Molecular Structure of the Cell Membrane

Faculty of Medicine Histology Department Principles of microscopic and macroscopic structures Lecture (2) The cell membrane Plasma membrane or Plasma lemma Prepared by: Prof. Dr. HEKMAT OSMAN ABDEL AZIZ The Plasma membrane Prepared by: Prof. Dr. HEKMAT OSMAN ABDEL AZIZ Objectives: By the end of this unit, the student should be able to:  under the light and electron microscope.  Correlate the molecular structure of the PM with the EM appearance.  Introduction Fig 1; Diagram of the cell components The cell (Fig.1) The cell is the structural and functional unit of the body. The cells are bound together to form tissues. The tissues are combined to form organs. Several organs having interrelated functions constitute the different systems of the body as: digestive system, urinary system 2 Size of Cells: They are variable in size. Some cells are very small as certain cells of the cerebellum, while others are very large as the muscle and ovarian cells. Shape of Cells: The different body cells vary in shape. Some cells are rounded, others are oval, flat, cubical or columnar in shape. Structure of Cells: Each cell is composed of two main parts:1. The Cytoplasm 2. The nucleus The Cytoplasm The cytoplasm is a complex structure. It is formed of three main components:1. Cytoplasmic Organelles (Cell Organoids): They Are Of Two Types : a. Membranous Cytoplasmic Organelles :  They are enclosed in membranes.  They perform essential metabolic functions for each cell.  Examples: The Cell membrane or plasma membrane, the Mitochondria, The Golgi apparatus,….. etc b. Non-membranous Cytoplasmic Organelles.  They are not enclosed in membranes.  Examples: Ribosomes and centrioles…. Cytoplasmic Matrix:  It is a colloidal solution of proteins, lipids, carbohydrates, minerals, enzymes, small molecules and ions. 3. Cytoplasmic Inclusions (Cell Inclusions):  They are temporary components of certain cells.  They are usually an accumulation of o stored food as : glycogen and fat o or an accumulation of pigments as : carbon and melanin. 2. The Membranous Cell Organelles 1. The Cell Membrane Definition:  It is a covering membrane which forms the outermost cover of the cytoplasm.  It is also named Plasmalemma or Plasma Membrane.  It regulates the passage of materials in and out of the cell. Thickness : Its thickness ranges 7 nm to 10 nm By The Light Microscope (L/M): It cannot be distinguished (not visible) because it is very thin.  its thickness is 7 – 10 nm which is below the Lr of the L.M (240 nm)  Its appearance by L.M. is due to its oblique cut of 2 contiguous cell membranes or due to condensation of the stain on the cell coat. By The Electron Microscope (E.M.) (Fig 2):; It appears as if it is formed of three layers, therefore it is called trilaminar membrane.  Its outer and inner layers appear as dark lines,  While its middle layer appears as light area. Fig.(2):The trilaminar appearance of the cell membrane by E.M. Molecular Structure Of The Cell Membrane(Fig 3): It is formed of lipids, proteins and carbohydrates. 60% proteins, 35% lipids and 5% carbohydrates  The Lipid, Protein and Carbohydrate Molecules Are Present In the Following Arrangement In The Cell Membrane:- Fig. (3): The molecular structure of the cell membrane 1. The Lipid Molecules They are formed of : Phospholipid and Cholesterol Molecules. 1. Phospholipid molecules (Fig. 4): is formed of Two Parts : Fig 4 Phospholipid molecule Hydrophilic heads Hydrophobic tails Hydrophilic Lipid Part. Hydrophobic Lipid Part. The Hydrophilic Lipid of The Phospholipid Molecule. It forms the heads of the lipid molecules. It is called hydrophilic because it has a great affinity for aqueous solutions (it likes water). It is composed of charged lipid and is called Polar Region. It is directed towards the surfaces of the cell membrane (intracellular and extracellular surfaces). The Hydrophobic Lipid Part of The Phospholipid Molecule. It forms the tail of the lipid molecule. It is called hydrophobic because it has no affinity for aqueous solutions (it dislikes water). a) b) a) — — — — b) — — — It is formed of non-charged lipid and is called Non Polar Region. — The hydrophobic ends of the lipid molecules are directed inwards, they face each other in the central part of the cell membrane. 