Cell Membrane Structure and Function PDF

Instructor: Assist Prof. Dr. Shaymaa Jasim Mohammed  The Cell Cell compartments Introduction The functions of the organels of the cell 2 Cell Physiology STRUCTURAL ORGANIZATION IN ORGANISM Characteristic features of living or...

Instructor: Assist Prof. Dr. Shaymaa Jasim Mohammed  The Cell Cell compartments Introduction The functions of the organels of the cell 2 Cell Physiology STRUCTURAL ORGANIZATION IN ORGANISM Characteristic features of living organisms ⚫ Metabolism The sum of chemical events in which large molecules break down into small molecules and large molecules are formed from small molecules. ⚫ Responding to stimuli = responsiveness Sensing and responding to changes in internal or external electrical signals, hormones and glands ⚫ Movement (at all structural levels) = Movement Ability to act throughout the body, at the level of an organ, at the cell level, or as a component of the cell ⚫ Growth / Differentiation Increase in the number and volume of substances or cells between cells Differentiation of cells for a specific task ⚫ Reproduction New cells or individuals forming  CELL The smallest structural and functional unit of the organism is the cell. Generally, all cells basically have the same structure. The cell is a unit that  provides internal dynamic balance with many chemical reactions managed by its own organic catalysts,  regenerates itself in an isothermal system with organic molecules,  forms its own structures and  adapts to its environment. However, they show some specializations depending on the tissues they are in and thus their functions (such as muscle cells, nerve cells, secretory gland cells). Each cell is covered by a plasma membrane (cell membrane, plasmalemma) that separates it from the external environment. Inside the cell is a dense fluid called the cytoplasm. In the middle of the cytoplasm is a double membrane- lined nucleus that contains most of the cell DNA.  The number,  volume,  structure, and  function of organelles vary from cell to cell. It consists of 3 basic structures. Plasma membrane, Cytoplasm Organelles MEMBRANES Membranous Tissues Cell Membrane (Plasmalemma) Membranous Tissues The inner surfaces of the body cavities and the outer surfaces of the organs these are surrounded by membranes. There are two types of membranes, 1. epithelial 2. connective tissue membranes (synovial membranes) 1. Epithelial membranes cutaneous, serous and mucous  Mucous membranes The membranes lining the body cavities that are directly or indirectly connected with the external environment and the interior of the hollow organs are called mucous membranes (tunica mucosa) or just mucosa. Special cells in mucous membranes secrete mucus. 11 Serous membranes: The inner face of the body cavities that are not connected to the external environment and the outer surfaces of the organs and these are lined with a serous membrane (tunica seroza). Serous membranes consist of a simple, squamous epithelium (mesothelium) lined up on a thin connective tissue. A serous fluid is secreted by the serous membranes that will wet the surface. peritoneal cavity thoracis pleura and the cavity inside the pericardium is lined with serous membranes called the pericardium. The leaves of the serous membranes that line the inside of the body's cavity are called lamina parietalis (parietal lamina), and the leaf surrounding the outer surface of the organs is called lamina visceralis (visceral lamina). Synovial membranes: The membranes lining the joint space (cavitas articularis) inside the movable joints (synovial joints) such as the shoulder, elbow, hip and knee are called synovial membranes (membrana synovialis). Synovial membranes do not contain a continuous epithelium; They are made up of flat connective tissue cells and adipose tissue elements. Synovial membranes secrete a dark, clear, straw- yellow liquid called synovia.  CELL MEMBRANE STRUCTURE AND FUNCTIONS All cells are surrounded by a plasma membrane or cell membrane called the plasmalemma. Microscopically, this structure, seen as a thin line, carries out many essential and necessary functions for the cell in an orderly and flawless manner with its extraordinary molecular sequence. There are proteins, lipids and carbohydrates in the structure of the cell membrane. Lipids make up 50% of the membrane mass in the animal cell. There are three types of lipids in the cell membrane. These: Phospholipids Cholesterol Glycolipids. These three lipid molecules have amphipathic properties. In other words, these molecules have hydrophilic=like water parts and hydrophobic=not like water tail parts.  A typical phospholipid molecule has a polar head group and two hydrophobic tail regions in its structure. One of the tails carries saturated and the other unsaturated fatty acids.  