Introduction to Cell Membrane Biology PDF

introduction By dr \demiana Physiology “The study of the functions of living things”. Human physiology: can be defined as: “The study of the functions of organs and systems and the way these functions are integrated.” The human body is made up of (75 - 100 trillion cells). Collectio...

introduction By dr \demiana Physiology “The study of the functions of living things”. Human physiology: can be defined as: “The study of the functions of organs and systems and the way these functions are integrated.” The human body is made up of (75 - 100 trillion cells). Collection of cells with similar properties forms tissues Different tissues combine to form organs Organs of complementary functions constitute the different systems.  Body compartments The body of normal adult male is composed of 18% proteins - 15% fat - 7% minerals - 60% water (Total Body Water) (TBW). N.B.: TBW: is 75% in infants and less than 60% in adult female and obese people due to high content of fat. TBW is present in 2 compartments: 1. The intracellular fluid compartment (I.C.F.): which a)Constitutes (2/3 TBW). b)Presents inside the cells. c) Equals 40% of Total Body Weight. d)It contains More K+ and less Na+, CL- and HCO3-. 2. The extra cellular fluid compartment (E.C.F.): which a)Constitutes (1/3 TBW). b)Presents outside the cells. c) Equals 20% of Total Body Weight. d)It contains less K+ and More Na+, CL- and HCO3-. The extra cellular fluid is distributed as follows: a. 5% inside the blood vessels called intravascular fluid (I.V.F.) which is the plasma. b.15% outside the blood vessels called the interstitial fluid (I.S.F.)  Homeostasis Definition: Keeping the conditions in the internal environment constant. e.g., O2, CO2, glucose, different ions, amino acids and fatty acids concentrations in the internal environment. Most of the systems in the body work either directly or indirectly to maintain homeostasis. The cell The cell is the structural unit of various tissues and organs in the human body. It consists of a mass of protoplasm surrounded by the cell membrane. The protoplasm comprises: 1)The cytoplasm 2)The cell organelles 3)The nucleus The cell membrane The thickness of the cell membrane is about 7.5nm (10-9 meter). It is made up of: Proteins 55% Lipids 29% (phospholipids & glycolipids) Cholesterol 13% Carbohydrates 3% Under electron microscope it is formed of lipid bilayer with globular proteins interspersed in the thin lipid film. Cell membrane  Structure of the cell membrane The shape of phospholipids molecule is that of a clothespin. It is formed of a polar head and two non-polar tails. A. The head: ★ Is formed of phosphate charged group. ★ It is hydrophilic (relatively soluble in water). ★ It is polarized. A. The tail part: Is formed of 2 fatty acid chains. It is hydrophobic (relatively insoluble in water). It is nonpolar. The phospholipids The phospholipids molecule is amphipathic: that is; it has one part hydrophilic and the other hydrophobic. In the membrane, the hydrophilic ends of the molecule are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm. While hydrophobic ends meet in the water poor interior of the membrane. The lipid bilayer is important for the flexibility and selective permeability of the cell membrane. Structure and function of the cell membrane proteins The cell membrane proteins are present as globular masses floating in the lipid bilayer. They have 2 portions: the hydrophobic uncharged one is present in the interior of the membrane, and the hydrophilic charged part is directed to the surface. There are 2 types of cell membrane proteins: a. Integral proteins: passes all the way of the membrane. b. Peripheral proteins: attached only to one surface of the membrane and do not penetrate it. The cell membrane proteins form complexes with glycolipids that are important as recognition sites and act as antigens for cell self-recognition immune system and differentiation. N.B.: The cholesterol molecules are completely hydrophobic and embedded in the phospholipid bilayer. They affect the permeability of the membrane and give toughness to it.  Functions of cell membrane proteins: 1) Structural proteins: They keep the integrity of the membrane and give it strength 1) Form passive channels: They form protein channels with various diameters, various shapes and electric charges along their surfaces. They are classified according to presence or absence of gates into: i. Gated channels closed during rest by a part of the protein molecule. ii. Non-gated channels open all the time. These gated channels are either: - a) Voltage gated: They open or close when the membrane potential is changed. b) Ligand gated: They open or close when a ligand binds a specific receptor on the cell membrane. Functions of cell membrane proteins: 1) Carriers in facilitated diffusion: For large molecules passively e.g. facilitated diffusion of glucose. They bind the molecule and then change their configuration, moving the bound molecule from one side of the cell membrane to the other 1) Carriers in active transport (= Pumps): This form of transport requires energy as it transports substances against their electrical and chemical gradients. They are of 3 types a) Uniport: this is a carrier that transports one substance in one direction e.g. Ca++ b) Symport: this is a carrier that transports two substances simultaneously in the same direction, e.g. carriers of glucose and Na+ from the intestinal lumen to inside of the cell. c) Antiport: this is a carrier that transports one substance in one direction and another substance in the opposite direction e.g., Na+ - K+ pump. N.B.: Symport and Antiport are called Cotransport carriers. 