2. The Cholesterol Molecules: They are mainly present in the cytoplasmic aspect of the cell membrane and have a stabilizing effect, helping to keep the phospholipid molecules packed together and thus increase the integration of the plasma membrane. 2. The Protein Molecules (Fig.5) The protein molecules are present in the cell membrane in these forms: Fig.5 plasma membrane a- Extrinsic or Peripheral Protein:— It is formed of protein molecules which are loosely bounded to both surfaces of the cell membrane. b. Intrinsic or Integral Protein: — The intrinsic protein molecules are present in the cell membrane in two forms: 1. Small intrinsic protein molecules which are present as small collections of protein molecules distributed among the lipid molecules. 2. Large intrinsic protein molecules. They are present as large globular masses of protein molecules. They may extend through its entire thickness known as transmembrane proteins. 3. The Carbohydrate Molecules  These are in the form of oligosaccharides attached to the lipids (glycolipids) or the proteins (glycoproteins).  They project to the outer surface of the cell to form the cell coat or the glycocalyx. Notes:  Fluid mosaic nature of the PM: At body temperature, the lipid bilayer is fluid allowing free movement of the integral proteins within the membrane.  Unit membrane theory: By E.M. all cellular membranes have the same trilaminar structure. Functions of the Cell Membrane The Main Function of The Cell Membrane Is To Control The Exchange of materials Between the Cell And Its Surroundings; This occurs By Several Ways:1. Passive Diffusion of Substances:The cell membrane can allow substances to enter or leave the cell depending on the relative concentrations inside and outside. Examples; dissolved gases, essential ions and oxygen to pass into the cell. It also allows carbon dioxide and other metabolic wastes to leave the cell. 2. Active Transport of Materials Large molecules of sugars, amino acids and fatty acids in order to pass through the cell membrane, they should be combined with some catalysts. This combination requires energy which is provided by the enzymatic reactions of the mitochondria. 3. Selective Transport of Materials Transport by the use of receptors. These receptors determine which materials to enter the cell and which substances to be shut out. Examples; transport of drugs, hormones, bacteria and viruses which act on their target cells. 4. Transport of Solid Materials By (Phagocytosis) The cell membrane can engulf or allow solid materials to enter into the cytoplasm by the process of phagocytosis. This solid particle after its entrance into the cytoplasm is called phagosome. 5. Transport Of Fluid Material By (Pinocytosis). By this process, the cell membrane can engulf droplets of fluid. The engulfed fluid becomes surrounded by a membrane to be changed into a Pinocytic Vesicle. 6. Expulsion Of Residual Bodies By (Exocytosis) : After the entrance of the solid or the liquid materials into the cytoplasm, they will be digested by the lysosomal enzymes. After digestion, the cell can extrude these bodies outside 4 the cell by the process of exocytosis. 7. Sodium-Potassium Pump Function : The cell membrane is continuously pumping sodium ions to the outside of the cell. As a result of this process the concentration of sodium is higher in the tissue fluid than in the cytoplasm. The cell membrane can also pick up potassium ions to the inside of the cell to make the concentration of potassium ions more in the cytoplasm than in the tissue fluid. Functions Of Cell Coat Or Glycocalyx: 8. Cell Adhesion: The cell coat help in holding adjacent cells together. 9. Cell Recognition: The cell coat enable cells to recognize cells of their own special kind. This is of importance in transplantation of kidney, heart and skin grafts. 10. Formation Of Cell Immunity : The cell coat plays an important role in the develpment of cell immunity against infections.. 11. The Cell Membrane may be modified to form the following structures which form specific functions in certain cells as : a) Micro-Villi b) Cilia c) Flagella