Phospholipid molecules move in the size of the single layer they are in on the membrane. Movement is either in the form of going left and right, opening and closing their tails or turning around themselves. Very rarely, a phospholipid molecule in one layer can jump to the other layer. Animal cells contain cholesterol in their membranes. (Plant cells have no cholesterol in their membranes.) In the membrane, cholesterol connects with the polar head of phospholipids. The functions of cholesterol in the cell membrane; 1. It gives strength to the cell membrane. 2. Adjusts the fluidity of the cell membrane (Makes it less fluid) 3. It decreases the permeability of small molecules dissolved in water through the lipid layer. Another lipid found in the cell membrane is glycolipid. The head part of the glycolipid molecules is oligosaccharide, the tail part has a polar head group and two hydrophobic tail regions in the structure of a typical phospholipid molecule. One of the tails carries saturated and the other unsaturated fatty acids. While the lipid bilayer forms the main structure of biological membranes, most of the special functions of the membrane are carried out by the proteins in the membrane.  The shape and amount of proteins in the membrane determine what function it will perform. It is amphipathic like lipid molecules located in the membrane. Inside the lipid bilayer, there are hydrophobic regions that interact with the hydrophobic tails of the lipid molecules and the hydrophilic regions in the parts of the membrane facing the inside or outside of the cell, facing the aqueous medium. Proteins in the cell membrane are divided into two classes, peripheral proteins and integral proteins, depending on their positions.  While some of the integral membrane proteins are completely embedded in the lipid bilayer, some have parts that protrude towards the surface. Some are embedded in the outer half of the lipid bilayer, and some are embedded in the inner half. Integral proteins that extend across the lipid bilayer and meet the aqueous microenvironment on both sides of the membrane are called transmembrane proteins.  Peripheral proteins are completely outside the lipid layer and do not constitute a continuous layer. Like membrane lipids, membrane proteins can rotate around their own axis or move laterally in the membrane where they are located.  The functions of the proteins in the membranes Transport: a.Some proteins along the membrane form a hydrophilic channel and play a role in selectively transporting some substances. b.Some transport proteins hydrolyze ATP, pumping substances across the membrane. Enzyme activity: A protein found in the membrane can act as an enzyme. In some cases, many enzymes on a membrane can act as a team in the metabolic pathway. Signal transduction: Some proteins in the membranes allow the first messenger molecules (signal molecules) to bind, such as hormones. A change in the protein causes the message to be transmitted to the cell. Binding between cells: Membrane proteins bind neighboring cells together by various types of connections. Cell-cell recognition: Some glycoproteins carrying the carbohydrate chain enable the cell to be recognized by other cells. Attachment to the extracellular matrix: Proteins play a role in the attachment of the microfilament and other elements of the cytoskeleton to the extracellular matrix.  ⚫ Carbohydrates of Membrane Proteoglycans are carbohydrates attached to the outer surface of the cell with small proteins. Functions They form the recognition regions of the cell (blood group, immune structure, etc.). They create a negatively charged cell surface. They make the cells stick together. They act as receptors (insulin etc.). The most important function of the cell membrane is to separate the cell from the external environment and to control the entry and exit of matter into the cell. Thus, substances are not allowed to enter the cell easily. This property of the cell membrane is defined as "selective permeability".  THE FUNCTIONS OF CELL MEMBRANE ⚫ A) The lipid layer, which has a solid structure with cholesterol, ensures cell and organelle integrity. ⚫ B) With carrier or channel proteins, molecules are transported from one side of the membrane to the other. ⚫ C) With receptors initiates metabolic reactions in the cell. ⚫ D) It regulates metabolic reactions by catalyzing specific chemical reactions with enzymes. ⚫ E) Binds cells together with the special connections they form with transmembrane proteins. ⚫ F) Transmembrane proteins provide support to the cell by binding to the filamentous proteins inside the cell and maintaining the cell shape. ⚫ G) Glycoproteins and glycolipids form cell and organelle identity. Other FUNCTİONS: Cell membranes have many other functions than those listed above. They provide mechanical power to the cells. Cell movement, Secretion, They are involved in Endocytosis and Exocytosis events. They carry nerve impulses. They act like electrical insulators. *It provides cell recognition and tissue specificity with the antigenic molecules it contains. * Ion pumps for regulation of the intracellular environment. * It has receptors for various hormones. * It takes the task of creating messenger molecules that will activate physiological responses to stimuli.  