Cytoplasm and Cellular Organisms Lecture Notes PDF

1 Prof. Hala EL-Tantawi Cytoplasm and the cellular organisms Prof. Hala EL-Tantawi The cytoplasm: 2 The cytoplasm contains a variety of organelles, many of which are bounded by membranes. The nucleus is the la...

1 Prof. Hala EL-Tantawi Cytoplasm and the cellular organisms Prof. Hala EL-Tantawi The cytoplasm: 2 The cytoplasm contains a variety of organelles, many of which are bounded by membranes. The nucleus is the largest organelle, and its substance is bounded by a membrane system called the nuclear envelope or membrane. The nucleus contains the genetic material of the cell in the form of deoxyribonucleic acid (DNA). 3 An extensive system of flattened membrane-bound tubules, saccules, and flattened cisterns, collectively known as the endoplasmic reticulum, is widely distributed throughout the cytoplasm. A second discrete system of membrane-bound saccules, the Golgi apparatus, is typically located close to the nucleus. Scattered free in the cytoplasm are several relatively large, elongated organelles called mitochondria, which have a smooth outer membrane and a convoluted inner membrane system. Prof. Hala EL-Tantawi  Prof. Hala EL-Tantawi In addition to these major organelles, the cell contains a variety 4 of other membrane-bound structures, including intracellular transport vesicles and lysosomes. The cytoplasmic organelles, are suspended in a gel-like medium called the cytosol, in which many metabolic reactions take place. Within the cytosol, there is a network of minute tubules and filaments, collectively known as the cytoskeleton, which provides structural support for the cell and its organelles, as well as providing a mechanism for the transfer of materials within the cell and movement of the cell itself.  Prof. Hala EL-Tantawi Thus, the cell is divided into several membrane-bound 5 compartments, each of which has its biochemical environment. Membranes, therefore, serve to separate incompatible biochemical and physiological processes. In addition, enzyme systems are found anchored in membranes, so that membranes are themselves the site of many specific biochemical reactions.  Prof. Hala EL Tantawi. All eukaryotic cells start life with a nucleus. 6 The Nucleus This double-membraned sac holds an eukaryotic cell’s DNA.  Prof. Hala EL Tantawi. The Nucleus Nuclear Pore-riddled double membrane that controls 7 envelope which substances enter and leave the nucleus. Nucleoplasm Semifluid interior portion of the nucleus. Nucleolus Rounded mass of proteins and copies of genes for ribosomal RNA used to construct ribosomal subunits. The subunits pass through nuclear pores into the cytoplasm, where they join and become active in protein synthesis. Chromatin Total collection of all DNA molecules and associated proteins in the nucleus; all of the cell’s chromosomes. Chromosome One DNA molecule and many proteins associated with it.  Dr. Hala EL Tantawi The Chromosomes 8 Chromatin is the name for all the DNA molecules, together with its associated proteins, in the nucleus. The genetic material is distributed among a specific number of DNA molecules. The number of DNA molecules is characteristic for the type of organism and the type of cell, but it varies widely among species. Prof. Hala EL-Tantawi The chromatin is two types: 9 Heterochromatin, it appears as aggregation of coarse granules in the EM, and as basophilic clumps in the LM. It is condensed chromatin. Euchromatin, it appears as finely dispersed granular materials in EM, and as lightly stained basophilic areas in LM. It is extended, uncoiled chromatin. Prof. Hala EL-Tantawi Mitochondria 10 Mitochondria vary considerably in size , shape and change shape over time but are most often elongated, sausage-shaped organelles. Mitochondria are very mobile, moving around the cell by means of microtubules. They tend to localize at intracellular sites of maximum energy requirement. The number of mitochondria in cells is highly variable; liver cells contain as many as 2000 mitochondria whereas inactive cells contain very few. The number of mitochondria in a cell are modified by mitochondrial division and fusion. Prof. Hala EL-Tantawi Structure of Mitochondria 11 The outer membrane is relatively permeable as it contains a pore- forming protein known as porin, which allows free passage of small molecules. The outer membrane contains enzymes that convert certain lipid substrates into forms that can be metabolized within the mitochondrion.  Prof. Hala EL-Tantawi The inner membrane, which is thinner than the outer, is thrown into 12 complex folds and tubules called cristae that project into the inner cavity. The inner cavity filled by the mitochondrial matrix. The matrix is the site of the mitochondrial DNA and ribosomes. The matrix also contains a number of dense matrix granules. Prof. Hala EL-Tantawi The intermembranous space between the two membranes also contains a 13 variety of enzymes. Aerobic respiration takes place within the matrix and on the inner membrane, a process enhanced by the large surface area provided by the cristae. The matrix contains most of the enzymes involved in oxidation of fatty acids and the Krebs cycle. The inner membrane contains the cytochromes, and the enzymes involved in ATP production. Prof. Hala EL-Tantawi The mitochondrial matrix contains one or more circular strands of DNA 14 resembling the chromosomes of bacteria. The matrix also contains ribosomes with a similar structure to bacterial ribosomes. Mitochondria synthesize thirteen of their own constituent proteins, others being synthesized by the usual protein synthetic mechanisms of the cell and imported into the mitochondrion. In addition, mitochondria undergo self-replication in a manner similar to bacterial cell division.  