1)Receptors: These are proteins or glycoproteins present mainly on outer cell membrane. They are inactive during rest, but when they combine with their specific ligand (neurotransmitter, drug or hormone) they become activate and start a series of cellular reactions to stimulate or inhibit a certain cellular function.  The number and sensitivity of the receptors may change: a)Down regulation: When a ligand is present in excess, the number of the receptors for this ligand decreases and the sensitivity to the ligand also decreases. a)Up regulation: When a ligand is deficient, the number of receptors for this ligand increases. 1) Enzymes: They are present mainly on the inner surface of cell membrane to catalyze certain reactions. 1) Identity proteins: They are mostly glycoproteins which give the cells the individual label of identity, so that they would not attacked by the immune system. Intercellular connections: Cells are connected by 2 types of junctions: A-Binding junctions: ★which bind cells together to make strong tissues. These include:Tight junctions: ridges project from certain cells and adhere strongly to similar ridges from neighboring cells. The intercellular space is obliterated. ★ Desmosomes: there is thickening of the membranes on two opposite cells. They are connected with fibrils bridging the intercellular space. a)Channel junctions (=Gap junctions): i. These are channels running across the intercellular space connecting one cell to another. ii. They allow the rapid passage of ions and other substances with molecular weight up to 1000 between cells without entering the intercellular space. iii. This permits the rapid propagation of electrical activity from one cell membrane to another. iv. At these junctions, hexagonal arrangement of protein units to form what is called "connexon" surrounding a channel that will be in line with the channel in corresponding connexon in the adjacent cell. v. Intracellular Ca++, PH, hormones and drugs regulate the diameter of the channel. Functions of cell membrane proteins 1) Cell adhesion molecules: They are formed of proteins responsible for adhesions of the cells to basal lamina and to each other. They are important in embryonic development of nervous system, in inflammation, wound healing and in the metastasis of tumors. 1) Fixation of parts of the cytoskeleton of the cell: The cytoskeleton is a system of fibers that maintains the structure of the cell and permits it to change shape and move. It is made up of microtubules, intermediate filaments and microfilaments. The three types of cellular transport  Transport through the cell membrane Mechanisms that control the transport through the cell membranes are so important to maintain the differences between ECF & ICF which are extremely important for life of the cell. I. Diffusion: Continual movement of molecules in liquids or in gases from regions of higher concentration to regions of lower concentration. Types of diffusion are: a) Simple diffusion: The molecules or ions will diffuse through the membrane, according to the concentration gradient, passively without binding with carrier protein in the membrane. Diffusion: It occurs through 2 pathways. 1. The lipid bilayer (can pass lipid soluble substances as O2 & N2 and Water & lipid insoluble substances as urea). 2. Gating of protein channels. The opening and closure of the gates is controlled by: a) Voltage gating → open by change in electric membrane potential. b) Ligand gating → open by binding to a specific ligand. a) Facilitated diffusion i. This is a passive mechanism but needs a carrier that has a receptor for the substance to be transported. i. The substance then is transported through the channel of the carrier molecule, part of the way but not all the way. i. This is followed by conformational changes in the carrier protein to allow passage of the substance to the opposite side. a) Osmosis Definition: ◎ Osmosis is diffusion of water molecules down their concentration gradient. The pressure necessary to prevent solvent migration is the osmotic pressure of the solution. ◎ The osmotic pressure is determined by the numbers of particles / unit volume of fluid but not the mass of the particles. It is measured in mmHg. Osmosis ◎ The Osmole is the number of particles (molecules) in one-gram molecular weight (Mole) of undissociated solute. e.g., one mole of glucose = 180 gm. contains or equals one osmole. On the other hand, if the solute dissociates into 2 ions e.g., NaCl. ◎ One mole of NaCl = 58.5 gm. = 2 osmoles Osmolality = number of osmoles dissolved in one kg of water. Osmolarity = number of osmoles in one liter of water. Because one kg of water = one liter of water so osmolality is nearly equal to osmolarity (difference