MATERIAL TRANSPORT IN CELL MEMBRANE A living cell must be able to deliver the micro and macromolecules it needs in order to survive to the desired regions. Molecular movement occurs from one side of the membrane to the other or from one region to another within the cell. Therefore, the cell membrane is the most important structure that controls the entry of the molecules necessary for the cell to nourish and carry out its metabolic activities, and the exit of waste materials from the cell as a result of this function.  Material transport through the cell membrane is generally divided into passive and active transport. If a molecule passes through the cell membrane without using cellular energy, this is called passive transport. It is called active transport if it uses energy. Passive Transport  Simple diffusion  Facilitated diffusion  Osmosis  Dialysis  Filtration  Simple Diffusion It is the continuousmovementof ions or molecules from high concentration to low concentration. This is called diffusion. Molecules tend to move towards decreasing concentration, that is, they move from dense concentration to less dense concentration. Diffusion depends on the specific random motion of the molecules. In body fluids, all molecules and ions, including water and substances dissolved in water, are in constant motion. Each particle moves in its own unique path. Diffusion rate depends on which factors? 1. Gas or liquid form, - gases are faster. 2. Heat, - the higher the temperature, the higher the diffusion. 3. The size of the molecules, - the little ones are faster. 4. Diffusion area, - the larger the area, the higher the speed. 5. Membrane thickness - diffusion distance, speed decreases as distance increases.  Osmosis Osmosis is defined as the flow of water through a semipermeable membrane between two solutes with different concentrations. The flow of water is from the portion where the solute concentration is lower to the portion where the solute concentration is higher. The water pressure that develops as a result of osmosis in a solution is called osmotic pressure. It is extremely important for cells to keep their pressure and volume in balance so that they can function in a healthy way. In the case of osmosis, the volume of one of the solution while the volume of the other  The cells in our body constantly try to keep their pressure and volume in balance.  Because intracellular fluid and extracellular fluid have the same osmotic pressure, this is called an isotonic medium. Red blood cells undergo no change in an experimental medium, in an isotonic solution. While the red blood cells shrink by losing water in a hypertonic medium, water enters the red blood cells in a hypotonic medium; As a result, these cells swell. As a rule, in the case of water osmosis, it always moves from the hypotonic medium to the hypertonic medium. Dialysis ⚫ In some special cases, water and small water-soluble particles pass through a selectively permeable membrane called dialysis. ⚫ As a result, small particles dissolved in water pass through the membrane, while large particles remain on the other part of the membrane. The use of artificial kidney in patients with renal dysfunction is called hemodialysis.  In this procedure, the patient's blood is passed through a thin, long curved tube made of cellophane with certain sized holes.  The holes, depending on their size, hold large molecules such as blood proteins in the tube and pass small molecules such as urea that can dissolve in water.  Thus, while water and water-soluble substances pass through the membrane, the larger ones remain inside. Filtration ⚫ Filtration is the passage of water and water-soluble substances through the membrane by hydrostatic pressure force. ⚫ Hydrostatic pressure is the force of the fluid pushed against a surface. ⚫ In filtration, molecules that will pass through the membrane pass from the high hydrostatic pressure region to the low hydrostatic pressure region. ⚫ Water and small molecules dissolved in water pass out of the vein through the thin and perforated walls of the capillaries due to the hydrostatic pressure created by the blood inside the vessel. ⚫ Large blood proteins remain in the vein. ⚫ Small molecules and water that filter out of the vein, that is, filter, form the tissue fluid there. ⚫ The most important organ in which filtration takes place is the kidneys. ⚫ Water and many substances dissolved in water are filtered out of the vein with blood pressure and enter the renal tubules. This is the first step in urine formation. ⚫ The processing mechanism of dialysis and filtration is the same. ⚫ Dialysis is a method used in special cases, ⚫ and filtration is a type of filtration that works in the natural state of the organism. Facilitated Diffusion In this system, substances are transported from the membrane in the direction of the concentration difference. The difference from simple diffusion is that a carrier molecule mediates the ability of substances to cross the membrane. Some molecules cannot easily cross the membrane due to their chemical properties and their size. For this reason, protein molecules in the structure of the membrane assume the role of carriers, carrying these substances from the high concentration side to the low side. Glucose molecules, an important energy source of cells, are transported into the cell in this way.  