proteins and rRNA and have two subunits. Ribosomes are involved in the synthesis of proteins Endoplasmic Reticulum Endoplasmic reticulum is the largest membranous system of the cell, It is a system of interconnected tubules and vesicles whose lumen is referred as the cistern.  It has two components: Rough endoplasmic reticulum (RER) and Smooth endoplasmic reticulum (SER) Rough Endoplasmic Reticulum RER functions through interaction with ribosomes and RNA in the synthesis of proteins destined to be transported out of the cell, and proteins of lysosomes and of the plasma membrane Smooth Endoplasmic Reticulum SER is the site for the synthesis and metabolism of fatty acids and phospholipids, detoxification of drugs in the liver, and storage of calcium in muscle Golgi Apparatus (Complex) Golgi apparatus is a membranous system involved in the synthesis of carbohydrates sorting, packaging and transporting of the cell products The Golgi apparatus consists of 3 to 8 flattened cisternae which form functionally distinct compartments Each golgi stack has 3 levels of cisternae cis-face ( cis Golgi network)  medial face ( intermediate face)  trans-face  The Golgi is polarized so that the cis region faces the endoplasmic reticulum and the trans region faces the secretory vesicles in cells manufacturing proteins for export The cisternae are not interconnected and transport vesicles move material from one cisterna to another Vesicles Cytoplasmic vesicles are membrane bound organelles which are involved in cellular transportation Transcytosis is transportation of material through the cell by means of vesicles Exocytosis is transportation of material from inside the cell to the exterior Endocytosis is transportation of material from the exterior to the internal environment of the cell Coated Pits and Vesicles The coated pits and vesicles function in receptor mediated endocytosis and in the pathway for cell secretion There are at least two types of coated vesicles: clathrin coated and nonclathrin coated Clathrin coated vesicles are associated with transport vesicles which function in: 1- The regulated secretory pathway; 2- In transport of lysosomal proteins 3- In receptor mediated endocytosis Polypeptides of clathrin form a basket-like structure which coats the cytoplasmic surface of the membrane  Nonclathrin coated vesicles are either those which move between the endoplasmic reticulum and the Golgi complex, those which are a part of the constitutive secretory pathway, or those which move between the Golgi cisternae Endocytosis, Endosome, Lysosome These are involved in the ingestion, sequestering and degradation of substances internalized from the extracellular space Endocytosis divided into two categories: Phagocytosis and Pinocytosis Phagocytosis is the process of engulfing large particulate matter such as microorganisms, and cell fragments Pinocytosis is used when cell take up small molecules and fluid.the two main vesicles involved in transport of substances into cells are derived from surface membrane invaginations called coated pits and caveoli Endosomes Endosomes are divided into two compartments: early endosomes, near the periphery of the cell, and late endosomes,situated deeper within cytoplasm Lysosomes Lysosomes have an acidic pH and contain hydrolytic enzymes and involved in degradation of proteins Peroxisomes (Microbodies) Peroxisomes are self-replicating organelles that contain oxidative enzymes such as catalase They are important in metabolism and oxidation of fatty acids and in the synthesis of long chain phospholipids Among the enzymes associated with peroxisomes is catalase which destroys hydrogen peroxide. Mitochondria Mitochondria are enclosed by two layers of cell membrane, the inner of which is folded into shelf-like cristae (mitochondria of steroid secreting cells have tubular cristae) Located on the cristae are the enzyme assemblies of oxidative phosphorylation and electron transport  Within the mitochondrial matrix between the cristae are the enzymes of fatty acid metabolism and the Krebs cycle  In addition, the matrix contains circular mitochondrial DNA and RNA and ribosomes indicating that mitochondria are capable of limited protein synthesis They perform oxidative phosphorylation produce ATP and participate in lipid synthesis Annulate Lamella Annulate lamella are parallel aggregate of membranes that enclose cistern-like spaces they resembling multiple copies of nuclear envelope. Their function and their significance is not understood Inclusions Inclusions are nonliving components Glycogen: is the storage form of glucose Lipids: are storage forms of triglycerides Pigments: include melanin, hemoglobin and lipofusin Crystals: are not common except Sertoli and leydig cells in testis and occasionally in macrophages Cytoskeleton It has three major components: thin filaments, intermediate filaments and microtubules Cytoskeleton brace the internal structure of the cell Thin Filaments: are actin filaments that interact with myosin to bring about intracellular or cellular movement Intermediate Filaments: together with their associated proteins assist in establishment and maintenance of the three dimensional framework of the cell Cytoskeleton Microtubules: Microtubules are long straight rigid tubular-appearing structures that act as intracellular pathways Microtubule- Associated Proteins: are motor proteins that assist in the translocation of organelles and vesicles inside the cell Centrioles: are small cylinderical structures composed of nine microtubules triplets; they constitute the core of the microtubule organizing center or the centrosome Nucleus Nucleus is the largest organelle and contains nearly all of the DNA, it bounded by two lipid membranes and houses: Chromatin, Nucleolus and Nucleoplasm Nuclear Envelope It is composed of two parallel unit membranes that fuse with each other at certain regions to form perforations known as nuclear pores Nuclear Pores they are interruption in the nuclear envelope,where the inner and outer membranes fuse with each other. They function in bi-directional nucleocytoplasmic transport Nuclear Pore Complex it is composed of the nuclear pore and its associated glycoproteins Chromatin Chromatin is a complex of DNA and proteins and represents the relaxed, uncoiled chromosomes of the interphase nucleus Chromosomes are chromatin that become so condensed and tightly coiled during mitosis and meiosis that they are visible with the LM Nucleoplasm Nucleoplasm consists of interchromatin and perichromatin granules RNPs(small nuclear ribonucleoprotein particle) and nuclear matrix Nucleolus Nucleolus is deeply staining non-membrane bounded structure within the nucleus that is involved in rRNA synthesis and in the assembly of small and large ribosomal subunits The Cell Cycle The cell cycle is a series of events within the cell that prepare the cell for dividing into two daughter cells Interphase: the time between mitotic events is subdivided into three phases, Gap1, Synthesis, and Gap2 G1 phase: is a period of cell growth,RNA synthesis, and other events in preparation for the next mitosis S phase: DNA synthesis occurs during this period G2 phase: is the period between the end of DNA synthesis and the beginning of mitosis Mitosis is the process of cell division that result in the formation of two identical daughter cells Meiosis is a special type of cell division resulting in formation of gametes, cells whose chromosome number reduced from the diploid(2n) to the haploid(1n) number Apoptosis (Programmed Cell Death) Apoptosis is a normal means of controlling dividing cell populations, it is a programmed and energy-dependent process During this process there is no injury no inflammation In this process the cell becomes fragmented and is recognized and ingested by adjacent cells This process regulated by a number of highly conserved genes These genes code for a family of enzymes known as caspases, which degrade regulatory and structural proteins in the nucleus and in the cytoplasm Units of Measure in Microscopy •millimeter = 10-3 meter •micrometer = 10-6 meter = 10-3 millimeter •nanometer = 10-9 meter = 10-3 micrometer Relative Sizes of Some Biological Structures phospholipid molecule = 2 - 3 nm long plasma membrane = 7 nm thick typical virus = 50-100 nm diameter mitochondria = 0.5 - 1 µm diameter red blood cell = 7 µm diameter typical eukaryotic cell = 12-20 µm diameter human oocyte = 150 µm diameter