Transport through plasma membrane Physiology -I PHL 215 PHL 215 Dr/Gamal Gabr Pharmacy College Pharmacy College 1

Membranes Membranes may be: 1- Impermeable membrane: Membrane through which nothing can pass. 2- Freely permeable membrane: Membrane through which any substance can pass. 3- Selectively permeable membrane: Membrane through which some substance can pass while others can not. Plasma membrane is selectively permeable Plasma membrane is selectively permeable 2

Mechanisms of transport through plasma membrane 3 basic mechanisms: I) Passive Transport: passive means no ATP energy is required II) Active Transport: ATP energy is required III) Vesicular Transport: NOTE:  There are 2 types of transport through cells: one requires energy and the other does not 3

I) Passive transport o Passive transport is the movement of molecules through a cell membrane without expending any energy. passive transport o In passive transport 1- Energy is not needed to move molecules across the membrane. 2- Molecules move from high to low concentration. Types: 1: Simple Diffusion. 2: Facilitated Diffusion. 3: Osmosis. 4

1: Simple Diffusion directly according to the concentration gradient  It is the passage of molecules directly through the lipid layer of plasma membrane (between phospholipid molecules) according to the concentration gradient  i.e: from high to low concentration.  Molecules diffuse from high to low concentration, so NO ATP energy is needed for this process to occur.  Diffusion continues until the concentration of the molecules on both sides of the membrane is equal (equilibrium state). What two elements are required for diffusion?  Concentration gradient and membrane permeability NOTES: o A cell that suddenly stopped producing energy could still carry out diffusion. o In the process of simple diffusion, No pores or openings are needed. 5

o What does the term Equilibrium mean? 1- The concentration of molecules is equal on both sides of a membrane. 2- The rate of movement of molecules across a membrane is equal in both directions. o What does the term Concentration gradient mean? It is the difference in the concentration of molecules on both sides of the membrane. Factors affecting the rate of diffusion: 1) Concentration gradient:  Increased concentration gradient increases the rate of diffusion 2) Molecular size:  Molecules of small size diffuse more easily through lipid layer of the membrane. 3) Lipid solubility:  Lipid soluble molecules (Non-polar) diffuse more easily than water soluble molecules (polar)  because Non-polar molecules are easily soluble in the lipid layer of the membrane. 6

Molecules that transport through plasma membranes by simple diffusion: 1) Hydrophobic (Non-polar) molecules as O2, CO2, Nitrogen and steroids  diffuses very quickly. 2) Polar molecules of very small size as H2O, glycerol, urea, ethanol  diffuses freely, but more slowly than non-polar molecules. NOTES:  Water molecules are polar, but are small enough to pass through cell membranes.  Because O2 and CO2 are soluble in lipids, they can diffuse directly through the phospholipid bilayer of the cell membrane.  Because the size and polarity of Glucose and sucrose, they cannot diffuse directly through the lipid bilayer of the cell membrane 7

2: Facilitated diffusion  Facilitated diffusion is a type of passive transport.  It is the passage of large molecules or ions through the plasma membrane with the aid of integral proteins (Channel proteins), according to the concentration gradient.  Because the process occurs from high to low concentration, NO energy is required. 8

Molecules that transport through plasma membrane by facilitated diffusion: Glucose molecules diffuse through the cell membrane by simple diffusion very slowly because glucose is not easily soluble in the phospholipid bilayer. However, glucose diffuses very quickly across a cell membrane by facilitated diffusion because the Channel proteins help the glucose molecule to cross into the cell. Ions such as Na + or Cl -. Water. 9

3: Osmosis aquaporins It is the movement of water through the plasma membrane from high to low concentration with the aid of specific type of channel proteins called aquaporins.  Osmosis continue until the concentration of water on both sides of the membrane are equal.  Osmosis is a completely passive process, so ATP energy is not required. NOTES: high water low water 1- Water moves from area of high water concentration to area of low water concentration. OR low solute high solute 2- Water moves from area of low solute concentration to area of high solute concentration. 10

o The pressure caused by the movement of water through the membrane is called osmotic pressure. o A solution of high osmotic pressure is the solution that contain excess solute and less water. SO, o Water moves from area of low osmotic pressure to area of high osmotic pressure. o Water flows until the osmotic pressures on both sides of the membrane are equal NOTE:  The greater the concentration of solutes in a solution, the greater the osmotic pressure of that solution. 11

Tonicity It is the ability of a solution to affect the volume of fluid and pressure inside the cell  cause the cell to shrink or swell  According to tonicity, there are 3 types of solution: 1- Hypertonic Solution (= More solute concentration) (+ Low water concentration) 2- Hypotonic Solution (= Less solute concentration) (+ High water concentration) 3- Isotonic Solution.  Tonicity and direction of water flow Water moves from area of hypotonic solution to the area of hypertonic solution. 12

1- Hypertonic Solution: It is the solution that has higher concentration of solutes than that of the cytosol.  In this case, concentration of solutes in the ECF is higher than in the ICF. ► So, water moves from inside the cell to the outside. ► The cell lose water & shrinks (crenation). 2- Hypotonic Solution: It is the solution that has lower concentration of solutes than that of the cytosol.  In this case, concentration of solutes in the ECF is lower than in the ICF. ► So, water moves from outside the cell to the inside. ► The cell absorb water, swells and may damaged, so release its contents (lysis). 13

3- Isotonic Solution: It is the solution with the same solute concentration as that of the cytosol.  In this case, concentration of solutes in the ECF and ICF are equal. ► So, no net water movement ► No change in cell volume or shape. NOTES:  Solution of NaCL 0.9% and glucose 5% are isotonic solutions, so they are important in clinical medicine because the cell neither swell nor shrink.  NaCL solution (0.9%) is called Normal saline. 14

II) Active transport pumping carrier proteins ()  It is the pumping of specific molecules through the plasma membrane against the concentration gradient, with the aid of carrier proteins (pumps).  i.e: from low to high concentration, therefore Energy is needed to move molecules across the membrane  The carrier proteins obtain the energy from ATP to pump the molecules.  Active transport stops if cellular respiration stops, since there is no energy. 15

Examples: 1- Na + - K + Pump:  3 Na + moves out of the cell and 2 K + moves in.  Both ions move from low to high concentration.  This maintain the concentration of sodium and potassium ions on either side of the nerve cell membrane at a certain level. 2- Reabsorption of valuable molecules from the urine such as glucose, amino acids and sodium ions. o Which energy molecule is needed to activate the sodium-potassium pump so it will work? ATP o How many Na + ions must be pumped out of the cell during each cycle of the sodium-potassium pump? 3 Na + ions o How many K+ ions must be pumped into the cell during each cycle of the sodium-potassium pump? 2 K + ions 16

III) Vesicular Transport It is the passage of large molecules (macromolecules) such as proteins or fluid droplets through the plasma membrane in vesicles. Types: 1: Exocytosis: Transport out of cell 2: Endocytosis: Transport into cell a) Phagocytosis: a) Phagocytosis: Engulfing solid particles b) Pinocytosis: b) Pinocytosis: Engulfing fluid droplets 17