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Free Lecture notes cell and subcellular organelles and membrane transport pdf


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  • 1. 1 Introduction Prokaryotes & Eukaryotes Components of cell  Cell membrane  Cytoplasm with its organelles  Nucleus Cell membrane  Components  Structure  Fluid Mosaic Model Cytoplasm & its organelles  Endoplasmic Reticulum  Golgi apparatus  Mitochondria  Lysosomes  Peroxisomes Nucleus  Structure & Functions Membrane transport Cell is the universal functional unit of all forms of life. On the basis of differences in cell structure, all life forms are divided into two major classes. They are prokaryotes and eukaryotes. INTRODUCTION PROKARYOTES & EUKARYOTES LECTURE NOTES ON CELL & MEMBRANE TRANSPORT Dr Vijay Marakala, MBBS. M.D. Assistant professor, Department of Biochemistry, SIMS & RC, MUKKA - SURATHKAL, MANGALORE.
  • 2. 2 A COMPARISON OF PROKARYOTES AND EUKARYOTES ORGANELLE PROKARYOTES EUKARYOTES Nucleus No definite nucleus; DNA present but not separate from rest of cell Present Cell membrane Present Present Mitochondria None; enzymes for oxidation reactions located on plasma membrane Present Endoplasmic reticulum None Present Ribosomes Present Present COMPONENTS OF THE CELL  The outermost structure of the cell that decides its contour is the cell membrane.  It is a lipid bi-layer. It also consist of proteins and small amounts of carbohydrates  Membranes are asymmetric. The outer and inner surfaces have different components and different enzymatic activities.  Membranes are fluid structures. The unsaturated fatty acids bound to phospholipids contribute to the fluid state of the membrane. At body temperature lipids are in a fluid state and this fluidity of the membrane is essential for the normal functioning to occur. Cell Membrane (Plasma Membrane)  Cell membrane  Cytoplasm with its organelles  Nucleus
  • 3. 3 FLUID MOSAIC MODEL OF CELL MEMBRANE  In 1972 Singer and Nicolson postulated a theory of membrane structure called the fluid mosaic model.  A mosaic is a structure made up of many different parts. Likewise, the plasma membrane is composed of different kind of macromolecules like phospholipid, integral proteins, peripheral proteins, glycoproteins, glycolipids and cholesterol.  According to this model the matrix or continuous part of membrane structure, is a polar lipid bilayer. Phospholipids are arranged in bilayers with polar head groups oriented towards the extracellular side and cytoplasmic side with hydrophobic (nonpolar) tails face each other at the core of the bilayer.  The lipid bilayer is fluid because of more number of unsaturated fatty acids. The cholesterol content of the membrane alters the fluidity (More cholesterol less fluid).  Proteins are interspersed in the lipid bilayer, of the plasma membrane, producing a mosaic effect.
  • 4. 4 FUNCTIONS OF CELL MEMBRANE 1. It is fluid and dynamic. 2. It is semipermeable; only selected compounds are allowed to pass through from out-side. The selective permeability is responsible for the maintenance of internal environment of the cell and for creating potential difference across the membrane. 3. The modification of the cell membrane results in formation of specialized structures like axon of nerves, microvilli of intestinal epithelium and tail of spermatids. MEMBRANE LIPIDS  Three major classes – phospholipids, glycolipids and cholesterol  They are all amphipathic molecules, that is, they have both hydrophobic and hydrophilic ends. PROTEINS OF THE CELL MEMBRANE Two major categories 1. Integral or intrinsic or transmembrane proteins: are either partially or totally (transmembrane proteins) immersed in lipid bilayer. They serve as channels (pores) and carrier proteins. 2. Peripheral or extrinsic proteins: function almost entirely as enzymes and receptors. Most of membrane proteins are glycoproteins. They have short chains of carbohydrates on the exterior side of the membrane. MEMBRANE CARBOHYDRATES (THE CELL GLYCOCALAYX)  Membrane carbohydrate is not free. It occurs as glycoproteins or glycolipids.  Many of the carbohydrates act as receptor substances for binding hormones such as Insulin
