Vesicular Transport, Osmosis and Filtration Lecture 4 PDF

## Vesicular Transport, Osmosis and Filtration ### Biophysics - Physiology Lecture 4 **Professor Sahar El Agaty | Professor of Physiology** **Galala University** **Powered By:** Arizona State University **gu.edu.eg** ### Intended Learning Outcomes - Identify vesicular transport across the cell...

## Vesicular Transport, Osmosis and Filtration ### Biophysics - Physiology Lecture 4 **Professor Sahar El Agaty | Professor of Physiology** **Galala University** **Powered By:** Arizona State University **gu.edu.eg** ### Intended Learning Outcomes - Identify vesicular transport across the cell membrane. - Recognize osmosis, osmotic pressure, osmolarity, osmolality and osmole. - Discuss the Gibbs Donnan effect and its influence on the distribution of ions. ### Transport Across Cell Membrane #### 2. Active Transport ##### Vesicular Transport Vesicular transport is the process by which large water-soluble molecules that cannot be transported by channels or carriers move between the ECF and the ICF. ##### 1. Exocytosis - Vesicles containing material for export, such as secretory granules synthesized in Golgi apparatus containing hormone or neurotransmitter, move to the cell membrane, where they fuse with cell membrane. - The area of fusion then breaks down, leaving the contents of the vesicle outside and the cell membrane intact. ##### 2. Endocytosis - Endocytosis is the reverse of exocytosis. **Types:** - These include phagocytosis and pinocytosis. **a) Phagocytosis (“cell eating”)** - It is the process by which bacteria, or dead cells, or other bits of microscopic material are engulfed by cells such as the white blood cells. - The material makes contact with the cell membrane, which then invaginates. The invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact. **b) Pinocytosis (“cell drinking”)** - Is a similar process with the vesicles much smaller in size and the substances ingested are in solution. **N.B.** - Both exocytosis and endocytosis need $Ca^{2+}$ and energy. - Exocytosis adds to the total amount of membrane surrounding the cell, and removal of cell membrane occurs by endocytosis, and such exocytosis-endocytosis coupling maintains the surface area of the cell at its normal size. ### Osmosis The membrane is impermeable to the solute and permeable to water. #### The Osmotic Pressure The osmotic pressure is the pressure, required to stop the osmosis. #### Important Terms Used to Define and Measure Concentration of Solutes **Osmosis** is the passive movement of water through a semipermeable membrane from an area of high-water concentration to an area of low-water concentration. - In other words, water moves from an area of low solute concentration to an area of high solute concentration through a membrane which is impermeable to the solute. - Osmosis is the net diffusion of water down its own concentration gradient. - The solute particles draw the water towards them. **Osmotic pressure (O.P.)** is the pressure applied on the concentrated solution to stop osmosis (movement of water). The O.P. of a solution is indirect measure of the osmotic concentration of the solution. O.P. is a pulling pressure. **The osmotic concentration (Osmolarity and osmolarity)** of a solution is determined by the number of osmotically active particles (molecules or ions) in the solution per unit volume. A solute that dissociates in a solution into ions (NaCl, CaCl₂) is more osmotically active than non-ionizing solute (Glucose). **Osmole** is a unit used to express the osmotic concentration of a solution depending on the numbers of particles. One osmole is 1-gram molecular weight of osmotically active solute. - 1 milliosmole = 1/1000 osmole - Thus, 180 grams of glucose (which is 1 gram molecular weight of glucose) is equal to 1 osmole of glucose because glucose does not dissociate into ions. - If a solute dissociates into two ions, 1 gram molecular weight of the solute will become 2 osmoles because the number of osmotically active particles is now twice as great as for the non-dissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride, 58.5 grams, is equal to 2 osmoles. **Osmolality** of a solution is the number of osmole of solute per kilogram of solvent. **Osmolarity** of a solution is the number of osmole of solute per liter of solution. - Because our body fluids are formed of solutes dissolved in water, osmolarity and osmolality are measured in osmoles/liter. The normal osmolality of the extracellular and intracellular fluids is about 290 milliosmoles per liter **Relationship of Osmolality to Osmotic Pressure** - At normal body temperature, 37 °C, a concentration of 1 milliosmole/per liter will cause 19.3 mm Hg osmotic pressure in the solution. - Therefore, the total osmotic pressure of body fluid = 290X19.3. It is nearly 5500 mmHg (, of which only 25 mmHg is effective osmotic pressure produced by plasma proteins and called colloidal osmotic pressure or oncotic pressure of plasma proteins. **Calculation of plasma osmolarity** Osmolarity (mOsm/L)= 2 [Na+] (mEq/L)+ 0.055[Glucose](mg/dL)+ 0.36 [BUN](mg/dL) - BUN is the blood urea nitrogen. - From this equation increased blood glucose (hyperglycemia) e.g., in diabetic patient increases plasma osmolarity. - Also, increased BUN in renal failure, increases plasma osmolarity. - The total plasma osmolality is important in assessing dehydration, overhydration, and other fluid and electrolyte abnormalities ** Tonicity** - The term tonicity is used to describe the osmolality of a solution relative to plasma. Solutions are classified according to tonicity into: - Isotonic solutions have the same osmolality as plasma - Hypertonic solutions have greater osmolality