Active Transport System In this way substances are transported from the low concentration region to the high side. For this reason, a carrier molecule mediates the transport of substances and has energy support. The force used for the passage of substances through the membrane in active transport is the energy produced by the cell as a result of its own metabolism. This is achieved by breaking down the energy-containing ATP (adenosine triphosphate) molecule. Transport proteins involved in active transport extend from one side of the cell membrane to the other. Each carrier protein binds to the ion or molecule it carries with a special region and transfers it to the other side of the membrane by changing its shape. Active transport systems in the body are as follows; Sodium potassium pump, It is commonly found in many cells, especially nerve and muscle cells. It is located where a sodium-potassium concentration gradient is created to create an electrical potential difference. Calcium pump, It is a pump that has a very important role in muscle contraction. It is used for calcium reuptake into the sarcoplasmic reticulum. Sodium dependent cotransport, It is used for the active transport of sugar and amino acids. Hydrogen dependent cotransport. It is used in the active transport of sugar. These 4 active transport mechanisms also work using similar events;  The molecule outside the cell binds to the carrier protein in the membrane,  The molecule carrier protein complex crosses the membrane,  In the presence of energy obtained from ATP with the help of at least one enzyme, the molecule and the carrier protein separate from each other and the molecule is released into the cell.  The carrier protein returns to its original position and these events are repeated for other molecules. In active transport When the transport proteins pass a molecule from one side of the membrane to the other, this one-way function is called uniport transport.; When a molecule passes through the membrane with the help of a carrier protein, and another molecule passes in the same direction, it is called symport transport; When a molecule passes through the membrane with the help of a carrier protein, and another molecule passes in the opposite direction, it is called antiport transport.  Transport of Big Molecules and Particles through the Cell Membrane: Exocytosis and Endocytosis While carrier proteins allow the passage of many small polar molecules through the cell membrane, they cannot transport large molecules such as proteins, polynucleotides or polysaccharides through the membrane. Hence, cells have developed other active mechanisms. These are exocytosis and endocytosis. While most cells excrete large molecules via exocytosis, they ingest them via endocytosis.  Pinocytosis means "cell drinking". Extracellular fluid is taken into the cell in small droplets. If there is any material dissolved in this liquid (low molecular weight nutrients, amino acids, glucose, vitamins and other substances), they are taken into the cell along with the liquid. Kidney cells are one of many cells that are an example of the use of pinocytosis.  Phagocytosis means "Eat of the Cell" as a word. In phagocytosis, the cell absorbs (swallows) solid substances such as bacteria and nutrients. Extensions of the cell membrane surround the substance or bacteria and pull it into the cell. Later, this vesicle, which breaks off the membrane, moves into the cell with the substance inside. It is digested by combining with lysosomes inside the cell. For example, red blood cells swallow bacteria like this, then lysosomes dissolve the cell membrane surrounding the bacteria and break down the bacteria with their enzymes. Receptor-mediated endocytosis  It occurs through a special receptor that recognizes and binds to the macromolecule outside the cell.  The substance that binds to the receptor is called a ligand.  The cell membrane region containing the receptor-ligand complex undergoes endocytosis. A specific example of transport in this way is the uptake of LDL (low molecular weight lipoprotein) into the cell.  Most of the cholesterol needed to form a new c e l l membrane is transported into the cell in this way as LDL. Protoplasm is the vital content of a cell surrounded by a cell membrane. General term is cytoplasm. Water Ions Protein Lipids Carbohydrates  Water is the basic liquid environment of cells, 75-80% of many cells are made up of water. Many chemicals in the cell are dissolved in water. Electrolytes form inorganic chemicals necessary for cellular reactions. The most important electrolytes in the cell;  potassium  magnesium  phosphate  sulfate  bicarbonate  sodium  chlorine  calcium  Proteins are the most abundant substance in the cell after water. Normally they make up 10-20% of the cell mass. Cell proteins Structural proteins It is divided into globular proteins that function as enzymes.  