  • 5. 5 CYTOPLASM AND ITS ORGANELLES The extra nuclear cell content that possess both organelles and other material constitutes cytoplasm. Material other than subcellular components in the cytoplasm makes up the cytosol. Cytosol contains mainly dissolved proteins, electrolytes and glucose. Five important organelles that are suspended in the cytoplasm are:  Endoplasmic reticulum  Golgi apparatus  Mitochondria  Lysosomes and  Peroxisomes. MITOCHONDRIA (Power house of the cell)  Mitochondria consist of outer and inner membranes.  The outer membrane is composed of equal amount of protein and lipids.  The outer membrane is freely permeable to many compounds.  The inner membrane consists of 75% protein and remainder is lipid.  The inner membrane is folded to form number of invaginations known as cristae extending to matrix. Cristae give large surface area and are the site of oxidative phosphorylation.  Mitochondrion is the power house of the cell. It is responsible for the production of energy in the form of ATP.  Mitochondrial matrix contains enzymes of the citric acid cycle and β – oxidation.  Mitochondria contains some DNA known as mitochondrial DNA
  • 6. 6 The endoplasmic reticulum (ER) is part of a continuous single-membrane system throughout the cell; the membrane doubles back on itself to give the appearance of a double membrane in electron micrographs. The endoplasmic reticulum is attached to the cell membrane and to the nuclear membrane. It occurs in two forms, rough and smooth. The rough endoplasmic reticulum is studded with ribosomes bound to the membrane. Ribosomes, which can also be found free in the cytosol, are the sites of protein synthesis in all organisms. The smooth endoplasmic reticulum (SER) does not have ribosomes bound to it ENDOPLASMIC RETICULUM FUNCTIONS OF ENDOPLSAMIC RETICULUM  Ribosomes and rough endoplasmic reticulum are involved in protein synthesis.  SER of intestinal cells is involved in formation of triglycerides.  In the adrenal cortex, SER is the site of steroid formation.  Cytochrome P450 dependent monooxygenases are present in liver cell SER.
  • 7. 7 GOLGI APPARATUS Golgi apparatus is separate from the endoplasmic reticulum but is frequently found close to the smooth endoplasmic reticulum. It is a series of membranous sacs. Functions 1. The Golgi apparatus is involved in secretion of proteins from the cell. Material produced in the cell for export is processed by Golgi body and is packaged as vesicle and is pinched off. The vesicles fuse with plasma membrane and their content is released to exterior by the process known as exocytosis. The digestive enzymes of pancreas and insulin are produced and released in this way. 2. Golgi apparatus helps in the formation of other subcellular organelles like lysosomes and peroxisomes. 3. Golgi apparatus is involved in protein targeting. LYSOSOMES They are small vesicles present in cytoplasm. They are surrounded by a membrane enclosed sacs containing hydrolytic enzymes that could cause considerable damage to the cell if they were not physically separated from the lipids, proteins, or nucleic acids that they are able to attack. Lysosomes are called as ‘Suicidal bags’ of the cell. Functions 1. Lysosomes are rich in hydrolytic enzymes, which are active at acidic pH. The lysosomal enzymes digest the molecules brought into the cell by phagocytosis. 2. Macrophages are rich in lysosomes. 3. Lack of one or more of lysosomal enzymes cause accumulation of materials in the cell resulting in lysosomal diseases.
  • 8. 8 PEROXISOMES Peroxisomes are similar to lysosomes; their principal characteristic is that they contain enzymes involved in the metabolism of hydrogen peroxide (H2O2), which is toxic to the cell. The enzyme catalase, which occurs in peroxisomes, catalyzes the conversion of H2O2to H2O and O2 MEMBRANE TRANSPORT Biological membranes are semipermeable membranes. Permeability is conferred by membrane proteins; these are called channel proteins and carrier proteins. There are two types of transport mechanisms 1. Passive transport or passive diffusion and 2. Active transport PASSIVE TRANSPORT OR PASSIVE DIFFUSION Transport of solute molecules from high concentration to low concentration across membrane is called passive transport. It is a spontaneous process because it is thermodynamically favourable. It is a downhill transport and requires no energy.  Two types of passive transport,  Simple diffusion and  Facilitated diffusion. Simple diffusion Transport of solute molecules from high concentration to low concentration across membrane is known as simple diffusion. Lipid soluble (lipophilic) molecules can pass through the cell membrane, without any interaction with carrier proteins.
