Transport through plasma membrane

1. Transport through plasma membrane Physiology -I PHL 215

2. 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 somesubstance can pass while others can not. Plasma membrane is selectively permeable

3. Mechanisms of transport through plasma membrane 3 basic mechanisms: I) Passive Transport: passive meansno 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

4. I) Passive transport • Passive transport is the movement of molecules through a cell membrane without expending any energy. • 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.

5. 1: Simple Diffusion • 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: • A cell that suddenly stopped producing energy could still carry out diffusion. • In the process of simple diffusion, No pores or openings are needed.

6. What does the term Equilibriummean? 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. • What does the term Concentration gradientmean? 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.

7. 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

8. 2: Facilitated diffusion • Facilitated diffusion is a type of passive transport. • Itis 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.

9. 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.

10. 3: Osmosis 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: 1- Water moves from area of high water concentration to area of low water concentration. OR 2- Water moves from area of low solute concentration to area of high solute concentration.

11. The pressure caused by the movement of water through the membrane is called osmotic pressure. • A solution of high osmotic pressure is the solution that contain excess solute and less water. SO, • Water moves from area of low osmotic pressure to area of high osmotic pressure. • 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.

12. 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.

13. 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).

14. 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.

15. II) Active transport • It is thepumping 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.

16. 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. • Which energy molecule is needed to activate the sodium-potassium pump so it will work? • ATP • How many Na+ions must be pumped out of the cell during each cycle of the sodium-potassium pump? • 3 Na+ions • How many K+ ions must be pumped into the cell during each cycle of the sodium-potassium pump? • 2 K+ ions

17. 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: Engulfing solid particles b) Pinocytosis: Engulfing fluid droplets