Lec 3 Cytology PDF

Molecular structure of cell membranes Cell membranes (biomembranes) are important part of all cells. Their discovery is closely related to the upgrading of microscopic techniques, especially with the design of transmission electron microscopy, by which was observed typical trilaminar structure.Furt...

Molecular structure of cell membranes Cell membranes (biomembranes) are important part of all cells. Their discovery is closely related to the upgrading of microscopic techniques, especially with the design of transmission electron microscopy, by which was observed typical trilaminar structure.Further observations showed that biomembranes in the cell are similar in structure and slight differences in chemical composition are due to cell differentiation and specialization.  Every cell is surrounded by the cytoplasmic membrane, which separates intracellular from extracellular space. Its average thickness is 60 – 100 nm. Functions of Cell membrane is selectively permeable boundary, cell which ensures the maintenance of dynamic equilibrium between cell and environment. membrane It contains enzymes, receptors, transport proteins, signalling systems and antigens. It performs different functions, e.g. intake of substances, interactions, recognition of signals, etc. The biological membrane is a part of many important cellular organelles. Dr. Ahmed Alazzouni  The main components of cell membranes are phospholipids. The molecule of phospholipids is composed of polar (hydrophilic) head and two non-polar (hydrophobic) fatty acids chains. In the aqueous environment the hydrophilic parts are oriented towards the water around them and fatty acids chains to each other, creating so-called phospholipid bilayer.Given that phospholipids are not chemically bound to each other, their lateral movement is possible. Dr. Ahmed Alazzouni  Other important components of biological membranes are proteins (Fig. 14). They may be: integral, which affect the hydrophobic parts of the phospholipid bilayer or transgress it. They are hardly separable from the biomembranes; peripheral, which lie outside the lipid bilayer. They are associated with electrostatic bonds and can be easily separated from biomembranes. Types and number of proteins in biomembrane is variable. It is dependent on cell differentiation and cell cycle phase. Specific protein composition of biomembranes is regulated by cell. Dr. Ahmed Alazzouni Functions of membrane protiens Membrane proteins perform various functions. They are part of biomembrane structure (structural proteins). Some of them are involved in the transport of ions across the membrane (pump and ion channels) or in the transfer of substances along the electrochemical gradient by facilitated diffusion. Many of them are part of the receptors that are able to specifically bind hormones,neurotransmitters and other signal molecules. Some have the role of enzymes. Proteins and glycolipids are part of the antigens. Dr. Ahmed Alazzouni  Membrane receptors Membrane receptors Membrane receptors are protein structures located in cell membrane, which are responsible for recognition and binding of signal molecules (e.g. hormones, neurotransmitters etc.). Through these receptors cell interacts with its surroundings. Membrane receptors may be classified into: Dr. Ahmed Alazzouni  receptors which are part of the ion channels – these are receptors for transport of cations (e.g. nicotine-acetylcholine receptor and receptor for excited amino acids) and anions (e.g. glycine receptor and receptor for gamma amino-butyric acid); Membrane receptors with enzyme activity are receptors with intracellular protein subunit which catalyzes receptors certain chemical reactions. These include receptors with tyrosine kinase activity, insulin receptor etc.; receptors coupled to G proteins – the largest group of membrane receptors (five families are known). Dr. Ahmed Alazzouni Transport of substances through the membrane Transfer of substances into cells and outside of cells is realized by two basic mechanisms. 1- passive transport. 2- active transport. Dr. Ahmed Alazzouni  The passive transport The passive transport ensures transfer of substances in the direction of concentration gradient without consumption of energy (diffusion and osmosis). The speed of transition depends only on the size of the gradient (difference between concentrations in the cell and outside). Given the selective permeability of the cytoplasmic membrane only a few substances with low molecular weight (e.g. water, oxygen, carbon dioxide, urea, methanol and ethanol) can be transported by this way. Dr. Ahmed Alazzouni  Diffusion Diffusion is an unordered movement of molecules in solution. It results in the movement of dissolved substances from the higher concentration to places of lower concentration. This movement will stop as soon as the concentration of the substance on both sides membrane equalized. Dr. Ahmed Alazzouni Osmosis Osmosis is a process in which water passes through the cytoplasmic membrane from the environment with a lower concentration in more concentrated environment. The process takes place until equalization of concentrations of both solutions. In case, that both solutions are isotonic to each other and cells that are in it perform no changes. For human cells the isotonic solutions are 0.9% NaCl saline and 5% glucose solution. If the solution in the extracellular environment is more concentrated than inside the cell, it is hypertonic solution. The cells lose water and shrink. In plant cells occurs plasmolysis (separation of the plasma membrane from the cell wall). If the solution outside the cell is of lower concentration as in the cell, it ishypotonic solution. Water penetrates into the cell. Animal cell