  • 9. 9 Simple diffusion can occur through the cell membrane by ways.  Through the interstices of the lipid bilayer if the diffusing substance is lipid soluble and  Through the channel proteins. Example: Transport of O2, CO2, N2, ethanol and urea Facilitated Diffusion (Carrier mediated diffusion) Carrier molecules present in membranes mediate transport of many solute molecules across membrane. Hence, mediated transport involves carrier molecules. They are known as permeases, translocases, transporters or pumps. Most of them are proteins.
  • 10. 10 Facilitated diffusion is faster than simple diffusion. It requires no energy. Facilitated diffusion by carrier molecule involves conformational change of carrier molecule. The carrier molecule exists in two conformations. It has binding site to solute molecule. In the native conformation the carrier is exposed to high concentration of solute. Then the solute molecules bind to the sites on carrier molecule. A conformational change in carrier molecule occurs. It exposes solute molecule to low concentration and solute molecules are released into the cell. The empty carrier returns to the native state to transport solute molecules once again. Example: Transport of glucose and most of amino acids. Facilitated diffusion displays saturation behavior while the rate of transport of a solute in simple diffusion across a membrane is directly proportional to its concentration.
  • 11. 11 Active transport It requires energy in addition to carrier molecules. It moves solute molecules from low concentration to high concentration or against concentration gradient.  Substances that are actively transported through cell membranes include, Na+ , K+ , Ca++ , Fe++ , H+ , Cl- , I- , several different sugars and most of amino acids.  Active transport is classified into two types according to the source of energy used as follows: 1. Primary active transport and 2. Secondary active transport. Primary active transport  Energy is derived directly from hydrolysis of ATP.  Example: , Na+ , K+ ,Ca++ , H+ and Cl- transport across the membrane. Primary active transport of Na+ and K+ / sodium-potassium pump Active transport by Na+ , K+ - ATPase. Three sodium ions bind to the transporter protein on the cytoplasmic side of the membrane. When ATP is hydrolyzed to ADP, the carrier protein is phosphorylated and undergoes a change in conformation that causes the sodium ions to be released into the extracellular fluid.
  • 12. 12 Two potassium ions then bind on the extracellular side. Dephosphorylation of the carrier protein produces another conformational change, and the potassium ions are released on the inside of the cell membrane. The transporter protein then resumes its original conformation, ready to bind more sodium ions. Importance of Na+-K+ pump  The Na+ -K+ gradient created by this pump in the cells, controls cell volume.  Renders neurons and muscles electrically excitable and  Drives the active transport of sugars and amino acids. Secondary active transport Are those active transport systems in which transport of molecules is indirectly linked to hydrolysis of ATP. The Na+ gradient, which is maintained by primary active transport, is used to power the transport of glucose, amino acids, and many other compounds into the cell through secondary active transport. An example is provided by the transport of glucose into cells of the intestinal epithelium in conjunction with Na+ ions
  • 13. 13 Secondary active transport of glucose by the Na+ -glucose cotransporter. Sodium ion binds to the carrier protein in the luminal membrane, stimulating the binding of glucose. After a conformational change, the protein releases Na+ and glucose into the cell and returns to its original conformation. Na+ -K+ ATPase in the basolateral membrane pumps Na+ against its concentration gradient into the extracellular fluid. Thus, the Na+ concentration in the cell is low, and Na+ moves from the lumen down its concentration gradient into the cell and is pumped against its gradient into the extracellular fluid. Glucose, consequently, moves against its concentration gradient from the lumen into the cell by traveling on the same carrier as Na+ . Glucose then passes down its concentration gradient into the extracellular fluid on a passive transporter protein.
  • 14. 14 UNIPORT, SYMPORT AND ANTIPORT UNIPORT: Carries single solute across the membrane. E.g. glucose transporter CO-TRANSPORT: If the transport of one molecule depends on simultaneous or sequential transfer of another molecule, it is called co-transport. The co- transport system may either be a symport or an antiport SYMPORT: The transporter system carries two solutes in the same direction across the membrane. E.g. Sodium dependent glucose transporter ANTIPORT: Carries two solutes or ions in opposite direction. E.g. Sodium pump or chloride- bicarbonate exchange in RBC.