Cell Physiology PDF

General physiology Dr. Shaimaa M. Abdelghany Assistant Prof. of Physiology  Structure of the cell Cells are the smallest structural units in the body 1) A mass of protoplasm: which consists of. a) Cytoplasm. b) Cell organelles. c) Nucleus. 2) Cell membrane. The Ce...

General physiology Dr. Shaimaa M. Abdelghany Assistant Prof. of Physiology  Structure of the cell Cells are the smallest structural units in the body 1) A mass of protoplasm: which consists of. a) Cytoplasm. b) Cell organelles. c) Nucleus. 2) Cell membrane. The Cell Membrane The cell membrane, also known as the plasma membrane, is a double layer of lipids and proteins that surrounds a cell and separates the cytoplasm from its surrounding environment. Function: 1) Protects the cell from its surroundings and gives it its shape. 2) Controls the movement of substances in and out of cells and organelles. it is selectively permeable to ions and organic molecules. 3) It contains receptors for chemical substances that stimulate or inhibit the cell functions.  Structure: The cell membrane is a lipid bilayer with embedded proteins and small amount of carbohydrates: Lipid 41% Proteins 55% Carbohydrate 4% This Photo by Unknown Author is licensed under CC BY-SA-NC Lipids: The lipids in the cell membrane are polar, mostly a mixture of phospholipids and cholesterol. Phospholipids are arranged in bilayer with the hydrophilic head of the phospholipid molecules arranged on the inner and outer surfaces of the cell membrane and the hydrophobic non- polar tail inside. Cholesterol is responsible for toughness of the cell membrane and the solidity of cell membrane is directly proportional with cholesterol/phospholipids ratio. Proteins: Proteins are embedded in the phospholipid bilayer. some are combined with lipids (lipoproteins) and some are combined with carbohydrates (glycoproteins). Types: Peripheral proteins Transmembrane or integral proteins Peripheral proteins: oThese are only attached to the surfaces of cell membrane (both from outside and inside) but they don’t penetrate. oActs mainly as enzymes. oBinds to substances to be transported. e.g. hormone receptors Transmembrane or integral proteins: ooccupy the whole thickness of the membrane and appear on both outer and inner surfaces. oProvide structural channels (or pores). o Act as carrier proteins. Functions of membrane proteins : Membrane receptors: relay signals between the cell and the external environment. Transport proteins: for transport of water and ions either through active (pumps) or passive transport (passive channels ). Membrane enzymes: catalyze reactions on inner and outer surface of the membrane. Adhesion molecules: that give the cell its label of identity. So that the cell can identify each other and interact , they are glycoproteins or glycolpids , e.g., proteins involved in the immune response. Structural proteins: that give the membrane its integrity Carbohydrates: Small in amount, less than 10% of the cell membrane mass. They are either combined with lipids (glycolipids) or with proteins (gycoproteins). Glycolipids and glycoproteins are present on the outer surface of the cell membrane Functions: Attach cells to each other. Act as a receptor substance. Some enter in the immune reaction. Give most of the cells -ve surface charge which affects interaction of regulatory molecules with the membrane Transport of substances across cell membrane TRANSPORT ACROSS CELL MEMBRANE The cell membrane is described as partially permeable or semi- permeable. It mostly consists of phospholipids which are composed of hydrophobic (water hating) tail and hydrophilic (water loving)heads. This tail prevents water soluble, polar ions and charged molecules to pass in and out of the cell. Those ions and molecules restricted by the phospholipids pass in and out of the cell via membrane proteins.  There are concentration differences between substances in the extracellular fluid and intracellular fluid. Transport of Note that the extracellular fluid contains a large amount of sodium but only a small amount of potassium. Exactly the opposite is true of the intracellular fluid. substances Also, the extracellular fluid contains a large amount of chloride ions, across cell whereas the intracellular fluid contains very little. membrane But the concentrations of phosphates and proteins in the intracellular fluid are considerably greater than those in the extracellular fluid. These differences in concentration are brought about by the transport mechanisms of the cell membranes.  Methods of Transport Passive Transport: Active Transport Endocytosis and Exocytosis  I. Passive Transport:  transport through the lipid bilayer or through the transmembrane proteins which create a water filled pores through which ions can pass by diffusion down a concentration gradient. It requires NO energy. Types of passive transport: Simple diffusion Facilitated diffusion osmosis Simple Diffusion: It is the net movement of substances from an area of higher concentration to an area of lower concentration. It requires NO energy. Factors which affect the rate of diffusion: Fick’s laws describe the factors affecting the rate of diffusion. This can be written as follows: Rate of diffusion = (surface area x concentration gradient)/(length of diffusion pathway) The greater the concentration difference (∆C), greater is the rate of diffusion. The greater the surface area (A) of the membrane in which diffusion takes place, the greater is the rate of diffusion. The greater the temperature (T), greater the molecular motion and greater the diffusion. The shorter the distance (d), the greater the rate of diffusion. It equals the thickness of the cell membrane. The less the square root of molecular weight ( MW), the greater is the rate of diffusion. Routes of diffusion across the cell membrane: By passing through the lipid bilayer: This route allows the passage of only small molecules and fat soluble substances. By passing through the protein channels: This route allows the passage of large molecules and water soluble substances. Types of protein channels: Non-gated channels (leak channels): Open all the time. Have smaller diameter and lesser number than other channels Diffusion through them is slow. Gated Channels Closed at rest by a part of protein molecule (a GATE). Rapid diffusion occurs upon opening of the gate. Mechanism of opening of the gate ??? Mechanisms of opening of the gate: @Voltage gated channels: are voltage dependent channels which open only when membrane is depolarized. @Ligand gated channels: a substance which binds to and activates a receptor on the cell membrane→ change in the configuration of the channel protein→ opening of the channel. @Stretch activated channels : open when the wall is stretched e.g. blood vessels. Facilitated Diffusion: It is the movement of molecules across the cell membrane down its concentration and electric gradient. It requires no energy It is concerned for transport of substances that have larger molecular diameter and insoluble in lipid, e.g., glucose and amino acids. It requires carrier protein. Characteristics of the carrier protein: Specificity: the carrier is specific to the substance bound to it. Saturation: increased concentration of the substance to be transported beyond a certain extent, no more carriers will be available for the transport process. Competition: similar substances compete for the same carrier and the transport of the substance depends on its relative concentration to the other substances and affinity of the carrier.  Simple diffusion Facilitated diffusion Mechanism Molecules move passively across Movement occurs passively down a the membrane without the aid concentration gradient; however, of a carrier protein the molecules require the help of carrier proteins to allow them to cross the lipid bilayer Examples of molecules  Hydrophobic molecules such  Charged molecules (ions) using this process as O2 and CO2  Large uncharged  Small uncharged polar molecules such as glucose polar molecules such as urea Osmotic pressure: The pressure needed to prevent movement of the solvent (such as water) across the semipermeable membrane caused by different osmolarity. The osmotic pressure of a solution depends on number of particles in the solution rather than the type of particles.  Osmosis: Movement of a solvent (such as water) through a semipermeable membrane (as the cell membrane) into a solution of higher solute concentration that tends to equalizes the concentration of solutes on the two sides of the membrane. Thank you