Lipids encompass several different types of substances whose common properties are to dissolve in fat solvents. The most important lipids in most cells are phospholipids and cholesterol and they constitute 2% of the cell mass. Apart from phospholipids and cholesterol, there are also triglycerides in the cell called neutral fat.  Apart from being part of glycoproteins, carbohydrates are not of much structural importance. However, they play an important role in the nutrition of the cell as they provide energy in an easy and anaerobic environment.  Membraneous Structures in the Cell Plasmalemma Nucleus Endoplasmic reticulum Mitochondria Lysosome Peroxisome Golgi complex  🟌 Endoplasmic reticulum; 🟌 They are found in all eukaryotic cells except Bacteria, mature white blood cells and platelets. 🟌 The endoplasmic reticulum is a closed system with no opening to the cell surface, and inside the cell the nucleus outer membrane continues as ER. 🟌 The endoplasmic reticulum (ER) is a type of organelle made up of two subunits – rough endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). 🟌 The general structure of the endoplasmic reticulum is a network of membranes called cisternae. 🟌 These sac-like structures are held together by the cytoskeleton. 🟌 The phospholipid membrane encloses the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol. The functions of RER 1) Participating in protein synthesis, 2) Performing protein glycosylation (adding sugar), 3) To make quality control of proteins, 4)Selectively sending some of the synthesized proteins (transmembrane proteins) to these organelles to form the structure of Golgi, lysosome, peroxisome, plasmalemma and other intracellular membranes. The functions of SER 1) Takes part in steroid hormone synthesis. (In adrenal cortex, testis, ovary and corpus luteum), 2)To take part in the production of glycogen in hepatocytes (glycogenesis) and degradation of glucose (glycogenolysis), 3) To function in the production of cholesterol and bile in the liver, 4)To perform detoxification with enzymes synthesized from RER (sit P450, G-6 phosphate dehydrogenase, G-6 phosphatase, hydroxylase, glucuronyl transferase etc.), 5) To remove Cl ions in the cells of the stomach that secrete HCl, 6)To combine monoglycerides with proteins to form lipoproteins (in small intestinal epithelial cells), lipoproteins first pass to the Golgi complex, form chylomicrons, and then are excreted from the cell by exocytosis. 7) Storing Ca ++ in striated muscle and cardiac muscle cells (sarcoplasmic reticulum).  Mitochondria It is one of the double-membrane organelles. Its number is variable depending on the cell's function. It contains unique DNA and ribosome in its structure. Functions  It acts as the energy (ATP) production center of the cell.  It stores the excess calcium in the cytosol in the cell and returns it to the cell when necessary.  Steroid plays an important role in hormone synthesis.  It is the organelles that decide cell survival or death by initiating apoptosis, which is a programmed cell death.  Ribosoms It is found individually or in groups on the surface of the cytosol-facing membrane of the RER, on the surface of the outer nucleus and in the cytoplasm. It is made up of protein and ribonucleic acid (RNA) molecules. The rRNAs synthesized in the nucleus come together in the nucleus with ribosomal proteins synthesized in the cytoplasm. Both bound and free ribosomes main function is to carry out protein synthesis. Golgi Apparatus It is not found in prokaryotes and red blood cells. Golgi exchanges the products synthesized in the cell and ensures that they are transported to their destination by packaging. It consists of three different parts; Flat sacs, secretory granules and vacuoles. All of these structures together are called dictyosomes. Functions of Golgi complex 1) To make secretion (Zimogen granules), 2) Synthesis of glycoproteins in the structure of plasmalemma and regulating the function of the cell membrane, 3)Glycosylation, sulphation, phosphorylation of secretory substances, limited proteolysis of proteins, concentration and packaging of the secretion, 4) Formation of lysosomes in the RER region, 5) Preservation of membrane integrity by the addition of vesicles to the cell membrane by exocytosis, 6) Golgi forms the acrosome in developing mammalian spermatids. Lysosomes  They are defense and digestive organelle.  It contains degrading enzymes called hydrolase (digesters) produced in the ER and Golgi complex.  Large molecule nutrients (glycogen etc.)  Bacteria, virus (phagocytic vacuole) toxins  Digests degenerated cell organelles (autophagic vacuole)  They release calsium in bone. Thank you