increases in size and burst (cytolysis); e.g. haemolysis of red blood cells. In plant cells only it increases their turgor – cell wall prevents them against breaking. Phenomena of osmosis in plant and animal cells are presented by figure 17. Dr. Ahmed Alazzouni  Facilitated diffusion Facilitated diffusion (Fig. 18) represents the transport of substances mediated by plasma membrane proteins without consumption of energy in the direction of concentration gradient. This process occurs so that the substance is bound to transport protein on the cell surface. It changes its conformation and the substance is released into the cytoplasm (e.g. transport of glucose). Dr. Ahmed Alazzouni  Active transport is the transfer of substances against concentration or electrochemical gradient without moving membrane. This process ensures due to consumption of energy obtained by dissociation of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) Active or up to AMP (adenosine monophosphate). transport This process is provided by special transportation systems (channels and pumps), protein complexes, which pass through the membrane. It is a rigorous selective and managed process, often controlled by receptors. There are two basic types of active transport – primary and secondary. Dr. Ahmed Alazzouni  Primary active transport is realized against concentration or electrochemical gradient with the energy consumption (obtained by hydrolysis of ATP). This process is performed by cyclic phosphorylation and dephosphorylation of Primary transport proteins. This also changes the affinity to the substrate – alternately on the active outside and inside the membrane. The whole process can be summarized as follows – transport transported substance (substrate) is attached to phosphorylated transport protein; protein is dephosphorylated to open the binding site toward the cytoplasm and the substrate is released. The function of Na+– K+ pump (Na +-K +-ATPase) and H+ pump (H+–ATP–ase) is realized by this way. Dr. Ahmed Alazzouni  Primary active transport Dr. Ahmed Alazzouni  Secondary active transport secondary active transport, the affinity of membrane transport protein is not changed by phosphorylation, but by the attachment of ions (e.g. Na+). These proteins have two sites, first one for connection with ion and second one for transported substrate. In the case of the substrate and the ions are transported in the same direction, it is cotransport. In the case of transport in the opposite direction, it is antiport. Dr. Ahmed Alazzouni  Endocytosis and exocytosis Transport of substances with high molecular weight is carried by Endocytosis endocytosis and exocytosis. Both processes are associated with active and participation of the cytoplasmic exocytosis membrane (changes in its structure or its movement). Dr. Ahmed Alazzouni  Endocytosis Endocytosis is the process by which substances are transported into cells. According to transported substances we distinguished pinocytosis (especially the transport of soluble substances) and phagocytosis (transport of solids). Dr. Ahmed Alazzouni  pinocytosis pinocytosis the transported substance is attached to receptor in the plasma membrane, which “creates” hollow inside the cytoplasmic membrane. The resulting cavity gradually increasing and surrounded transported substance. Finally, it is closed and creates a pinocytotic vesicle, which is released into cell and travels to the place of further processing. After that plasma membrane integrity is Dr. Ahmed Alazzouni restored  Phagocytosis Phagocytosis represents transport of solid particles, for example phagocytosis of bacteria. The cell generates plasma membrane processes (pseudopodia) which surround the transported material. It enters cells in vesicles and is processed. Dr. Ahmed Alazzouni  exocytosis exocytosis the substances are transported from the cell into the extracellular environment. Secreted material is located in vesicles, which usually arise from endoplasmic reticulum and Golgi apparatus. Vesicle approaches the plasma membrane, touched her, and merging with it and the substances are released into the environment. Dr. Ahmed Alazzouni Endocytosis and exocytosis Dr. Ahmed Alazzouni  Cytoplasm represent basic inner environment of the cell. It is composed mainly of water (70 – 80%). It also includes a considerable number of ions, inorganic and organic compounds (e.g. fatty acids, amino acids, lipids, carbohydrates, nucleic acids, proteins). Cytoplasm is a semi-liquid mass, which should be in form of sol (liquid) or gel. Protein content and their ability to bind water influences its viscosity. The cytoplasm of eukaryotic Cell cells contains a variety of cell organelles. According to their composition cell organelles are organelles distinguished into three basic types: membrane, which are composed from one membrane (endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, vacuoles and other vesicles) or from two membranes (nuclear envelope, mitochondria and chloroplasts); composed of proteins – cytoskeleton (microtubules, microfilaments, intermediate filaments, flagellas, cilia etc.); composed of proteins and nucleic acids – ribosomes, nucleolus. Dr. Ahmed Alazzouni  Mitochondria are organelles inevitable for the life of eukaryotic cells. They are involved in generating energy for cells by breaking down of saccharides, lipids and other energy- rich organic compounds. Mitochondria can vary in number and shape according to the type of cell. Their number is in a direct proportion to the intensity of cell’s energy metabolism. They consist of two biological membranes. Mitochondria The outer membrane encloses it while the inner one is folded into the mitochondrial cristae expanding its surface. The enzyme system (H+ATP