than plasma. - Hypotonic solutions have lower osmolality than plasma. #### Normal O.P. and Osmolality - Normally the O.P. and osmolality of ECF are equal to those of ICF, so that the cell volume is kept constant. - ↑ O.P. or osmolarity of ECF→ movement of water from ICF to ECF Decreased cell volume (shrinkage of cells). - ↑ O.P. or osmolarity of ICF→ movement of water from ECF to ICF Increased cell volume (Swelling of cells → rupture). - Saline (0.9 % NaCl solution) is used as a vehicle for delivering drugs intravenously or for expanding plasma volume without affecting the cells. (Sometimes hypotonic or hypertonic fluids are injected therapeutically to correct osmotic imbalances.) ### Gibbs-Donnan Effect - Presence of non-diffusible ion on one side of a membrane affects the distribution of other diffusible ions in a predictable way. - The Gibbs Donnan effect on the distribution of ions has three effects on our body: 1. At equilibrium, ICF contains higher number of osmotically active molecules compared to ECF. However, osmosis from ECF to ICF can not occur normally, because of presence of Na+-K+ pump which keeps low levels of Na+ inside the cell to maintain the cell volume. 2. Normally, there is electric differences between ICF and ECF with more negativity inside the cell. 3. Presence of non-diffusible plasma proteins inside blood, allows flow of diffusible water from ISF to blood inside the capillaries (reabsorption) which is essential to prevent loss of water of plasma in ISF. ### Filtration **Definition:** It the movement of water and solutes through a porous membrane under the force of hydrostatic pressure ( from area of high pressure to area of low pressure) - Filtration and osmosis are the main processes by which exchange is accomplished across capillary walls. - The hydrostatic pressure in the capillaries tends to force fluid and its dissolved substances through the capillary pores into the interstitial spaces. - Conversely, osmotic pressure caused by the plasma proteins (called colloid osmotic pressure) tends to cause fluid movement by osmosis from the interstitial spaces into the blood. This osmotic pressure exerted by the plasma proteins normally prevents significant loss of fluid volume from the blood into the interstitial spaces. #### Fluid Exchange Across Capillary Membrane **Starling Forces:** - **Filtering Forces:** - 1. The capillary hydrostatic pressure (Pc) - 2. The interstitial fluid colloid osmotic pressure (Πif) - **Reabsorbing Forces:** - 3. The capillary plasma colloid osmotic pressure (Пр) - 4. The interstitial fluid hydrostatic pressure (Pif) **Net filtration pressure (NFP)= (Pc+ Πif)- (Пр+Pif)** - Filtering forces > reabsorbing forces Filtration - Filtering forces < reabsorbing forces Reabsorption - **Hydrostatic and Colloid Osmotic Forces Determine Fluid Movement Through the Capillary Membrane.** - 1. The capillary hydrostatic pressure (Pc), which tends to force fluid outward through the capillary membrane - 2. The interstitial fluid hydrostatic pressure (Pif), which tends to force fluid inward through the capillary membrane when Pif is positive but outward when Pif is negative - 3. The capillary plasma colloid osmotic pressure (Пр), which tends to cause osmosis of fluid inward through the capillary membrane - 4. The interstitial fluid colloid osmotic pressure (Πif), which tends to cause osmosis of fluid outward through the capillary membrane - If the sum of these forces—the net filtration pressure- is positive, there will be a net fluid filtration across the capillaries. - If the sum of the Starling forces is negative, there will be a net fluid absorption from the interstitial spaces into the capillaries. The net filtration pressure (NFP) is calculated as follows: Net filtration pressure= (Pc+ Πif)- (Пр+Pif). **Factors affecting filtration:** 1. Pressure gradient across membrane. 2. Surface area of the membrane. 3. Permeability of the membrane. **Filtration rate = NFPX Filtration coefficient.** - Filtration coefficient is determined by surface area and permeability of the membrane **Ultrafiltration:** - It is filtration through a semipermeable membrane that separates colloid solutions from crystalloids e.g., filtration at glomeruli of kidneys. **Bulk flow (Solvent drag)** - Solvent drag is the process whereby bulk movement of solvent drags some molecules of solute with it. ### Movement of Water Across Membranes **Water move inside the body by:** 1. Osmosis is the net diffusion of water down its own concentration gradient. - a) Through the intermolecular spaces of phospholipid bilayer: - Water molecules can permeate the plasma membrane. Although, water molecules are strongly polar, they are small enough to move through spaces between the phospholipid molecules. However, this type of water movement across the membrane is slow. - b) Through water channels called aquaporins, which are channels specific for the passage of water. 2. Filtration ### Student Activity **True and False** - Phagocytosis is a passive process. **F** - Placing RBCs in Hypotonic solution causes cell swelling and rupture. **T** - Normal plasma osmolarity is 290 osmol/L. **T** - Water move through cell membrane by osmosis. **T** - Normally the osmolality of ICF is greater than ECF. **F** - Filtration is directly related to membrane permeability. **T** ### References - **Barrett KE, Barman SM, Brooks HL, and Yuan JX. (2019). Ganong's Review of Medical Physiology. 26th ed. ebook by McGraw-Hill Education.** - **Hall JE, and Hall ME. (2021). Guyton and Hall Textbook of Medical Physiology. 14th ed. eBook by Elsevier, Inc.** - **Sherwood L, (2016). Human Physiology From Cells to Systems. 9th ed. eBook by Nelson Education, Ltd.** ### Thank You For Listening!