Prof. Hala EL-Tantawi 15 Endoplasmic reticulum ▪Endoplasmic reticulum is a complex network of membranes/tubules. It considers a transport system of the cell that surrounds the nucleus. It is two types: rough (granular) and smooth (agranular) endoplasmic reticulum.  Prof. Hala EL-Tantawi 16 Rough Endoplasmic reticulum (RER) Structure: It consists of parallel flattened cisternae studded with ribosomes on its surface. It is connected to the outer layer of the nuclear envelope. Function: Its function is protein synthesis through its ribosomes. Moreover, it is involved in the glycosylation of protein (adding certain sugars to proteins). It is prominent in cells that synthesize high amount of protein, such as plasma cell (secretes antibodies), fibroblast (secretes collagen fibers), etc.  Prof. Hala EL-Tantawi 17 Smooth endoplasmic reticulum (SER): Structure: It consists of a branched network of tubules connected together with no ribosomes. Function: Its function is lipids and steroid synthesis and destruction of toxic substances. It is abundant in cells that synthesize lipid or steroid, such as cells of adrenal gland, Leydig cells in testis.  Prof. Hala EL-Tantawi 18 Ribosomes They are the site of protein synthesis inside the cell. Structure: They are small non-membrane-bounded organelle with granular appearance. They are present on surface of RER and also free in cytoplasm. They made of 2 subunits (large and small), each one made of proteins and ribosomal RNA (rRNA). They contain 85% RNA of cell. Function: They are the site of protein synthesis inside the cell.  Prof. Hala EL-Tantawi 19 Golgi apparatus Structure: It is a packaging house of cell. It was described by Camillo Golgi. It consists of series of flattened cisternae/sacs and peripheral vesicles (incoming and outcoming). The cisternae have two faces (cis/concave and trans/convex). This apparatus is continuous with ER. Function: They are responsible for packaging and sorting of protein, formation of lysosomal enzymes, and glycosylation of proteins.  Prof. Hala EL-Tantawi 20 Lysosomes Lysosome are considered recycling center in the cell. Structure: They are small membrane bound vesicles containing a variety of enzymes. The enzymes are synthesized in the RER, modified and packed in the Golgi apparatus, and bud off small vesicles (lysosomes). They are three types: 1ry, 2ry, and residual bodies. Function: They are responsible for intracellular digestion, killing and removing foreign bodies, breaking of bacteria and cell debris from dead cells that have been engulfed by a cell. They are abundant in cells involved in phagocytic activity such as macrophage and neutrophils.  Prof. Hala EL-Tantawi 21 Prof. Hala EL-Tantawi Peroxisomes 22 Structure: They are small membrane bound organelle in the cell formed by self-replication or budding from SER. They are essentially containing two types of enzymes: oxidase (active in oxidation of lipids) and catalase (act on hydrogen peroxide to liberate oxygen). Prof. Hala EL-Tantawi Function: 23 They are responsible for metabolism of lipids, destruction of toxin and drugs metabolites in the liver and kidney. They degrade hydrogen peroxide by catalase. In first, several oxidases combine oxygen and hydrogen to form H2O2. Then this H2O2 is oxidized by catalase into H2O and O2. They are very similar to lysosomes; they have digestive enzymes to break down the toxic materials inside the cells. But the difference is that lysosomes have enzymes that work in poor- oxygen level and low pH but peroxisomes have enzymes that require oxygen (oxidative enzymes). So, it protects cell from toxins. Their enzymes are synthesized by the free ribosomes in cytoplasm. Prof. Hala EL-Tantawi Cytoskeleton 24 Structure: It is a network of filaments and tubules. It extends from the plasma membrane to the nuclear envelope and even interior of the nucleus. It is made of proteins. It has three types: microfilaments, intermediate filaments, and microtubules. Prof. Hala EL-Tantawi Function: 25 It provides the shape and support for the cells; it also provides the organelles to move around them. Microfilaments: Thin thread-like structures. They resemble twisted necklace. Made of a protein called actin. Contractile, so they are found in motile cells. They are important in movement, pinocytosis, and phagocytosis. Prof. Hala EL-Tantawi Intermediate filaments: Intermediate in size between actin filaments and microtubules. 26 Rope-like assembly of fibrous polypeptides (Vimentin, Keratin, Lamin, Neurofilaments, or glial fibrillary acidic protein (GFAP)). Vary in nature from tissue to tissue and from time to time. They support nuclear envelope, and anchor cell-cell junctions. Microtubules: Hollow, rigid tube-like structures. Made of proteins known as tubulin. Their functions: ▪ Maintain shape of some cells. ▪ Form centrioles and spindle fibers for separating chromosomes during mitosis (animal cells only). ▪ Form projections such as flagella and cilia that enable some cells to move. Prof. Hala EL-Tantawi Centrioles 27 Short, hollow cylinders. Found in pairs near the nucleus in animal cells. Made of two bundles of microtubules at right angle to each other. Each bundle arranged into nine overlapping triplets. Appear during cell division. May give rise to basal bodies of cilia and flagella. Prof. Hala EL-Tantawi Cilia and flagella 28 They are hair-like structures that project from the cell. They assist in movement. They consist of 9 doublet units of microtubules arranged as a ring. Cilia are short, and numerous and hair-like. They move in coordinated waves. e. g. in trachea and bronchi to move dust, particles and bacteria away from the lungs. Flagella are much longer, fewer, and are whip-like. They move like a propeller or cork screw. e. g. in sperm to swim to the egg.  Prof. Hala EL-Tantawi 29