synthase) is localized in the inner membrane and is responsible for cellular respiration (oxidative phosphorylation). This is where the glucose is broken down with the freed energy bounding into ATP molecules (ADP + E + P = ATP). It also contains other important enzymes, e.g. cytochromes, NADP dehydrogenases etc. Dr. Ahmed Alazzouni  The space between the cristae is filled with mitochondrial matrix made up of phospholipoproteins and ions of calcium and magnesium. Moreover it also contains enzymes of the citric acid cycle (the Krebs cycle). Mitochondria have their own circular DNA and ribosomes of the prokaryotic type. This enables their auto-reproduction and synthesis of their own proteins. Most of mitochondrial enzyme substance is however coded by nuclear genes. They are synthesized on endoplasmic reticulum, modified in the endoplasmic reticulum and completed in Golgi apparatus. Consequently, they are transported into mitochondria where they are enhanced with proteins synthesized in mitochondria and become functional. Dr. Ahmed Alazzouni  Endoplasmic reticulum Endoplasmic reticulum (ER) is a system of tubules, cisterns and flat vesicles from the biological membrane closely communicating with the nucleus and via transport vesicles with the Golgi apparatus and cytoplasm. ER is responsible for synthetic processes. Dr. Ahmed Alazzouni  Smooth ER From the morphological and functional aspect, we distinguish between two types of endoplasmic reticulum. Smooth ER which is the place of synthesis of saccharides and lipids (including parts of the biomembranes), steroid hormones and cholesterol. The detoxication function of the smooth ER is also significant – excessive cell‘s intoxication leads into apoptosis. The muscle cells contain a special form of endoplasmic reticulum – the sarcoplasmic reticulum. It ensures the transport of Ca2+ cations that are inevitable for muscle contraction. Dr. Ahmed Alazzouni Rough endoplasmic reticulum Rough endoplasmic reticulum (Fig. 25b) contains ribosomes on its surface and is the place of protein synthesis. Formed proteins penetrate its structure and are modified for the first time. For further processing they are transported in a vesicle from the membrane into the Golgi apparatus. The ratio of the rough and smooth ER in a cell depends on its function and products it creates. Dr. Ahmed Alazzouni Endoplasmic reticulum Dr. Ahmed Alazzouni Golgi apparatus It is made up of a system of flat cisterns whose sides are from the functional aspect divided into cis and trans. Proteins synthesized on the endoplasmic reticulum, usually enclosed in a transport vesicle, come to the cis side. Their posttranslational modification continues inside of the Golgi apparatus. They leave Golgi apparatus from the trans side in the form of vesicles to cytoplasm. Some of them fulfill specific tasks in a cell, others join the cytoplasmic membrane and their content comes into the extracellular environment. Some vesicles only take part in “circulation” of cell membranes. Dr. Ahmed Alazzouni Functions of Golgi Apparatus The Golgi apparatus is a complex of stacked membranous sacs that is crucial for the processing, packaging, and further distribution of proteins and lipids coming from the ER. The formation of secretory vesicles also takes place here. Its main function is the packaging and secretion of proteins. It receives proteins from Endoplasmic Reticulum. It packages it into membrane-bound vesicles, which are then transported to various destinations, such as lysosomes, plasma membrane or secretion. The Golgi apparatus is responsible for transporting, modifying, and packaging proteins and lipids into vesicles for delivery to targeted destinations. As the secretory proteins move through the Golgi apparatus, a number of chemical modifications may transpire. Dr. Ahmed Alazzouni  Lysosomes and other vesicles Lysosomes are vesicles from single biological membrane created by being detached from the Golgi apparatus as a primary lysosome. They contain digestive enzymes (e.g. hydrolases for protein digestion). Following connection with an unneeded organelle or pinocytotic (fagocytotic) vesicle creates a secondary lysosome and lysosomal enzymes break down the content of these structures. In plant cells and some protists, the function of lysosomes and many other tasks is performed by vacuoles. Dr. Ahmed Alazzouni  Peroxisomes belong to the large family of vesicles in cytoplasm, the role of which is to ensure activity of various Peroxisomes enzymes. Peroxisomes contain catalase eliminating the aggressive hydrogen peroxide. Enzymes in the cytosolic vesicles are thus available for the cell; however, they are not directly contained in the cytoplasm, but used when the cell needs them. Dr. Ahmed Alazzouni  Ribosomes These might be one of the smallest cell organelles, but they are crucial, because of protein synthesis. They are made up of a large and small sub-unit that is connected only during translation (the protein synthesis). The sub-units are made up of proteins and ribosomal RNA. The main function of ribosome is protein synthesis. Dr. Ahmed Alazzouni  ribosomes of eukaryotic cells The ribosomes of eukaryotic cells are larger and their gravitation density is 80S. They consist of four types of rRNA and 82 proteins. They are present freely in cytoplasm, but their prevailing majority participates in the creation of rough ER. This enables the eukaryotic cell to effectively manage the course of protein synthesis. In the eukaryotic cells there are also ribosomes of the prokaryotic type, specifically in mitochondria and chloroplasts. These organelles synthesize their own proteins needed in order to activate their enzymes and auto-reproduction Dr. Ahmed Alazzouni Dr. Ahmed Alazzouni Dr. Ahmed Alazzouni